R2019-099 2019-04-08RESOLUTION NO. R2019-99
A Resolution of the City Council of the City of Pearland, Texas, accepting the
Master Drainage Plan Report prepared by Halff and Associates in cooperation
with Brazoria Drainage District No. 4.
BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF PEARLAND, TEXAS:
Section 1. That certain Master Drainage Plan Report, prepared by Halff and Associates,
a copy of which is attached hereto as Exhibit "A" and made a part hereof for all purposes, is hereby
accepted.
PASSED, APPROVED and ADOPTED this the 8th day of April, A.D., 2019.
TOM REID
MAYOR
ATTEST:
.� QEP, LqN
221 2 �--
YNG T MC
Y SE&RETARY
APPROVED AS TO FORM:
10. . 0
DARRIN M. COKER
CITY ATTORNEY
own
CITY OF PCARLAND
BRAZORIA DRAINAGE DISTRICT NO. 4
MASTER DRAINAGE
PLAN UPDATE
FINAL REPORT
HALFF ASSOCIAl LS, INC.
FEBRUARY 2019
TERRY M. BARR
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HALFF
Pearland / Brazoria Drainage District No 4 Master Drainage Plan ■■• HALFF ,
Final Report—February 2019 MEN
Executive Summary
Over the last several decades, the City of Pearland (City) and Brazoria Drainage District No. 4 (BDD4) have
experienced significant growth. This growth and the accompanying rise in development have resulted in
an increased risk of flooding, which can present hazards to the public and property. To more effectively
identify flood risks, plan drainage improvements, and consider regulatory measures aimed at minimizing
negative development impacts, the City and BDD4 initiated a two-phase effort with Halff Associates, Inc.
(Halff) to update and combine the master drainage plans for Pearland and BDD4 into one document, along
with updated hydrologic and hydraulic modeling for all major streams within the study area and provide
a capacity analysis for more than 90 miles of local drainage ditches.
ES.1 Master Plan Background
The previous Master Drainage Plans were developed in the late 1990's. Within the nearly 20 years since
the current plans were completed, the City and BDD4 have successfully implemented numerous projects,
including channel improvements, channel diversions, regional detention facilities, and storm sewer
improvements. The intent of this master plan update is to leverage the success of these efforts and to
modernize the plan based on physical changes to the area, changes in City and BBD4 goals, changes in
modeling technologies, and changes in the CIP funding of both the City and BDD4 since the last plan.
The master planning update effort considered these past drainage improvements but focused on how to
manage drainage issues along the bayous and major ditches as the area continues to develop. Updating
the current hydrologic and hydraulic modeling will facilitate future development growth by better
informing citizens of their potential flood risk as well as identifying and prioritizing drainage improvements
needed to reduce flooding risk. The evaluation of local drainage infrastructure (roadside ditches or storm
sewers) was not included in this master plan, where the emphasis was on channel capacity and detention
projects needed to meet the desired level of service (i.e. 100 -year or other) for the major streams.
Pearland and BDD4 has long history of flooding, with damages dating back to the 1970s. The region has
experienced several significant flooding events, with the most recent event being Hurricane Harvey in
2017. According the City of Pearland's Hurricane Harvey Drainage Assessment Report, the rainfall ranged
from 30 to 49 inches for the City and surrounding area over 4 days, which resulted in widespread damage.
Approximately 1,080 properties experienced minor damage while 641 properties had major damage.
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report— February 2019
C: HALFF
ES.2 Study Area Description
The City and BDD4 Master Plan area lies in the northern part of Brazoria County and consists of the shared
boundary of the City and BDD4, which encompasses a combined area of nearly 97 square miles. The area
is generally bound by Clear Creek to the north and FM 521 to the west. The Galveston County line serves
as the primary eastern boundary. A majority of BDD4 is located north of SH6, except for a small area near
the Fort Bend County line. Major streams to be included in the study will be Clear Creek, Hickory Slough,
Mary's Creek, Cowart Creek, Chigger Creek, West Fork of Chocolate Bayou, and Mustang Bayou. The
western half of the study area is generally flat while the eastern half has more elevation change as the
ground slopes down to Clear Creek. The dominant flow direction across the study area is west to east,
with the exception of Chocolate Bayou, which generally flows from north to south. The majority of the
City of Pearland is developed with a majority of residential, commercial development along major
roadways, pockets of industrial use, and some institutional and parks/open space. There is significantly
less development in the Cowart and Chigger Creek watersheds, with large rural tract predominant in the
area. Mustang is similar to Cowart; however, there are some residential areas surrounding SH288.
Chocolate Bayou has a mix of newly developed residential areas, mostly north of SH6, and undeveloped
open space to the south.
Pearland / Brazoria Drainage District No 4 Master Drainage Plan •■• HALFF'
Final Report — February 2019 OEM
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Figure ES1. Master Drainage Plan Area
ES.3 Data Collection
Information relevant to the watershed and beneficial to this study was collected including previous
drainage reports, the FEMA hydrologic and hydraulic models (effective modeling), historical rainfall and
gage information, and field data. Updated FEMA mapping, developed through Risk Map 6, was included
as well as FEMA flood claim and Hurricane Harvey damage data. A significant field investigation effort
was conducted to document the existing conditions drainage in the watersheds. The field reconnaissance
and collected data were reviewed and used to create a comprehensive database of available drainage
information from various sources. The model and field data inventory were utilized during the modeling
update process and helped facilitate the City and BDD4 managing their data more effectively.
ESA Existing Conditions Model Updates
A major component of this study was updating the existing conditions modeling. Halff, in conjunction
with the City and BDD4, updated the hydrologic models to reflect changes in the development and major
channel and detention projects. Updates to the hydrology were performed in accordance with HCFCD
methodology to maintain consistency with previous HCFCD studies for Clear Creek. Differences in peak
flow rates between the revised existing conditions (Halff) and the current FEMA models can be attributed
to significant differences in development levels and storage routing.
The FEMA effective hydraulic models for the major streams were converted to the most current version
of HEC -RAS and then revised to account for changes in terrain (based on 2008 LiDAR) and new hydraulic
structure data. New hydraulic models were developed for the areas where no previous models were
available, which included Mustang Bayou, East Fork Chocolate Bayou, and West Fork Chocolate Bayou.
Hickory Slough and Mary's Creek were modeled using unsteady HEC -RAS to more effectively capture the
impact of multiple regional detention ponds.
The existing modeling results showed that most of the major streams within the study area have less than
a 3 -year capacity. Mary's Creek provides approximately 5 -year capacity and portions of Cowart have
upwards of 25- to 5 -year capacity. There a few ditches or major bayou segments higher capacity, but the
majority of the existing channels provide limited conveyance. The flat terrain and low existing capacity of
the major streams result in wide floodplains, resulting in large areas experiencing flooding from relatively
frequent storms. In additional, the existing conveyance capacity of local drainage ditches located
throughout the study area was evaluated, and the capacity estimations indicated that many of these local
ditches do not have the capacity necessary to adequately convey runoff to receiving channels.
Pearland / Brazoria Drainage District No 4 Master Drainage Plan NowHALFF
Final Report—February 2019 mom ■■■
ES.S Flood Reduction Analysis
Two flood reduction alternatives were evaluated for the entire study area based on a desired carrying
capacity or level of service (LOS). Alternative 1 was developed to provide the 100 -year LOS for each of
the major watersheds. Alternative 2 was developed to provide a lower cost improvement, but still show
significant flood reduction benefits. The Alternative 2 LOS varied by channel and depended on the existing
channel capacity; the goal being to provide an increase of 1-2 levels of service. For example, the goal for
a channel with less than 3 -year capacity would be to get 5- to 10 -year capacity.
The focus of the flood reduction alternative analysis was on structural improvements throughout the
study watersheds, specifically channel conveyance improvements and regional detention. Channel
improvements consisted of widening the existing channel and providing a uniform, trapezoidal shape.
Detention was provided for both reduction of peak discharges as well as for mitigation of flow increases
associated with channel conveyance modifications. Bridges and culverts were upsized where necessary
to reflect the widened top width of the channels due to proposed channel conveyance improvements and
to reduce hydraulic restrictions, which contribute to flooding along the major streams.
While the intent of the flood reduction measures is to address existing flooding concerns within the
Pearland BDD4 area, the planning effort considered the in the future development conditions (i.e.
ultimate build -out) to ensure that the improvements provide the necessary protection for the long-term.
The future conditions hydrology accounted for increases to impervious cover associated with expected
development, which would result in increased runoff volumes. The future conditions flows were used to
size the proposed channel conveyance improvements and detention ponds for the two different flood
reduction alternatives.
ES.6 Flood Reduction Project Recommendations
The study resulted in the recommendation of major projects in 29 separate stream segments, which will
address flood reduction needs across the entire project area. Each of these projects is comprised of
multiple components including detention and channel conveyance improvements. Given the diverse
development conditions across the watershed, a variety of metrics were used to prioritize projects.
Traditional benefit -cost analyses (BCA) were considered for project prioritization using information from
a Flood Damage Assessment; however, the limited number of structures in watersheds like Cowart,
Chigger, and Mustang, would result in those projects being pushed way down the priority list. A
"Prioritization Based on Need" assessment was developed, which scored projects based on the number
of structures at risk for the 10- and 100 -year rainfall events, the number of flood insurance claims, and
Pearland / Brazoria Drainage District No 4 Master Drainage Plan ■■■ HALFF ,
Final Report — February 2019 :::
the channel level of service. In addition, an evaluation was performed of the inundation removed from
structures, land acreage, and roadway miles. These metrics provided a bit more balanced comparison of
the projects in the different watersheds, but those areas with higher levels of development are still heavily
favored. Finally, the development potential and projected future buildout timeframes were considered,
this measure gave some weight to those watersheds that are still largely undeveloped but may experience
growth in the future. The various metrics were weighted to provide a single score for each project, which
was used to set the priority. It should be noted that many of the more rural projects would fall under the
jurisdiction of BDD4 alone, and as such should be considered separately from projects in the City of
Pearland. Recommended projects are listed in the table below.
ES.7 Implementation Planning
An implementation plan was developed to help outline a path forward for the recommended projects
provided in the study. The plan includes the development of a project prioritization methodology and
identification of the projects to be completed both for major creeks and bayous as well as local ditches.
Specific projects are listed in the tables below. Projects were divided into 4 categories:
• Large CIP Projects: Top 10 scoring main channel segment projects, will need to be phased
• Reserve CIP Projects: Main channel projects that rank from 11-15 and may provide benefits
• Small CIP: Local ditch projects from the Top 50 that cost more than $500k
• Small O&M: Local ditch projects from the Top 50 that cost less than $500k
The primary challenges to implementation of flood reduction measures include project costs, ROW
acquisition, environmental constraints, and utility conflicts. ROW availability was a significant issue in
developing proposed alternatives throughout the watershed, particularly within the City of Pearland
where high levels of urbanization limit the amount of undeveloped land for drainage improvements.
Another major concern is potential utility conflicts with large oil and gas pipelines that are located
throughout the watershed. Environmentally sensitive areas or areas with identified cultural resources
need to also be considered and avoided when possible.
The work completed as part of this master drainage plan study represent a major effort in streamlining
and modernizing the drainage analysis for the City and BDD4. The updated hydrologic and hydraulic
modeling will help facilitate more efficient updates to the master plan in the future as the area continues
to develop in order to more accurately identify and manage flooding risks. The development of flood
reduction alternatives and supporting analysis and cost estimates will benefit the City and BDD4 as they
plan out their short-term and long-term Capital Improvement Planning.
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report - February 2019
Table ES.1 - Large Capital Improvement Project (CIP) Recommendations
;;; HALFF
PROJECT
WATERSHED
PROJECT COSTS (M)
PROJECT DESCRIPTION
CHANNEL
DETENTION
ROW
TOTAL
PRIORITY
SEGMENT
Hickory Slough
100 -year LOS; Channel modifications from Cullen Blvd. to Garden
$6.7
$19.2
$17.3
$43.2
1
Middle Segment
Rd. and 1010 ac -ft mitigation. Max ROW width of 170 ft.
2t
Cowart Creek
10 -year LOS; Channel modifications from Wells Dr. to BNSF
$2.1
-
$5.2
$7.3
Segment 16
Railroad. Max ROW width of 200 ft.
West Fork Chocolate
100 -year LOS; Channel modifications from Rio Lindo St. to Hwy 6
$6.4
$10.4
$4.2
$21.0
3
Cold River Ranch Ditch
and 580 ac -ft mitigation. Max ROW width of 180 ft.
100 -year LOS; Channel modifications from Pearland Site Rd. to
4
Cannon Ditch
Amoco Industrial St. and 9800 ac -ft mitigation. Max ROW width
$4.8
$37.5
$4.2
$46.5
Segment 2
of 120 ft.
Mary's Creek
25 -year LOS; Channel modifications from B129-01-00 to McLean
$10.5
$4.5
$7.9
$22.9
5
Upper Segment
Rd. and 240 ac -ft mitigation. Max ROW width of 250 ft.
6 t
Mary's Creek
25 -year LOS; Channel modifications from Magnolia Dr. to SH35
$10.7
$17.6
$3.1
$31.4
Middle Segment
and 1000 ac -ft mitigation. Max ROW width of 250 ft.
Mustang Bayou
25 -year LOS; Channel modifications from CR521 to Airline Rd and
$10.7
$44.4
$46.9
$102.0
7
Upper Segment
890 ac -ft mitigation. Max ROW width of 240 ft.
25 -year LOS; Channel modifications from SH35 to downstream of
8
Mary's Creek
Pearland Pkwy. and 1670 ac -ft mitigation. Max ROW width of
$14.8
$55.2
$51.8
$121.8
Lower Segment
220 ft.
9
Mustang Bayou
25 -year LOS; Channel modifications from Airline Rd. to SH288
$5.8
$31.9
$22.8
$60.5
Middle Segment
and 1070 ac -ft mitigation. Max ROW width of 260 ft.
10
Hickory Slough
10 -year LOS; Channel modifications from Garden Rd. to SH35 and
$12.4
$24.7
$15.2
$52.3
Lower Segment
1310 ac -ft mitigation. Max ROW width of 170 ft.
t Detention is included in downstream segment; however, mitigation will be required for conveyance improvements and should be evaluated in the PER Phase
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report — February 2019
Table ES.2 - Reserve Capital Improvement Project (CIP) Recommendations
HALFF'
PROJECT
WATERSHED
PROJECT COSTS
PROJECT DESCRIPTION
CHANNEL
DETENTION
ROW
TOTAL
PRIORITY
SEGMENT
5 -year LOS; Channel modifications from E101-02-00 to
West Chocolate Bayou
11
confluence with West Fork Chocolate Bayou and 1230 ac -ft
$8.90
$27.60
$215.70
$252.20
CR 383 Ditch
mitigation. Max ROW width of 190 ft.
5 -year LOS; Channel modifications from county boundary to
West Fork Chocolate
12
confluence with E101-00-00 and 3310 ac -ft mitigation. Max
$16.10
$69.50
$17.90
$103.50
Bayou
ROW width of 260 ft.
100 -year LOS; Channel modifications from CR94 to confluence
Hickory Slough
13
with H126-00-00 and 2800 ac -ft mitigation. Max ROW width of
$2.40
$19.60
$34.70
$56.70
Upper Segment
170 ft.
10 -year LOS; Channel modifications from SH288 to confluence
East Chocolate Bayou
14 t
with Rodeo Palms Ditch and 2210 ac -ft mitigation. Max ROW
$1.70
-
$0.70
$2.40
E103-00 00
width of 220 ft.
5 -year LOS; Channel modifications from Hwy 6 to confluence
West Fork Chocolate
15 t
with West Fork Chocolate Bayou and 1230 ac -ft mitigation.
$8.70
-
$1.20
$9.90
Cold River Ranch Ditch
Max ROW width of 250 ft.
t Detention is included in downstream segment; however, mitigation will be required for conveyance improvements and should be evaluated in the PER Phase
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report - February 2019
Table ES.3 - Small Capital Improvement Project (CIP) Recommendations
HALFF
DITCH
PRIORITY
WATERSHED
DITCH
3 -YR LOS
10 -YR LOS
TOP WIDTH
COST
TOP WIDTH
COST
1
Cowart Creek
C123-00-00
56
$ 918,000
66
$ 1,161,000
2
Hickory Slough
H123-00-00
311
$ 3,017,000
451
$ 4,356,000
3
Chocolate Bayou
E100-01-01
72
$ 1,490,000
91
$ 1,946,000
4
Cowart Creek
C118-00-00
37
$ 961,000
41
$ 1,199,000
5
Cowart Creek
C122-00-00
71
$ 1,069,000
83
$ 1,332,000
6
West Chocolate
E101-01-06
66
$ 806,000
80
$ 1,029,000
7
Chigger Creek
J101-02-00
146
$ 2,401,000
171
$ 2,920,000
8
Cowart Creek
C128-00-00
34
$ 671,000
40
$ 864,000
9
Chigger Creek
J102-05-01
50
$ 1,492,000
60.
$ 1,910,000
10
Cowart Creek
C120-01-00
26
$ 632,000
26
$ 776,000
11
Cowart Creek
C124-01-00
42
$ 551,000
49
$ 701,000
12
Clear Creek
A105-05-00
83
$ 847,000
101
$ 1,074,000
13
Hickory Slough
H125-02-00
158
$ 718,000
158
$ 772,000
14
Cowart Creek
C107-03-01
35
$ 784,000
39
$ 984,000
15
Mary's Creek
8117-00-00
33
$ 1,545,000
37
$ 1,929,000
16
Hickory Slough
H114-00-00
34
$ 1,124,000
38
$ 1,421,000
17
West Chocolate
E101-01-01
52
$ 648,000
60
$ 810,000
18
Clear Creek
A113-00-00
34
$ 665,000
34
$ 799,000
19
Mary's Creek
B102-01-01
56
$ 499,000
66
$ 631,000
20
Chocolate Bayou
E102-00-00
50
$ 1,009,000
67
$ 1,373,000
21
Clear Creek
A115-00-00
39
$ 1,132,000
44
$ 1,420,000
22
Cowart Creek
C124-00-00
34
$ 669,000
34
$ 805,000
23
Cowart Creek
C119-00-00
28
$ 698,000
30
$ 875,000
24
Hickory Slough
H123-01-00
311
$ 3,017,000
451
$ 4,356,000
25
Clear Creek
A116-00-00
25
$ 870,000
33
$ 1,181,000
26
Cowart Creek
C120-00-00
91
$ 1,216,000
110
$ 1,534,000
27
Cowart Creek
C100-00-00
28
$ 532,000
31
$ 676,000
28
Clear Creek
Alll-00-00
31
$ 989,000
34
$ 1,241,000
29
Chigger Creek
J101-02-01
94
$ 1,094,000
114
$ 1,382,000
30
Mary's Creek
B114-01-01
37
$ 660,000
43
$ 843,000
31
Cowart Creek
CR 414 Ditch
21
$ 775,000
23
$ 993,000
32
Cowart Creek
C101-00-00
60
$ 2,659,000
79
$ 3,580,000
33
Chigger Creek
J102-00-00
116
$ 3,312,000
160
$ 4,525,000
34
Hickory Slough
H125-01-00
50
$ 1,141,000
50
$ 1,330,000
35
Hickory Slough
C103-03-00
39
$ 535,000
42
$ 657,000
36
Cowart Creek
Cowart's Creek Ditch
35
$ 784,000
39
$ 984,000
37
Chigger Creek
J102-00-00
91
$ 1,606,000
118
$ 2,121,000
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report —February 2019
Table ESA - Small O&M Project Recommendations
MEE HALFF
DITCH
PRIORITY
WATERSHED
DITCH
3 -YR
LOS
10 -YR LOS
TOP WIDTH
COST
TOP WIDTH
COST
1
Chigger Creek
J101-01-01
61
$
190,000
71
$
238,000
2
Chigger Creek
.1101-01-00
22
$
233,000
24
$
298,000
3
Clear Creek
A122-00-00
34
$
479,000
35
$
583,000
4
Chigger Creek
J101-01-01
23
$
239,000
26
$
309,000
5
Cowart Creek
C105-01-00
34
$
77,000
41
$
100,000
6
Cowart Creek
B102-01-03
33
$
217,000
38
$
278,000
7
Cowart Creek
C107-10-01
62
$
429,000
71
$
546,000
8
Clear Creek
A121-01-00
42
$
397,000
50
$
511,000
9
Cowart Creek
C125-00-00
22
$
193,000
23
$
241,000
10
Cowart Creek
C107-01-02
24
$
244,000
28
$
319,000
11
Cowart Creek
C107-04-01
31
$
361,000
34
$
453,000
12
Hickory Slough
H111-00-00
30
$
85,000
30
$
103,000
13
Chocolate Bayou
E100-01-01
56
$
430,000
72
$
572,000
Pearland / Brazoria Drainage District No 4 Master Drainage Plan SON HALFF,
Final Report — February 2019 mom ■■■
Table of Contents
1.0 Introduction......................................................................................................................................1
1.1 Project Goals.................................................................................................................................2
2.0 Study Area Overview......................................................................................................................... 3
2.1 General Description of Study Area............................................................................................... 3
2.2 Historical Flooding........................................................................................................................3
2.3 Drainage and Flooding..................................................................................................................4
2.3.1 Regulatory Floodplain...........................................................................................................4
2.3.2 Local Drainage.......................................................................................................................4
2.3.3 Flood Claims..........................................................................................................................4
3.0 Data Collection and Inventory..........................................................................................................5
3.1 Plan Report and Inventory ............................................................................................................5
3.1.1 Inventory Development Process..................................................................................................5
3.2 Model Inventory ...........................................................................................................................6
3.2.1 Model Inventory Development....................................................................................................7
3.3 Topographic Survey......................................................................................................................8
4.0 Data and Model Review....................................................................................................................9
4.1 Previous Master Plans...................................................................................................................9
4.1.1 Flood Protection Plan for Brazoria Drainage District No. 4 ..................................................9
4.1.2 Master Drainage Plan for the City of Pearland.....................................................................9
4.2 Report and Plan Review..............................................................................................................10
4.3 Model Review.............................................................................................................................10
4.3.1 Existing Models and Previous Studies.................................................................................10
4.4 Drainage Criteria Review............................................................................................................11
4.5 Current CIP and Funding Review................................................................................................12
5.0 Field Verification.............................................................................................................................14
5.1 Field Reconnaissance Process.....................................................................................................14
5.2 Field Data Collected....................................................................................................................15
5.3 Findings.......................................................................................................................................16
6.0 Existing Conditions Modeling..........................................................................................................17
6.1 Hydrologic Model Updates.........................................................................................................18
6.1.1 Watershed and Subbasin Delineation.................................................................................18
6.1.2 Watershed Parameter Updates..........................................................................................18
Pearland / Brazoria Drainage District No 4 Master Drainage Plan ■E■ HALF F ,
Final Report —February 2019 mom
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6.1.3 Modified -Puts Storage Routing...........................................................................................19
6.1.4 Existing Conditions HEC -HMS Model Updates and Results................................................19
6.2 Hydraulic Model Updates...........................................................................................................
22
6.2.1 Hydraulic Modeling Update Process...................................................................................22
6.2.2 Updated Hydraulic Model Sources.....................................................................................23
6.2.3 New Hydraulic Models........................................................................................................23
6.2.4 Existing Regional Detention Basins.....................................................................................25
6.3 Existing Conditions Hydraulic Model Results..............................................................................25
6.3.1 Chigger Creek......................................................................................................................26
6.3.2 Chocolate Bayou.................................................................................................................27
6.3.3 Country Place Ditch.............................................................................................................28
6.3.4 Cowart Creek.......................................................................................................................28
6.3.5 Hickory Slough.....................................................................................................................28
6.3.6 Mary's Creek.......................................................................................................................
29
6.3.7 Mary's Creek Tributaries.....................................................................................................30
6.3.8 Mustang Bayou...................................................................................................................31
6.3.9 Shadow Creek Ranch Ditch.................................................................................................31
6.3.10 Town Ditch..........................................................................................................................
31
6.4 Local Ditch Capacity Analysis......................................................................................................
32
7.0 Existing Flooding Issues..................................................................................................................33
7.1 Country Place Ditch.....................................................................................................................33
7.2 Hickory Slough............................................................................................................................34
7.3 Mary's Creek...............................................................................................................................34
7.4 Shadow Creek Ranch Ditch.........................................................................................................35
7.5 Town Ditch..................................................................................................................................
35
7.6 Cowart Creek...............................................................................................................................35
7.7 Chigger Creek..............................................................................................................................36
7.8 Mustang Bayou...........................................................................................................................36
7.9 Chocolate Bayou.........................................................................................................................37
8.0 Flood Reduction Alternative Analysis.............................................................................................38
8.1 Potential Improvement Options.................................................................................................38
8.1.1 Project Challenges...............................................................................................................39
8.2 Future Conditions Hydrology......................................................................................................40
8.2.1 Impervious Cover................................................................................................................41
mom
Pearland / Brazoria Drainage District No 4 Master Drainage Plan mom H ALF F '
Final Report — February 2019 ■■■
8.2.2
Future Conditions Hydrologic Results.................................................................................41
8.3
Potential Flood Reduction Measures..........................................................................................41
8.3.1
Trapezoidal Channel Conveyance Improvements..............................................................42
8.3.2
Regional Detention.............................................................................................................42
8.4 Alternatives for Flood Reduction Measures...............................................................................43
8.4.1
Chigger Creek......................................................................................................................44
8.4.2
East Chocolate Bayou..........................................................................................................46
8.4.3
West Chocolate Bayou........................................................................................................47
8.4.4
Country Place Ditch.............................................................................................................49
8.4.5
Cowart Creek.......................................................................................................................49
8.4.6
Hickory Slough.....................................................................................................................51
8.4.7
Mary's Creek.......................................................................................................................52
8.4.8
Mustang Bayou...................................................................................................................53
8.4.9
Shadow Creek Ranch...........................................................................................................54
8.5
Local Ditch Capacity Improvements...........................................................................................55
9.0 Costs
and Benefits...........................................................................................................................56
9.1
Project Cost Components...........................................................................................................
56
9.1.1
Construction Costs..........................................................................................:...................56
9.1.2
ROW Acquisition Costs........................................................................................................57
9.1.3
Utility Relocation Costs.......................................................................................................58
9.2 Total Flood Reduction Alternative Costs.....................................................................................58
9.2.1
Chigger Creek...........................................................................................................................
58
9.2.2
East Chocolate Bayou...............................................................................................................59
9.2.3
West Chocolate Bayou.............................................................................................................
59
9.2.4
Cowart Creek............................................................................................................................60
9.2.5
Hickory Slough..........................................................................................................................60
9.2.6
Mary's Creek............................................................................................................................60
9.2.7
Mustang Bayou........................................................................................................................61
9.2.8
Capacity Analysis Ditches.........................................................................................................61
9.3
Benefit -Cost Analysis..................................................................................................................61
9.3.1
Flood Damage Assessment.................................................................................................62
9.3.2
Benefit Cost Ratio (BCR)......................................................................................................65
9.3.3
Local Ditch BCR...................................................................................................................66
9.4
Non -Monetary Project Benefits..................................................................................................66
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9.4.1 Reduction of Inundated Structures.....................................................................................67
9.4.2 Reduction of Inundated Acreage........................................................................................
67
9.4.3 Reduction of Inundated Roadway Miles.............................................................................68
9.4.4 Watershed Development Potential....................................................................................69
9.4.5 Local Ditch Non -Monetary Benefits....................................................................................69
10.0 Flood Risk Reduction Recommendations.......................................................................................
70
10.1 Project Prioritization................................................_..................................................................70
10.1.1 Priority Based on Need Assessment...................................................................................
70
10.1.2 Flood Reduction Benefits....................................................................................................71
10.1.3 Development Potential.......................................................................................................72
10.1.4 Prioritization Scoring...........................................................................................................72
10.2 Tier 1 Project Rankings................................................................................................................75
10.2.1 Hickory Slough (Cullen Boulevard to Garden Road)...........................................................75
10.2.2 Cowart Creek (Wells Drive to BNSF Railroad).....................................................................76
10.2.3 West Chocolate Bayou — Cold River Ranch Ditch (Upstream of Hwy 6) .............................77
10.2.4 Cannon Ditch (Pearland Site Road to Amoco Industrial Street).........................................78
10.2.5 Mary's Creek (Confluence with B129-01-00 to McLean Road)...........................................79
10.2.6 Mary's Creek (Magnolia Drive to SH 35).............................................................................80
10.3 Tier 2 Project Rankings................................................................................................................81
10.3.1 Mustang Bayou (CR 521 to Airline Road)............................................................................81
10.3.3 Mary's Creek (Downstream of SH 35).................................................................................82
10.3.3 Mustang Bayou (Airline Road to SH 288)............................................................................83
10.3.4 Hickory Slough (Garden Road to SH 35)..............................................................................84
10.4 Tier 3 Project Rankings................................................................................................................85
10.4.1 West Chocolate Bayou — CR 383 Ditch................................................................................85
10.4.2 West Fork Chocolate Bayou................................................................................................86
10.4.3 Hickory Slough (CR 94 to Cullen Boulevard).......................................................................87
10.4.4 East Chocolate Bayou — E103-00-00...................................................................................
88
10.4.5 West Chocolate Bayou — Cold River Ranch Ditch (Downstream of Hwy 6) ........................89
10.5 Tier 4 Project Rakings..................................................................................................................90
10.6 Ditch Capacity Analysis Rankings................................................................................................90
11.0 Implementation Plan......................................................................................................................92
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1.0 Introduction
Over the last several decades, the City of Pearland (City) and Brazoria Drainage District No. 4 (BDD4) have
experienced significant growth and development with the last decade alone seeing a two -fold increase in
the area's population. With development comes an increased risk of flooding from streams as well as
local sources, which can present hazards to the public and impede growth. To more effectively identify
flood risks, plan drainage improvements, and consider regulatory measures aimed at minimizing negative
development impacts, the City and BDD4 initiated a two-phase effort with Halff Associates, Inc. (Halff) to
update and combine the master drainage plans for Pearland and BDD4 into one document, along with
updated hydrologic and hydraulic modeling for all major streams within the study area
The previous Master Drainage Plans, which are currently being used by the City and BDD4 were developed
in the late 1990's. The level of growth in the area has changed along with many of the assumptions and
modeling parameters that were developed at that time. The plans currently utilized include:
• Flood Protection Plan for Brazoria Drainage District No. 4, Rust Lich liter/Jameson, November 1997
• Master Drainage Plan for the City of Pearland, Rust Lich liter/Jameson, February 1998
Several other planning efforts, including detention master planning and sub -regional plans for drainage
improvements have been completed. Within the nearly 20 years since the current plans were completed,
the City and BDD4 have successfully implemented numerous projects, including channel improvements,
channel diversions, regional detention facilities, and storm sewer improvements. The intent of this master
plan update is to leverage the success of these efforts and to modernize the plan based on physical
changes to the area, changes in goals on the part of Pearland and BDD4, new technologies, and changes
in the CIP funding of both the City and BDD4 since the last plan.
The master planning effort will consider these past successes and the growth that the City and BDD4 have
experienced but the focus will now be shifted to how to manage drainage issues along the bayous and
ditches as the area continues to develop. The area that encompasses the City and BDD4 is continuing to
develop at a healthy rate, with new businesses and residents moving into the area every day. This effort
will facilitate that growth by better informing current and future citizens of their potential flood risk, and
identifying the current bayous, ditches, and detention needs to accommodate this future growth.
Both the City and BDD4 maintain MS4 permits, follow drainage criteria for development aimed at
mitigating negative impacts, identify and address areas of concern, and construct Capital Improvement
Projects. Jurisdictions within BDD4 including Pearland, participate in the National Flood Insurance
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Program (NFIP); however, since BDD4 is considered a district and not a community, BDD4 is not eligible
to directly participate in the NFIP's Community Rating System (CRS) like the jurisdictions within BDD4. The
City of Pearland participates in the CRS and has implemented standards that exceed the NFIP minimums,
resulting in a CRS rating of 6. The City's rating provides residents within the Special Flood Hazard Area
(SFHA) with a 20% discount on their flood insurance premiums and 10% for those not in the SFHA. This is
among the highest rated programs in Texas.
1.1 Project Goals
The study focused on identifying and addressing the flood hazards along the main streams within the
study area. As part of the study, Pearland, BDD4 and Halff established certain goals and objectives. The
goals include:
• Update or develop new hydrologic modeling for all watersheds contributing to the study area
using HEC -HMS and updated land use data.
• Update or develop new hydraulic modeling for major bayous within the study area using HEC -RAS
and updated terrain information
• Identify high-level comprehensive plan to provide 1% (100 -year) level of service as well as a
reduced level of service option along the modeled streams.
• Evaluate the capacity of local ditches that flow into the modeled streams and estimate the cost
to achieve 3 -year and 10 -year level of service in the ditches.
• Develop a priority list of projects to reduce flood risks within the watersheds. This includes major
channel improvement and detention projects as well as improvements to smaller ditches to
improve local drainage.
It is important to note that the focus of this Master Drainage Plan is on channel capacity and detention
projects needed to meet the desired level of service (i.e. 100 -year or other) for the major streams and
larger local ditches. It does not evaluate local drainage infrastructure (roadside ditches or storm sewers)
for residential or other developed areas. However, local systems drain to the receiving streams being
evaluated and, as such, it is likely that these improvements could result in a reduction in flood levels for
these areas. Ultimately, the projects recommended are intended to contain stormwater runoff and
protect people and property to the maximum extent practicable. It is likely that there will still be
structural and street flooding for low lying areas, even after projects are implemented. These areas
should consider elevation or other methods for reducing flood risk.
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2.0 Study Area Overview
2.1 General Description of Study Area
The City and BDD4 Master Plan area consists of the shared boundary of the City and BDD4, which
encompasses a combined area of nearly 97 sq. mi (Exhibit 1). The area is generally bound by Clear Creek
to the north and FM 521 to the west. The eastern and southern boundaries are less clearly demarcated.
A portion of the City extends into Harris County on the east side of Clear Creek. The Galveston County
line serves as the remainder of the eastern boundary. A majority of BDD4 is located north of SH6, except
for a small area near the Fort Bend County line, which includes portions of Iowa Colony and Manvel.
Major streams to be included in the study will be Clear Creek, Hickory Slough, Mary's Creek, Cowart Creek,
Chigger Creek, West Fork of Chocolate Bayou, and Mustang Bayou. A map of the master plan area with
the major watersheds are included in Exhibit 2. Numerous drainage ditches, diversions, and tributaries
feeding these streams will be included as well. The study area lies within fourteen (14) FEMA FIRM panels
in three counties; Brazoria, Harris, and Fort Bend. The panels include those in Brazoria (48039C-00101,
0020H, 00301, 00351, 00401, 00451, 0110H), in Harris (48201C -1010L, 1030L, 1035L, 1055L), and in Fort
Bend (48157 0305L, 0315L, 0455L). There have been multiple revisions and amendments for each panel
since the effective dates in 1989-1999. Revised FEMA maps were recently released with a preliminary
effective date of June 29, 2018, but as of this report, the maps have not been finalized. There are several
revisions to the maps throughout the study area. Exhibit 3 includes the FEMA FIRM panels for the master
plan area.
2.2 Historical Flooding
Pearland and BDD4 has long history of flooding, with damages dating back to the 1970s. The region has
experienced several significant flooding events including the following: Tropical Storm Claudette,
Hurricane Alicia, October 1994 storm, Tropical Storm Francis, Tropical Storm Allison, Hurricane Ike, and in
2017 Hurricane Harvey. Pearland data indicates that 272 flood claims were filed between 2005 and 2011.
The largest number of flood claim claims, before Hurricane Harvey, followed Hurricane Ike, which caused
183 additional claims. No loss data was available outside of Pearland.
In 2017 Hurricane Harvey produced widespread flooding in Harris County and the surrounding area.
According the City of Pearland's Hurricane Harvey Drainage Assessment Report, the rainfall ranged from
30 to 49 inches for the City and surrounding area over 4 days. This storm greatly surpassed the rainfall
for all other historical storms in the area. The rainfall for the area exceeds the 0.2% (500 -year) annual
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exceedance probability throughout much of the county. Peak channel water surface elevation
frequencies for Clear Creek and its tributaries range from 1% (100 -year) to greater than 0.2% (500 -year)
and record level floods were recorded throughout. Water levels for Hurricane Harvey surpassed the
previous record storm (Tropical Storm Claudette) by 2 to 3 feet along Clear Creek and its portions of its
major tributaries. However, based on field observations and stream gauge data, portions of the study
area may not have experienced a 100 -year storm. Based on reported flood damage for Pearland, 1080
residencies experienced minor damage and 641 major damage. While 22 business recorded minor losses
and 11 recorded major losses.
2.3 Drainage and Flooding
2.3.1 Regulatory Floodplain
Federal Emergency Management Agency (FEMA) effective floodplains were obtained through various
sources and are shown in Exhibit 3. FEMA FIRM panels that encompass the watershed are listed above.
Chigger Creek, Chocolate Bayou and Clear Creek are all previously studied but do not have detailed studies
within the boundaries of Pearland/BDD4. Mary's Creek, Hickory Slough, Mustang Bayou and Cowart Creek
all have regulatory floodplain within the study area. During the master planning process, preliminary
updated maps were provided. The effective date for the maps is listed as June 29, 2018.
2.3.2 Local Drainage
Local drainage throughout a majority of the watershed consists of roadside ditches and storm sewer.
These roadside ditches provide drainage for residential areas and large agricultural tracts and discharge
into larger streams. The roadside ditches and culverts are in various states of repair, but generally have
capacity that is below the design storm specified in the local criteria (3 -year). The capacity for local ditches
is shown in Exhibit 9. In many instances there is debris present in the culverts, limiting their capacity.
Several of the tributaries have ponding upstream of the channel headwaters due to limited access to the
channel. These are not intended to be evaluated in detail as part of this Master Drainage Plan.
2.3.3 Flood Claims
The study area has a history of flooding and associated flood damages. Flood loss data provided by
Pearland for this study included the flood claim data between 2005 and 2012 as well as data collected in
the aftermath of Hurricane Harvey. Approximate locations where flood claims have been filed are shown
in Appendix C.36.
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3.0 Data Collection and Inventory
The initial effort for Phase I of the study consisted of a significant amount of data collection. The City and
BDD4 provided all available reports, Geographical Information Systems (GIS) data, hydrologic and
hydraulic models, and other data. This data was a critical part of establishing the current benchmark for
information required for alternatives modeling conducted in Phase 11. Modeling data collected included
the current effective models for Mary's Creek, Hickory Slough, Cowart Creek, as well as other modeling
data developed as part of smaller master planning efforts and development projects. The current master
plans were collected as well as current CIP information. Each of these components will be discussed
below.
3.1 Plan Report and Inventory
The first of those mechanisms is the drainage report inventory, which consists of a GIS database of each
unique document provided by the City. The City of Pearland and Brazoria Drainage District No. 4 provided
a total of 128 documents. In most instances the complete report was provided including narrative,
exhibits, and appendices. In some cases, digital data including models were provided. Some of the data
provided was in digital format only. Some of the data provided was only small pieces of information or
exhibits from reports with no accompanying data. Additional documents may be added once they are
received or the City/BDD4 may provide regular updates once they take over management of the database.
3.1.1 Inventory Development Process
All hard copy reports were scanned to Adobe Acrobat (.pdf) format and catalogued. This information was
combined with other available information and organized by type of project or study. In order to simplify
the inventory, six (6) study/project categories were used, including:
• CIP Improvements — Any design report or PER associated with a design project including
roadways, channel improvements, detention improvements, storm sewer improvements, etc.
• Detention Analysis — Includes planning efforts (Sub -Regional or otherwise) for detention, which
may be aimed at specific locations or a larger area and include adjustments to existing ponds.
• Flood Mitigation Study — Aimed at flood mitigation efforts for large developments, typically
master -planned residential communities
• Letters of Map Change (LOMC) — Includes all letters of map change such as LOMR-F (based on fill
placement) and CLOMR-F (conditional based on fill placement), LOMR (based on a physical change
or updated information) and CLOMR (conditional based on a physical change)
• Master Plan — Planning efforts for major watersheds or inclusive of a jurisdiction (City, MUD, etc.)
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• Drainage Report — Drainage analyses for smaller areas or developments. May include smaller
residential areas, commercial areas, etc.
For each of the reports provided, a unique GIS element was generated in polygon format to represent
that information. The specific shape was based on the limits of the analysis. For example, a watershed
master plan would be identified by watershed drainage area. A drainage report would include the limits
of the development or improvement being evaluated. For detention studies, some of the reports may be
represented by several polygons where recommendations for detention ponds have been made. Each
report was catalogued using several data points as shown in Figure 1. These include the following:
• Project or Study Name
• Year of Submittal
• Engineer of Record
• Name of Engineering Firm
• Watershed Name
• Project or Study Type (based on classifications shown above)
Figure 1. Field Verification Data
Proleet/study Name
- g/4YIi
. EnginaarlK qnn
2009 Hickory Slough (HI100-00-00) Update
2009
Lawrence Lopez
4Site
Hickory Slough
Flood Mitigation Study
523 -Acre Southern Trails Master Drainage Plan Clear Creek Watershed
2004
Lee Clark Lennard
Brown & Gay
Clear Creek
Master Plan
Brazoria County Mapping Update Clear Creek
2005
Brett Sachtleben
Dannenbaum
Clear Creek
Flood Mitigation Sh*
Brazoria County MUD No 22
2009
Robert Bardin
UA
Chocolate Bayou
Master Plan
Brazoria County MUD No 23 Master Drainage Plan and Impact Analysis
2005
Robert Bardin
UA
Clear Creek
Master Pbn
Broadway at Woodcreek LOMR
2002
Jennifer 1. Walker
Lentz Engineering
Marys Creek
LOMR
City of Manvel Master Drainage Plan
2015
Philip Bullock
iaotz
Chocolate Bayou
Master Plan
City of Pearland Sub -Regional Detention Plan
2010
Dennis Todd Miller
laotz
Detention Analysis
City of Pearland Tropical Storm Allison Flood Study
2002
Gary Wayne Bezemek
taotz
Clear Creek
Flood Waigation Study
Clear Creek From Station 1691+93 to Station 1776+79 Request for
2002
Carolyn Gilligan
UA
Clear Creek
CLOMR
Clear Creek Watershed Modeling Update
2009
Dannenbaum
CIP Improvement
3.2 Model Inventory
The City and BDD4 provided numerous hydrologic and hydraulic models as part of the data collection
process. Because the master plan update focuses specifically on the open channel systems within the
study area, those models related to storm sewer analysis were not included in the inventory. The models
received included both effective Flood Insurance Study (FIS) models as well as model updates that were
not considered effective at the time of the model review, such as those completed for channel
improvement projects or updates based on better information or physical changes to the watershed.
There are two sets of FEMA Effective models for the City of Pearland. The effective models for the
portions of Pearland in Brazoria County are associated with the Brazoria County FIS conducted in the
1980's. The models for that FIS were created in HEC -1 and HEC -2. Electronic versions of the HEC -1 model
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of the Clear Creek watershed and the HEC -2 models for Hickory Slough, Mary's Creek, and Cowart Creek
were provided by the City/BDD4. No copies of the effective HEC -2 model of Clear Creek or the effective
models for Mustang Bayou were obtained. Electronic versions of HEC -1 and HEC -2 models of Mustang
Bayou that were created as part of the Brazoria County Master Drainage Plan in 1998 were provided;
however, electronic versions of the effective HEC -2 model of Chigger Creek were not available. No
hydrologic or hydraulic modeling for the West Fork of Chocolate Bayou was available.
The effective models for the portions of Pearland in Harris County are those associated with the Tropical
Storm Allison Recovery Project (TSARP) conducted by Harris County Flood Control District (HCFCD). These
models were created in HEC -HMS and HEC -RAS. Digital versions of these models were obtained from the
HCFCD Model Management (M3) website and include HEC -HMS and HEC -RAS models for Clear Creek.
In addition to the FEMA Effective models collected, hydraulic models prepared by Dannenbaum
Engineering Corporation (DEC) as part of the TSARP hydrologic study for Hickory Slough, Mary's Creek,
and Cowart Creek were also obtained. Dannenbaum was the TSARP hydrologic study contractor for the
Clear Creek Watershed. Part of their scope was to create tributary hydraulic models to facilitate storage -
routing computations in HEC -HMS. This effort provided the base HEC -RAS models for Hickory Slough,
Mary's Creek and Cowart Creek.
DEC updated the models for Hickory, Mary's and Cowart in 2006. The models were provided along with
a report Clear Creek Watershed Modeling Update for the Clear Creek Watershed Steering Committee and
the City of Pearland. The reports provide detailed documentation of the methodology used to create the
models as well as modeling results. This effort also produced updated hydrologic modeling of the Hickory,
Mary's and Cowart watersheds.
Based on a draft report to the Clear Creek Watershed Steering Committee, there appears to have been
another update of the Clear Creek tributary models in 2009. These updates were made by modifying the
effective Clear Creek HEC -HMS model. Updates included the addition of some small regional detention
ponds on the tributary streams (Hickory, Mary's, Cowart and Chigger) and some channel improvements
on Mary's Creek. The DEC models are the basis for the FEMA mapping updates for Brazoria County along
the aforementioned streams.
3.2.1 Model Inventory Development
Similar to the report and plan inventory, each of the models provided was organized by classification (i.e.
Effective, 2006 Update, 2009 Update, etc.) and then by watershed. The models were then reviewed to
determine their applicability for Phase II. The review of these models will be discussed in Section 4 of this
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report. Information regarding each model was gathered, if available, including effective date (in the case
of FEMA Effective models) or date of development, watershed, stream name, etc. During the Phase I
effort, it was determined that the GIS Model Inventory would be more effective following the
development of the existing conditions models that will be done in Phase 11. After the models are
developed, they will be compressed in a .zip format and linked to a feature in GIS, as was done with the
report and plan inventory. Hydrologic models will be attached to a watershed polygon feature. Hydraulic
models will each be attached to a linear stream feature.
As previously mentioned, some of the models were effective, some were updated versions, and others
were related to specific projects. Those models that were project specific will not be included in the
inventory. The FEMA Effective models will be attached to the associated watershed and stream features
as well as the Dannenbaum model updates as best available information. For regulatory purposes, the
FEMA effective models should be utilized such that submittals for LOMR's and CLOMR's meet the FEMA
requirements.
3.3 Topographic Survey
A limited field survey was performed to collect structure data at selected locations. For each surveyed
location photographs of the upstream and downstream face of the structure and channels were provided
along with field sketches with culvert and bridge data included. Surveyed cross sections, with centerlines,
tops of bank, and at least two other points were provided. The survey was performed to aid in the
development of new hydraulic models, which is discussed in later sections. Field survey information is
included in Appendix E.
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4.0 Data and Model Review
A thorough review of the data, including reports and modeling, was needed to identify those projects that
have been completed, those that are no longer appropriate, and those that should be considered moving
forward. For the latter case, reviewing the specifics such as the project configuration, costs, effectiveness
and constraints aided in new model development for the study. As this is a Master Drainage Plan update,
the previous master planning efforts were reviewed, as well as the current drainage criteria, the current
CIP, and funding sources.
4.1 Previous Master Plans
As mentioned in Section 1, the City and BDD4 both had Master Drainage Plans prepared by Rust
Lich liter/Jameson in the late 1990's. Since that time there have been several smaller plans, including
watershed plans and a sub -regional detention plan. The plans currently utilized include:
• Flood Protection Plan for Brazoria Drainage District No. 4, Rust Lich lite r/Jameson, November 1997
• Master Drainage Plan for the City of Pearland, Rust Lichliter/Jameson, February 1998
Both plans were comprehensive and covered major flood protection projects as well as smaller local storm
sewer projects. A brief discussion of each of these plans and recommendations relating to the master
plans is provided below.
4.1.1 Flood Protection Plan for Brazoria Drainage District No. 4
The 1997 Flood Protection Plan for BDD4 included a thorough evaluation of the primary system for Mary's
Creek, Hickory Slough, Cowart Creek, Chigger Creek, and looked at Clear Creek ditches within the Brazoria
County limits. While these streams were updated as part of this master plan, Clear Creek was not
modeled. Clear Creek has a shared jurisdiction with Harris County Flood Control District (HCFCD) and is
the subject of a significant proposed U.S. Army Corps of Engineers (USACE) project, which is to be funded
using federal money allocated in the wake of Hurricane Harvey. In addition, the Mustang Bayou and
Chocolate Bayou watersheds within the study boundary were included in the evaluation and planning
process.
4.1.2 Master Drainage Plan for the City of Pearland
A heavy focus of the 1998 City of Pearland MDP was the evaluation of roadside ditches and storm sewers.
While the intent of this Master Plan Update was to focus on the primary systems, consideration of the
flooding history in the area related to local drainage was considered as well. A comparison of the revised
flood inundation mapping to those areas outside the "floodplain" that have a record of flooding help
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determine if flooding in those areas is a function of the local drainage system or due to open channel
flooding. In many instances, those areas that have historical flood complaints have been addressed
through detention, ditch improvements, storm sewer projects identified in the 1998 master plan and
subsequent planning efforts.
In addition to historical flood complaints, public input gathered during the study helped identify areas
that continue to flood as a result of local drainage deficiencies. A detailed analysis of neighborhood
drainage is not included with Phase II. Recommendations for small area studies are included in
subsequent sections.
4.2 Report and Plan Review
Each of the plan sets and reports received and inventoried was subjected to a two-step review process.
The first step included a cursory review conducted to determine the document's applicability to the larger
master planning effort. Drainage reports for small developments, small projects that were constructed
prior to 2008 (date of LiDAR flight), and others were labeled as "Archive" and no detailed review was
conducted. For those that were more recent and of a sufficient scale to impact the master planning effort,
a detailed review was done. Each of the plans was reviewed for project recommendations, costs, and
constraints as well as other information that was helpful during the modeling and planning process.
4.3 Model Review
The models received were reviewed to determine what information could be maintained moving forward
and what should be updated to ensure that current conditions are reflected in the models. The review
was conducted for both the hydrologic and hydraulic models.
4.3.1 Existing Models and Previous Studies
Past master planning efforts have resulted in several iterations of comprehensive modeling for the Clear
Creek watershed and its tributaries. This includes efforts by the USACE, the City, BDD4 and HCFCD.
However, these efforts did not include Mustang Bayou or Chocolate Bayou. There are two sets of effective
FIS models for the City of Pearland. The effective models for the portions of Pearland in Brazoria County
are associated with the Brazoria County FIS conducted in the 1980's. The models for that FIS were created
in HEC -1 and HEC -2. Halff has obtained electronic versions of the HEC -1 model of the Clear Creek
watershed and the HEC -2 models for Hickory Slough, Mary's Creek, and Cowart Creek. Halff does not
have copies of the effective HEC -2 model of Clear Creek or the effective models for Mustang Bayou. Halff
does have electronic versions of HEC -1 and HEC -2 models of Mustang Bayou that were created as part of
the Brazoria County Master Drainage Plan in 2000. Further, Halff has an electronic version of the HEC -2
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model of Chocolate Bayou created in 2003. Halff does not have an electronic version of the effective HEC -
2 model of Chigger Creek. The HEC -1 and HEC -2 models were used for informational purposes only.
The effective models for the portions of Pearland in Harris County are those associated with the Tropical
Storm Allison Recovery Project (TSARP) conducted by Harris County Flood Control District (HCFCD). These
models were created in HEC -HMS and HEC -RAS. Halff has obtained the HEC -HMS and HEC -RAS models
for Clear Creek from the HCFCD website. After Tropical Storm Allison in 2001, Dannenbaum Engineering
Corporation (DEC) was selected as the TSARP hydrologic study contractor for the Clear Creek Watershed.
Part of their scope was to create tributary hydraulic models to facilitate storage -routing computations in
HEC -HMS. This effort provided the base HEC -RAS models for Hickory Slough, Mary's Creek and Cowart
Creek. These models formed the primary basis in the development of the new hydraulic models. The
calculated water surface elevations in the DEC models vary from those in the effective models. The
percent difference in elevation ranges from -12% to 0% change.
The existing FEMA effective hydraulic models for the study area were HEC -RAS v. 3.0.1 and the hydrologic
models were HEC -HMS v. 3.3. These models were utilized as a starting point for the model update effort,
which is discussed below. Per the scope for the Master Drainage Plan, the hydrologic models were
converted to HEC -HMS v. 4.2.1 and hydraulic models were updated to HEC -RAS v. 5.0.5, which are the
most current versions of the software. In addition to better computational and visual capabilities, the 5
The newer versions of HEC -HMS and HEC -RAS fix errors from previous versions. The most recent version
of HEC -RAS has 2D modeling capability, which may be beneficial for Preliminary Engineering analysis of
recommended projects, and provides better tools for terrain modification and inundation mapping.
Using the DEC models as a base, new hydraulic modeling was developed for each of the streams that drain
to Clear Creek using previous model data wherever possible. Specific modeling information is provided
in the sections below. Completely new models were developed for Chocolate Bayou, Mustang Bayou,
and several smaller sub -tributaries.
4.4 Drainage Criteria Review
As part of the Phase I reviews, Halff evaluated the current drainage criteria for the City of Pearland and
Brazoria Drainage District No. 4. In general, nearly all the needed updates for the City's manual apply to
the BDD4 manual since they have nearly identical criteria. At the time of the initial review, there were
some updates to the C -value calculations and times of concentration that warranted further investigation.
It should be noted that an update to the Engineering Design Criteria Manual (EDCM) was submitted in
September 2016, parallel with the Phase I MDP effort. The City requested a subsequent review, which
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Final Report— February 2019 mom
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indicated that many of the initial recommendations for changes had been incorporated into the updated
criteria. This included the few discrepancies between the City and BDD4 criterion. Future criteria updates
may be made as the area grows as long as there is a consistent drainage criteria to clarify acceptable
methodologies for developers and engineers working within the jurisdiction.
One concern that was presented during discussions with Pearland and BDD4 staff was the need to ensure
that both parties have a way to review or, at a minimum, be aware of submittals along BDD4 channels
within the City. It is not uncommon for platting applications to be made to the City where improvements
or alterations are proposed along a BDD4 ditch. While both jurisdictions currently sign off on applications
where the jurisdictions overlap, a more formal mechanism by which each agency is alerted when a project
impacting both jurisdictions may be beneficial.
Another area of concern was how to handle the development of small sites. Where regional detention is
not a viable option, on-site detention will be required based on 0.65 ac -ft per acre of increased impervious
cover. Requiring detention on very small sites could restrict the desired growth in the area. An evaluation
of the cumulative effects of small site development could be considered as part of a future study or
implementation phase consideration as well as some guidance on how to handle these types of site
projects.
4.5 Current CIP and Funding Review
A review of the current CIP Plan and funding sources was also conducted. Per the City of Pearland's Capital
Improvement Program (2018-2022), the five-year program totals $566,193,226. The CIP is funded from
several sources including general obligation bonds, water/sewer revenue bonds, impact fees for water
and wastewater, and other funding sources. Of the nearly $566.2M in the CIP, only about $12.4M (2%) is
planned for drainage projects, with nearly all of that from general obligation bonds. The City has an
additional $124.21VI in CIP needs; however, to have a manageable program over the next 3-5 years and
manage debt, a bond election is planned in 2019 for $70.8M. The four drainage projects listed in the
2018-2022 CIP are:
• Cullen/FM518 Detention Pond - $4.61VI
• Southeast Quadrant of Old Townsite Drainage - $1.51VI
• PER for Future Bond Referendum - $500k
• D.L. Smith Detention Pond Expansion (Phase 1) - $5.8M
The last bond referendum for the City was conducted in 2007 for a total of $162M was scheduled to be
completed over a 10 -year period; however, it is still ongoing. Several of these projects were completed
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Pearland / Brazoria Drainage District No 4 Master Drainage Plan mmmo omHALF F
Final Report — February 2019 .■■
in cooperation with BDD4. Drainage projects included in the 2007 bond referendum that have been
subsequently completed include:
• East Mary's Creek Ditch Improvements — September 2010
• Town Ditch Improvements — February 2011
• Veterans/Walnut Street Storm Sewer—January 2012
• Cowart Creek Diversion — April 2013
• Hickory Slough Detention (Pearland Pkwy) —July 2013
BDD4 is funded from property taxes within its jurisdiction. The tax rate for 2018 is $0.146/$100 appraised
valuation. BDD4 does not currently use bonds or other sources of funding. Per discussion with BDD4,
there is some capacity for drainage projects that may be recommended as part of this MDP. Specific
information is needed on the cost and schedule of the proposed projects before BDD4's CIP capacity can
be determined. That information is provided as part of this plan and is included in subsequent sections
of the report. Projects that are specific to the City, specific to BDD4, and opportunities to partner will be
identified as such in the implementation plan based on jurisdictional boundaries, including the ETJ.
Other mechanisms could potentially be employed to generate additional revenue for drainage projects.
Some of these include a Storm Water Utility (SWU), development impact fees, public-private partnerships,
TWDB low-interest loans, and FEMA grant programs. The City is currently evaluating the implementation
of a SWU. The potential may exist for negotiations with private developers relating to the funding of
drainage projects. This approach could be along the lines of an impact fee, where the developer, or group
of developers pays a fee in lieu of onsite drainage improvements, and the City and/or BDD4 builds
improvements that will benefit the developer as well as the other property owners within the area of
influence. Another option could include cost-sharing between the City and private interests to build
infrastructure, ostensibly in support of development interests. While these will not be determined by this
study, the plan will identify drainage infrastructure needs and, as development interests approach the
City, partnership opportunities should be discussed.
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Final Report — February 2019 ONE
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5.0 Field Verification
A significant part of effort for this study was the field verification process. This effort included the
development of a systematic approach to the site work and the data collection process. In addition, the
data collected was done through the use of mobile GIS applications, enabling Halff to upload data to the
database and web map in real-time. A discussion of the field work process and the data collected is
provided below.
5.1 Field Reconnaissance Process
During the field data collection process, an effort was made to gather data on as many of the ditches as
possible. Specific locations that were targeted included detention ponds, culverts, bridges, storm outfall
and channel sections where access was available. It was necessary to develop a systematic method by
which the data could be collected and progress tracked. Halff developed a field work grid in order to
facilitate that process. Each grid covers an area of approximately 2600 acres (^'4.0 sq. mi.) Many of the
grids overlap the study area boundary, but very little data was collected outside the boundary. The field
work grid was available in the mobile data collection apps as well as the web map. As each grid area was
visited and the data collected, the grid cell was marked as complete and colored green, indicating that
data collection was finished in that area pending further comments. (Figure 2)
Figure 2. Field Reconnaissance Grid
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Final Report — February 2019 mom
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There is a comment feature in the web map available for the City and BDD4 to identify those areas where
any additional data is needed.
5.2 Field Data Collected
Most of the focus was on ditches in the Cowart Creek, Mary's Creek, Hickory Slough, Chigger Creek,
Mustang Bayou, and Chocolate Bayou watersheds. Less data was collected along Clear Creek because
there is an effective HCFCD model for Clear Creek (A100-00-00) and it was not modeled.
Each data point collected included specific information, including the feature type (bridge, culverts,
channel, outfall, etc.), a general condition assessment, the date of collection, any specific notes relating
to that location, and the person who collected the data. In addition, photos were taken at each location
and attached to the data points. The data was placed on the web map for viewing by the City and BDD4.
As an example, Figure 3 shows a zoomed in view of the Mary's Creek data points along Southfork Dr. and
the surrounding area. The field verification database was delivered to the City and BDD4 at the
completion of Phase I in March of 2017. Some additional data was added as part of this Phase II MDP
process.
Figure 3. Field Verification Data
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Pearland / Brazoria Drainage District No 4 Master Drainage Plan MINN
Final Report—February 2019 mom HALFF'
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5.3 Findings
In general, the majority of the bayous and ditches in the project are in good condition. They are regularly
mowed and are free of debris both along the ditch as well as at most of the structures. The storm sewer
outfalls are typically visible and in good condition. There are a few areas along the ditches where light to
moderate erosion was observed, often downstream of bridges or culverts where the erosion protection
ends. Conversely, there were also some areas of deposition, typically upstream of bridges or culverts;
however, these issues were not determined to be pervasive. Based on these observations, it is more likely
that any flooding issues are a result of a lack of channel capacity or bridge/culvert capacity than overgrown
or blocked ditches or hydraulic structures.
There were several bridges and culvert that could not be approached due to their location. Some of these
were on private land and others, particularly the culvert siphons under the canals, could not be measured
because they were either completely inundated or surrounded by water. For these, visual estimates were
made when possible, but several bridges and culverts have no recorded field (or survey) data. When these
areas were encountered, they were marked as 'No Access'. A query of the points in the database reveals
there are numerous such locations with at least one structure that is inaccessible.
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6.0 Existing Conditions Modeling
One of the primary tasks of the study was to update and/or develop baseline conditions modeling for the
major streams in the study area. Existing models provided by the City of Pearland and Brazoria Drainage
District No. 4 are listed below. All the existing hydrologic models listed are included in the Clear Creek
hydrologic model available through the HCFCD M3 website. Each individual stream has its own hydraulic
models provided by the City of Pearland or BDD4, except Clear Creek, which was downloaded from the
M3 website as well. a
Existing Hydrologic Models
• Clear Creek
• Chigger Creek
• Cowart Creek
• Hickory Slough
• Mary's Creek
• Country Place Ditch
• Shadow Creek Ranch Ditch
• Town Ditch
Existing Hydraulic Models
• Clear Creek
• Cowart Creek
• Hickory Slough
• Mary's Creek
• Corrigan Bypass Ditch
• Town Ditch
Several of the tributaries had no effective model available, requiring that new models to be developed.
These include the following:
New Hydrologic Models
• Mustang Bayou
• East Chocolate Bayou
• West Chocolate Bayou
New Hydraulic Models
• Mustang Bayou
• East Chocolate Bayou
• West Chocolate Bayou
• Clear Creek
• Country Place Ditch
• Shadow Creek Ranch Ditch
• Chigger Creek
• Mary's Creek (except Corrigan)
The new hydrologic and hydraulic models developed were based on 2008 LiDAR data, along with some
survey at bridge or culvert crossings. They are not considered FEMA effective and should be used for
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Report—January 2019
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planning purposes. While the 2018 LiDAR has very recently been released, it was not available during the
model development or alternatives analysis process. The same is true forthe NOAA rainfall updated (Atlas
14). Future updates and planning efforts should consider both the terrain and rainfall information.
General information relating to the hydrologic and hydraulic modeling process is included in the sections
below.
6.1 Hydrologic Model Updates
Several updates were made to the hydrologic models to best reflect current conditions in the study area.
Among these changes were updates to the watershed and subbasin boundaries, updates to the loss and
hydrograph transform parameters, and the storage routing. All Clear Creek tributaries were previously
modeled as part of the HCFCD study and, as such, their existing hydrology is based on HCFCD methods.
Updates to the hydrology for Clear Creek tributaries used the same methods as the existing Clear Creek
HEC -HMS model. Hydrologic models for Mustang Bayou, East Fork of Chocolate Bayou, and West Fork of
Chocolate Bayou used the HCFCD methodology to maintain consistency.
6.1.1 Watershed and Subbasin Delineation
Both the studied stream centerlines and the drainage subbasin delineations were updated using the
available 2018 NearMap aerial imagery, 2008 HGAC LiDAR data, and field reconnaissance data.
Preliminary boundaries were based upon existing subbasin boundaries provided by the City of Pearland
and Brazoria Drainage District No 4. Revisions to the effective model subbasin boundaries were made at
locations where the boundaries appeared to vary significantly based on the LiDAR, plans, or discussion
with Pearland and BDD4.. There are a few locations where the Clear Creek watershed boundary was
updated, which added or removed drainage area into Clear Creek. The total decrease in area from the
effective model to the revised existing model is 0.06 square miles. The Clear Creek, Mustang Bayou, and
Chocolate Bayou drainage areas were subdivided for the purposes of developing new tributary hydraulic
models (see Exhibit 4).
6.1.2 Watershed Parameter Updates
Percent impervious cover and percent land urbanization (DLU) were determined using 2018 NearMap
aerial imagery and City of Pearland and Brazoria Drainage District No 4 land use categories. These
categories consist of various land uses including undeveloped, residential, light industrial/commercial,
and high-density development. Each category has an associate value for percent imperious cover (IMP)
and DLU. To account for future development, Halff assumed the area would improve to near fully -
developed conditions. These changes in the watershed were represented by increasing the percent
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impervious. Major thoroughfares identified in the HGAC data have a 100 -foot buffer placed on each side
of the major roadway centerline to be reserved for transportation improvement and right-of-way (ROW)
development. Roadway and ROW were assumed to have 90% IMP. Another 200 -foot buffer was placed
on each side of thoroughfares and 1000 -foot buffer at major intersections to represent high density
development which was assumed to have 85% IMP. Previously vacant areas were classified as small lot
resident development with 40% IMP. Exhibit 5 shows the existing conditions land use, while Exhibit 7
shows the future conditions land use.
Percent ponding (DPP) and on-site detention (DET) were also determined using 2018 NearMap aerial
imagery and 2008 HGAC LiDAR. The remaining watershed parameters; percent channel conveyance
(DCC), percent channel improvement (DCI), watershed length (L), length to centroid (Lca), channel slope
and watershed slope were determined in accordance with the standard HCFCD methodology. These
values were used to update the values for TC and R. The flowpath and DCI lengths for revised existing
conditions subbasins are shown in Exhibit 6. The flowpath and DCI lengths for the future conditions
subbasins are shown in Exhibit 8. Appendix A includes calculation tables for each of the updated
hydrologic parameters.
6.1.3 Modified-Puls Storage Routing
In addition to the loss and unit hydrograph parameters, storage routing parameters were also updated
for those watersheds that did not use unsteady HEC -RAS modeling (Hickory, Mary's). A range of flows
based on the preliminary 100 -year discharges was run through the hydraulic models to iteratively
determine storage -discharge relationships for hydrologic routing. In addition, the number of subreaches
was determined using the average velocity in the reach calculated from HEC -RAS. The routing was used
to determine the hydrograph attenuation due to storage for each stream.
6.1.4 Existing Conditions HEC -HMS Model Updates and Results
The effective HEC -HMS model for Clear Creek was updated from version 3.3 to version 4.2.1.
Modifications to the HEC -HMS model included adding drainage subbasin components and connecting
subbasins to appropriate junctions, representing the revised subbasins. Several "reservoirs" were added
to account for ponding upstream of roadways with limited culvert capacity and were connected to the
drainage subbasins draining toward those structures. In addition, diversion relationships were added
where overflow potential from one stream to another was identified using the hydraulic models.
Modified -Puts routing relationships and subreach values were updated in the model. Appendix B provides
the tabulated Modified -Pull routing parameters entered into the HEC -HMS model.
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Log-linear interpolation was performed for each tributary to develop flow values for use in the hydraulic
model. Error! Reference source not found. provides a comparison of the effective model (FEMA) peak
discharges with that of the DEC modeling, the Risk Map 6 modeling, and the revised existing conditions
(Halff) peak discharges at various locations along Clear Creek. The revised existing conditions discharges
show a maximum 18% increase from that of the effective model. This large increase is due to significant
differences in the watershed parameter determination between the previous FIS study and the revised
existing conditions analysis. These differences in watershed parameters consist of: watershed length (L),
length to centroid (Lca), channel slope (S), overland slope (So), percent urban development (DLU), percent
channel improvement (DCI), percent channel conveyance (DCC), percent ponding (DPP), DLU affected by
detention (DET), and percent impervious. DLU and percent impervious values generally increased, but
the other values did not have a consistent pattern to the change, they were just updated based on the
new terrain. The Green & Ampt soil loss parameters were not changed as it was assumed that the general
soil characteristics had not changed. It should be noted that the revised flows shown in Error! Reference
source not found. are for use as a comparison point for the MDP update. They are not currently being
submitted to FEMA for mapping updates or to USACE for use in the Clear Creek Federal Project.
The differences between FEMA Effective, Risk Map 6, and the Halff revised existing conditions varied by
reach and cross section (Error! Reference source not found.). The lettered cross section is the reference
cross section as identified by the FIS and shown on the FIRM panels. Mary's Creek, Mary's Creek Bypass,
Hickory Slough, West Chocolate Bayou, Mustang Bayou, Chigger Creek, Chigger Creek Bypass and Cowart
Creek all have at least one cross section in a FIS study. The FIS study stationing was compared with defined
FIS cross sections to determine the nearest appropriate cross section and junction in Halff's modeling. The
changes in peak discharge between the FIS data and the revised existing HMS model ranged from -35% to
177%. The largest change, in Chigger Bayou Bypass, is due to a change in the split flow at the top of the
bypass. If the Chigger Bayou and Chigger Bayou Bypass flows are combined, the total difference is 57%.
Both Mary's and Hickory Creek exhibit decreases, this is due to improved routing and consideration of
inline detention in unsteady HEC -RAS. Some of the increases in flows are due to increased development
in the area since the previous version of the model was completed. East Chocolate Bayou did not have
any FIS flooding source locations within the study area.
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Table 1. FIS Effective Hydrologic Modeling Comparison
OEM HALFF
Stream
Lettered
FIS Cross
Section
Peak 100 -yr Discharge (cfs)
Effective
Modeling
DEC Modeling
Risk Map 6
Revised
Existing
Chigger Creek
S
640
1060
416
Chigger Creek Bypass
D
490
490
1360
Chocolate Bayou - West Fork
N
1377
1470
1060
Cowa rt Creek
G
828
1970
977
1281
Hickory Slough
A
1328
1569
1495
1249
Mary's Creek
A
1541
1558
1238
1231
Mustang Bayou
BR
1284
1284
1885
Clear Creek
Confl uence w/Hi ckory SI ough
BS
5376
5166
5376
5956
Confl uence w/Bea mer Ditch
CP
7901
7921
7901
9357
C6nfl uence w/Mary's Creek
CD
16162
15453
21060
16658
22534
Confl uence w/Ch i gger Creek
BF
22891
Egret Bay Blvd
AA
24535
22997
23223
23979
23941
23949
24220
Confluence w/Cow Bayou
y
24557
Above Cl ear Lake
V
24879
Confluence w/Armand Bayou
M
42012
41939
46278
43021
47500
Mouth of Clear Creek
A
46342
Freese and Nichols, Inc. (FNI) are currently completing a study of Cowart Creek, including updates to the
hydrologic and hydraulic models and floodplain mapping within the watershed. Several of the subbasins
and parameters changed from the DEC models as FNI progressed through their study. Halff used the FNI
preliminary existing models for Cowart with only a few minor changes, which were made to facilitate the
proposed improvements analysis. These served as the baseline for the proposed conditions that will be
discussed in subsequent sections. Some minor changes to the models include:
• The flow split at Baker Rd. was modified to reduce artificially high flow through the new box
culvert system
• A junction was added at the confluence of Hood and Diversion Ditches so that the flow
distribution for Hood Ditch did not account for Diversion Ditch's flows
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MEN HALFF
• The pond owned by Clear Creek Flood Control District (Galveston County) reflected in the LiDAR
at the Pearland city limit along Cowart Creek was added to the model
For Chocolate Bayou (East and West) and Mustang Bayou, Halff created a new HEC -HMS model to model
the existing conditions. In order to maintain a consistent methodology, these models were also built using
the same HCFCD methodology used for the Clear Creek Model updates.
6.2 Hydraulic Model Updates
Information from the effective hydraulic model geometry and parameters was leveraged from the
effective models whenever possible to create an updated set of models for those streams that were
previously modeled. New hydraulic models were developed for the previously unstudied tributaries. Each
of the streams and their status (updated, new) are listed in Table 2 . For those streams that were updated,
the effective HEC -RAS models were converted from v. 3.0.1 to the most recent version, HEC -RAS v. 5.0.5.
The majority of the streams were modeled using steady HEC -RAS modeling. However, Hickory Slough and
Mary's Creek were modeled using unsteady HEC -RAS. Unsteady HEC -RAS allowed for a single model to
perform the water surface calculations as well as the storage routing. This was useful for these two
streams because they both had several existing regional detention ponds which unsteady HEC -RAS can
model more effectively than the iterative process between HEC -HMS and steady HEC -RAS.
The 3 -year, 5 -year, 10 -year, 25 -year, 50 -year, 100 -year and 500 -year storm events were run for each od
the hydraulic models. Resulting flow rates, water surface elevations, and velocities were examined from
these models. The resulting inundation maps for the 100 -year event, are provided in Appendix C.
Modeling specifics for each tributary are detailed in subsequent sections.
6.2.1 Hydraulic Modeling Update Process
The modeling updates included extracting the terrain information from the current LiDAR (2008) dataset
such that the modeling and mapping terrain is consistent. The newly extracted cross sections were
adjusted where necessary to account for features in the terrain which were inadequately represented
existing conditions. Many bridges and culverts in the study area were updated based on survey and field
observations. Further, ineffective flow areas and blocked obstructions, which were initially based on the
effective models, were adjusted where appropriate. Lateral structures were added where needed to
calculate diversion relationships or to accommodate offline detention basins where necessary. Flow
change locations and values were also updated based on the revised HEC -HMS modeling.
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Ems HALFF'
In the unsteady HEC -RAS models for Hickory Slough and Mary's Creek, storage areas were added into the
models. After completing the unsteady HEC -RAS modeling for Mary's Creek and Hickory Slough, the
models were converted into steady models to maintain consistency with the modeling in other
watersheds. The steady state versions are included in the model deliverables; however, the unsteady
versions were used for the proposed flood reduction alternatives analysis.
6.2.2 Updated Hydraulic Model Sources
All Cowart Creek existing models, except Cannon Ditch, were provided by FNI in November 2018. Cannon
Ditch is based on the DEC models from 2006. Hickory Slough, Mary's Creek, and Chigger Creek were based
on DEC models from 2006 and 2009. The model for Corrigan Ditch was updated from a study by Halff
Associates, Inc. in 2016. The model for Town Ditch was provided by the City of Pearland based on recent
updates to the channel.
6.2.3 New Hydraulic Models
New hydraulic models for Mustang Bayou, East Fork Chocolate Bayou, and West Fork Chocolate Bayou,
as well as the Clear Creek tributaries were constructed utilizing a methodology similar to the updated
models. As with the updated models, cross-section geometries extracted were extracted from the 2008
LiDAR data using HEC-GeoRAS. Cross sections were placed along each tributary to provide sufficient
coverage and capture changes in the channel geometry. At the upstream end, the cross sections stopped
at the study area boundary. At the downstream end, cross sections were placed downstream of the study
area boundary such that the boundary conditions did not interfere with analysis inside the study area.
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Table 2. HEC -RAS Models
HALFF
Watershed
Stream Name
Updated
Hydraulic
Model
New
Hydraulic
Model
Chigger Creek
Chigger Creek
x
Chigger Creek
Chigger Creek Bypass
x
Chigger Creek
Moore Road Ditch
x
Chigger Creek
Old Chigger Creek
x
Chigger Creek
Resort Park Ditch
x
Chocolate Bayou (East)
E103-00-00
x
Chocolate Bayou (East)
East Fork of Chocolate Bayou
x
Chocolate Bayou (East)
Rodeo Palms Ditch
x
Chocolate Bayou (West)
Cold River Ranch Ditch
x
Chocolate Bayou (West)
CR 383
x
Chocolate Bayou (West)
McCutchen
x
Chocolate Bayou (West)
Villarreal
x
Chocolate Bayou (West)
West Fork of Chocolate Bayou
x
Country Place Ditch
Country Place Ditch
x
Cowart Creek
C107-03-00
x
Cowart Creek
Cannon
x
Cowart Creek
Cowart Creek
x
Cowart Creek
Cowart Creek (CR 413 Ditch)
x
Cowart Creek
CR 413 Ditch
x
Cowart Creek
Dare Ditch
x
Cowart Creek
Diversion Ditch
x
Cowart Creek
Hood Ditch
x
Cowart Creek
LeClair Ditch
x
Hickory Slough
Hickory Slough
x
Mary's Creek
Mary's Creek
x
Mary's Creek
Mary's Creek Bypass
x
Mary's Creek
New Corrigan Ditch
x
Mary's Creek
North Fork Mary's
x
Mary's Creek
South Fork Mary's
x
Mary's Creek
Weatherford Ditch
x
Mustang Bayou
Mustang Bayou
x
Mustang Bayou
Mustang Spur
x
Shadow Creek Ranch
Shadow Creek Ranch
x
Town Ditch
Town Ditch
x
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6.2.4 Existing Regional Detention Basins
There are several regional detention ponds in the study area particularly in Hickory Slough and Mary's
Creek. The following table (Table ) describes water surface elevation and storage volume for existing
regional detention during a 100 -year storm based on unsteady HEC -RAS modeling.
Table 3. Regional Detention Pond Storage Volume
Stream
Basin Name
Water Surface
Elevation (ft)
Modeled Storage
Volume (ac -ft)
Mary's Creek
East Mary's
40.43
191.0
Mary's Creek
Independence
43.86
56.0
Mary's Creek
SWEC
48.52
299.0
Mary's Creek
Veterans
46.47
134.0
Mary's Creek
West Mary's
51.42
544.0
Hickory Slough
Cullen
52.51
81.3
Hickory Slough
SH 35
44.75
9.1
Hickory Slough
SportsPlex
52.11
143.8
6.3 Existing Conditions Hydraulic Model Results
There were noticeable differences in the effective and updated models along Hickory and Mary's Creek,
likely due to the change between steady and unsteady modeling. A comparison of the steady vs unsteady
differences is included in Table 4 below. The table shows an average difference of about +0.1 ft or less
with variations ranging from -0.15' to +0.41'.
Table 4. Existing Conditions Unsteady vs Steady Modeling WSEL Comparison
Stream
River
Station
Existing Conditions WSEL* (ft)
Unsteady
Steady
Difference
Hickory Slough
Average
0.06
34986
54.23 54.08
-0.15
20256
49.29 49.23
-0.06
1295
39.47 39.49
0.02
Mary's Creek
Average
0.09
60725
53.02 53.21
0.19
13244
30.66 31.07
0.41
2387
16.88 16.88
0.00
*2018 Field Data and Survey with 2008 LiDAR
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These differences were due to a couple of factors, including differences in the computational method.
Where steady state modeling only considers conveyance, unsteady modeling includes volume and timing.
Storage routing for steady state models uses storage -discharge relationships whereas in the unsteady
model, the routing is done with the hydraulic model. Because the unsteady model considers volume, it
recognized significant overbank storage along both channels. The water surface elevations change
between steady and unsteady likely also included differences at bridges and culverts. As previously
discussed, the intent of this analysis is not to submit the modeling to FEMA for map update purposes, but
to provide a reasonable basis for comparison during the proposed flood reduction alternatives analysis.
Specific information about the existing conditions findings for each of the modeled streams is provided in
the following sections. Each section will include a discussion of the streams included, the original
modeling source if there is one, data collected, modeling specifics, and general results. In addition, some
general comparisons to the effective FIRM mapping, completed as part of FEMA Risk Map 6 (RM6) is
provided for informational purposes. The Revised Preliminary date for the mapping is June 29, 2018.
Specific Information regarding the proposed flood reduction alternatives is provided in Section 8.
6.3.1 Chigger Creek
The Chigger Creek system modeling was developed as a system including the following streams: Old
Chigger Creek, Chigger Creek Bypass, Resort Park Ditch, Moore Road Ditch, and Old Chigger Creek (Exhibit
2). These tributaries were modeled together because they represent a complex network of streams with
numerous overflows that can't accurately be modeled with individual models.
Although Chigger Creek and Chigger Creek bypass have an existing FIS model, Halff was unable to obtain
the model or model documentation. As such, Halff developed a new HEC -RAS model with survey data
collected at several locations, including the three roadway crossings of SH 35 and numerous county roads.
Additional field observation data was collected at many of the publicly accessible crossings on the stream
network. There was no previous modeling for Resort Park Ditch, Moore Road Ditch and Old Chigger Creek,
which were developed as part of this study with geometric information based on 2008 LiDAR data and
field observations; no survey was included.
A lateral weir along the right bank of Chigger Creek accounts for the diversion between Chigger Creek and
the Chigger Creek Bypass. Two additional lateral weirs account for overflow out of the system into
Dickinson Bayou from Old Chigger Creek. The lateral weirs were optimized through several iterations
during Modified-Puls routing process.
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With the revised modeling approach, the inundation for this area changed as compared to the floodplain
shown on the new Revised Preliminary FIRM (RM6). The inundation width at the upper end of the reaches
tend to be wider than FEMA RM6 mapping but the lower reaches tend to be the same or smaller. The
floodplain along Chigger Creek Bypass maintains a similar inundation shape to the RM6 mapping. The
Resort Park and Moore Road Ditches did not have an effective floodplain and so any inundation associated
with these streams is new. Other areas experience more ponding than the effective floodplain suggests.
Upstream of the BNSF Railway, the floodplain extends further south and west toward the levee associated
with the water supply canal than in the effective models. Chigger Creek, Old Chigger Creek and Resort
Park Ditch upstream of the railway are significantly inundated in both the 10-year and 100-year storm
events. Resort Park Ditch flows into both Moore Ditch Road and Dickinson Bayou, which is out of the
system. Significant flooding occurs at the diversion between the two receiving streams. Revised existing
inundation mapping for the Chigger Creek system can be found in Appendix C.1-C.3.
6.3.2 Chocolate Bayou
The Chocolate Bayou system was modeled as two interconnected stream models divided into the East
and the West. The West Chocolate Bayou system includes the following streams: West Fork Chocolate
Bayou, E101-00-00, E101-01-00, Villarreal, Cold River Ranch Ditch, McCutchen Ditch and E101-02-00 (CR
383 Ditch) (Exhibit 2). The East Chocolate Bayou system includes the following streams: E103-00-00,
Rodeo Palms Ditch and East Fork Chocolate Bayou. There is significant overflow between the two systems
due to the very flat terrain and significant ponding caused by restrictions along the channel. As a result,
these streams experience a 2-dimensional flow problem that is modeled using 1 -dimensional flow
techniques. As such, the interaction between HMS and RAS models were developed iteratively.
Chocolate Bayou has an existing FIS model developed using HEC -2, but the detailed study is largely
downstream of the modeling for this master drainage plan. As such, Chocolate Bayou is a new model,
which includes some survey data collected at the stream crossing at County Road (CR) 383 and West Fork
Chocolate Bayou. Culvert/bridge data from the existing FIS model was used to supplement field data for
several of the bridges in the downstream reach of the models. Halff collected additional field observation
data at many of the public crossings on the stream network.
Several lateral weirs aided in balancing the overflows between the two systems and were optimized
through several iterations during Modified -Puts routing. Further, the HEC -HMS model has several
diversions which represent overflows caused by limited culvert capacity, particularly through the siphons
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that run under raised raw water canals. Ponding behind the canals is accounted for using "reservoirs" in
HEC -HMS, such that the volume was considered.
The revised modeling indicated that the inundation for this area changed as compared to the flooding
shown on the FEMA RM6 mapping. The model shows a large increase in inundated area due in large part
to the backup created by the canal siphons. The USGS regression curves which were used to determine
peak discharges on Chocolate for the previous FIS study do not properly account for the runoff trapped
behind the siphons seen in the Halff models, which show extensive ponding throughout the upper
Chocolate Bayou watershed. Inundation mapping for the Chocolate Bayou systems can be found in
Appendix C.4 -C.9.
6.3.3 Country Place Ditch
Country Place Ditch is tributary of Clear Creek. A new model was developed, which incorporates a limited
amount of survey data. This tributary is modeled from about 1000 feet upstream of Hughes Ranch Road
to the confluence with Clear Creek. Three bridge crossings are included in this model as well as several
existing offline detention ponds, which were included in the cross sections as a blocked obstruction, as
was done with previous models. The most downstream portion of the tributary is overlapped by the
floodplain from Clear Creek. Inundation mapping for the Country Place Ditch can be found in Appendix
C.10.
6.3.4 Cowart Creek
Cowart Creek is a modified model with geometry and parameters based on models created by FNI. The
models created by FNI were provided in November 2018 as preliminary models to their current study for
the City of Pearland. FNI's work did not include Cannon Ditch. Cowart Creek is modeled from the top of
the watershed to the confluence with Clear Creek. Halff changed the flow distribution from FNI's in several
places to better reflect flow conditions and facilitate the proposed flood reduction alternatives analysis.
Based on FNI's hydrology methods, Halff updated the flows and floodplains. Floodplain information for
Cowart Creek can be found in Appendix C.11 -C.16.
6.3.5 Hickory Slough
The Hickory Slough model was based on models created by Dannenbaum Engineering (DEC) and was
updated with new terrain data (2008 LiDAR) and hydraulic parameters. Adjustments were made to bridges
and culverts where necessary based on engineering judgement, survey data, and field observation. The
updated flows were based on the revised hydrology and inundation mapping was performed using the
2008 LiDAR. The model was also extended to begin at CR 94 whereas the current model ends upstream
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of Cullen Blvd. Hickory Slough was modeled using unsteady HEC-RAS in order to better capture the
significant overbank storage and the interaction with several offline regional detention basins. The use of
unsteady HEC-RAS allowed for optimization of ponds without iterating in HEC-HMS. Several culvert and
bridge crossings were updated or added into the model. The bridge at Old Alvin Road was modeled based
on the 2006 DEC model but aerial imagery indicates the bridge has been updated since then. Halff used
the 2006 DEC model geometry as neither survey nor plans were obtained.
In addition, all existing offline detention ponds were added into the model using storage areas. In order
to capture potential overbank storage on the far side of the detention ponds, cross sections were
extended to the limit of the bayou watershed and those areas with ponds were modeled as a blocked
obstruction. This was the same approach taken with the Dannenbaum models.
Inundation is prevalent between Cullen Boulevard and Oday Road, where low-lying areas exist in the
terrain along the original channel alignment. Aerial imagery shows that by 1944, the channel had been
realigned; however, the original channel is still visible. This imagery is consistent with channel remnants
detected in the LiDAR. The most downstream portion of the tributary is overlapped by the floodplain from
Clear Creek. Inundation Mapping for Hickory Slough can be found in Appendix C.17 -C.20.
6.3.6 Mary's Creek
Mary's Creek is an updated model with geometry and parameters based on models created by
Dannenbaum Engineering. The model was also extended to begin at Southwyk Road and incorporated the
online storage area adjacent to Magnolia Rd. near Cullen Pkwy. As with Hickory Slough, unsteady HEC -
RAS modeling was used to more effectively capture the stream interaction with offline detention ponds.
In order to capture potential overbank storage on the far side of the detention ponds, cross sections were
extended to the limit of the bayou watershed and those areas with ponds were modeled as a blocked
obstruction. This was the same approach taken with the Dannenbaum models. Adjustments were made
to bridges and culverts where necessary based on survey data and field observation. The updated flow
rates were based on the revised hydrology. A large percentage of the ineffective flow areas in the model
were updated to represent the observed conditions along the stream.
Revised inundation mapping was done using the 2008 LiDAR. It is important to note that the most
downstream portion of the Mary's Creek is overlapped by the floodplain from Clear Creek. Floodplain
information for Mary's Creek can be found in Appendix C.21 -C.25.
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6.3.7 Mary's Creek Tributaries
Mary's Creek includes four tributaries: North Fork Mary's Creek, South Fork Mary's Creek, Weatherford
Ditch, and Corrigan Ditch (Exhibit 2). None of these was previously modeled and are not mapped in the
newly effective FIRM panels (June 29, 2018). As such no comparison to the effective mapping is provided.
North Fork and South Fork of Mary's Creek
For the North Fork and South Fork new models were developed and incorporated a limited amount of
survey data. These tributaries are modeled from the top of their channel to their confluences with Mary's
Creek. The North Fork joins Mary's Creek immediately downstream of Southfork Dr. just west of Versaille
Dr. The South Fork joins Mary's Creek near Magnolia Ave, near Charles Ave. Two bridge/culvert crossings
on each of the tributaries are included in this model as well as any existing offline detention ponds were
included in the cross sections as a blocked obstruction. The most downstream portion of both the
tributaries are controlled by the floodplain from Mary's Creek. Floodplain information for these tributaries
can be found in Appendix C.26 -C.27
Corrigan Bypass Ditch
The model for Corrigan Ditch was updated model based on a previous model developed by Halff. The
bypass ditch runs from W. Broadway south to Mary's Creek and confluences between Wagon Trail Rd.
and Hatfield Rd. The bypass was built to divert water around the Corrigan Subdivision to reduce flooding.
Field observation indicates that the connection between the bypass and Old Corrigan Ditch has been cut
off and a wall separates the two ditches. The flows were updated based on the revised hydrology, as
discussed in Section 4.2. Three bridge crossings are included in this model as well as any existing offline
detention ponds were included in the cross sections as a blocked obstruction, as done with previous
models. The entirety of the tributary is overlapped by the 100 -year floodplain from Mary's Creek.
Floodplain information for Corrigan Ditch can be found in Appendix C.28. Corrigan Ditch appears to have
a 100 -year LOS so no improvements will be recommended.
Weatherford Ditch
The Weatherford ditch model was a new model based on the 2008 terrain. Weatherford Ditch is
overlapped by the Mary's Creek cross sections, but the Mary's Creek Floodplain does not appear to
inundate the channel during the 100 -year. Apart from a few low-lying areas below the bank elevations,
Weatherford Ditch appears to have 100 -year capacity. As such, no improvement recommendations will
be made. Floodplain information for Weatherford Ditch can be found in Appendix C.29.
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6.3.8 Mustang Bayou
For Mustang Bayou a new model was developed based on the 2008 HGAC LiDAR. This channel is modeled
from about 2000 feet downstream of Fort Bend Parkway Toll Road to about 500 feet downstream of Del
Bello Rd. (CR90). Eleven culvert/ bridge crossings are included in this model as well as all existing offline
detention ponds were included in the cross sections as a blocked obstruction. Two of the bridge
geometries were determined based on the HEC -2 model. Similar to other bayous, a lateral weir was added
downstream of SH288 to account for overflow from Mustang Bayou into the adjacent watershed (East
Fork Chocolate Bayou — Cooper Ditch). The modeling indicates a wide shallow floodplain downstream of
FM 521. The floodplain is narrower until the channel approaches SH 288. The downstream portion of
this tributary appears to have limited capacity, with the 3 -year storm exhibiting significant ponding in low-
lying areas all the way from SH288 to the lower limit of the modeling outside the study area. Floodplain
information for Mustang Bayou can be found in Appendix C.30 -C.33.
6.3.9 Shadow Creek Ranch Ditch
The Shadow Creek Ranch ditch, also referred to as the Clear Creek Relief, is a tributary of Clear Creek for
which a new model was developed and incorporates a limited amount of survey data. This tributary is
modeled from FM 521 to the confluence with Clear Creek. Five bridge/culvert crossings are included in
this model as well as any existing offline detention ponds were included in the cross sections as a blocked
obstruction. Half of the modeled tributary is overlapped by the floodplain from Clear Creek but,
independent of Clear Creek, the ditch appears to have 100 -year capacity. Floodplain information for
Shadow Creek Ranch can be found in Appendix C.34.
6.3.10 Town Ditch
Town Ditch is an updated model based on a model from Carter & Burgess. The model was not geo-
referenced, so the previous model was used as a guide for cross section locations. Further, the model was
extended to begin at Cherry Street and continue to the confluence with Clear Creek. The flows were
updated based on the revised hydrology, as discussed in Section 4.2. Revised inundation mapping was
done using the 2008 LiDAR. Six bridge crossings are included in this model as well as any existing offline
detention ponds were included in the cross sections as a blocked obstruction, as done with previous
models. The most downstream portion of the tributary is overlapped by the floodplain from Clear Creek.
Town Ditch appears to have 100 -year capacity. Any potential improvements to Mykawa Rd. are not
included in this analysis as no information was available at the time of this study. Floodplain information
for Town Ditch can be found in Appendix C.35.
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6.4 Local Ditch Capacity Analysis
A channel capacity analysis was performed for roadside ditches located throughout the study area in order
to determine the level of service. Flow capacity was determined using the Rational Method; no hydrologic
or hydraulic models were prepared for these reaches. A total of 91.8 miles were evaluated in this capacity
analysis as shown in Exhibit(s) 8A -8E. Several ditch sections showed alignments through recent
developments or appear to have recent improvements that include storm sewer sections. These ditches
were excluded from the analysis because they are assumed to have adequate capacity because of the
recent improvements.
A drainage area was delineated for each ditch in the analysis using 2008 LiDAR and 2018 NearMap
imagery. For each drainage area a runoff coefficient, percent impervious, and rainfall intensity was
calculated according to BDD4 criteria. If the drainage area was less than 250 acres, Rational Method was
used to compute the ditch capacity. If the drainage area was greater than 250 acres, Harris County Flood
Control District (HCFCD) Site Runoff Curves were used to compute the capacity. To estimate the existing
channel capacity, a typical cross section was identified. The geometry of this cross section was estimated
using GIS 3D analyst tools and the WAR dataset. Channel slope was estimated between stream points
approximately 10% and 85% of the channel length. Manning's equation was used to determine the
discharge that would fit in the existing channel geometry. Discharge calculations for each ditch are
provided as Appendix D.2.A.
The calculate discharge was compared to the discharges for the 3-, 5-, 25-, 50-, and 100 -year storm events
calculated using the Rational Method or Site Runoff curves. The approximate Level of Service (LOS) for
each ditch was estimated based on a comparison to determine for which storm event the ditches had
adequate capacity. Exhibit(s) 8A -8E provides a graphical estimate of the level of service for the various
channels and ditched for the existing conditions. Peak discharge calculations for each of the storms as
well as the LOS determination are provided in Appendix 13.2.13. In general, most of the local ditches
provide a level of service below the 3 -year storm and require additional capacity. A brief discussion of
the improvements is provided in Section 8.5.
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7.0 Existing Flooding Issues
The stream modeling and local ditch capacity analysis indicated that a majority (65%) of the streams (both
major bayous and local ditches) in the City/BDD4 area have less than a 3-year capacity. The lack of
conveyance capacity is widespread with very limited capacity in every watershed, particularly those in the
southern portion of the area (Chigger, Cowart, Chocolate, Mustang). Given the limited channel capacity
and the generally flat terrain, even frequent storms like the 5-year and 10-year can cause significant out-
of-bank flooding. The capacity limitation is particularly evident for the local ditches, 70% of which have
less than a 3-year capacity. Of the major creeks and bayous, about 60% have 3-year capacity or less. Clear
Creek was not included in the capacity analysis; however, it should be noted that the downstream portions
of Hickory Slough, Town Ditch, Country Place Ditch, Shadow Creek Ranch Ditch, and numerous other small
ditches are influenced by the Clear Creek floodplain.
At both the main stem and tributary levels, the limited channel capacities are evident by the number of
flood claims in the reported area. The locations with reported instances of flooding or flood damage are
predominantly within developed areas because the flood data is only provided within the City of Pearland
and its Extraterritorial Jurisdiction (ETJ). Exhibit 9 shows the concentrations of flood claims/damages,
which are distributed along the main channel and tributaries and will be discussed in the sections below.
These include information from Hurricane Harvey in August 2017. The information provided does not
disclose specific flood damaged properties, only the relative concentration in the area. Several clusters
throughout the study area showed evidence of repeated losses and losses during Hurricane Harvey. Most
of these were within the defined revised preliminary FEMA 100- or 500 -year floodplains. There were a
few areas where flood claims were found outside of the defined floodplain. These could be the result of
either rainfall exceeding the amounts corresponding to those storm events or inadequate capacity in the
local ditches, culverts, storm sewers, or inlets.
There are no recorded flood claim locations within Chigger Creek, Chocolate Bayou, and Mustang Bayou
because the watersheds are outside of the City of Pearland or its ETJ. Cowart Creek has a few flood FEMA
flood claims within the City of Pearland's ETJ. No losses due to Hurricane Harvey were provided for the
Cowart Creek, Chigger Creek, Mustang Bayou, or Chocolate Bayou watersheds.
7.1 Country Place Ditch
This section of the study is predominantly residential. There is one recorded FEMA flood claim from
Hurricane Ike within the Country Place Ditch drainage area. Country Place Ditch itself has an approximate
100 -year capacity; however, the portion to the north of the ditch is inundated by the Clear Creek
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floodplain per the Halff mapping as well as the Risk Map 6 (RM6) mapping. A majority of the structures
that flooded during Harvey were within the floodplain limits. Considering the neighborhood's proximity
to Clear Creek, there were relatively few incidents of flooding.
7.2 Hickory Slough
Hickory Slough has a high number FEMA flood claims as well as damages from Hurricane Harvey. More
than 500 total claims or damage reports have occurred in the Twin Wood, Twin Creek Woods, and Clear
Creek Estates neighborhoods; however, these are entirely within the Clear Creek Floodplain. Flood claims
in the Brookland Acres area are also within the Clear Creek floodplain. There are several FEMA Claims
and Harvey damages in the Hickory Creek Place subdivision, the majority of which are in the Hickory
Slough 100 -year floodplain. There are also isolated instances of flood damages upstream of Garden Rd.,
a majority of which appear to be floodplain related. There is a small pocket of flood damages near Garden
Rd. at the watershed boundary between Hickory Slough and Mary's Creek. These are most likely the result
of limited local drainage capacity, which is not unexpected given the capacity analysis findings detailed in
Section 6.5 and the typically flat terrain found at watershed boundaries. Hickory slough has an estimated
capacity of 5 -year or less.
7.3 Mary's Creek
Mary's Creek has a significant number of flood claims and Harvey damages throughout the watershed.
The section downstream of Pearland Pkwy. has about 250 historical claims, with Harvey damages
accounting for about 65% of those. A majority of the flood claims are outside of the delineated floodplain
(both Halff and RM6), indicating that internal storm sewers in these neighborhoods may not have
adequate capacity.
Upstream of Pearland Pkwy, there are pockets of flood claims located within the floodplain, in particular
the Corrigan Subdivision, which has a long history of flooding. Records indicate more than 460 claims in
the neighborhood including Harvey and dating back to the 1970's. The Corrigan Bypass ditch was
constructed in the mid -2000's, directing flow from the north around the neighborhood to Mary's Creek.
There does not appear to be a connection between the bypass and the neighborhood drainage any longer.
The Corrigan Bypass has 100 -year capacity but is backed up by the Mary's Creek floodplain. The Corrigan
Subdivision's internal drainage now goes to a detention pond and is pumped out. The FEMA flood claims
appear to cease after the bypass ditch construction, indicating that it helped. However, the magnitude of
rainfall during Hurricane Harvey as well as the Mary's Creek floodplain seem to have overwhelmed the
system and flooded virtually the entire neighborhood.
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Further upstream, the West Lea subdivision has experienced significant flooding, all of which is within the
100- or 500-year floodplain. There are approximately 14 properties who have filed claims, some of them
multiple times. After Harvey, that number jumped to nearly 100 flood damage claims. In the upper
reaches, there are a few sporadic FEMA flood claims but no Harvey damages. Mary's Creek has an
estimated 5- to 10-year channel capacity.
The North Fork and South Fork have very few flood claims between the two of them, all in residential
areas. They are likely isolated events due to a temporary blockage in the drainage system given the lack
of claims by surrounding property owners. The area surrounding Weatherford Ditch has a handful of
Harvey flood claims, all of which are in the River Mist neighborhood to the north of the ditch. These are
likely caused by capacity limitations of the local drainage system. Mary's Creek Tributaries have an
approximate 50- to 100 -year capacity.
7.4 Shadow Creek Ranch Ditch
There is one isolated FEMA repetitive loss claim in Shadow Creek Ranch from Hurricane Ike. There are
approximately 40 Harvey flood claims, mostly clustered along a low-lying area south of the ditch and
within the Clear Creek backwater area. The Shadow Creek Ranch Ditch itself has approximately 100 -year
capacity based on the modeling data. The 100 -year inundation mapping shows some ponding in the
streets but is otherwise contained within the ditch.
7.5 Town Ditch
The lower portion of Town Ditch is within the Clear Creek floodplain; however, there are no Harvey flood
claims and only a few FEMA loss claims. The highest concentration of flood claims (mostly Harvey) is
upstream of Mykawa Rd in the Willow Crest and Mimosa Acres subdivisions. This area is outside of the
delineated floodplain (Halff and RM6) and is likely due to inadequate capacity in the local drainage system.
Town Ditch has been improved in the last several years and has an approximately 100 -year capacity.
7.6 Cowart Creek
Cowart creek has only a few flood claims within the watershed, making it difficult to assess the flooding
extents during Harvey. Cowart is a complex network of ditches with diversions and numerous crossings,
both roads and railroads. The revised existing conditions modeling was done by Freese & Nichols with
Halff making only minor changes to the existing conditions models. The lower portion (downstream of
the RR crossing) of Cowart Creek and Leclair Ditches have 25- to 50 -year capacity, as does Dare Ditch, a
portion of which is referred to as the Cowart Creek Diversion Ditch (50 -year capacity). Beyond that, the
majority of the ditches in the system have 3 -year capacity or less, with most having less. Inundation
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Mapping confirms that Cowart Creek has good capacity downstream of the SH35 crossing, which is similar
to the RM6 mapping. The FNI/Halff mapping shows significant flooding along the LeClair ditch all the way
up to the railroad crossing, which differs from the RM6 mapping which shows LeClair flooding contained
in the ditch up to SH35. The revised existing mapping shows the diversion ditch contains most of the flow
for the 100 -year; however, the RM6 mapping does not include the diversion ditch.
North of McKeever Rd. (CR100) the revised existing mapping shows flooding for a majority of the area
from the railroad west to Berry Rd. (CR879C). There are 3 properties in this area with flood claims
between 2000 and 2002. The RM6 mapping does not include the diversion ditch and, as such, shows the
area flooded all the way to Manor Rd. There are no flood claims in that area. The area along Bailey Rd. is
also flooded as shown in both the revised existing and RM6 mapping. A storm sewer was recently
constructed along Bailey Rd. and a majority of the flow directed to the diversion ditch. For larger storms,
such as the 100 -year, the culvert does not have capacity to contain the flows and there is shallow flooding
both north and south of the road from the railroad to west of Manvel Rd.
Cannon ditch shows limited flooding downstream of the railroad, which differs from the RM6 mapping
which shows more flooding, including a floodplain for a ditch that does not appear to be there anymore.
Upstream of the railroad, there is significant flooding shown in both the revised existing and RM6
mapping, though the revised existing mapping shows greater inundation. The estimated capacity for the
upper portion of Cannon Ditch is less than 3 -year.
7.7 Chigger Creek
There is no flood claim information for Chigger Creek, including for Hurricane Harvey. The lower reaches
of Chigger have a 3- to 5- year estimated capacity, including the Chigger Creek Bypass, which takes the
majority of flow off of Old Chigger Creek. Upstream of the Bypass split, the channels have less than a 3 -
year estimated capacity. The floodplain mapping for Chigger Creek and Old Chigger Creek is similar,
except where Old Chigger Creek bends at Moore Rd. The RM6 mapping is much wider at this location,
which is potentially the result of modifications to the Chigger Creek Bypass diversion done in order to
improve the water surface balance in the modeling. The Resort Park Ditch is not mapped in the RM6
version but shows a wide floodplain. The estimated capacity Resort Park Ditch is less than 3 -year.
7.8 Mustang Bayou
There is no flood claim information for Mustang Bayou, including for Hurricane Harvey. All of Mustang
Bayou within the study area has less than a 3 -year estimated capacity. There are several new
developments that have been built right up to channel along the north bank, much of which is shown as
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inundated based on the 2008 LiDAR. Continued development pressure in the area will make finding
detention locations a challenge, but significant detention is needed to reduce the flood risk to the
neighboring drainage district caused by improvements in BDD4jurisdiction. The revised existing and RM6
floodplain mapping are relatively consistent with the RM6 being wider at the lower end and narrower at
the upper end. The raw water canal east of Airline Rd. is where the change occurs. The revised existing
mapping extends further upstream than the RM6 and shows a large portion of the area between Airline
Rd, and FM521 as flooded.
7.9 Chocolate Bayou
Both East and West Chocolate Bayou have similar ditch capacity issues, with both streams having a 3 -year
capacity or less in all but a few ditch segments. The channel siphons under the raw water canal south of
SH6 are a major contributor to flooding in the area. They restrict flow, resulting in significant ponding
throughout the area, including over SH6. This results in a majority of the area upstream of the siphons
being in the floodplain. In addition, the backup allows water to overflow from one watershed to the other.
The most effective solution would be to remove the siphons and have them added to the canal as opposed
to the drainage channel; however, significant detention would be required to mitigate the conveyance
increases downstream. Most of the RM6 mapping ends before the revised existing inundation starts, so
there is no comparison. The mapping along the West Fork of Chocolate Bayou is fairly consistent between
the two mapping sources.
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8.0 Flood Reduction Alternative Analysis
As discussed in Section 1, there were a few principal goals of the Pearland/BDD4 Master Drainage Plan:
• Identify high-level comprehensive plan to provide 1% (100 -year) level of service as well as a lower
level of service option along the modeled streams.
• Evaluate the capacity of local ditches that flow into the modeled streams and estimate the cost
to achieve 3 -year and 10 -year level of service in the ditches.
• Develop a priority list of projects to reduce flood risks within the watersheds. This includes major
channel improvement and detention projects as well as improvements to smaller ditches to
improve local drainage.
Several flood reduction alternatives were considered and evaluated for these streams to accomplish the
goals of the study. An Alternative includes the complete solution for all the watersheds within the study
area. Each 'Alternative' is comprised of channel conveyance and detention improvements that will be
broken into many projects over the implementation lifetime. The alternatives evaluated can be grouped
into two major categories as described below.
• Alternative 1: 100 -year LOS (Contain flow in the channel with some exception for low-lying areas)
• Alternative 2: Additional LOS based on Existing Channel Capacity
This section details the types of improvements that were considered as well as the challenges that could
potentially be faced during implementation. The alternatives are discussed in detail in the sections below.
For this study, the LOS is defined as the storm event that.is contained within the banks of the channel
and/or detention basins. If the channel and/or detention basins are designed to contain the 100 -year
storm event, then the surrounding areas should only flood during storm events greater than the 100 -year.
However, the surrounding neighborhoods may not experience reduced flooding with channel and/or
detention basin improvements until the drainage infrastructure within the neighborhoods is improved as
well. Also, structures and property at elevations below the proposed water surface elevation for a given
LOS may continue to experience flooding, though it may be reduced. These structures must be raised
above the water surface elevation to avoid flooding during the LOS storm event.
8.1 Potential Improvement Options
The focus of the flood reduction alternative analysis was on structural improvements throughout the
study watersheds, specifically channel capacity improvements and regional detention. Detention was
provided for both reduction of peak discharges where channel conveyance improvements were infeasible
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as well as for mitigation of flow increases associated with channel conveyance improvements. Trapezoidal
sections were modeled for several streams to provide a 1% (100 -year) and an alternate LOS. The alternate
LOS was selected based on the current channel capacity of each of the modeled streams. Bridges and
culverts were assumed to be upsized where necessary to reflect the widened top width of the channels
due to proposed channel conveyance improvements. Improvements are addressed specifically for each
of the alternatives below.
The proposed channel alignments generally followed the existing channel alignments and additional ROW
needs were identified based on the recommended channel configuration. Several of the tributaries and
streams in the study are roadside ditches located along public roadways and within their ROW.
Another option to reduce flood damages in the watershed would be to consider buyouts of flood prone
properties. While buyouts could remain an option for existing flood damaged properties, it will not
address the flooding issues in the watershed or provide adequate drainage infrastructure for future
development in the area. Conversely, floodplain preservation of undeveloped property is an option that
could prevent future flood damages but will not address existing damages. However, no analysis of
buyouts for floodplain preservation was included with the flood reduction alternative analysis. If property
buyouts are considered feasible, those properties could potentially be used for detention or conveyance
improvements as projects are implemented.
8.1.1 Project Challenges
While the proposed alternatives could significantly reduce the flood potential throughout the watershed,
they are not without certain challenges. Among these are utility conflicts, property ownership,
environmental constraints, and other factors that could influence project implementation. These
challenges have been identified for each project evaluated and were considered during the
recommendation process.
One of the primary challenges is property ownership. ROW availability was a significant issue in
developing proposed alternatives throughout the watershed, particularly within the City of Pearland. This
area is highly urbanized with residential, commercial, and light industrial development. As part of the
alternative analysis, ultimate ROW widths were determined for channel improvement projects and
required detention volumes were determined to estimate how much property would be needed to
accommodate mitigation. The required ultimate ROW locations and widths are included in the individual
alternative descriptions within Section 8.4 and shown on the maps in Exhibits 10-23 for all watersheds.
Detention needs are indicated using circles of proportional size, which relates to the approximate total
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detention footprint needed in that area to achieve the desired Level of Service for the specific alternative
being evaluated (i.e. acres of detention ponds needed to provide the 100 -year LOS in Alternative 1). The
desired level of service alternatives are specified in Section 8.4, which discusses alternatives for flood
reduction measures. Specific properties or pond locations were not identified for detention as part of the
alternative analysis.
Utility crossings are another constraint that could potentially influence the implementation process. The
main utility concern is large oil and gas pipelines that are located throughout the watershed. These
crossings were considered during the alternative analysis; however, in many cases they are unavoidable,
and relocations must be done if the project is to be constructed. Approximate locations of major pipelines
have been included in the ROW maps (Exhibits 10-23). In addition, there are other utility crossings
including power line easements and several major raw water canal crossings.
Environmentally sensitive areas or areas with identified cultural resources may also pose a challenge to
the implementation of flood reduction improvements. Disturbance of these environmental areas could
require mitigation in the form of constructing new environmental areas to replace the disturbed areas or
purchasing credits. These areas have been identified on the ROW maps and efforts were made during the
alternative analysis process to avoid these areas. In those areas where potential impacts to wetlands
were unavoidable, mitigation costs have been included in the cost estimates.
8.2 Future Conditions Hydrology
While the intent of the flood reduction measures is to address existing flooding concerns within the
Pearland BDD4 area, the planning effort considered the in the future development conditions (i.e.
ultimate build -out) to ensure that the improvements provide the necessary protection for the long-term.
The future conditions hydrology accounted for increases in impervious cover associated with expected
development, which would primarily result in increased runoff volumes. Future conditions represent the
situation where current undeveloped areas become developed and is based on the ultimate build -out
expectation. Per BDD4 policy and City design standards, increases in peak flow due to future development
are required to be mitigated onsite and the future conditions hydrology maintained that assumption by
not adjusting any of the TC and R parameters. The specific updates to the hydrologic model were
discussed with Pearland/BDD4 and are detailed in the sections below. Due to changes in hydrology and
channel geometry, the Modified-Puls storage routing was updated for each of the channel improvement
alternatives, with the exception of those done for Hickory Slough and Mary's Creek. Both streams were
modeled using HEC -RAS Unsteady.
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8.2.1 Impervious Cover
The increase in imperviousness due to future development was estimated by updating the land use of the
study area to approximate fully -developed (future) conditions. For the City and its incorporated areas,
future development was based on the City's future land use classification from the comprehensive plan
information. For areas outside the City and its incorporated areas, future development was approximated
based on future thoroughfare information, which was provided by HGAC. The approximation based on
future thoroughfares added a 200 -ft buffer of high-density development (85% impervious) to each side of
proposed major thoroughfares and highways, which would consist mainly of commercial development.
The remaining area was represented as residential small lot development (40% impervious). Appendix
A.5 provides a map showing the estimated future land use. The updated percent impervious values were
added to the HEC -HMS model. The overall composite increase in percent impervious was 24%. While on-
site detention would offset any increases for the TC & R parameters, the change in impervious cover
slightly increased the peak discharges because of the way that the Green & Ampt Method calculates
infiltration losses. This results in a slight increase in peak discharges of around 3% on average with a
maximum of 7%.
8.2.2 Future Conditions Hydrologic Results
Appendix D includes a comparison of the existing and future development conditions discharges for each
drainage subbasin within the study area. When compared to existing development, the future
development subbasin discharges are, on average, about 3% higher than the existing development
discharges for the 100 -year event. The maximum increase in flow discharge from existing to future
conditions discharges is 7% while the minimum increase is 0%. The larger increases occur in subbasins
located in the southern portion of the watershed which is currently rural, and this area is assumed to
experience the greatest change in levels of development in the future. The smaller increases occur in the
northern portion where the watershed is highly developed and would not have much opportunity for
significant future development. The future development conditions discharges were used to size the
proposed channel conveyance improvements and detention ponds for multiple Levels of Service based
on the two different alternatives evaluated.
8.3 Potential Flood Reduction Measures
Using the peak discharges developed from the future development conditions hydrology, hydraulic
modeling was developed for several proposed flood reduction projects along the studied streams. These
alternatives include channel conveyance improvements and associated mitigation, regional detention
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basins, and storm sewers in areas where the channel is confined. The following are discussions of the
various flood reduction methods.
8.3.1 Trapezoidal Channel Conveyance Improvements
A trapezoidal channel section was evaluated for most of the streams for both Level of Service alternatives.
The trapezoidal sections include side slopes of 4:1 or 3:1 depending on the level of development, a 30'
maintenance berms on each side, and a minimum longitudinal slope of 0.001 ft/ft. Because of the
available space and limited development, a 4:1 was used on Cowart Creek and Cannon Ditch, while others
focused on 3:1 because of existing channel encroachments. Channel flowlines were lowered along the
streams wherever possible to gain depth. For the starting channel elevation on the proposed stream
improvements, the flowline of the receiving stream or higher was used. The channel improvements
resulted in a significantly wider channel sections than existing and, in some cases, the width was limited
due to property ownership. Detention required to mitigate the LOS storm event was provided on each
tributary such that those projects may stand on their own without creating negative impacts along the
main stem. Adverse impacts were monitored, and improvements were adjusted to prevent increased
water surfaces for all the modeled storms. Hydraulic summary and comparison tables are provided in
Appendix D.
Currently, there is a significant amount of volume that is stored in the channel overbanks for all of the
studied streams; this natural existing storage helps to attenuate peak flows downstream. The channel
conveyance improvements result in a more efficient system that can convey more water; however, the
loss of floodplain storage increases the peak discharge rates. This increase was mitigated for by using
detention. To account for this change, Modified-Puls routing was updated for each channel reach and
LOS alternative (Alternative 1, Alternative 2). Alternative specifics will be discussed in subsequent
sections. The Modified -Puts routing parameters are tabulated in Appendix B and include an update to
the storage discharge curves as well as the subreach calculations.
8.3.2 Regional Detention
Beyond just conveyance mitigation needs, regional detention was a significant part of the proposed
improvements because, as mentioned in Section 8.1, the existing channel depths could not always be
changed, and conveyance improvements were not feasible in all the reaches. Detention was used in a
similar fashion to reduce flows where channel conveyance improvements were avoided or minimized. IN
some instances, detention pond depths were limited because the receiving channels were shallow with
little vertical room to increase depth. This resulted in large detention footprint requirements for a given
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storage volume. Proposed ponds were modeled as diversions in HMS, which followed the procedure used
for the effective Clear Creek models.
As discussed in Section 8.1.1, one the biggest challenges in developing proposed alternatives was limited
ROW availability, particularly for improvements along Mary's Creek, Hickory Slough, and Mustang Bayou.
These areas are already developed or currently developing, with few undeveloped areas remaining. This
limits the potential detention pond locations. To communicate the necessary volume and approximate
footprint without identifying specific parcels, the acreage required was estimated using an assumed depth
and a general location of the pond(s), or reach along which ponds should be constructed, was determined
for a given LOS. Estimated detention ROW acreage was based on a 4:1 side slope, a 30 -foot maintenance
buffer from top of bank and a square pond. Pipeline and other utility crossings were avoided where
possible, and additional costs when utilities or environmentally sensitive areas could not be avoided are
included in the project cost estimates.
The detention volume listed in the sections below for each alternative are partly based on the LOS storm
event and the volume required to contain the flows from the LOS event within the channel banks. In
addition to checking that the WSEL was contained within the banks for that storm, any adverse impacts
to the stream were checked and mitigated across a range of storm events from the 3 -year to the 500 -
year. In many situations, this resulted in increased detention requirements. For example, the detention
requirement for 10 -year LOS may only be 500 ac -ft, but in order to remove impacts from storms above
the 10 -year, 700 ac -ft was needed
8.4 Alternatives for Flood Reduction Measures
During the existing conditions analysis, several streams were determined to have a capacity to provide
approximately a 3 -year LOS or less. The second LOS to be evaluated (Alternative 2) was selected based
on the existing channel capacity and a reasonable target LOS that could provide flood reduction benefits
but at a significantly lower cost than the 100 -year LOS (Alternative 1). The modeled alternatives were
largely selected based on the existing inundation limits and recorded flooded properties. Table 5 provides
the target LOS for each stream segment for Alternatives 1 and 2. It should be noted that in several
instances the channel improvements for both Alternatives 1 and 2 were the same or close in size, but
there was a substantial difference in the detention requirement. That is because many of the alternatives
rely on detention to bring flows down to a level where the channels can be sized reasonably. However,
even with the lower LOS, the detention may still be very large because of the impacts that were considered
for all storm events as discussed in Section 8.3.2 above.
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Table S. Existing Conditions Unsteady vs Steady Modeling WSEL Comparison
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Modeled Streams
Watershed
Streams/Tribs
Existing LOS
Alternative 1
AlternativeLOS* 2 LOS*
Chocolate
East Chocolate
Syr - 5yr
100yr
10yr
Rodeo Palms
3yr-5yr
100yr
10yr
E103
< 3yr
100yr
10yr
West Chocolate
< 3yr
100yr
Syr
Cold McCutchen
< 3yr
100yr
Syr
CR 383
< 3yr
100yr
Syr
Mustang
Mustang
< 3yr
100yr
25yr
Mustang Spur
< 3yr
100yr
25yr
Chigger
Old Chigger
Syr - 10yr
100yr
10yr
Resort Park
< 3yr
100yr
10yr
Chigger
< 3yr
100yr
10yr
Chigger Trib
< 3yr
100yr
10yr
Clear Creek Tribs
Country Place
100yr
N/A
N/A
Shadow Creek
50yr-100yr
N/A
N/A
Hickory
_
Hickory
3yr-5yr
100yr
10yr
Marys
Marys Main Stem
5yr - 10yr
100yr
25yr
Marys North Fork
50yr - 100yr
N/A
N/A
Marys South Fork
50yr - 100yr
N/A
N/A
Weatherford
50yr-100yr
N/A
N/A
Corrigan
100yr
N/A
N/A
Cowart
Cannon
< 3yr
100yr
10yr
Diversion Ditch
50yr - 100yr
100yr
N/A
Hood Ditch
< 3yr
100yr
10 -yr
Cowart Tributary
3yr - Syr
100yr
10 -yr
Dare Ditch
25yr-50yr
100yr
N/A
Cowart Creek - Upper
< 3yr
100yr
10yr
Cowart Creek - Lower
25yr-50yr
100yr
N/A
* N/A Indicates that existing capacity was high enough that improvements were not proposed
8.4.1 Chigger Creek
Much of the flooding along Chigger Creek and its tributaries occurs upstream of Highway 35 with a wide
area of inundation upstream of the BNSF Railway crossing. The existing channels west of Highway 35 all
provide a 3 -year LOS or less except for Old Chigger, which provides a 10 -year LOS or less. While most of
the area west of the BNSF Railway crossing is undeveloped, the area between the railroad and Highway
35 is more urbanized with residential areas to the south and more industrial land use to the north.
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Due to the low existing LOS, all studied streams besides Old Chigger required channel conveyance
improvements. Channel improvements were not proposed for Old Chigger due to the presence of multiple
residential properties adjacent to the stream and limited availability of undeveloped land for channel
expansion. There were multiple culvert improvements proposed for Old Chigger aimed at removing
hydraulic restrictions along the channel by upsizing culverts and increasing the number of barrels. Several
bridges and culverts, which adequately conveyed flows based on existing conditions, will need to be
increased to convey the increases in flow due to channel improvements and future conditions.
Detention is provided both in the upstream and downstream portions of the watershed to lower flows in
upstream portion and to reduce water surface elevations and mitigate for increase conveyance resulting
from the proposed channel improvements.
8.4.1.1 Chigger Creek :100 -year LOS
Exhibit 10 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $149 million and includes the following:
• Channel Bottom Width: 20-40 feet
• Channel Depth: 3.5 —15.5 feet
• Channel ROW: 100 —190 feet
• Bridge/Culvert Replacement: Ramirez Road, Highway 35 (2x), Hastings Road, CR 294, CR 191, Britt
Oaks Drive Ware Diary Road, St. Cloud Drive, 5 Private Roads, 6 Driveways, and 8 Private Crossings
• Detention Volume: 4,790 acre-feet
8.4.1.2 Chigger Creek: 10 -year LOS
Exhibit 17 A -C shows the project layout, ultimate ROW, and 100 -year inundation map for the 10 -year LOS
scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $75 million and includes the following:
• Channel Bottom Width: 20-40 feet
• Channel Depth: 3.5 —15.5 feet
• Channel ROW: 100 —190 feet
• Bridge/Culvert Replacement: Ramirez Road, Highway 35 (2x), Hastings Road, CR 294, CR 191, Britt
Oaks Drive Ware Diary Road, St. Cloud Drive, 5 Private Roads, 6 Driveways, and 8 Private Crossings
• Detention Volume: 1,700 acre-feet
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8.4.2 East Chocolate Bayou
Most of East Chocolate Bayou is heavily inundated, particularly upstream of the raw water canals, which
force the drainage across via siphons. The channel's upstream portion begins near the Palm Desert Drive
and Rodeo Drive intersection and the studied area ends between Mason Road and Burnett Road. There is
a shorter channel, E103, that confluences with Rodeo Palms Ditch, and at the most southern position of
East Chocolate Bayou, the Rodeo Palms Ditch merges with the main stem. The existing channels in this
area all provide a LOS between 3 to 5 -years, and the northern portion of Rodeo Palms Ditch is mostly
surrounded by residential development which limits the available ROW. Channel conveyance
improvements were investigated for all studied streams in the East Chocolate Bayou watershed for the
100 and 10 -year LOS. Several bridges and culverts, which adequately conveyed flows based on existing
conditions, will need to be increased to convey the increases in flow due to channel improvements and
future conditions.
Currently, the drainage siphons along the channel (crossing under the intersecting canals) are the biggest
contributor to flooding in the area, due to the flow restriction. Therefore, the removal of these siphons
was considered for both levels of service (10- and 100 -year). These alternatives would require that the
siphons along the channel be removed and the channel widened at the crossings. In order to allow the
canals to continue functioning, siphons would need to be built on the canals that cross under the channel.
As the canals have a more controlled discharge rate than the drainage, they would function more
effectively using siphons than the drainage channels do. To account for the costs of the new siphons, it
was assumed from other nearby siphons, that there would be 4 -60 -inch RCP pipes. Each pipe would also
have 2 headwalls. In addition to the removal of the siphons along the drainage channels and channel
conveyance improvements, detention was added to reduce the flooding in the East Chocolate Bayou
watershed and mitigate impacts due to conveyance increases.
8.4.2.1 East Chocolate Bayou: 100 -year LOS
Exhibit 11 A -C shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $203 million and includes the following:
• Channel Bottom Width: 45 — 75 feet
• Channel Depth: 5 - 11 feet
• Channel ROW: 160 — 260 feet
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• Bridge/Culvert Replacement: Oakland Park Drive, Palmero Way, US 288, Highway 6, Railroad, CR
81, CR 418, CR 80, 1 Private Road, 6 Driveways, and 2 Private Crossing Removals
• Remove existing drainage siphons and replace with canal siphons
• Detention Volume: 5,550 acre-feet
8.4.2.2 East Chocolate Bayou: 10 -year LOS
Exhibit 18 A -C shows the project layout, ultimate ROW, and 100 -year inundation map for the 10 -year LOS
scenario for East Chocolate. East Chocolate's 10 -year LOS maintained the same channel improvements
but decreased the detention. The channels maintained the same bottom widths, depths, ROW and bridge
replacements as the 100 -year LOS. The detention was reduced enough to not allow impacts on the water
surface elevation and flow. To not create impacts for all storm events including the 500 -year, more
detention was needed than for the target level of service, 10 -year. See Appendix D.6 for details on
structure improvements or changes. The total cost for this scenario is $161 million and includes the
following:
• Channel Bottom Width: 45 — 75 feet
• Channel Depth: 5 - 11 feet
• Channel ROW: 160 — 260 feet
• Bridge/Culvert Replacement: Oakland Park Drive, Palmero Way, US 288, Highway 6, Railroad, CR
81, CR 418, CR 80, 1 Private Road, 6 Driveways, and 2 Private Crossing Removals
• Remove existing drainage siphons and replace with canal siphons
• Detention Volume: 4,220 acre-feet
8.4.3 West Chocolate Bayou
Similar to East Chocolate Bayou, West Chocolate Bayou is also heavily inundated, particularly upstream
of the channel siphons used to cross the raw water canals. The channel has three sections: the western
section begins near the intersection of Fenn Road and FM 521 Road, the middle section begins near
County Road 383, and the eastern section starts by the Old Airline Road and Mars Drive intersection. The
western, middle and eastern channels were named West Chocolate, CR383 and Cold McCutchen,
respectively. CR383 confluences onto West Chocolate near the southern portion of West Chocolate, and
Cold McCutchen and West Chocolate merge at the most downstream potion of the channel. The existing
channels in this area all provide a 3 -year or less LOS. Channel conveyance improvements for the 100 -year
and 5 -year LOS (Alternative 2) were evaluated for each of the three streams. Several bridges and culverts,
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which adequately conveyed flows based on existing conditions, will need to be increased to convey the
increases in flow due to channel improvements and future conditions.
As with the East Chocolate Bayou alternative, the siphons on the West Chocolate streams were converted
to siphons on the raw water canals wherever stream crossings occurred. The removal of the drainage
siphons and replacement with canal siphons will allow the streams to flow without the significant
restriction that is currently in place. To account for the costs of the new siphons, it was assumed that
there would be 4 -60 -inch RCP pipes. Each pipe would also have 2 headwalls. In addition to the removal
of the siphons along the drainage channels and channel conveyance improvements, detention was added
to reduce the flooding in the East Chocolate Bayou watershed and mitigate impacts due to conveyance
increases.
8.4.3.1 West Chocolate Bayou: 100 -year LOS
Exhibit 12 A -C shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. A detention pond is necessary in the upstream section of CR383 to decrease the flooding
from the upstream contributing drainage area. A constraint to the detention pond is that the area nearest
to the channel is all residential development. The area north of the residential area is more rural and less
developed. Building the pond in the residential area would increase the cost of the pond , whereas
building the pond in the rural area would be more economical. While calculating the improvements costs,
the detention area was estimated to be one third within the residential area and two thirds in the more
rural area. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $458 million and includes the following:
• Channel Bottom Width: 35 — 80 feet
• Channel Depth: 4 —18 feet
• Channel ROW: 120 — 280 feet
• Bridge/Culvert Replacement: Rio Lindo Street, Rio Ramos Street, Highway 6 (2x), Oak Street (2x),
Railroad (2x), Sanders Street, North Pine Road, Coen Road, South Pine Road, CR 383, 4 Driveways,
and 3 Private Crossing Removals
• Remove existing drainage siphons and replace with canal siphons
• Detention Volume: 6,220 acre-feet
8.4.3.2 West Chocolate Bayou: 5 -year LOS
Exhibit 19 A -C shows the project layout, ultimate ROW, and 100 -year inundation map for the 5 -year LOS
scenario. The same channel improvements that were created for the 100 -year LOS were utilized for this
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second alternative. To not create impacts for all storm events including the 500-year, more detention was
needed than for the target level of service, 5-year. See Appendix D.6 for details on structure
improvements or changes. The total cost for this scenario is $390 million and includes the following:
• Channel Bottom Width: 35 — 80 feet
• Channel Depth: 4 —18 feet
• Channel ROW: 120 — 280 feet
• Bridge/Culvert Replacement: Rio Undo Street, Rio Ramos Street, Highway 6 (2x), Oak Street (2x),
Railroad (2x), Sanders Street, North Pine Road, Coen Road, South Pine Road, CR 383, 4 Driveways,
and 3 Private Crossing Removals
• Remove existing drainage siphons and replace with canal siphons
• Detention Volume: 4,780 acre-feet
8.4.4 Country Place Ditch
Country Place Ditch revised existing conditions modeling shows the ditch having almost a 100 -year LOS;
however, there is ponding in the streets. In order to remove the street ponding for the future
development condition, approximately 156 ac -ft of detention would need to be added upstream of SH
288.
8.4.5 Cowart Creek
Much of the flooding along Cowart Creek and its tributaries occurs upstream BNSF Railway crossings. The
existing channels west of the BNSF railway provide a 5 -year LOS or less except for the upper portion of
Diversion Ditch, which provides a 50 -year LOS or less. While most of the area east of the BNSF Railway
crossing is industrial land use (oil fields), the area upstream of the railroad is more urbanized with
residential areas to the north and more industrial land to the south. The upstream end of Cannon Ditch
and in the south-west corner of Diversion Ditch is largely undeveloped. Downstream of the confluence of
Cowart Creek and Cannon Ditch, the landuse is predominately residential. Several bridges and culverts,
which adequately conveyed flows based on existing conditions, will need to be increased to convey the
increases in flow due to channel improvements and future conditions.
Due to the low existing LOS, all studied streams required channel conveyance improvements. There were
multiple culvert improvements proposed aimed at removing hydraulic restrictions along the channel by
upsizing culverts and increasing the number of barrels. Further, the existing storm sewer along Bailey road
requires a significant increase in size and flow line adjustment upstream of Diversion Ditch to provide a
100 -year LOS. The storm sewer downstream of Diversion Ditch requires an increase in size in several
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reaches. Detention is provided throughout the watershed to lower flows in upstream portion and to
reduce water surface elevations and mitigate for increase conveyance resulting from the proposed
channel improvements.
8.4.5.1 Cowart Creek: 100 -year LOS
Exhibit 13 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $343 million and includes the following:
• Channel Bottom Width: 20 — 50 feet
• Channel Depth: 5 —16 feet
• Channel ROW: 70 — 290 feet
• Bridge/Culvert Replacement: CR 143, Amoco St (2x), Railroad (4x), Ramirez Road, CR 128, Main
Street, CR 327, CR 143, CR 175C, CR 176C, CR 104, CR829, CR 143, CR 115 (2x), Moore Road, CR
130, CR 129, Baker Road, 6 Private Roads, 4 Driveways, and 5 Private Crossing Removals
• Weir Adjustment: Lower Weir on Diversion Ditch
• Storm Sewer Replacement: 1900 LF of 4'x3.5' RCBs, 900 LF of 6'x4' RCBs, 600 LF of 4'x2.5' LF, 900
LF of 5'x3.5', 5800 LF of 2-6'x5'RCBs
• Detention Volume: 6,470 acre-feet
8.4.4.2 Cowart Creek: 10 -year LOS
Exhibit 20 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 10 -year LOS
scenario. Unlike other streams, the lower LOS does not match the 100 -year LOS proposed geometry. The
10 -year LOS does not change the existing storm sewer system along Bailey Road nor does it change the
diversion channel upstream of CR 143. See Appendix D.6 for details on structure improvements or
changes. The total cost for this scenario is $325 million and includes the following:
• Channel Bottom Width: 20 — 50 feet
• Channel Depth: 5 —16 feet
• Channel ROW: 70 — 290 feet
• Bridge/Culvert Replacement: CR 143 (2x), Amoco St (2x), Railroad (4x), Ramirez Road, CR 128,
Main Street, CR 327, CR 143, CR 175C, CR 176C, CR 104, CR829, CR 115 (2x), Moore Road, CR 130,
CR 129, Baker Road, 6 Private Roads, 4 Driveways, and 5 Private Crossing Removals
• Detention Volume: 5,475 acre-feet
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8.4.6 Hickory Slough
The majority of flooding along Hickory Slough not related to Clear Creek occurs upstream of Mykawa
Road. The entirety of the channel has residential and light industrial development which limits the
available ROW. The channel runs from just downstream of CR 94 to the confluence with Clear Creek.
Channel conveyance improvements run from CR 94 to 3500 ft upstream of Cullen Road and from Garden
Road to the Burlington Northern and Santa Fe railway crossing. Further, two bypass channels were added
between Roy Road and Oday Road to add additional capacity to Hickory Slough away from the road -side
ditch. The current stream is constrained by the roadway and several structures and cannot be adequately
improved within these constraints. The bypass allows flow to follow a path similar to the original Hickory
Slough alignment, which is still visible in the terrain. Several bridges and culverts, which adequately
conveyed existing conditions flows, will need to be upsized to convey the increases in flow due to channel
improvements and future development conditions. Due to uncertain land availability, Halff did not lay out
specific detention pond locations for Alternatives 1 and 2, but instead identified required detention as
well as the general location in which it would be required.
8.4.5.1 Hickory Slough: 100 -year LOS
Exhibit 14 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. For the 100 -year LOS, two bypass channels in close succession between Roy and Oday roads
were required. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $166 million and includes the following:
• Channel Bottom Width: 5 — 30 feet
• Channel Depth: 7 —15 feet
• Channel ROW: 130 — 230 feet
• Bridge/Culvert Replacement: Fair Oaks Street, Miller Ranch Road, Garden Road, Oday Road,
Hatfield Road, Woody Drive, Mykawa Road, 1 Private Road, 2 Pedestrian Bridges, and 2 Driveways
• New Bridge: Roy Road at proposed channel bypass
• Detention Volume: 3,800 acre-feet
8.4.5.2 Hickory Slough: 10 -year LOS
Exhibit 21 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 10 -year LOS
scenario. For the 10 -year LOS, two bypass channels in close succession between Roy and Oday roads were
required. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $133 million and includes the following:
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• Channel Bottom Width: 5 — 30 feet
• Channel Depth: 7 —15 feet
• Channel ROW: 130 — 230 feet
MEN HALFF'
• Bridge/Culvert Replacement: Fair Oaks Street, Miller Ranch Road, Garden Road, Oday Road,
Hatfield Road, Woody Drive, Mykawa Road, 1 Private Road, 2 Pedestrian Bridges, and 2 Driveways
• New Bridge: Roy Road at proposed channel bypass
• Detention Volume: 2,850 acre-feet
8.4.7 Mary's Creek
Most of the flooding along Mary's occurs upstream of the split between Mary's Creek and Mary's Creek
Bypass. While much of the channel downstream appears to have near adequate capacity to carry existing
100 -year discharges. The entirety of the channel has residential development which limits the available
ROW. The channel runs from just downstream of Southwyk Parkway to the confluence with Clear Creek.
Channel conveyance improvements run from upstream of Manvel Road to downstream of Pearland
Parkway. Several bridges and culverts, which adequately conveyed flows based on existing conditions, will
need to be increased to convey the increases in flow due to channel improvements and future conditions.
Due to restrictions on land availability, Halff did not lay out specific detention pond locations but instead
identified required detention and the approximate location it is needed.
8.4.7.1 Mary's Creek: 100 -year LOS
Exhibit 15 A-E shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. Note that the 500 -year flows on Mary's Creek control the required detention so several
detention basins were significantly increased to mitigate for the 500 -year impacts. See Appendix D.6 for
details on structure improvements or changes. The total cost for this alternative is $294 million and
includes the following:
• Channel Bottom Width: 30-40 feet
• Channel Depth: 11— 20 feet
• Channel ROW: 160 —250 feet
• Bridge/Culvert Replacement: Manvel Road, Magnolia Street, Harkey Road, McLean Road,
Veterans Drive, Railroad, HWY 35/ Main Street, Old Alvin Road, Pearland Parkway, John Lizer
Road, Liberty Drive, Longherridge Drive, and 1 Pedestrian Bridge.
• Detention Volume: 5,770 acre-feet
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8.4.7.2 Mary's Creek: 25 -year LOS
Exhibit 22 A-E shows the project layout, ultimate ROW, and 100 -year inundation map for the 25 -year LOS
scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $176 million and includes the following:
• Channel Bottom Width: 30 — 40 feet
• Channel Depth: 11— 20 feet
• Channel ROW: 160 —250 feet
• Bridge/Culvert Replacement: Manvel Road, Magnolia Street, Harkey Road, McLean Road,
Veterans Drive, Railroad, HWY 35/ Main Street, Old Alvin Road, Pearland Parkway, John Lizer
Road, Liberty Drive, Longherridge Drive, Dixie Farm Road, and 1 Pedestrian Bridge.
• Detention Volume: 3,480 acre-feet
8.4.8 Mustang Bayou
Significant flooding occurs throughout Mustang Bayou; however, this study only addresses flooding
downstream of the Fort Bend — Brazoria County Line to 700 ft downstream of CR 90. Proposed channel
conveyance improvements run from FM 521 to CR 90. Several bridges and culverts, which adequately
conveyed flows for the existing development conditions, will need to be increased to convey the increases
in flow due to channel improvements and future development conditions. Due to restrictions on land
availability in much of the Mustang Bayou watershed, Halff did not lay out specific detention pond
locations for the two alternatives, but instead identified the detention recommended to achieve the
desired LOS as well as the approximate location in which it would be needed.
Upstream of FM521, within Fort Bend County, no improvements were recommended, but potential
impacts caused by the proposed improvements were avoided for the future development condition. The
area along Mustang Bayou surrounding SH288 has experienced development, which does not appear in
the LiDAR dataset; However, those areas were filled above BFE and have been removed from the
floodplain. The area downstream of Highway 288, but within Brazoria County has areas adjacent to the
channel that are very low as compared to the existing channel. As such, improvements to the level of
service desired could not obtained without improvements outside the BDD4 jurisdiction. As such, this
area was treated as a "no -impact" area, meaning that the proposed improvements upstream would not
negatively impact the drainage, but no significant flood reduction benefits were achieved. Additional
benefits could be achieved by considering joint projects with the neighboring jurisdictions.
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8.4.8.1 Mustang Bayou: 100 -year LOS
Exhibit 16 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 100 -year
LOS scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $303 million and includes the following:
• Channel Bottom Width: 100 feet
• Channel Depth: 5 —15 feet
• Channel ROW: 200 — 280 feet
• Bridge/Culvert Replacement: CR 564, CR 48, CR 84, SH 288, and 2 Private Crossing Removals
• Detention Volume: 5,380 acre-feet
8.4.8.2 Mustang Bayou: 25 -year LOS
Exhibit 23 A -D shows the project layout, ultimate ROW, and 100 -year inundation map for the 25 -year LOS
scenario. See Appendix D.6 for details on structure improvements or changes. The total cost for this
scenario is $160 million and includes the following:
• Channel Bottom Width: 100 feet
• Channel Depth: 5 —15 feet
• Channel ROW: 200 — 280 feet
• Bridge/Culvert Replacement: CR 564, CR 48, CR 84, SH 288, and 2 Private Crossing Removals
• Detention Volume: 2,950 acre-feet
8.4.8.3 Mustang Bayou Extension
The area between Highway 288 and CR 90 has a shallow, flat channel, which prevents significant flood
reduction in the area. One way to address flooding in this vicinity is a bypass channel that would begin
downstream of CR 90 and end near Lira Road, which would allow the channel flow line to be lowered.
This channel would be outside of the BDD4/Pearland boundaries and would require coordination with
Brazoria Drainage District No. 3 as well as acquisition of property along the proposed alignment. Based
on preliminary calculations, a bypass channel would allow for a 5 -year LOS where currently there is less
than a 3 -year LOS. The estimate channel would require an approximately 235 ft ROW. A more detailed
investigation of the extension is necessary for any cost estimates or detailed explanation of benefits.
8.4.9 Shadow Creek Ranch
Shadow Creek Ranch has near to a 100 -year LOS with the exception of some street ponding in low-lying
areas adjacent to the channel. The area is currently fully developed in Pearland, but based on the future
development conditions hydrologic analysis, development in Fort Bend County could result in ponding
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increases along the ditch if it is not mitigated. Coordination with Fort Bend Drainage district on
development and drainage projects in their jurisdiction is recommended in order to avoid these increases.
Using the same methodology as discussed in Section 8.2, an onsite detention requirement in Fort Bend
County could still result in the need for 290 ac -ft upstream of FM 521 and culvert size along the ditch. A
demonstration of no impact for projects in Fort Bend County would allay these concerns.
8.5 Local Ditch Capacity Improvements
A procedure similar to what was discussed in Section 6.4 was completed to determine the proposed
dimensions for local ditch improvements. Using the Manning's equation and the 3 -year and 10 -year
estimated discharges, the cross-sectional geometry needed to carry the respective flow rate was
determined. Proposed cross sections were assumed to have 4:1 side slopes to meet BDD4 design criteria.
The ditch bottom width, and depth were updated so that the calculated capacity of ditch matched or
exceeded the drainage area discharges. Ditches were sized to contain the 3 -year and 10 -year discharges
within the ditch banks. Appendix D.3 - D.4 provides tables summarizing the proposed ditch geometries
for the 3 -year and 10 -year levels of service. Conceptual level engineering cost estimates for local ditch
improvements are provided in Appendix F. Recommendations for local ditch improvements are discussed
in Section 10.6 and 11.0 as well as in the Implementation Plan (Appendix G).
Figure 4. General Proposed Cross Section Layout
TOP WIDTH
------------
-
----------------
----- EXISTING
DITCH
DEPTH
SIDE SLOPE f ROUGNESS (N)
Z
I_ BOTTOM _I
I� WIDTH �I
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9.0 Costs and Benefits
The recommendation and prioritization of flood risk reduction alternatives was based on a combination
of estimated implementation costs and evaluation of general project benefits. A cost/benefit analysis was
performed to help prioritize the different proposed flood reduction solutions. The methodology used for
estimating costs and performing the cost/benefit analysis is described in the following sections. In
addition, other benefits were considered in order to better balance the recommendations for those areas
where development is not as dense, or property values are lower due to socio-economic conditions. Each
of the alternative benefits are discussed in this section.
9.1 Project Cost Components
Project cost estimates were prepared for each of the proposed alternatives. Cost estimates included
several items such as conveyance and detention improvements, utility relocation, ROW acquisition,
Wetlands Mitigation, and other costs associated with implementation of the proposed alternatives.
Detailed cost estimates for each scenario that was evaluated are provided in Appendix F.
9.1.1 Construction Costs
Estimates for channel conveyance improvements and detention included overhead costs for several work
items in the spreadsheet such as:
• Mobilization — 5% of Direct Construction Costs
• Planning, Engineering, Design — 12% of Direct Construction Costs
• Construction Management— 10% of Direct Construction Costs
• Contingencies — 20% of Direct Construction Costs
Channel improvement cost estimates included site preparation, earthwork, pipeline and utility conflicts,
and installation of structures (such as bridges and culverts/headwalls). Channel excavation volumes were
determined using the HEC -RAS Channel Modification tool, which calculated cut/fill quantities by
comparing the proposed channel cross-section to the existing channel one.
Structure costs were further broken down into installation and removal costs to account for the cost of
both removing the existing structure and constructing the new one. Bridge costs were based on an
assumed average cost per square foot of bridge deck, estimated from the existing bridge deck width and
the new span length needed for the widened proposed channel. The bridge removal and installation costs
were assumed to be $25/SF and $85/SF respectively.
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The installation costs for major culverts were based on the modeled configuration required to achieve the
specific LOS for each alternative. An average cost per linear foot of RCP or RCB was used to estimate the
total cost for each structure based on the culvert size, length, and number of barrels. The removal cost
for the major culverts was estimated at $40/LF.
Many small, private bridges and culverts (such as driveway crossings) across various streams were not
modeled. A list of structures and the general type of improvement are provided in Appendix D. To
estimate the installation cost, each crossing was assumed to be 25 feet long and would utilize dual 60"
RCPs. This is an estimate and should not be regarded as a recommendation for construction. The removal
cost for the small bridges and culverts was $85/SF and $40/LF, respectively.
Detention cost estimates were also prepared, which included similar items. Detention volume was
measured up to the estimated peak water surface elevation in the ponds, which was related to the
maximum water surface elevation in the adjacent channel. In most cases, this maximum storage elevation
was several feet below the existing natural ground. This required additional excavation above the
maximum storage elevation, which is why the excavation volumes were based on the necessary mitigation
volume plus additional earthwork needed to reach existing grade. Further refinement of the pond designs
will be completed during subsequent H&H analyses as part of future individual projects. Detention pond
outfall and overflow structure costs are included in the detention estimates. For the purposes of cost
estimating, a 48" RCP outfall pipe was assumed. Given the relatively small cost of the pipe and headwall
versus other cost items (primarily earthwork and ROW acquisition), the costs associated with variations
in actual proposed outfall could reasonably be considered to be within the 20% contingency. Large
overflow weir structures were estimated using an assumed depth and estimated peak flow rate from
modeling for each alternative. The assumed depth was used to determine the required length and
associated surface area required to convey a given peak flow. The surface area was assumed to be
concrete paved and used for costing the weir structures.
9.1.2 RDW Acquisition Costs
Ultimate ROW widths were determined for each channel improvement alternative using the proposed
top -width and a 30' maintenance easement on either side of the channel. This information was overlaid
onto the Harris Galveston Area Council (HGAC) parcel data to determine potential property to be
acquired. In order to estimate detention ROW acquisition costs, an average cost/acre was determined
using the properties surrounding the potential detention pond location. ROW widths for all the evaluated
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projects are shown in Exhibits 10 through 23. Detention ROW needs were estimated based on the
calculated acreage and placement of the proposed detention ponds.
Using the ultimate channel and detention ROW information, the cost of acquisition was determined using
the area and estimated value per acre for each property. The estimated cost per acre for each detention
basin and channel improvement is provided by segment in Appendix F. For each parcel, any publicly -
owned land (i.e. existing city, or county) was subtracted from the total area required since land acquisition
costs were assumed to be $0 for publicly -owned parcels. Additional closing costs were added, including
a 35% value markup and other associated fees to account for potential relocation and demolition costs
when structures could not be avoided. Structures being acquired were priced at the "condemned" cost
(2X the voluntary cost) for the purpose of these estimates.
9.1.3 Utility Relocation Costs
For the proposed alternatives, pipeline and overhead utility relocation were considered but some public
utility information, such as water and sewer lines, was not available. In lieu of available utility data, water
and sewer lines quantities to be relocated were estimated by using road lengths within the well-developed
areas. Water and sewer relocation costs were determined by identifying the total road length within each
specific ROW area and multiplying by the unit prices. Pipeline locations were based on information
provided by COP and BDD4. Relocation lengths were estimated based on the length of the pipeline
located within the identified project ROW. Cost per linear foot of pipeline relocation were based on the
diameter and linear footage to be relocated. Power transmission lines were based on information
provided by COP and BDD4. The total number of power lines to be relocated within each specific ROW
area was determined and multiplied by the unit price. The unit prices for the pipeline and power line
relocations were based on costing information provided from recent bid tabulations.
9.2 Total Flood Reduction Alternative Costs
A detailed breakdown of costs for each scenario is provided in Appendix F. These costs are considered a
planning level estimate and will need to be refined as projects are implemented.
9.2.1 Chigger Creek
The 100 -year LOS scenario has a total cost of $149 million. The 10 -year LOS scenario has a total cost of
$75 million. Table 6 provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
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Table 6. Chigger Creek Cost Estimate Summary
HALFF
9.2.2 East Chocolate Bayou
The 100 -year LOS scenario has a total cost of $203 million. The 10 -year LOS scenario has a total cost of
$161 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 7. East Chocolate Bayou Cost Estimate Summary
Detention
Channel
ROW Acquisition
Scenario
Detention
Improvement
Costs ($M)
Total Cost
Detention
Channel
Costs ($M)
Costs ($M)
($M)
Chigger Creek Alternative 1- 100 -yr LOS
$117
$32
$16
$3
$149
Chigger Creek Alternative 2 - 10 -yr LOS
$43
$32
$5
$3
$75
9.2.2 East Chocolate Bayou
The 100 -year LOS scenario has a total cost of $203 million. The 10 -year LOS scenario has a total cost of
$161 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 7. East Chocolate Bayou Cost Estimate Summary
9.2.3 West Chocolate Bayou
The 100 -year LOS scenario has a total cost of $439 million. The 5 -year LOS scenario has a total cost of
$373 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 8. West Chocolate Bayou Cost Estimate Summary
Detention
Channel
ROW Acquisition
Scenario
Costs
Improvement
Costs ($M)
Total Cost
($M)
($M)
Costs ($M)
Detention
Channel
East Chocolate Bayou Alternative 1 - 100 -yr
$172
$31
$52
$14
$203
LOS
East Chocolate Bayou Alternative 2 - 10 -yr
$130
$31
$36
$14
$161
LOS
9.2.3 West Chocolate Bayou
The 100 -year LOS scenario has a total cost of $439 million. The 5 -year LOS scenario has a total cost of
$373 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 8. West Chocolate Bayou Cost Estimate Summary
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Detention
Channel
ROW Acquisition
Scenario
Costs
Improvement
Costs ($M)
Total Cost($ )
M
($M)
Costs ($M)
Detention
Channel
West Chocolate Bayou Alternative 1 - 100 -yr
$388
$51
$257
$11
$439
LOS
West Chocolate Bayou Alternative 2 - 5 -yr
$322
$51
$225
$11
$373
LOS
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9.2.4 Cowart Creek
::C HALFF
The 100 -year LOS scenario has a total cost of $343 million. The 10 -year LOS scenario has a total cost of
$325 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 9. Cowart Creek Cost Estimate Summary
Scenario
Detention
Costs
($M)
Channel
Improvement
Costs ($M)
ROW Acquisition
Costs ($M)
Total Cost
($M)
Detention
Channel
Cowart Creek Alternative 1- 100 -yr LOS
$226
$116
$69
$45
$343
Cowart Creek Alternative 2 - 10 -yr LOS
$216
$109
$69
$43
$325
9.2.5 Hickory Slough
The 100 -year LOS scenario has a total cost of $166 million. The 10 -year LOS scenario has a total cost of
$133 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 10. Hickory Slough Cost Estimate Summary
Scenario
Detention
Costs
($M)
Channel
Improvement
Costs ($M)
ROW Acquisition
Costs ($M)
Total Cost
($M)
Detention
Channel
Hickory Slough Alternative 1 - 100 -yr LOS
$131
$35
$59
$13
$166
Hickory Slough Alternative 2 - 10 -yr LOS
$98
$35
$43
$13
$133
9.2.6 Mary's Creek
The 100 -year LOS scenario has a total cost of $294 million. The 25 -year LOS scenario has a total cost of
$176 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 11. Mary's Creek Cost Estimate Summary
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Channel
ROW Acquisition
Scenario
Detention
Improvement
Costs ($M)
Total Cost
Detention
Channel
Costs ($M)
Costs ($M)
($M)
Mary's Creek Alternative 1 - 100 -yr LOS
$241
$53
$118
$17
$294
Mary's Creek Alternative 2 - 25 -yr LOS
$123
$53
$46
$17
$176
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9.2.7 Mustang Bayou
The 100 -year LOS scenario has a total cost of $216 million. The 25 -year LOS scenario has a total cost of
$129 million. Table provides a breakdown of the total costs by channel conveyance improvements,
detention requirements, and ROW acquisition.
Table 12. Mustang Bayou Cost Estimate Summary
Scenario
Detention
Costs
($M)
Channel
Improvement
Costs ($M)
ROW Acquisition
Costs ($M)
Total Cost
($M)
Detention
Channel
Mustang Bayou Alternative 1- 100 -yr LOS
$186
$30
$86
$6
$216
Mustang Bayou Alternative 2 - 25 -yr LOS
$99
$30
$44
$6
$129
9.2.8 Capacity Analysis Ditches
Ditch improvement costs were estimated based on linear footage of improvement because no modeling
was conducted for these ditches. The cost for ditch improvements used for the 3 -year LOS is $7.60 per
linear foot. This value is an average cost based on TxDOT and Harris County published bid prices. There
is no pricing that considers the size of the channel when basing the cost on linear footage alone. To
account for the difference in size between the 3 -year LOS and 10 -year LOS ditch improvements, a ratio of
the average channel bottom widths was determined. This ratio along with the 3 -year LOS improvement
cost was then used to calculate the 10 -year LOS improvement cost of $10.60 per linear foot.
The overhead and contingency costs are the same as those for the main streams in the study. The cost of
ROW acquisition is estimated to be approximately 33% of the project subtotal. This value is the average
percentage of the ROW costs compared to the total project cost for the main streams in the study. This
ROW cost was added to the project subtotal to calculate the total cost for each ditch segment. Appendix
F provides a summary of the cost estimates for these ditch improvements.
9.3 Benefit -Cost Analysis
A benefit -cost analysis (BCA) of the proposed improvements was also conducted to determine the value
of the improvements relative to estimated construction and ROW costs. A traditional benefit -cost (BC)
ratio was calculated based on project costs vs. benefits. The benefits were determined by subtracting the
present value damages for the proposed project scenarios from the existing present value damage
estimates. BC ratios were calculated for the main stems of Mary's Creek, Hickory Slough, Cowart Creek,
Chigger Creek, Chocolate Bayou, and Mustang Bayou.
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9.3.1 Flood Damage Assessment
As part of the study, a damage assessment was conducted using the HEC -FDA. HEC -FDA is a flood damage
reduction analysis software developed by the USACE. This program assists in analyzing the economics of
flood risk management measures and formulating a flood risk management plan by visualizing data and
results and computing the expected and equivalent annual damages.
The main stem of Hickory Slough, Mary's Creek, Cowart Creek, East Chocolate and West Chocolate Bayou,
Chigger Creek, and Mustang Bayou were analyzed for damages. Some tributaries were also analyzed,
particularly in the Chocolate Bayou watershed. However, for areas where the main stem of a stream and
its tributary had areas of overlapping inundation, the main stem took precedence. The streams were
divided into damage reaches, which were defined based on major roadway crossings and are used to
calculate the annualized damages in HEC -FDA.
The Harris -Galveston Area Council (HGAC) parcel data was used to create a structure module to estimate
annualized and present value damages. For every parcel, a data point representing the structure was
placed at the centroid of the parcel and a ground elevation extracted from the chosen LiDAR dataset. In
several cases this method placed data points far from the actual home location according to the aerial
imagery. These data points were edited manually to better represent the true structure location and
elevation. Each structure was assigned to the closest cross section from the appropriate hydraulic model.
HEC -FDA uses finished -floor elevations for damage calculations which were assumed at 0.5 feet above the
ground elevation for slab structures. Because HGAC does not differentiate between slab and pier
structures, all structures were assumed slab for the damage assessment. The only exception to this was
where mobile home communities were identified. Mobile home finished -floor elevations were assumed
at 3 feet above the ground elevation.
No information about specific structure improvements was provided. The same structure module was
used for both existing conditions and future conditions damages. Along with the value of the structures,
structure contents were also required for HEC -FDA. Damage curves used for the contents were selected
from the appropriate category in the depth damage tables developed by USACE New Orleans District. This
calculation does not include costs associated with damaged vehicles or lost productivity.
A 50 -year project lifetime was assumed for this flood damage analysis. HEC -FDA requires the assignment
of a "Most Likely Future Year' which was defined individually and is the estimated year when full buildout
is reached for the watershed. Buildout timelines were discussed with the City and BDD4 and were based
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HALFF*
on the current level of development in the watershed and development pressure. The assumed project
durations in Table below were used to develop the "Most Likely Future Year" within HEC -FDA.
Table 13. Most Likely Future Year Estimate
Watershed
Most Likely Future
(years)
Hickory Slough
20
Mary's Creek
20
Cowart Creek
30
Chocolate Bayou
40
Chigger Creek
30
Mustang Bayou
30
HEC -FDA uses the results from the hydraulic models to calculate the depth of flooding each structure
experiences and calculates annualized damages for the stream segment. A "Without -Project" plan was
analyzed and uses the existing conditions hydraulic model. "With -Project" plans were analyzed and use
the proposed Alternatives 1 and 2 hydraulic models. HEC -FDA results are calculated and provided as
annualized damages for the 'Base Year" flows (2018) and the "Most Likely Future Year" flows (See Table
) based on a total analysis period of 50 years. The present values damages were calculated using the
annualized values for the period from the Base Year to the End of Analysis (50 years). Present value
calculations used the total analysis period (50 years) and a discount rate of 4%. The structures and
resulting damages were assigned to the appropriate watershed. Error! Reference source not found.A-14C
summarizes the expected damage costs for each major watershed for existing as well as improved
conditions.
Table 3A. Existing Flood Damage Assessment Summary
631 Page
Without Project
Stream
Annualized Damages
Present Value
Damages
Current Year
(2018)
Most Likely
Future Year
Mary's Creek
$
2,059,490
$
2,207,130
$
45,407,291
Hickory Slough
$
1,975,560
$
2,170,590
$
43,978,306
East Chocolate Bayou
$
208,280
$
522,700
$
5,005,949
West Chocolate Bayou
$
758,140
$
954,220
$
16,618,040
Chigger Creek
$
35,720
$
48,120
$
819,283
Mustang Bayou
$
1,645,470
$
1,899,370
$
36,411,442
Cowart Creek
$
1,078,040
$
1,427,280
$
24,621,481
Total
$
7,760,700
$
9,229,410
$
172,861,792
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MEN HALFF
Note: This table provides the annualized damage costs for the Base Year flows and the Future
flows as well at the calculated present value damage costs for the existing conditions.
The current annualized damages Shown in Table 14A are the average damages that are expected from
flooding based on the current condition, with none of the recommended flood reduction projects in place.
The "Most Likely Future Year" values represent the estimated increase in annualized damages that is
expected when each watershed reaches its ultimate (full build -out) condition if no flood reduction
projects are implemented.
The Present Value Damages represent the current estimated cost of damages over the 50 -year period in
today's dollars. As expected, the highest expected damages are in those areas where the watershed is
more heavily developed, including Hickory Slough, Mary's Creek, and Mustang Bayou. At the other end
of the spectrum, Chigger Creek has limited expected damages due to its sparse development.
Similarly, the values shown in Table 14B represent the expected damages if Alternative 1 were fully
implemented. The improvements would result in expected damages to $24M, a reduction of nearly
$150M over the 50 -year analysis period, representing an 86% drop in economic costs due to flooding.
141B: Alternative 1 Flood Damage Assessment Summary
Note: This table provides the annualized damage costs for the Base Year flows and the Future flows as well at
the calculated present value costs for improvements associated with Alternative 1.
Table 14C shows a similar pattern for Alternative 2; however, because the level of service for Alternative
2 is not as high as for Alternative 1, the expected damages are a little higher. The improvements would
result in expected damages to $38M, a reduction of nearly $135M over the 50 -year analysis period,
representing an 78% drop in economic costs due to flooding.
641 Page
With Project - Alternative 1
Stream
Level of
Service
Annualized Damages
Present Value
Damages
Current Year Most Likely
(2018) Future Year
Mary's Creek
100 -year
$
397,870
$
445,640
$
8,924,075
Hickory Slough
100 -year
$
78,590
$
93,100
$
1,802,790
East Chocolate Bayou
100 -year
$
20
$
970
$
2,035
West Chocolate Bayou
100 -year
$
550
$
4,670
$
18,776
Chigger Creek
100 -year
$
-
$
-
$
-
Mustang Bayou
100 -year
$
544,530
$
712,680
$
12,401,997
Cowart Creek
100 -year
$
37,230
$
38,670
$
805,801
Total
$
1,058,790
$
1,295,730
$
23,955,473
Note: This table provides the annualized damage costs for the Base Year flows and the Future flows as well at
the calculated present value costs for improvements associated with Alternative 1.
Table 14C shows a similar pattern for Alternative 2; however, because the level of service for Alternative
2 is not as high as for Alternative 1, the expected damages are a little higher. The improvements would
result in expected damages to $38M, a reduction of nearly $135M over the 50 -year analysis period,
representing an 78% drop in economic costs due to flooding.
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Table 14C: Alternative 2 Flood Damaee Assessment Summary
mom HALFF
Note: This table provides the annualized damage costs for the Base Year flows and the Future flows as well at
the calculated present value costs for improvements associated with Alternative 2.
These reduced damages do not account for any reduction in vehicle or infrastructure costs or lost
productivity. These reductions compared to the project costs were one of several considerations used
when prioritizing projects
9.3.2 Benefit Cost Ratio (BCR)
A benefit -cost analysis (BCA) of the proposed improvements was conducted to determine the value of the
flood reduction improvements relative to estimated construction and ROW costs. Benefits were
determined by subtracting the present value damages from the existing conditions present value damages
for each scenario. The Benefit -Cost Ratios (BCR) were determined by dividing the benefits by the
Estimated Project Costs. The BCR aided in determining economic feasibility and project prioritization.
Estimated project costs, benefits and BCRs for each watershed are provided in Tables 15A and 15B below
for Alternatives 1 and 2, respectively.
Table 4A. Alternative 1 Benefit Cost Ratios
Stream
With Project - Alternative 2
Alternative 1
Stream
LOS
BC Ratio
Annualized Damages
$
Present Value
Damages
Current Year
(2018)
Most Likely
Future Year
Mary's Creek
25 -year
$
782,290
$
852,350
$
17,358,126
Hickory Slough
10 -year
$
290,070
$
321,920
$
6,482,666
East Chocolate Bayou
10 -year
$
780
$
960
$
17,060
West Chocolate Bayou
5 -year
$
22,330
$
26,470
$
486,699
Chigger Creek
10 -year
$
310
$
3,480
$
19,941
Mustang Bayou
25 -year
$
576,960
$
704,370
$
12,927,960
Cowart Creek
d
10 -year
$
49,600
$
53,680
$
1,082,592
Total
$
1,722,340
$
1,963,230
$
38,375,044
Note: This table provides the annualized damage costs for the Base Year flows and the Future flows as well at
the calculated present value costs for improvements associated with Alternative 2.
These reduced damages do not account for any reduction in vehicle or infrastructure costs or lost
productivity. These reductions compared to the project costs were one of several considerations used
when prioritizing projects
9.3.2 Benefit Cost Ratio (BCR)
A benefit -cost analysis (BCA) of the proposed improvements was conducted to determine the value of the
flood reduction improvements relative to estimated construction and ROW costs. Benefits were
determined by subtracting the present value damages from the existing conditions present value damages
for each scenario. The Benefit -Cost Ratios (BCR) were determined by dividing the benefits by the
Estimated Project Costs. The BCR aided in determining economic feasibility and project prioritization.
Estimated project costs, benefits and BCRs for each watershed are provided in Tables 15A and 15B below
for Alternatives 1 and 2, respectively.
Table 4A. Alternative 1 Benefit Cost Ratios
Stream
Alternative 1
Est. Cost ($M) Benefits ($M)
BC Ratio
Mary's Creek
$
294.0
$
36.5
0.124
Hickory Slough
$
165.9
$
42.2
0.254
East Chocolate Bayou
$
203.4
$
5.0
0.025
West Chocolate Bayou
$
457.5
$
16.6
0.036
Chigger Creek
$
148.7
$
0.8
0.006
Mustang Bayou
$
302.6
$
24.0
0.079
Cowart Creek
$
342.9
$
23.8
0.069
Total
$
1,915.0
$
148.9
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Table 15B. Alternative 2 Benefit Cost Ratios
::C HALFF
Stream
Est. Cost (M)
Alternative 2
Benefits (M)
BC Ratio
Mary's Creek
$
176.1
$
28.0
0.159
Hickory Slough
$
132.6
$
37.5
0.283
East Chocolate Bayou
$
161.1
$
5.0
0.031
West Chocolate Bayou
$
389.8
$
16.1
0.041
Chigger Creek
$
74.8
$
0.8
0.011
Mustang Bayou
$
160.1
$
23.5
0.147
Cowart Creek
$
324.5
$
23.5
0.073
Total
$
1,419.0
$
134.5
It is important to note that the BCR is only one of the metrics used for project prioritization and are a
means of comparison. While many federally -funded (FEMA, USACE) projects are required to have a BCR
of 1.0 or greater, those are for a specifically defined project with more detailed information about the
damage potential, including surveyed finished floor elevations. For this planning study, the BCR
considered large segments, often including multiple projects over a diverse area with information about
the area based on UDAR, not detailed survey information. The value of the BCR from a planning
standpoint is that the various alternatives can be compared at a watershed level, which can inform
prioritization decisions. A project BCR less than 1.0 doesn't make the project infeasible or of limited value.
9.3.3 Local Ditch BCR
Neither benefits nor damages were calculated for the local ditches because no modeling was conducted.
As such there is no BCR available for these projects. A ranking of the projects based on the existing
capacity will be discussed in Section 10.6 as well as in the implementation plan in Appendix G.
9.4 Non -Monetary Project Benefits
In addition to the monetary value of the of the benefits for each scenario, non -monetary metrics were
examined. The non -monetary metrics considered items such as how many structures are expected to no
longer flood, how many acres of land are expected to no longer be inundated, and how many miles of
roadway are expected to no longer be inundated by the 10- and 100 -year storms. These were considered
because the projects with lower B/C ratios are skewed to those areas that have less dense development
levels and/or are socio -economically disadvantaged. This metric provides a comparison that is
independent of assessed property value.
In addition, the development potential for each watershed was considered. If a watershed has a
considerable amount of developable property, then the potential exists that significant development may
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occur in the watershed, which could influence the project priority. Information regarding each metric is
discussed below.
9.4.1 Reduction of Inundated Structures
The number of structures from which inundation was removed was calculated using GIS, the resulting
inundation mapped from RAS Mapper, and the HEC -FDA information. The number of structures
inundated for Alternative 1 (100 -year LOS) and the Alternative 2 (LOS varies) were examined to help
estimate the benefit of each scenario. The FDA analysis indicates that approximately 3474 of the
structures analyzed are inundated by the 100 -year storm. The same analysis showed that inundation is
removed from all known structures within the study area for Alternative 1. For Alternative 2 a total
reduction of 2783 structures (80%) that are expected to be inundated during a 100 -year event throughout
the entire study area. This is a significant reduction, but not complete removal given the lower levels of
service provided. Table 16 summarizes the reduction number of structures inundated.
Table 16. Summary of Inundated Structures
Stream
Total
Structures
Existing
100 -year
Alternative 1
LOS (YR) 100 -year
Alternative 2
LOS 100 -year
Mary's Creek
12385
947
100
0
25
75
Hickory Slough
3950
438
100
0
10
68
East Chocolate Bayou
1008 1
34
100
0
10
2
West Chocolate Bayou
1344
290
100
0
5
47
Chigger Creek
308
114
100
0
10
6
Mustang Bayou
1471
497
100
0
25
140
Cowart Creek
4788
1154
100
0 1
10
353
Total
25254
3474
0 1
691
9.4.2 Reduction of Inundated Acreage
Area of inundation was also considered in the benefits. The total acreage was calculated using GIS and
the resulting inundation mapped from RAS Mapper. This acreage includes area within the channel. The
streams in the study area show 17230 acres (26.9 sq mi) of inundated area, which accounts for more than
25% of the total project area. With the proposed improvements, a reduction of 83% in the area expected
to be inundated was shown for Alternative 1 with most of the streams containing flows up to the 100 -
year event. Alternative 2 provides approximately 64% reduction in the inundated area for the 100 -year
event. Table 17 summarizes the reduction inundated area.
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Table 17. Inundated Area Estimates (Acreage)
ONO HALFF
Stream
Existing
100 -year
Alternative 1
LOS (YR) 100 -year
Alternative 2
LOS (YR) 100 -year
Mary's Creek
2858
100
666
25
1038
Hickory Slough
1847
100
236
10
683
East Chocolate Bayou
2209
100
132
10
238
West Chocolate Bayou
1778
100
213
5
638
Chigger Creek
2174
100
82
10
549
Mustang Bayou
2457
100
961
25
1663
Cowart Creek
3907
100
586
10
1373
Total
17230
2876
1 1
6182
9.4.3 Reduction of Inundated Roadway Miles
The number of roadway miles flooded during the 100 -year storms was also considered. Inundation limits
for the 100 -year storm for both Alternative 1 and Alternative 2 were compared to all roadways within the
watershed to determine the extent of flooding impacts to the transportation network. Approximately
150 miles of roadway are estimated to experience flooding during the 100 -year storm. Alternative 1
shows a major reduction, with only 14 miles inundated during the 100 -year event while Alternative 2
shows 48 miles inundated for the same event, which is still a significant drop. Table summarizes the
reduction of inundated roadway miles.
Table 18. Roadway Inundation Estimates (Miles)
Stream
Existing
100 -year
Alternative 1
LOS (YR) 100 -year
Alternative 2
LOS (YR) 100 -year
Mary's Creek
53.7
100
4.4
25
20.1
Hickory Slough
22.5
100
0.9
10
11.8
East Chocolate Bayou
8.3
100
0.3
10
0.7
West Chocolate Bayou
11.4
100
0.9
5
2.2
Chigger Creek
15.3
100
1.3
10
1.3
Mustang Bayou
11.0
100
4.6
25
5.1
Cowart Creek 1
28.0
100
2.0
10
7.1
Total 1
150.2 1
1 14.4
1 1
48.3
Each of these metrics was considered during the prioritization and implementation process that will be
discussed in Section 11.0, along with other considerations. Like BCR, the non -monetary benefits are
intended to provide an additional basis of comparison for the alternatives.
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HALFF'
9.4.4 Watershed Development Potential
The development potential for each watershed was also evaluated and was used as part of the
prioritization process. The area of land available for future development as compared to the total area in
each watershed was determined and, the percentage was considered the development potential.
9.4.5 Local Ditch Non -Monetary Benefits
None of the non -monetary metrics were calculated for the local ditches because no modeling was
conducted. A ranking of the ditch projects based on the existing capacity will be discussed in Section 10.6
as well as in the implementation plan in Appendix G.
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10.0 Flood Risk Reduction Recommendations
::C HALFF
The recommendations provided in this section represent a combination of several factors, which were
discussed in Section 9 above. These include the benefit -cost analysis based on damages calculated using
HEC -FDA as well as estimated reductions in the number of inundated structures, acreage of inundated
area, and the number of roadway miles inundated. Development potential in the watershed is also
considered, based on the proportion of undeveloped property to the total watershed area. One of the
primary factors influencing the project recommendations is a Priority Based on Need assessment, which
considers several factors related to existing flooding. Each of these factors was considered in the
selection, order and Level of Service to be provided in these recommendations. The project selection
factors are discussed in this section. Information relating to the project selection is included in the
Implementation Plan discussed in Section 11.0 and in Appendix G.
10.1 Project Prioritization
Project prioritization for the COP/BDD4 implementation plan relied on a number of factors that looked at
costs, damages, needs and challenges, and development potential in the watershed. Each of these factors
was considered and the project recommendations were determined. Each of the selection factors is
discusses in this section.
10.1.1 Priority Based on Need Assessment
The prioritization based on need considered a handful of criteria for project scoring, including the number
of structures in the 10- and 100 -year inundation areas, channel level of service, and recorded flood losses
in the watershed. Records for the structures impacted by the 10% and 1% AEP storms were taken from
the HEC -FDA information and were used to set scoring ranges for the prioritization. The 'Current Level of
Service' was based on the modeling developed as part of the Pearland/BDD4 MDP. In addition, single
FEMA claims as well as FEMA repetitive loss information were utilized in the scoring. Table 19 shows the
scoring matrix for the projects evaluated as part of the MDP.
Table 19. Roadway Inundation Estimates (Miles)
Studied Stream Weighting
Criteria
Weight
Structure Inventory -10% (10 Year)
0.3
Structure Inventory -1%(100 Year)
0.1
Level of Service
0.3
FEMA Repetitive Loss
0.2
Historical Flooding (FEMA Single)
1 0.1
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Table 20: Priority Based on Need Assessment
HALFF
Chemel Name
Channel Segment
Structures
I Impacted
by 10%AEP
Event
Structures
ImpactedCurrent
by 1%aAEP
Event
Level of
Service
FEMA
Repetitive
Loss
claims
HatoricalWeighted
Flood palms
:Average
Rounded
Score
Segment 2
4
4
4
1
1
3.10
3
Cannon Ditch
Segment 3
0
1
4
1
0
1.50
2
Segment 4 0 1 4 0 0 1.30
1
Chigger - Bypass C1
2
2
4
0
0
2.00
2
Chigger - Bypass C2 0 2 4 0 0 1A0
1
Chigger Creek
Chigger- Upper
0
0
4
0
0
1.20
1
Old Chigger- U/S 2 2 3 0 0 1.70
2
Old Chigger - DI 0 0 2 0 0 0.60
1
Segment 16
4
3
4
2
1
3.20
3
Segment 17 0 1 4 2 0 1.70
2
Cowart Creek
Segment 19
2
3
1
2
0
1.60
2
Segment 18
0
0
1
0
0
0.30
0
E103
3
2
4
0
0
2.30
2
East Chocolate
Rodeo Palms - U/S
2
2
4
0
0
2.00
2
Bayou
Rodeo Palms - DIS
2
2
4
0
0
2.00
2
East Chocolate
0
0
4
0
0
1.20
1
Upper
3
3
4
0
2
2.60
3
Hickory Slough
Middle
4
4
3
2
3
3.20
3
Lower
3
3
3
4
4
3.30
3
Upper
1
4
2
3
4
2.30
2
Mary's Creek
Middle
2
4
2
4
4
2.80
3
Lower
3
4
2
4
4
1 3.10
3
Upper
4
4
4
0
0
2.80
3
Mustang Bayou
Middle
4
3
4
0
0
2.70
3
Louver
2
2
4
0
0
2.00
2
Cold McCutchen - U/S
4
3
4
0
0
2.70
3
West
Cold McCutchen - D/S
3
2
4
0
0
2.30
2
Chocolate
Bayou
West Chocolate
3
3
4
0
0
2.40
2
CR 383 Ditch
4
3
4
0 1
0
1 2.70
3
Table 20 shows the scoring matrix for the projects evaluated as part of the MDP. Priority Based on need
scores ranged from a high of 3.3 on Hickory Slough to a low of 0.3 on Cowart Creek. Different weights
were assigned to each of the criteria as shown in Table 5.1.A below. The weighting can be adjusted in the
future based on the needs of the City and/or BDD4; however, the weights shown were used for the current
prioritization.
10, 1.2 Flood Reduction Benefits
In addition to the Priority Based on Need assessment, additional flood mitigation benefits were
considered in the prioritization. The reduction of inundated structures, reduction in acreage of
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inundation, and reduction in miles of roadway inundation were considered. Specific information
regarding those reductions is provided below. Alternative 1, which provides 100-year LOS, show the
removal of inundation from all structures up to the 100-year. While there are still quite a few structures
expected to experience some flooding inundated for the 100-year, Alternative 2 provides a reduction of
nearly over 2230 structures (81%) that are expected to be inundated during a 100-year event.
There is a substantial reduction in the area expected to be inundated for both the 10 -year and 100 -year
events. Alternative, which provide 100 -year LOS, show the greatest reduction of inundation area, with
flow up to the 100 -year being contained in the channel. Alternative 2 provides a substantial reduction of
inundated area as well, containing the 10 -year within the channel and allowing for a 36% reduction in the
inundated area for the 100 -year event.
Approximately 128 miles of roadway are estimated to experience flooding during the 100 -year storm and
nearly 33 miles during the 10 -year event for the current drainage system. Alternative 1 would remove
inundation from more than 28 miles of roadway for the 10 -year storm and almost 115 miles for the 100 -
year event throughout the entire study area. Alternative 2 would result in inundation being removed for
more than 27 miles for the 10 -year storm and approximately 85 miles for the 100 -year storm.
10.1.3 Development Potential
The future development potential of the watersheds were also included in the prioritization. The area of
land available for future development as compared to the total area in each watershed was determined
and, the percentage was considered the development potential. Watersheds with more currently
undeveloped land show a greater potential for future development and get ranked higher than those
watersheds that are currently densely developed. The Most Likely Future Year, which was determined by
COP/BDD4 and Halff, is the predicted year by which projects would be completed, was also used in the
prioritization.
10.1.4 Prioritization Scoring
Each of the factors discussed above were combined and weighted to determine an overall score for the
various projects. Each parameter examined was assigned a ranking 0 through 4, and each was weighted
according to Table 21 to determine the overall project ranking. Scoring weights were varied with the
greatest emphasis on the priority based on need at 40%. The BC Ratio and other flood reduction metrics
made up about 45%, with the development potential accounting for the last 15%. The scoring system is
set up to allow for changes to the weighting of each category if needed.
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Table 21— Prioritization Parameter Weights
Parameter
Weight Factor
Priority Based on Need Score
0.40
BC Ratio
0.15
Reduction in Inundated Structures
0.15
Area of Inundation Removed
0.10
Reduction in Inundated Roadway Miles
0.05
Development Potential
0.10
Most Likely Future Year
0.05
HALFF+
Based on the scoring, the projects were divided into priority tiers. The scoring and tier classifications are
provided in Table 22. More detail is provided in the Implementation Plan in Appendix G.
Based on the weighted scores, the Tier 1 (top 6), Tier 2 (Next 4), and Tier 3 (Last 5) ranked projects were
identified. The remaining projects are classified as Tier 4. Tier 1 project scores ranged from a 2.8 to 2.6
out of a maximum 4.0. Several of the Tier 1 projects are in the Mary's Creek and Hickory Slough
watersheds, likely because the highest concentration of structures is along the main stem of these
channels. Tier 2 projects ranged from 2.5 to 2.1. The Tier 3 projects that round out the top 15 have a
scoring range between 2.0 and 1.7. Additional projects have been evaluated and a cost determined, but
their priority is lower.
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Table 22 - City of Pearland and BDD4 Project Priority
::C HALFF
Channel Name
Cannon Ditch
Channel Segment
Segment 3
Priority Based on
Need Score
1.50
BC Ratio
100 -year
4.00
Structures
Removed
100 -year
0.00
Area ofRoadway
InundationRemoved
100 -year
'
0.00
Miles
Removed
100 -year
2.00
0.00
Development
Potential
2.00
Most Likely
Future Year
1.00
Total Weighted
Score
1.45
Rounded
Score
1
Segment 4
1.30
0.00
0.00
0.00
0.00
2.00
1.00
0.77
1
Chigger - Bypass C1
2.00
0.00
0.00
2.00
1.00
2.00
1.00
1.30
1
Chigger - Bypass C2
1.40
0.00
1.00
3.00
3.00
2.00
1.00
1.41
1
Chigger Creek
Chigger - Upper
1.20
0.00
1.00
1.00
2.00
2.00
1.00
1.08
1
Old Chigger - U/S
1.70
0.00
1.00
1.00
1.00
2.00
1.00
1.23
1
Old Chigger - D/S
0.60
0.00
0.00
0.00
1.00
2.00
1.00
0.54
1
Cowart Creek
3.20
4.00
2.00
2.00
1.00
2.00
1.00
2.68
3
Segment 17 1.70 0.00 0.00 0.00 0.00 2.00 1.00 0.91
Segment 19
1.60
1.00
0.00
0.00
0.00
2.00
1.00
1.04
Segment 18
0.30
0.00
0.00
1.00
1.00
2.00
1.00
0.52
E103
2.30
0.00
0.00
4.00
2.00
3.00
0.00
1.72
2
East Chocolate
Rodeo Palms - U/S
2.00
0.00
1.00
2.00
2.00
3.00
0.00
1.55
2
Bayou
Rodeo Palms - D/S
2.00
0.00
1.00
1.00
0.00
3.00
0.00
1.35
1
East Chocolate
1.20
0.00
0.00
0.00
0.00
3.00
0.00
0.78
1
Upper
2.60
0.00
3.00
1.00
2.00
0.00
2.00
1.79
2
Hickory Slough
Middle
3.20
3.00
3.00
4.00
3.00
0.00
2.00
2.83
3
Lower
3.30
0.00
2.00
2.00
4.00
0.00
2.00
2-12
2
Upper
2.30
3.00
4.00
3.00
4.00
0.00
2.00
2.57
3
Mary's Creek
Middle
2.80
1.00
4.00
4.00
4.00
0.00
2.00
2.57
3
Lower
3.10
0.00
3.00
3.00
4.00
0.00
2.00
2.29
2
Upper
2.80
0.00
4.00
4.00
3.00
2.00
1.00
2.52
3
Mustang Bayou
Middle
2.70
2.00
2.00
3.00
0.00
2.00
1.00
2.23
2
Lower
2.00
0.00
0.00
1.00
0.00
2.00
1.00
1.15
1
C.IdWi tchen - U/S
2.70
3.00
3.00
2.00
2.00
4.00
0.00
2.68
3
West Chocolate
Cold McCutchen - D/5
2.30
0.00
1.00
2.00
1.00
4.00
0.00
1.72
2
Bayou
West Chocolate
2.40
0.00
2.00
2.00
1.00
4.00
0.00
1.91
2
CR 383 Ditch
2.70
0.00
1.00
3.00
1.00
4.00
0.00
1.98
2
Priority Tiers Based on Scoring Tier 1 Tier 2 Tier 3 Tier 4
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10.2 Tier 1 Project Rankings
Tier 1 projects include those that were ranked between 1 and 6 based on the Priority Based on Need
assessment score. The projects scored between 2.57 and 2.83 out of a possible maximum score of 4.0
and a minimum score of 0.0. These projects are classified as Large CIP.
10.2.1 Hickory Slough (Cullen Boulevard to Garden Road)
PROJECT RECOMMENDATIONS: Construct channel conveyance improvements that provide 100 -year LOS
(Alternative 1) along the channel and associated mitigation. Priority Score: 2.83
PROJECT DESCRIPTION: The project includes channel conveyance improvements from Roy Road to
Garden Road. The channel conveyance improvements and detention will contain the 100 -year inundation
and remove inundation from all structures for the 10- and 100 -year event. Project specifics include:
• Trapezoidal channel with 30 ft bottom width and 4:1 side slope; channel depth 11 ft
• Channel ROW of 170 ft
• Approximately 1010 ac -ft of stormwater detention needed, likely split into multiple basins (— 130
acres)
• Bridge Replacement: Private Crossing and CR115
PROJECT BENEFITS
• Remove inundation from all the structures for 100 -yr and 10 -yr
• Contain the 100 -year future conditions flows within channel
• Estimated reduction of $36.5 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Potential wetlands south of channel that could require permitting and mitigation if they are
impacted
• Availability of property; Currently heavily developed and would be difficult to acquire ROW
without impacting current development
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10.2.2 Cowart Creek (Wells Drive to BNSF Railroad)
PROJECT RECOMMENDATIONS: Construct channel conveyance improvements that provide 10 -year LOS
(Alternative 2) along the channel and associated mitigation. Priority Score: 2.68
PROJECT DESCRIPTION: The project includes channel conveyance improvements from XS 45221 which is
—330 ft south of Bailey Rd and Wells Dr intersection to BNSF railroad. The Alternative 2 (10 -year LOS)
channel conveyance improvements will provide a significant reduction in the inundated area and remove
inundation from all the structures currently estimated to be flooded by the 100 -year and 10 -year event.
Project specifics include:
• Trapezoidal channel with 40 ft bottom width and 4:1 side slope; channel depth 11-16 ft
(Alternative 2)
• Channel ROW range from 115 - 185 ft (10 -yr LOS).
• Bridge Replacement: Private Crossing and CR115
PROJECT BENEFITS
• Remove inundation from all the structures for 100 -yr and 10 -yr
• 85+% reduction for 100 -year inundation
• Estimated reduction of $10 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Railroad bridge replacements will require coordination with railroad authority
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10.2.3 West Chocolate Bayou — Cold River Ranch Ditch (Upstream of Hwy 6)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 100 -year LOS (Alternative 1) along the channel. Acquire approximately 101 acres of
property for mitigation. Priority Score: 2.68
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from Rio
Lindo Street to Hwy 6, and stormwater detention. The Alternative 1 (100 -year LOS) channel conveyance
improvements will provide a significant reduction in the inundated area and remove inundation from all
structures currently estimated to be flooded by the 100 -year event and the 10 -year event. Project
specifics include:
• Trapezoidal channel with 70 ft bottom width and 3:1 side slope; channel depth 6-7 ft
• Approximately 580 ac -ft of stormwater detention needed, likely split into multiple basins (-100
acres)
• Channel ROW range from 170 - 180 ft
• Bridge Replacement— Rio Lindo Street, Rio Bravo Street, Hwy 6, and Private crossings
PROJECT BENEFITS
• Remove inundation from up to 104 structures (10 -yr) and 128 structures (100 -yr)
• Contain the 100 -year future conditions flows within channel
• Estimated reduction of $12.6 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Bridge replacement will require coordination with TXDOT
• Natural gas lines cross Cold River Ranch Ditch on downstream of CR 714
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OEM
10.2.4 Cannon Ditch (Peorlond Site Road to Amoco Industrial Street)
PROJECT RECOMMENDATIONS: Construct channel conveyance improvements that provide a 100 -year
LOS (Alternative 1) along the channel and associated mitigation. Acquire approximately 138 acres of
property for mitigation. Priority Score: 2.59
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements from the
confluence of Cannon Ditch and C101-12-03 to the railroad crossing near Amoco Industrial Street. The
channel conveyance improvements and detention will contain the 100 -year inundation and remove
inundation from all structures for the 10- and 100 -year event. Project specifics include:
• Trapezoidal channel with 20 ft bottom width and 4:1 side slope; channel depth 8-9 ft
• Approximately 980 ac -ft of stormwater detention needed
• Channel ROW range from 105 - 120 ft
• Bridge Replacement: 3 private crossings and Amoco St
PROJECT BENEFITS
• Remove inundation from up to 272 structures (10 -yr) and 353 structures (100 -yr)
• Contain the 100 -year future conditions flows within channel
• Estimated reduction of $2 M in present value damages
• Facilitates improvements to Trevino Ditch (C101-00-00)
PROJECT CHALLENGES/CONSIDERATIONS:
• Railroad bridge replacements will require coordination with railroad authority
• Multiple pipelines cross the channel
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10.2.5 Mary's Creek (Confluence with 8129-01-00 to McLean Road)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 25 -year LOS (Alternative 2) along the channel. Acquire approximately 25 acres of property
for mitigation. Priority Score: 2.57
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from
confluence with B129-01-00 to McLean Road, and stormwater detention. The Alternative 2 (25 -year LOS)
channel conveyance improvements will provide a significant reduction in the inundated area and remove
inundation from 249 of 251 structures currently estimated to be flooded by the 100 -year event and 2
structures for the 10 -year event. Project specifics include:
• Trapezoidal channel with 30 ft bottom width and 3:1 side slope; channel depth 12-15 ft
• Approximately 240 ac -ft of stormwater detention needed (-25 acres)
• Channel ROW range from 160 — 250 ft
• Bridge Replacement— Manvel Road, Magnolia Street, Harkey Road, and McLean Road
PROJECT BENEFITS
• Remove inundation from up to 2 structures (10 -yr) and 249 structures (100 -yr)
• Significant reduction for 100 -year inundation
• Estimated reduction of $18 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Availability of property; Currently heavily developed and would be difficult to acquire ROW
without impacting current development
• Several natural gas line crossings
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10.2.6 Mary's Creek (Magnolia Drive to SH 35)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 25 -year LOS (Alternative 2) along the channel. Acquire approximately 71 acres of property
for mitigation. Priority Score: 2.57
PROJECT DESCRIPTION: The project includes channel conveyance improvements from Magnolia Drive to
Highway 35 and detention pond. The Alternative 2 (25 -year LOS) channel conveyance improvements will
provide a significant reduction in the inundated area and remove inundation from 445 of 456 structures
currently estimated to be flooded by the 100 -year event and all the structures for the 10 -year event.
Project specifics include:
• Trapezoidal channel with 100ft bottom width and 3:1 side slope; channel depth 13-16 ft
• Approximately 1000 ac -ft of stormwater detention needed, likely split into multiple basins (-70
acres)
• Channel ROW range from 160 - 250 ft
• Bridge Replacement—Harkey Road, McLean Road, Pedestrian Bridge, Veterans Drive, and AT&SF
Railroad
PROJECT BENEFITS
• Remove inundation from up to 3 structures (10 -yr) and 445 structures (100 -yr)
• 75+/ reduction for 100 -year inundation
• Estimated reduction of $6.1 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Potential wetlands along channel that could require permitting and mitigation if they are
impacted
• Railroad bridge replacements will require coordination with railroad authority
• Availability of property; Currently heavily developed and would be difficult to acquire ROW
without impacting current development
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10.3 Tier 2 Project Rankings
Tier 2 projects include those that were ranked between 7 and 10 based on the Priority Based on Need
assessment score, which ranges from 2.12 to 2.52. These projects are classified as Large CIP.
10.3.1 Mustang Bayou (CR 521 to Airline Rood)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 25 -year LOS (Alternative 2) along the channel. Acquire approximately 178 acres of property
for mitigation. Priority Score: 2.52
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from
Almeda Road downstream to Airline Road, and stormwater detention. The Alternative 2 (25 -year LOS)
channel conveyance improvements will provide a significant reduction in the inundated area and remove
inundation from 278 of 393 structures currently estimated to be flooded by the 100 -year event and 144
structures for the 10 -year event. Due to restriction of land availability, improvements are limited along
Mustang Bayou. Project specifics include:
• Trapezoidal channel with 100ft bottom width and 3:1 side slope; channel depth 5-8 ft
• Approximately 890 ac -ft of stormwater detention needed, likely split into multiple basins (-180
acres)
• Channel ROW range from 200 - 240 ft
• Bridge Replacement — CR 564, CR 48, and private crossings
PROJECT BENEFITS
• Remove inundation from up to 144 structures (10 -yr) and 278 structures (100 -yr)
• 55% reduction for 100 -year inundation
• Estimated reduction of $4.5 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Wetlands along the channel may require permitting and mitigation
• Availability of property; Several large ponds already exist and would be difficult to acquire ROW
without impacting current development
• Natural gas line crosses Mustang Bayou downstream of CR 564
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10.3.3 Mary's Creek (Downstream of SH 35)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 25 -year LOS (Alternative 2) along the channel. Acquire approximately 140 acres of property
for mitigation. Priority Score: 2.29
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from SH 35
downstream to approximately 500 feet downstream of Pearland Parkway, and stormwater detention.
The Alternative 2 (25 -year LOS) channel conveyance improvements will provide a significant reduction in
the inundated are and remove inundation from 178 of 240 structures currently estimated to be flooded
by the 100 -year event and 19 structures for the 10 -year event. Project specifics include:
• Trapezoidal channel with 40 ft bottom width and 3:1 side slope; channel depth 15-17 ft
• Approximately 1670 ac -ft of stormwater detention needed, likely split into multiple basins (-140
acres)
• Channel ROW range from 170 - 320 ft
• Bridge Replacement — Highway 35, Old Alvin Road, Pearland Parkway (upstream), Pearland
Parkway (downstream), John Lizer Road, Liberty Drive, and Longherridge Drive
PROJECT BENEFITS
• Remove inundation from up 19 structures (10 -yr) and 178 structures (100 -yr)
• Significant reduction in 100 -year inundation
• Estimated reduction of $3.6 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Availability of property; Currently heavily developed and would be difficult to acquire ROW
without impacting current development
• Several bridge replacements will require coordination with TXDOT
• Natural gas line crosses the Mary's Creek and Mary's Creek Bypass on downstream of Dixie Farm
Road
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10.3.3 Mustang Bayou (Airline Road to SH 288)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 25 -year LOS (Alternative 2) along the channel. Acquire approximately 165 acres of property
for mitigation. Priority Score: 2.23
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from Airline
Road downstream to SH 288, and stormwater detention. The Alternative 2 (25 -year LOS) channel
conveyance improvements will provide a significant reduction in the inundated area and remove
inundation from 79 of 96 structures currently estimated to be flooded by the 100 -year event and 54
structures for the 10 -year event. Project specifics include:
• Trapezoidal channel with 100ft bottom width and 3:1 side slope; channel depth 8-13 ft
a Approximately 1070 ac -ft of stormwater detention needed, likely split into multiple basins (-165
acres)
• Channel ROW range from 240 - 260 ft
• Bridge Replacement — CR 84 and SH 288
PROJECT BENEFITS
• Remove inundation from up to 54 structures (10 -yr) and 79 structures (100 -yr)
• 55% reduction for 100 -year inundation
• Estimated reduction of $19 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Wetlands south of the channel may require permitting and mitigation
0 Availability of property; Much of the area north of the channel is highly developed
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10.3.4 Hickory Slough (Garden Road to SH 35)
PROJECT RECOMMENDATIONS: Construct channel conveyance improvements that provide 10 -year LOS
(Alternative 2) along the channel and associated mitigation. Priority Score: 2.12
PROJECT DESCRIPTION: The project includes channel conveyance improvements from Garden Road to
SH 35. The channel conveyance improvements and associated mitigation will provide a significant
reduction in the inundated area and remove inundation from 68 of 79 structures currently estimated to
be flooded by the 100 -year event and 22 structures for the 10 -year event. Project specifics include:
• Trapezoidal channel with 30 ft bottom width and 4:1 side slope; channel depth 11-13 ft
• Channel ROW range from 140 - 170 ft
• Approximately 1310 ac -ft of stormwater detention needed, likely split into multiple basins (— 170
acres)
• Bridge Replacement: Mykawa Rd, Hatfield Rd, Oday Rd, and Garden Rd
PROJECT BENEFITS
• Remove inundation from up 22 structures (10 -yr) and 68 structures (100 -yr)
• Significant reduction in 100 -year inundation
• Estimated reduction of $550 K in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Potential wetlands downstream of SH35 that could require permitting and mitigation if they are
impacted
• Availability of property; Currently heavily developed and would be difficult to acquire ROW
without impacting current development
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10.4 Tier 3 Project Rankings
Tier 3 projects include those that were ranked between 11 and 15 based on the Priority Based on Need
assessment score. The projects ranked between 1.70 and 2.00. These projects are classified as Reserve
CIP.
10.4.1 West Chocolate Bayou — CR 383 Ditch
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 5 -year LOS (Alternative 2) along the channel. Acquire approximately 276 acres of property
for mitigation. Priority Score: 1.98
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from
upstream end near confluence with E101-02-00 to confluence with West Fork of Chocolate Bayou, and
stormwater detention. The Alternative 2 (5 -year LOS) channel conveyance improvements will provide a
significant reduction in the inundated area and remove inundation from 93 of 94 structures currently
estimated to be flooded by the 100 -year event and 91 structures for the 10 -year event. Project specifics
include:
• Trapezoidal channel with 60 ft bottom width and 3:1 side slope; channel depth 8-9 ft
• Approximately 1230 ac -ft of stormwater detention needed, likely split into multiple basins (-280
acres)
• Channel ROW is 190 ft
• Bridge Replacement — Hwy 6, Oak Street, and Railroad at Oak Street
• Remove siphon at CWA canal crossing
PROJECT BENEFITS
• Remove inundation from up to 91 structures (10 -yr) and 93 structures (100 -yr)
• Significant reduction for 100 -year inundation
• Estimated reduction of $955 K in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Bridge replacement will require coordination with TXDOT
• Coordination with CWA for siphon removal
• Natural gas lines crossing at upstream end
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10.4.2 West Fork Chocolate Bayou
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 5 -year LOS (Alternative 2) along the channel. Acquire approximately 451 acres of property
for mitigation. Priority Score: 1.91
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from
upstream end near confluence with E101-02-00 to confluence with West Fork of Chocolate Bayou, and
stormwater detention. The Alternative 2 (5 -year LOS) channel conveyance improvements will provide a
significant reduction in the inundated area and remove inundation from 40 of 46 structures currently
estimated to be flooded by the 100 -year event and 38 structures for the 10 -year event. Project specifics
include:
• Trapezoidal channel with 80 ft bottom width and 3:1 side slope; channel depth 8-9 ft
• Approximately 3310 ac -ft of stormwater detention needed, likely split into multiple basins (-450
acres)
• Channel ROW range from 190 — 260 ft
• Bridge Replacement —CR 383 (Karsten Road)
PROJECT BENEFITS
• Remove inundation from up to 38 structures (10 -yr) and 40 structures (100 -yr)
• Significant reduction for 100 -year inundation
• Estimated reduction of $1.4 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Natural gas lines crossing upstream of CR 383
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10.4.3 Hickory Slough (CR 94 to Cullen Boulevard)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 100 -year LOS (Alternative 1) along the channel. Acquire approximately 42 acres of property
for mitigation. Priority Score: 1.79
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from
upstream end near CR 94 (Smith Ranch Road) to confluence with H126-00-00, and stormwater detention.
The Alternative 1 (100 -year LOS) channel conveyance improvements will provide a significant reduction
in the inundated area and remove inundation from all structures currently estimated to be flooded by the
100 -year event and the 10 -year event. Project specifics include:
• Trapezoidal channel with 15 ft bottom width and 3:1 side slope; channel depth 12-13 ft
• Approximately 280 ac -ft of stormwater detention needed, likely split into multiple basins (-40
acres)
• Channel ROW range from 130 —170 ft
• Bridge Replacement— Fair Oaks Street, Miller Ranch Road, and Private crossing
PROJECT BENEFITS
• Remove inundation from up to 12 structures (10 -yr) and 147 structures (100 -yr)
• Contain the 100 -year future conditions flows within channel
• Estimated reduction of $5.1 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Availability of property; Currently heavily developed and would be difficult to acquire ROW
without impacting current development
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mom HALFF'
mom
10.4.4 East Chocolate Bayou — E103-00-00
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 10 -year LOS (Alternative 2) along the channel. Acquire approximately 347 acres of property
for mitigation. Priority Score: 1.72
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from
upstream end near SH 288 to confluence with Rodeo Palms Ditch, and stormwater detention. The
Alternative 2 (10 -year LOS) channel conveyance improvements will provide a significant reduction in the
inundated area and remove inundation from 17 of 17 structures currently estimated to be flooded by the
100 -year event and 13 structures for the 10 -year event. Project specifics include:
• Trapezoidal channel with 60 ft bottom width and 3:1 side slope; channel depth 4-6 ft
• Approximately 2210 ac -ft of stormwater detention needed, likely split into multiple basins (-350
acres)
• Channel ROW range from 160 — 220 ft
• Bridge Replacement — CR 418, CR 80, CR 81, and Private crossings
PROJECT BENEFITS
• Remove inundation from up to 13 structures (10 -yr) and 17 structures (100 -yr)
• Significant reduction for 100 -year inundation
• Estimated reduction of $750 K in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Land availability; Much of the available land is outside Brazoria County/BDD4jurisdiction
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10.4.5 West Chocolate Bayou — Cold River Ranch Ditch (Downstream of Hwy 6)
PROJECT RECOMMENDATIONS: Construct regional detention and channel conveyance improvements
that provide a 5 -year LOS (Alternative 2) along the channel. Acquire approximately 276 acres of property
for mitigation. Priority Score: 1.72
PROJECT DESCRIPTION: The project includes trapezoidal channel conveyance improvements, from Hwy
6 to confluence with West Fork of Chocolate Bayou, and stormwater detention. The Alternative 2 (5 -year
LOS) channel conveyance improvements will provide a significant reduction in the inundated area and
remove inundation from 16 of 22 structures currently estimated to be flooded by the 100 -year event and
16 structures for the 10 -year event. Project specifics include:
• Trapezoidal channel with 60 ft bottom width and 3:1 side slope; channel depth 7-10 ft
• Approximately 1230 ac -ft of stormwater detention needed, likely split into multiple basins (-280
acres)
• Channel ROW range from 170-250 ft
• Bridge Replacement — Oak Street, Railroad at Oak Street, Sanders Street,
• Remove siphon at CWA canal crossing
PROJECT BENEFITS
• Remove inundation from up to 16 structures (10 -yr) and 16 structures (100 -yr)
• Significant reduction for 100 -year inundation
• Estimated reduction of $1.3 M in present value damages
PROJECT CHALLENGES/CONSIDERATIONS:
• Railroad bridge replacements will require coordination with railroad authority
• Coordination with CWA for siphon removal
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10.5 Tier 4 Project Rankings
Based on the study, there are several additional projects (Tier 4) that could be completed to provide flood
reduction. However, the remaining projects scored a 2.0 or less on the priority and are not among the
top 15. In general, they are not included because there are very few structures that are subject to
damages or they have a better than average level of service; however, they could potentially be built if a
grant or partnership opportunity were to present itself. In most cases, the BC Ratios and benefits were
very low, making it difficult to justify allocating resources to those areas. The identified projects not
included in the recommendations are:
• Chigger Creek (Amoco Dr. to intersection of SH 35 and Greenhouse Rd.)
• Mustang Bayou (Downstream of SH 288)
• East Chocolate Bayou - Rodeo Palms Ditch (Palm Desert Rd. to Hwy 6)
• East Chocolate Bayou - Rodeo Palms Ditch (Hwy 6 to confluence with East Fork Chocolate Bayou)
• Cowart Creek (Railroad at James St. to SH 35)
• Chigger Creek (Ramirez Rd. to downstream confluence with Chigger Bypass)
• Cowart Creek (Confluence near Pearland Regional Airport to confluence near Hastings
Friendswood Rd.)
• Cannon Ditch (Amoco Industrial St. to Hastings Cannon Rd.)
• Chigger Creek (Intersection of SH 35 and Greenhouse Rd. to confluence near Chigger Creek Dr.)
• Cannon Ditch (Hastings Cannon Rd. to confluence by confluence near Hastings Friendswood Rd.)
• Chigger Creek (CR 143 to Amoco Dr.)
• East Chocolate Bayou (Bissell Rd. to confluence with Rodeo Palms Ditch near Mason Rd.)
• Chigger Creek (Confluence with downstream end Chigger Bypass to Windsong Ln.)
• Cowart Creek (Confluence near Hastings Friendswood Rd. to confluence with Clear Creek near
Deepwood Dr.)
10.6 Ditch Capacity Analysis Rankings
For the over 190 ditch capacity segments throughout the COP/BDD4 study area, a ranking was determined
based on the what percentage the existing ditch was undersized. This was based on the existing ditch
capacity and the calculated subbasin discharges. These channels would need the most improvement to
reach the desired level of service. The top 50 ditch segments were selected as a point for which the City
and BDD4 could assess flooding concerns and needs for improvements. Appendix G.4 provides a table
showing the top 50 ditch sections, the proposed ditch top widths, and estimated costs for both the 3 -year
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and 10 -year LOS proposed ditches. More detailed cost estimates are provided in Appendix F of the main
report for all ditch capacity segments.
As with the main channel segments, the local ditches were divided into 2 categories for implementation;
Small CIP projects and Small 0&M projects. The distinction is based on cost. Small 0&M projects are
those local ditch projects that are estimated to cost less than $500k for the 3 -year LOS. These may be
more likely to be implemented by BDD4 staff. Small CIP projects are those local ditch improvements that
are expected to cost mode than $500k, with some of them reaching above $4M. Appendix G.3 provides
a series of maps that show which ditches have a potential project. A complete list of projects, including
those that are not ranked in the top 50 are provided in Appendix D.S. A summary of the local ditch
recommendations for both categories is provided in Section 11.0.
911 Page
Pearland / Brazoria Drainage District No 4 Master Drainage Plan MMEN
EN HALF F `
Report—January 2019 ■■i
11.0 Implementation Plan
As part of the Pearland and Brazoria Drainage District No. 4 Master Drainage Plan Update, an
implementation plan was prepared to help prioritize the projects and provide guidance on the size, scope
and order of projects moving forward. The plan is intended to be a stand-alone document and, as such,
include some of the same information presented in this report including:
• A discussion of the study background and goals
• Some of the limitations of the plan
• Existing Flood Damages
• Flood Reduction Strategies
• Flood Reduction Metrics and Project Prioritization
• Project Recommendations
• A breakdown of recommended projects into smaller projects
• A general timeline for implementation
The implementation plan is based on current conditions and information and will likely need to be adapted
over time. The full implementation plan, including project fact sheets for the recommended Large CIP
projects, is provided in Appendix G. The tables provided below include recommendations for each of the
four project categories discussed:
• Large CIP Projects: Top 10 scoring main channel segment projects, will need to be phased
• Reserve CIP Projects: Main channel projects that rank from 11-15 and may provide benefits
• Small CIP: Local ditch projects from the Top 50 that cost more than $500k
• Small O&M: Local ditch projects from the Top 50 that cost less than $500k
Each of these project categories should be considered in the CIP planning proceed for both the City of
Pearland and BDD4. It should be noted that the Large CIP and Reserve CIP projects will most likely need
to be phased in order to implement within budget. The Large CIP fact sheets have a potential phasing
breakdown and cost estimate for each of the Top 10 recommended projects.
921 Page
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report- February 2019
Table 23 - Large Capital Improvement Project (CIP) Recommendations
;;; HALFF
PROJECT
WATERSHED
PROJECT COSTS (M)
PROTECT DESCRIPTION
CHANNEL
DETENTION
ROW
TOTAL
PRIORITY
SEGMENT
1
Hickory Slough
100 -year LOS; Channel modifications from Cullen Blvd. to Garden
$6.7
$19.2
$17.3
$43.2
Middle Segment
Rd. and 1010 ac -ft mitigation. Max ROW width of 170 ft.
2 t
Cowart Creek
10 -year LOS; Channel modifications from Wells Dr. to BNSF
$2.1
-
$5.2
$7.3
Segment 16
Railroad. Max ROW width of 200 ft.
3
West Fork Chocolate
100 -year LOS; Channel modifications from Rio Lindo St. to Hwy 6
$6.4
$10.4
$4.2
$21.0
Cold River Ranch Ditch
and 580 ac -ft mitigation. Max ROW width of 180 ft.
100 -year LOS; Channel modifications from Pearland Site Rd. to
4
Cannon Ditch
Amoco Industrial St. and 9800 ac -ft mitigation. Max ROW width
$4.8
$37.5
$4.2
$46.5
Segment 2
of 120 ft.
5
Mary's Creek
25 -year LOS; Channel modifications from B129-01-00 to McLean
$10.5
$4.5
$7.9
$22.9
Upper Segment
Rd. and 240 ac -ft mitigation. Max ROW width of 250 ft.
6t
Mary's Creek
25 -year LOS; Channel modifications from Magnolia Dr. to SH35
$10.7
$17.6
$3.1
$31.4
Middle Segment
and 1000 ac -ft mitigation. Max ROW width of 250 ft.
7
Mustang Bayou
25 -year LOS; Channel modifications from CR521 to Airline Rd and
$10.7
$44.4
$46.9
$102.0
Upper Segment
890 ac -ft mitigation. Max ROW width of 240 ft.
25 -year LOS; Channel modifications from SH35 to downstream of
8
Mary's Creek
Pearland Pkwy. and 1670 ac -ft mitigation. Max ROW width of
$14.8
$55.2
$51.8
$121.8
Lower Segment
220 ft.
9
Mustang Bayou
25 -year LOS; Channel modifications from Airline Rd. to SH288
$5.8
$31.9
$22.8
$60.5
Middle Segment
and 1070 ac -ft mitigation. Max ROW width of 260 ft.
10
Hickory Slough
10 -year LOS; Channel modifications from Garden Rd. to SH35 and
$12.4
$24.7
$15.2
$52.3
Lower Segment
1310 ac -ft mitigation. Max ROW width of 170 ft.
t Detention is included in downstream segment; however, mitigation will be required for conveyance improvements and should be evaluated in the PER Phase
93 1 Page
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report — February 2019
Table 24 - Reserve Capital Improvement Project (CIP) Recommendations
;;; HALFF
PROJECT
WATERSHED
PROJECT COSTS
PROJECT DESCRIPTION
CHANNEL
DETENTION
ROW
TOTAL
PRIORITY
SEGMENT
5 -year LOS; Channel modifications from E101-02-00 to
11
West Chocolate Bayou
confluence with West Fork Chocolate Bayou and 1230 ac -ft
$8.90
$27.60
$215.70
$252.20
CR 383 Ditch
mitigation. Max ROW width of 190 ft.
5 -year LOS; Channel modifications from county boundary to
12
West Fork Chocolate
confluence with E101-00-00 and 3310 ac -ft mitigation. Max
$16.10
$69.50
$17.90
$103.50
Bayou
ROW width of 260 ft.
100 -year LOS; Channel modifications from CR94 to confluence
13
Hickory Slough
with H126-00-00 and 2800 ac -ft mitigation. Max ROW width of
$2.40
$19.60
$34.70
$56.70
Upper Segment
170 ft.
10 -year LOS; Channel modifications from SH288 to confluence
14 t
East Chocolate Bayou
with Rodeo Palms Ditch and 2210 ac -ft mitigation. Max ROW
$1.70
-
$0.70
$2.40
E103-00-00
width of 220 ft.
5 -year LOS; Channel modifications from Hwy 6 to confluence
15t
West Fork Chocolate
with West Fork Chocolate Bayou and 1230 ac -ft mitigation.
$8.70
$1.20
$9.90
Cold River Ranch Ditch
Max ROW width of 250 ft.
t Detention is included in downstream segment; however, mitigation will be required for conveyance improvements and should be evaluated in the PER Phase
94 1 Page
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report- February 2019
Table 25 - Small Capital Improvement Project (CIP) Recommendations
... HALFF
DITCH
PRIORITY
WATERSHED
DITCH
3 -YR LOS
10 -YR LOS
TOP WIDTH
COST
TOP WIDTH
COST
1
Cowart Creek
C123-00-00
56
$ 918,000
66
$ 1,161,000
2
Hickory Slough
H123-00-00
311
$ 3,017,000
451
$ 4,356,000
3
Chocolate Bayou
E100-01-01
72
$ 1,490,000
91
$ 1,946,000
4
Cowart Creek
C118-00-00
37
$ 961,000
41
$ 1,199,000
5
Cowart Creek
C122-00-00
71
$ 1,069,000
83
$ 1,332,000
6
West Chocolate
E101-01-06
66
$ 806,000
80
$ 1,029,000
7
Chigger Creek
J101-02-00
146
$ 2,401,000
171
$ 2,920,000
8
Cowart Creek
C128-00-00
34
$ 671,000
40
$ 864,000
9
Chigger Creek
J102-05-01
50
$ 1,492,000
60
$ 1,910,000
10
Cowart Creek
C120-01-00
26
$ 632,000
26
$ 776,000
11
Cowart Creek
C124-01-00
42
$ 551,000
49
$ 701,000
12
Clear Creek
A105-05-00
83
$ 847,000
101
$ 1,074,000
13
Hickory Slough
H125-02-00
158
$ 718,000
158
$ 772,000
14
Cowart Creek
C107-03-01
35
$ 784,000
39
$ 984,000
15
Mary's Creek
B117-00-00
33
$ 1,545,000
37
$ 1,929,000
16
Hickory Slough
H114-00-00
34
$ 1,124,000
38
$ 1,421,000
17
West Chocolate
E101-01-01
52
$ 648,000
60
$ 810,000
18
Clear Creek
A113-00-00
34
$ 665,000
34
$ 799,000
19
Mary's Creek
B102-01-01
56
$ 499,000
66
$ 631,000
20
Chocolate Bayou
E102-00-00
50
$ 1,009,000
67
$ 1,373,000
21
Clear Creek
A115-00-00
39
$ 1,132,000
44
$ 1,420,000
22
Cowart Creek
C124-00-00
34
$ 669,000
34
$ 805,000
23
Cowart Creek
C119-00-00
28
$ 698,000
30
$ 875,000
24
Hickory Slough
H123-01-00
311
$ 3,017,000
451
$ 4,356,000
25
Clear Creek
A116-00-00
25
$ 870,000
33
$ 1,181,000
26
Cowart Creek
C120-00-00
91
$ 1,216,000
110
$ 1,534,000
27
Cowart Creek
C100-00-00
28
$ 532,000
31
$ 676,000
28
Clear Creek
Alll-00-00
31
$ 989,000
34
$ 1,241,000
29
Chigger Creek
J101-02-01
94
$ 1,094,000
114
$ 1,382,000
30
Mary's Creek
8114-01-01
37
$ 660,000
43
$ 843,000
31
Cowart Creek
CR 414 Ditch
21
$ 775,000
23
$ 993,000
32
Cowart Creek
C101-00-00
60
$ 2,659,000
79
$ 3,580,000
33
Chigger Creek
J102-00-00
116
$ 3,312,000
160
$ 4,525,000
34
Hickory Slough
H125-01-00
50
$ 1,141,000
50
$ 1,330,000
35
Hickory Slough
C103-03-00
39
$ 535,000
42
$ 657,000
36
Cowart Creek
Cowart's Creek Ditch
35
$ 784,000
39
$ 984,000
37
Chigger Creek
J102-00-00
91
$ 1,606,000
118
$ 2,121,000
95 1 Page
Pearland / Brazoria Drainage District No 4 Master Drainage Plan
Final Report —February 2019
Table 26 - Small O&M Project Recommendations
NEE HALFF
DITCH
PRIORITY
WATERSHED
DITCH
3 -YR LOS
10 -YR LOS
TOP WIDTH
COST
TOP WIDTH
COST
1
Chigger Creek
1101-01-01
61
$ 190,000
71
$ 238,000
2
Chigger Creek
1101-01-00
22
$ 233,000
24
$ 298,000
3
Clear Creek
A122-00-00
34
$ 479,000
35
$ 583,000
4
Chigger Creek
1101-01-01
23
$ 239,000
26
$ 309,000
5
Cowart Creek
C105-01-00
34
$ 77,000
41
$ 100,000
6
Cowart Creek
B102-01-03
33
$ 217,000
38
$ 278,000
7
Cowart Creek
C107-10-01
62
$ 429,000
71
$ 546,000
8
Clear Creek
A121-01-00
42
$ 397,000
50
$ 511,000
9
Cowart Creek
C125-00-00
22
$ 193,000
23
$ 241,000
10
Cowart Creek
C107-01-02
24
$ 244,000
28
$ 319,000
11
Cowart Creek
C107-04-01
31
$ 361,000
34
$ 453,000
12
Hickory Slough
H111-00-00
30
$ 85,000
30
$ 103,000
13
Chocolate Bayou
E100-01-01
56
$ 430,000
72
$ 572,000
96 1 Page