R2022-190 2022-08-22RESOLUTION NO. R2022 -190
A Resolution of the City Council of the City of Pearland, Texas, amending
Chapter 5, Stormwater Drainage, of the City’s Engineering Design Criteria
Manual, to reflect the National Oceanic and Atmospheric Administration
(NOAA) Atlas 14 rainfall values for infrastructure design.
BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF PEARLAND, TEXAS:
Section 1. That the City Council hereby amends Chapter 5, Stormwater Drainage, of
the City’s Engineering Design Criteria Manual, to reflect National Oceanic and Atmospheric
Administration (NOAA) Atlas 14 rainfall values for infrastructure design.
Section 2. The City’s Engineering Design Criteria Manual is hereby amended in
accordance with Exhibit “A” attached hereto.
PASSED, APPROVED, AND ADOPTED this 22nd day of August, A.D., 2022 .
________________________________
J. KEVIN COLE
MAYOR
ATTEST:
______________________________
FRANCES AGUILAR, TRMC, MMC
CITY SECRETARY
APPROVED AS TO FORM:
______________________________
DARRIN M. COKER
CITY ATTORNEY
DocuSign Envelope ID: 2BED3E07-4DC7-44A0-AA5C-D8B4A12C4614
CITY OF PEARLAND
CHAPTER 5
STORMWATER DESIGN CRITERIA
ENGINEERING DESIGN CRITERIA MANUAL
August 2022
EXHIBIT A
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CHAPTER 5
STORMWATER DESIGN CRITERIA
5.1 GENERAL
This chapter includes criteria for the design of storm drainage improvements for the City
of Pearland, Texas. These Storm Drainage Design Requirements shall be effective within
the City of Pearland and in the subdivisions located within its extraterritorial jurisdiction.
All drainage work proposed for design within these limits are to adhere to these criteria
explicitly. Any questions regarding their use or function should be addressed to the City
Engineer.
5.1.1 Background
Over the years, a number of methods have been used in Brazoria County and
adjacent counties for discharge determination in the design and analysis of flood
control facilities. The methods included various forms of the Rational Method,
U.S. Soils Conservation Service (SCS) synthetic unit hydrograph analysis using
existing stream gaging records and computer programs developed by the U.S. Army
Corps of Engineers, and U.S. Geological Survey generalized regression equations
developed for the area.
In the mid-1960's, the Harris County Flood Control District (HCFCD) and the City
of Houston commissioned a detailed hydrologic study of Harris County which
resulted in the development of the Site Runoff Curve (discharge versus drainage
area relationships) and unit hydrograph methodologies used for the design of flood
control and drainage facilities.
In June of 2001, Tropical Storm Allison came ashore on the Upper Texas Coast and
produced record rainfall amounts and pervasive flooding in Harris and surrounding
counties, including the Clear Creek Watershed. In October of 2001, through a joint
effort between FEMA and HCFCD, Harris County began the Tropical Storm
Allison Recovery Project (TSARP). Flood Insurance Studies including for Clear
Creek Watershed were finalized in June 2007.
In September 2018, National Oceanic and Atmospheric Administration (NOAA)
released Atlas 14, a rainfall study that predicts the frequency of rainfall for an area.
Based on Atlas 14, the depth of rainfall for various events such as 2-year, 10-year,
and 100-year differ from what has been used for the analysis and design of drainage
systems. For example; 24 hours rainfall depth for 100-year storm event changed
from 13.5 inches to 17.0 inches.
Special care must be taken to make sure that the correct models and methodologies
are used for projects that require FEMA approval. In any case, for projects
requiring FEMA approval, design engineers should contact FEMA representatives
to get directions before selecting an applicable H&H effective model and
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methodology to satisfay FEMA requirement. For modification requirements to
FEMA floodplain and/or floodway, refer to Chapter 2.
In the case that FEMA approval is not required for the project, design engineers
should use the methodology presented in this manual to design drainage facilities
in the City of Pearland.
5.1.2 Previous Design Requirements
The criteria of this Manual supersedes the previous document of the same name
dated October 2021. All items listed herein are intended to supersede those
documents, so all designs of drainage facilities within the City of Pearland,
including all subdivisions within its extraterritorial jurisdiction, shall be based on
the criteria of this Manual from this time forward, until such time as it may be
revised or replaced.
5.2 DRAINAGE POLICY
5.2.1 Design Requirements
The drainage criteria administered by the City of Pearland for newly designed areas
provides protection of habitable areas from flooding by large storm events. This is
accomplished with the application of various drainage enhancements such as storm
sewers, roadside ditches, open channels, detention and overland (sheet) runoff. The
combined system is intended to prevent flooding of houses by extreme events up to
the level of a 100-year storm.
Recognizing that each site has unique differences that can enhance proper drainage,
the intent of these criteria is to specify minimum requirements that can be modified,
provided the objective for drainage standards is maintained and such modifications
are made with the approval from the office of the City Engineer.
5.2.2 Street Drainage
Street ponding of short duration is anticipated and designed to contribute to the
overall drainage conveyance capacity of the system. Storm sewers and roadside
ditch conduits are designed as a balance of convenience and economics. These
conduits are designed to convey less intense, more frequent rainfalls with the intent
of allowing for traffic movement during these events. When rainfall events exceed
the capacity of the storm sewer system, the additional runoff is intended to be stored
or conveyed overland in a manner that reduces the threat of flooding habitable
structures.
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5.2.3 Floodplain Management
A. The City of Pearland is a participant in the National Flood Insurance
Program’s (NFIP’s) Community Rating System (CRS). As a CRS
community, the City is required to develop and implement various
programs intended to reduce flood risk such as identifying drainage problem
areas, maintenance of existing drainage system, construction of drainage
project, outreach to the community, etc. As a result of the community’s
commitment to reduce the flood risk, the residents of the City are entitled
to receive discounted flood insurance premiums based on the City’s CRS
ranking as evaluated by CRS.
B. All runoff impacts created by development shall be mitigated, so post-
project runoff rates and flooding levels are equal to or less than equivalent
pre-project conditions. Stormwater detention requirements are presented in
Section 5.8.
C. All fill placed in the 500-year floodplain, as designated on the Flood
Insurance Rate Map shall be mitigated by the removal of a like amount (i.e.
1 cubic yard fill to 1 cubic yard removal) of compensating cut in the vicinity
of the fill, while maintaining hydraulic connectivity to the existing
floodplain. No fill mitigation is required for the fill placed above the 500-
year floodplain.
D. See Chapter 2, Section 2.12 Floodplain Management for additional details
and requirements.
5.2.4 Relationship to the Permitting and Platting Process
Approval of storm drainage is part of the review process for planning and platting
new development. The review of storm drainage is conducted by the Engineering
Department.
5.2.5 Final Drainage Plan and Plat
A detailed drainage plan for each proposed development shall be prepared by a
Registered Professional Engineer and shall be presented to the City Engineer for
review and approval. The plan shall consist of the detailed design drawings for all
drainage improvements and structures, rainfall-runoff and impact data, and notes
as applicable on the Final Drainage Plan as specified in Sections 5.5 through 5.9.
The following items, at a minimum, shall be shown on a plan for development:
1. Name, address, and phone number of the engineer that prepared the plans.
2. Scale of drawing with a minimum scale of 1” = 100’.
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3. Benchmark and reference benchmark with year of adjustment.
4. Location or vicinity map drawn to a scale.
5. Date on all submittals with date of all revisions, including month, day, and
year.
6. Contour lines at 0.5-foot intervals or greater with a minimum of 2 contours
covering the entire development and extending beyond the development
boundaries at least 100 feet on all sides for developments over 5 acres and
50 feet for developments under 5 acres.
7. Lot grading plan, which provides for the passage of sheet flow from
adjacent property. The lot grading plan shall include the proposed elevation
of each corner of the lot.
8. A 100-year sheet flow analysis that provides direct access to the detention
facility or main outfall.
9. Drainage area divides for project area, with peak runoff rates for each inlet,
structure, or drainage area.
10. Locations of pipelines, drainage structures, buildings, or other physical
features on the property and adjacent rights-of-way.
11. True locations of existing creeks, bayous, streams, gullies, and ditches, as
determined by actual ground survey current within one year of approval of
the Preliminary Plan. Show stream alignment 200 feet upstream and 200
feet downstream of development.
12. Cross sections of detention facility, ditches, property lines, and earthworks.
13. Details of all ditches, which are to convey rainfall runoff from a subdivision
and/or through a subdivision to the appropriate drainage artery and location
of that drainage artery.
14. Drainage easements and dedicated right-of-way along all creeks, bayous,
streams, gullies, and ditches.
15. Bridges which span any creek, bayou, stream, gully, or ditch and
maintenance responsibility and/or ownership of such structures.
16. Culvert type and size shall be shown. No culvert shall be less than 18” in
diameter, without special permission by the City Engineer.
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17. Copy of TxDOT permit approval (driveway, drainage connection, utility
crossing), if applicable.
18. Copies of letters of approval from entities holding easements or rights-of-
way to be crossed.
19. An erosion control plan acceptable to both the City and the TCEQ must be
presented with all plans. Copies of all submittals to the TCEQ shall be
delivered to the City.
20. Signatory requirement is applicable as noted on Chapter 2, Section 2.3.
21. Refer to Chapter 2-section 2.11.3 and this Chapter for engineering grading
and drainage plans requirement for the small residential infill lots.
5.3 REFERENCES
All projects that are required to conform to these standards shall also be in compliance with all
applicable city ordinances current versions. Relevant related laws and regulations include but are
not limited to the following.
1. Brazoria County Drainage Criteria Manual, May 2022.
2. Brazoria Drainage District No. 4, “Rules, Regulations, & Guidelines”, November
5, 1997 (BDD4 Regulations), amended April 2013.
3. Harris County Flood Control District, Policy Criteria Procedure Manual (PCPM)
for approval and acceptance of Infrastructure, July 2019.
4. Harris County Flood Control District, “Hydrology & Hydraulics Guidance
Manual”, December 2009 (HCFCD Criteria).
5. Applicable Portions of the City of Houston Design Manual, Chapter 9, “Storm
Sewer Design Requirements”, July 2021.
6. Ordinances of the City of Pearland (as currently amended).
7. U.S. Army Corps of Engineers. HEC-RAS River Analysis System User’s Manual
Version 4.1, The Hydrologic Engineering Center, Davis, California, January 2010.
8. U.S. Weather Bureau. “Rainfall Frequency Atlas for the United Stated for
Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years,”
Technical Paper No. 40, January 1963.
9. U.S. Army Corps of Engineers. Civil Works Bulletin 52-8.
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10. U.S. Army Corps of Engineers. HEC-HMS Hydrologic Modeling System User’s
Manual Version 4.0, The Hydrologic Engineering Center, Davis, California,
December 2013.
11. Chow, V. T., Open-Channel Hydraulics, McGraw-Hill, 1959.
12. Ramser, C. E., Flow of Water in Drainage Channels, U.S. Department of
Agriculture, Technical Bulletin No. 129, November 1929.
13. Engineer Handbook, Hydraulics, Section 5, U. S. Department of Agriculture, Soil
Conservation Service, 1955.
14. Barnes, Harry H., 1967, Roughness Characteristics of Natural Channels, U. S.
Geological Survey Water Supply Paper, 1849.
15. King, H. W. and E. F. Brater, Handbook of Hydraulics, 6th Edition, McGraw-Hill,
1976.
16. Research Studies on Stilling Basins, Energy Dissipators, and Associated
Appurtenances, Bureau of Reclamation, Hydraulic Laboratory Report No. Hyd-
399, June 1, 1955.
17. Texas State Department of Highways and Public Transportation standard
specifications for Construction of Highways, Streets, and Bridges latest edition.
18. ASTM A796 Structural Design of Corrugated Steel Pipe, Pipe Arches, and Arches
for Storm and Sanitary Sewers.
19. Liscum, F., and B. C. Massey. “Technique for Estimating the Magnitude and
Frequency of Floods in the Houston, Texas, Metropolitan Area,” Water Resources
Investigations 80-17, U. S. Geological Survey, April 1980.
20. Federal Emergency Management Agency. Flood Insurance Study - Brazoria
County, Texas and Incorporated Areas, June 5, 1989.
21. Federal Emergency Management Agency. National Flood Insurance Program and
Related Regulations, Index 44 CFR, October 1, 2002.
22. Hare, G. “Effects of Urban Development on Storm Runoff Rates,” U. S. Army
Corps of Engineers, Galveston District, September 1970.
23. Malcolm, H. R. A Study of Detention in Urban Stormwater Management, Report
No. 156, Water Resource Research Institute, University of North Carolina, Raleigh,
North Carolina, July 1980.
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24. Texas Water Development Board. “Study of Some Effects of Urbanization on
Storm Runoff from a Small Watershed,” Report 23, August 1966.
25. Hydraulic Design of Stilling Basins and Energy Dissipators, Engineering
Monograph No. 25, U. S. Department of the Interior, Bureau of Reclamation, 1964.
26. Viessman, Jr., Warren; John W. Knapp; Gary L. Lewis and Terence Harbaugh,
Introduction to Hydrology, Harper & Row, 1977.
27. Coenco, Inc. Master Drainage Plan, April 11, 1980, Revised 1988.
28. Rust Lichliter/Jameson. Flood Protection Plan for Brazoria Drainage District No.
4, Brazoria County, Texas, November 5, 1997.
29. U.S. Geological Survey, Water Resources Investigations Report 98-4044, Depth-
Duration Frequency of Precipitation for Texas, Austin, Texas 1998.
30. U.S. Geological Survey & Texas Department of Transportation, Atlas of Depth-
Duration Frequency of Precipitation Annual Maxima for Texas, June 2004.
31. USACE EM 1110-2-1417.
32. McCuen, Richard H., Prentice Hall, Hydrologic Analysis and Design, 1989.
33. City of Pearland Plat Amendment Ordinance No. 421.
34. City of Pearland Flood Damage Prevention Ordinance No. 532-6.
35. City of Pearland Flood Mitigation Exempt Ordinance No. 817-1.
36. City of Pearland City Right-of-Way Management Ordinance No. 669-3.
37. City of Pearland Amending Chapter 27 for Sidewalk City Ordinance No. 741-5.
38. City of Pearland Maintenance of Stormwater Storage Facilities Ordinance No 1059.
39. City of Pearland Discharge Ordinance No 1570.
40. Landuse and Urban Development Ordinance 509.
5.4 DEFINITIONS
Backslope Drain: A drain or swale that collects overland peak discharge from
channel overbanks and other areas not draining into the
storm sewer collection system. These may be to prevent
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unplanned runoff from entering a detention system, or from
entering a drainage ditch. They are also used to prevent
overland discharge from eroding the sides of a ditch or pond.
Benchmark: A point of known exact elevation, set and used by Surveyors
to start from to obtain elevations on other points of unknown
elevation. The known elevation is usually based on “mean
sea level” and is referenced to a “Year of Adjustment”.
BDD4: Brazoria Drainage District No. 4.
cfs: Cubic feet per second.
City Engineer: An Engineer licensed in the State of Texas who is
responsible for reviewing drainage plans or plats under the
authority of and in the employment of the City of Pearland.
CMP: Corrugated Metal Pipe.
Coefficient of Roughness: A number used to measure and compare the roughness of
pipe interior or open channel sides and bottom.
Commercial: Development of real estate for any purpose other than
“residential” as defined herein.
Conduit: Any open or closed device for conveying flowing water.
Construction: The building of a planned or designed project.
Continuity Equation: Q = VA
Where, Q = discharge (cfs or cms)
V = velocity (ft/sec or m/sec)
A = cross sectional area of conduit in
square feet or square meters.
Contour Line: A line on a map, chart or plan that follows a continuous line
of a certain known elevation.
Culvert: One or more pipes that carry the flow of water from one
point in a ditch or channel to another point in a ditch or
channel.
Design Storm Event: The rainfall intensity and/or depth upon which the drainage
facility will be sized.
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Detention Control Structure: The outlet pipe or weir, and high-level spillway that limits
the discharge from a detention facility.
Detention Facility: A reservoir, dam, pond or other area where storm water
collects and is held temporarily. The collected storm water
is released at a calculated rate through a control structure.
Developer: A person or entity that develops land.
Development: A tract of land that has been improved or subdivided,
exclusive of land being used and continuing to be used for
agricultural purposes. Improvement of land includes
grading, paving, building structures, or otherwise changing
the runoff characteristics of the land.
Development Engineer: An Engineer licensed in the State of Texas who is
performing work for a Developer.
Disturbed Area An area of the land or tract that changes the drainage
characteristics. This include any changes in land grading,
paving, building structures, or otherwise changing the runoff
characteristics of the land.
Drainage Area Map: Area map of watershed which is subdivided to show each
area served by each storm drainage subsystem.
Drainage Arteries: Natural or man-made ditches or channels that intercept and
carry stormwater on to a larger major creek, bayou or stream.
Drainage Plan: An engineering representation of the peak discharge of
rainfall runoff on or onto a particular area, and off of that
same area. It may also include systems that will be used to
detain or control runoff and provide flood control for a
development, subdivision, or structure.
Drainage System: A series of swales, storm sewers, ditches and creeks which
function to collect and convey stormwater runoff in a
watershed.
Easement: An area designed and dedicated for a specific use but
remains the property of the owner out of which it is a part.
The uses may be for drainage, maintenance, access, future
widening of channel or ditch, or other specific uses.
FEMA: The Federal Emergency Management Agency, which
administers the National Flood Insurance Program.
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FIRM: Flood Insurance Rate Maps published by a Federal
Emergency Management Agency.
Flood Plain Administrator: Person identified by the governing municipality or County
as responsible for administering the National Flood
Insurance Program for the City or County in accordance with
guidelines established by FEMA.
HCFCD: Harris County Flood Control District.
HDPE: High Density Polyethylene.
HEC-HMS: “Hydrologic Modeling System” computer program written
by the U.S. Army Corps of Engineers.
HEC-RAS: “River Analysis System” computer program written by U.S.
Army Corps of Engineers.
Hydraulic Analysis: The study and/or definition of the movement of stormwater
through a drainage system.
Hydraulic Grade Line: A line representing the pressure head available at any given
point within the drainage system.
Hydrologic Analysis: The study and/or definition of the properties, distribution
and circulation of stormwater runoff over land or in the soil.
Hydromulching: A process that prevents or helps to prevent erosion of the
soil. When sprayed on an exposed slope, it seals the surface
and seeds it with vegetation.
ICPR: Interconnected Channel and Pond Routing computer
program by Streamline Technologies, Inc. Computes
unsteady gradually varied flow.
Impact: The effect of a proposed development on the hydrology or
hydraulics of a subarea or watershed as defined by an
increase or decrease in peak discharges or water surface
elevations.
Impact Data: Data required to support the Developer’s Engineer to show
the effect the proposed development will have on the rainfall
runoff rates, rainfall concentration times and the surface
level of the affected creek, stream, gully, or ditch into which
proposed development runoff drains.
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Impervious Cover: A land surface cover which does not allow the passage of
stormwater into the underlying soil. Used in hydrologic
analysis to calculate the amount of stormwater runoff from
an area.
In-Fill Development: Development of open tracts of land in areas where the storm
drainage infrastructure is already in place and takes
advantage of the existing infrastructure as a drainage outlet.
Manning's Equation: V = (K/n) R 2/3 Sf1/2
Where K = 1.49 for English units,
1.00 for metric units
V = velocity (ft/sec or m/sec)
R = hydraulic radius (ft or m)
(area/wetted perimeter)
Sf = friction slope (headloss/length)
n = 0.013 for concrete pipes,
0.011 for HDPE pipes,
0.028 for CMP
0.035-0.05 for earthen channels
Metering Device: A device or structure containing pipe, V-notch weir, slots
and other configurations designed to measure or regulate the
outflow.
Mitigate: To lessen or eliminate the impact of a proposed development
on the hydrology or hydraulics of a subarea or watershed.
NAVD: North American Vertical Datum or Mean Sea Level,
pertaining to base elevations.
Outfall Structures: A structure made to contain the outfall pipe or peak
discharge, with necessary weir, concrete slope paving,
interlocking concrete block, or other methods to control
velocity and prevent erosion, and may contain the metering
device.
Outflow: The final peak discharge from the development system into
another or existing drainage system.
Overflow: The peak discharge that will not pass through the design pipe
or structure and must go over a weir or some other relief
structure.
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Peak Discharge: The maximum rate of stormwater runoff from a tract of land
or in a ditch or channel, as determined from the maximum
point in cubic feet per second of the calculated hydrograph
for the study area.
Plat: A formal drawing of property lines and spaces that may, or
may not, be recorded.
Rainfall Data: Data pertaining to the amount of rainfall in a certain area and
occurring over a certain specified period of time.
Rainfall Frequency: The probability of a rainfall event of defined characteristics
occurring in any given year. Information on rainfall
frequency is published by the National Weather Service. For
the purpose of storm drainage design, the following
frequencies are applicable:
2-year frequency - a rainfall intensity having a 50%
probability of being equaled or exceeded in any given year.
5-year frequency - a rainfall intensity having a 20%
probability of being equaled or exceeded in any given year.
10-year frequency - a rainfall intensity having a 10%
probability of being equaled or exceeded in any given year.
25-year frequency - a rainfall intensity having a 4%
probability of being equaled or exceeded in any given.
50-year frequency - a rainfall intensity having a 2%
probability of being equaled or exceeded in any given year.
100-year frequency - a rainfall intensity having a 1%
probability of being equaled or exceeded in any given year.
500-year frequency - a rainfall intensity having a 0.2%
probability of being equaled or exceeded in any given year.
Rational Formula: A method for calculating the peak runoff for a storm
drainage system.
Redevelopment: A change in land use that alters the impervious cover from
one type of development to either the same type or another
type and takes advantage of the existing infrastructure in
place as a drainage outlet.
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RCP: Reinforced Concrete Pipe.
Regional Detention Facility: A detention facility that collects and holds stormwater from
more than one development or from one of the major creeks
or tributaries in the City of Pearland.
Residential: Of or pertaining to single family detached dwelling(s) not
including multi-family townhomes, condominium,
duplexes, or apartments.
Right-of-Way: A strip of land that is set aside and reserved for certain
purposes including drainage and maintenance, and possibly
future widening of a drainage channel.
Runoff: That part of rainfall on property that does not soak in or
evaporate, and ultimately reaches drainage arteries.
Runoff Coefficient: A comparative measure of different soils, slopes and
growths, and improvements, for their capability of allowing
the peak discharge of water to move along and over them.
Sheet Flow: Overland storm runoff that is not conveyed in a defined
conduit and is typically in excess of the capacity of the
conduit.
Site: A space of ground occupied or to be occupied by a building
or development.
Spillway: The part of the outfall structure that allows and controls the
“overflow” that does not go through the outfall structure.
Subdivide: To divide a tract of land into two or more smaller tracts or
building lots.
Subdivision: A tract of land which has been separated from surrounding
tracts and has been divided into two or more lots.
Swale: A very shallow ditch that usually has very long sloping sides,
in some cases not much more that a small depression that
allows water to peak discharge in a somewhat controlled
manner.
TSARP: Tropical Storm Allison Recovery Project. Federally funded
flood study managed by Harris County Flood Control
District begun in October of 2001.
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U.S.G.S. SIR 2004-5041: 2004 U.S.G.S Publication regarding rainfall depth-duration
frequency relationships for Texas.
Variance: A one-time formal exception to a particular rule or rules
granted for extenuating circumstances, approved by City
Engineer. Engineering Variance as mentioned in EDCM
Chapter 1 is applicable.
Watershed: A region or area bounded peripherally by a ridge of higher
elevation and draining ultimately to a particular watercourse
or body of water.
5.5 STORM SEWER AND ROAD-SIDE DITCH DESIGN REQUIREMENTS
Storm sewer structures shall be per City of Pearland Standard Details. The City of Pearland
also adopts the hardware requirements of the TxDOT Specifications and Standard
Drawings as necessary. Manhole covers shall include the City of Pearland designation as
shown in Standard Details.
Furthermore, all outfall pipes, ditches, and structures that enter District Channels shall also
be designed in accordance with BDD4 Regulations or HCFCD Criteria. In such instances
wherein a conflict of these criteria arises, the most stringent requirements of these shall be
utilized for the design.
To distinguish the adequacy of road-side ditches to be designed by the following
requirements of this section, it is important to note that these ditches DO NOT include
channels that receive runoff flows from any other outfall drainage sources other than direct
overland runoff flows. The design of channels that do receive outfall system drainage can
be found in Section 5.7 of this Manual.
5.5.1 Determination of Runoff
The stormwater runoff (peak discharge) shall be determined for each inlet, pipe,
roadside ditch, channel, bridge, culvert, outfall, or other designated design point by
using the following standards applicable to the above requirements.
A. Application of Runoff Calculation Models
a. Acceptable Methodology for Areas Less Than 200 Acres
For areas up to 200 acres served by storm sewer or roadside ditch, peak
discharges will be based on the Rational Method. If the modeling is
associated with establishing a flood-prone area for purposes of a FEMA
submittal, the models to be used must be acceptable to that agency.
b. Acceptable Methodology for Areas Greater Than 200 Acres
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Rainfall runoff modeling will be applied to areas greater than 200 acres
in size. Again, if the modeling is associated with establishing a flood-
prone area for purposes of a FEMA submittal, the models to be used
must be acceptable to that agency.
B. Rainfall Durations for Hydrologic Modeling
For design using the HEC-HMS model, the 24-hour design storm
isohyetograph will be used for rainfall data for drainage areas greater than
200 acres.
C. Application of the Rational Method
Use of the Rational Method for calculating the peak runoff for a storm
drainage system involves applying the following formula to runoff:
Q = CIA
where: Q = peak discharge (cfs)
C = runoff coefficient
A = area (acres)
I = rainfall intensity (inches per hour)
a. Calculation of Runoff Coefficient
The runoff coefficient "C" values in the Rational formula will vary
based on the land use. Land use types and "C" values which can be used
are as follows:
Land Use Type Runoff Coefficient
Paved Areas/Roofs* 1.00
Compacted gravel, limestone 0.80
Residential Districts
Lots more than ½ acre 0.40
Lots 1/4 - ½ acre 0.50
Lots 8,000 sf. – 1/4 acre 0.55
Lots 5,000 sf. – 8,000 sf. 0.60
Lots less than 5,000 sf. 0.70
Multi-Family areas
Less than 20 DU/AC 0.75
20 DU/AC or Greater 0.85
Business Districts 0.95
Industrial Districts 0.95
Railroad Yard Areas 0.30
Parks/Open Areas 0.30
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Rice Fields/Pastures 0.20
Lakes/Detention Ponds** 1.00
Dry Detention Ponds 0.85
*Includes concrete and asphalt
**Includes wet detention facilities. Area will be computed from top of
slope.
Composite “C” values for mixed-use drainage areas are allowed for use
in the Rational Formula. These values are to be obtained by calculating
a weighted average of all the different “C” values of the sub-areas
contributing to each mixed-use drainage area. Any calculations of these
Composite “C” values are to be provided as part of the drainage
calculations.
C = (C1A1 + C2A2+C3A3+ …CnAn)
(A1+A2+A3+ …An)
The calculations and an exhibit of land use types for used for composite
“C” values shall be included with the drainage calculations and provided
in plans.
b. Determination of Time of Concentration
The following method shall be used for determining the time of
concentration:
Tc = D/(60*V) + Ti
where: Tc = Time of concentration (minutes)
Ti = Initial time (minutes)
Use 10 minutes for developed flows
Use 15 minutes for undeveloped flows
D = Travel distance on flow path (feet)
V = Velocity (ft/sec)
The time of concentration shall be calculated for all inlets and pipe
junctions in a proposed storm drainage system or other points of runoff
entry to the system. Time of concentration shall be based upon the
actual travel time from most remote point in the drainage area to the
point of runoff. The design engineer shall provide a sketch of the travel
path with the computations.
The following minimum and maximum velocities shall be used when
calculating the time of concentration:
Page 18 of 63 Stormwater Design Criteria
SURFACE
TYPE
UNDEVELOPED
FLOWS MIN V (fps)
DEVELOPED
FLOWS MIN V (fps)
Storm sewer 3.00 3.00
Ditch / channel 2.00 2.50
Paved area 1.50 1.50
Bare ground 0.50 1.00
Grass 0.35 0.50
Vegetation 0.25 0.35
For storm sewers, time of concentration for other analysis points shall
be the highest time of concentration of the previous upstream
contributing area(s) plus time of flow in the pipe. For drainage areas of
one acre or less the time of concentration need not be calculated and a
storm duration of 10 minutes for mostly impervious area or 15 minutes
for mostly pervious area may be used as the basis of design.
Note: Sheet flow usually becomes shallow concentrated flow after
around 100 feet.
c. Rainfall Intensity
The time of concentration of the runoff will be used to determine the
rainfall intensity component of the Rational Method Formula. The
rainfall intensity shall be computed as follows:
I = b/(Tc +d)^e
where: I = Rainfall intensity (in/hr)
Tc = Time of Concentration (min)
b, d, e = Coefficient per table below
d. Sample Calculation Forms
Appendix A has a sample calculation form for storm sewer systems.
5.5.2 Design of Storm Sewers
A. Design Frequency
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year
B 57.44 58.019 57.515 52.78 49.157 46.316 47.179
D 11.511 9.236 7.777 5.022 3.081 1.555 0.322
E 0.754 0.712 0.676 0.618 0.574 0.533 0.474
Page 19 of 63 Stormwater Design Criteria
a. Newly Developed Areas
The design storm event for sizing storm sewers will be 2-year rainfall
intensity. The storm sewer should be designed so that the design
hydraulic grade line shall be at or below the curb gutter grade for a curb
and gutter section, and six inches below the shoulder of a roadside ditch
section. For major thoroughfares and secondary thoroughfares, the
design storm event will be 5-year rainfall intensity.
b. Redevelopment or In-fill Development
The existing storm drainage system will be evaluated using 2-year
rainfall, assuming no development takes place. The same system will
then be evaluated with the development in place. Modifications to the
existing drainage system are to be considered based on the following:
1. If the proposed redevelopment has a lower or equal runoff
potential, no modifications to the existing storm drainage system
are required.
2. If the hydraulic gradient of the affected existing storm drainage
system is below the curb gutter grade (or six inches below the
shoulder of a roadside ditch section), no improvements to the
existing storm drain are required.
3. If the hydraulic gradient is above the gutter grade (or the edge of
shoulder of a roadside ditch section), the drainage system must
be analyzed for the impact of the 100-year storm event. If the
100-year event is at or one foot below the floor levels of adjacent
existing habitable structures and exceeds the top of curb (or the
roadway centerline elevation in a roadside ditch section) by
twelve inches or less, no improvements to the existing system
are required.
If none of these conditions are met by the proposed development
changes, improvements to the existing system will be required.
In all cases of development (a development that increases the runoff
potential of the site), mitigation in the form of onsite or offsite detention
must be in compliance with section 5.8 of this chapter.
c. City of Pearland Projects (Capital Improvement Programs)
Proposed City of Pearland Capital Improvements Program may indicate
that a larger diameter storm sewer is planned in the area for drainage
improvements. The City Engineer will provide information on planned
Page 20 of 63 Stormwater Design Criteria
capital improvements and should be consulted as to its impact on new
development.
d. Private Drainage Systems
Drainage facilities draining private areas shall be designed in
conformance with appropriate design standards. The City of Pearland
will not approve nor accept for maintenance a drainage system on
private property unless it drains public stormwater and is located in a
drainage easement. The connection of any storm sewer, inlet, ditch, or
culvert to a public drainage facility will be reviewed and approved by
the City of Pearland. Stormwater shall not be discharged or flow over
any public sidewalk or adjoining property except to existing creeks,
ditches, streets, or storm sewers in public right of way or easements.
Drainage to an existing Texas Department of Transportation (TxDOT)
ditch, road, or storm sewer, must be approved or documented with a
permit, letter or note of no objections to the plan by TxDOT. Similarly,
drainage to an existing BDD4 or HCFCD ditch/creek must be approved
or documented with a permit, letter or note of no objections to the plan
by BDD4 or HCFCD respectively.
B. Velocity Considerations
a. Minimum velocities should not be less than 3 feet per second with the
pipe flowing full, under the design conditions.
b. Maximum velocities should not exceed 8 feet per second without use of
energy dissipation before release to natural or cultivated grass channels.
C. Pipe Sizes and Placement
a. Use storm sewer and inlet leads with at least 18-inch inside diameter or
equivalent cross section. Box culverts shall be minimum of 2' x 2'.
Closed conduits (circular, elliptical, or box) shall be selected based on
hydraulic principles and economy of size and shape. For inlets and leads
carrying 5 cfs or more, 24-inch inside diameter is the minimum.
b. Larger pipes upstream should not flow into smaller pipes downstream
unless construction constraints prohibit the use of a larger pipe
downstream, the improvements are outfalling into an existing system,
or the upstream system is intended for use in detention.
c. Match crowns of pipe at any size change unless depth constraints or
other conditions justify matching flowlines.
Page 21 of 63 Stormwater Design Criteria
d. Locate storm sewers in public street rights-of-way or in approved
easements of adequate width as shown in standard details. Back lot and
side lot easements are discouraged and must be approved by the City
Engineer.
e. A straight line shall be used for inlet leads and storm sewers.
f. Minimum vertical clearance between the exterior of any storm pipe or
other appurtenances (manhole, inlet etc.) shall be at least 18 inches from
exterior of the existing or proposed utilities.
g. Cast-in-place concrete storm sewers are not allowed within the public
right-of-way.
D. Starting Water Surface and Hydraulic Gradient
a. Tailwater elevation selections for Hydraulic Gradient Line (HGL) analysis:
(1) If the receiving channel for the storm system being analyzed is less than
2,000 feet from the project limits, then the starting tailwater shall be
determined from outfall at the receiving channel according to the criteria
noted below:
· For the 2-year design rainfall event with non-submerged outfall to the
receiving channel, the starting tailwater shall be the top of the pipe.
· For the 100-year extreme rainfall event and outfall to the receiving
channel, the starting tailwater shall be the 10-year water surface
elevation (WSE) or 2-feet below the top of bank, whichever is higher.
(2) If the receiving channel for the storm system being analyzed is greater than
2,000-feet from the project limits, then the starting tailwater may be
determined from an outfall point, or truncation, downstream of the project
interconnect point, as noted below:
· For the 2-yr design rainfall event the starting HGL, shall be the top of
pipe 2,000-feet downstream of the project interconnect point assuming
pipes are connected at the soffit. If pipes are connectedthe at flow line,
the top of the larger receiving pipe must be used. If a starting tailwater
other than the topthe of pipe is chosen, the consultant shall analyze the
storm system from outfall at the receiving channel upstream to the
point of interconnect to demonstrate the alternate starting HGL value.
· For the 100-year extreme rainfall event the starting HGL shall be 2-
feet above the top of pipe 2,000-feet downstream of the project
interconnect point. If a starting tailwater other than 2-ft above the top
Page 22 of 63 Stormwater Design Criteria
of pipe is chosen, the consultant shall analyze the storm system from
outfall at the receiving channel upstream to the point of interconnect to
demonstrate the alternate starting HGL value.
· The design engineer can also use a stage/time variable tailwater
developed from effective HEC-RAS and HEC-HMS model for
extreme event analysis.
(3) For the hydraulic impact analysis, a variable tailwater at the downstream
end of the model may be used. A variable tailwater condition is
recommended for use for detention analyses.
b. At drops in pipe invert, where the top of the upstream pipe be higher than the
HGL, then the HGL shall be recalculated assuming the starting water surface to
be at the top of pipe at that point.
c. For the Design Rainfall Event, the hydraulic gradient shall at all times be below
the gutter line for all newly developed areas.
d. For the 100-year design storm, the maximum depth of ponding allowed will be
9-inches above top of curb for minor collector and local streets, and 3-inches
above top of inside curb for major and secondary thoroughfare roads which is
also known as one lane passable criteria.
E. Manhole Locations
a. Use manholes for precast conduits at the following locations:
(1) Size or cross section changes.
(2) Inlet lead and conduit intersections.
(3) Changes in pipe grade.
(4) Street intersections.
(5) A maximum spacing of 600 feet measured along the conduit run.
(6) Manholes and inlets shall not be located in driveway areas.
F. Inlets
a. Locate inlets at all low points in gutter.
b. Valley gutters across intersections are not permitted without approval
from the City Engineer.
c. Inlet spacing is a function of gutter slope, contributing drainage area,
and ponding width and height. For minimum gutter slopes, the
maximum spacing of inlets shall result in a gutter run of 350 feet from
Page 23 of 63 Stormwater Design Criteria
high point in pavement, with a maximum total of 700 feet of pavement
draining towards any one inlet.
d. Use the following standard inlets as detailed in standard details:
Inlet Type Application Capacity
Type A Parking Lots/Small Areas 2.5 cfs
Type B-B Residential/Commercial 5.0 cfs
Type C Residential/Commercial 5.0 cfs
Type C1 Residential/Commercial 10.0 cfs
Type C2 Residential/Commercial 15.0 cfs
Type D Parking Lots 2.0 cfs
Type E Roadside Ditches 20.0 cfs
e. Do not use “Beehive” grate inlets or other “specialty” inlets without
approval from City Engineer.
f. Do not use grate top inlets in unlined roadside ditches.
g. Place inlets at the end of proposed pavement, if drainage will enter or
leave pavement.
h. Do not locate inlets adjacent to esplanade openings.
i. Place inlets on side streets intersecting major streets, unless special
conditions warrant otherwise.
j. For lots 65’ in width or greater, place inlets at mid lot.
G. Outfalls
Storm sewer and open street ditch outfalls to BDD4 or HCFCD ditches shall
be per BDD4 or HCFCD criteria, as approved by the respective District and
City Engineer.
5.5.3 Consideration of Overland Flow
All storm drainage designs will take into consideration the overland flow of runoff
to account for the possibility of system inundation, obstruction, failure, or events
that exceed the design storm. A representation of the overland flow scheme must
be submitted with the system design.
A. Design Frequency
The design frequency for consideration of overland sheet flow will focus on
extreme storm events which exceed the capacity of the underground storm
Page 24 of 63 Stormwater Design Criteria
sewer system resulting in ponding and overland sheet flow through the
development to the primary outlet or detention basin. Unless otherwise
accepted by the City Engineer, the default storm event for this type of
analysis is the 100-year storm.
B. Relationship of Structures to Street
The relationship of structures to streets shall comply with EDCM chapter 2.
C. Calculation of Flow
a. Streets will be designed so that consecutive high points in the street will
provide for a gravity flow of sheet flow drainage to the ultimate outlet.
b. Sheet flow between lots will be provided only through a defined
drainage easement, through a separate instrument, or on the plat.
c. A map shall be provided to delineate extreme event flow direction
through a proposed development and how this flow is discharged to the
primary drainage outlet or detention basin.
d. In areas where ponding occurs and no sheet flow path exists, then a
calculation showing that runoff from the 100-year event can be
conveyed (or stored) and remain in compliance with the other terms of
this paragraph must be provided.
e. Selective reaches of the proposed storm sewer may need to be increased
in size to adjust the elevation of the 100-year HGL to not exceed the
desired HGL with respect to roadway top of curb.
f. Analysis using the Stormwater Management Model will be acceptable
to the City.
5.5.4 Design of Roadside Ditches
Open ditch subdivisions and asphalt streets are prohibited unless in an area
conforming with “RE” zoning as specified in the City’s Land Use and Urban
Development Ordinance or unless a variance has been granted by the City. In either
of these exceptions, the following shall apply to the design of roadside ditches.
A. Design Frequency
a. The design storm event for the roadside ditches shall be 2-year rainfall
for local streets and 5-year for thoroughfare streets.
Page 25 of 63 Stormwater Design Criteria
b. The 2- year storm design water surface elevations for a roadside ditch
shall be no higher than six inches below the edge of the shoulder or the
natural ground at the right-of-way line, whichever is lower.
The design must include an extreme event analysis to indicate that habitable
structures will not be flooded.
B. Velocity Considerations
1. For grass lined sections, the maximum design velocity shall be 3.0 feet
per second during the design event.
2. A grass lined or unimproved roadside ditch shall have side slopes no
steeper than three horizontal to one vertical (3:1).
c. Minimum grades for roadside ditches shall be 0.1 foot per 100 feet
(0.1%) unless approved by the City Engineer.
d. Calculation of velocity will use a Manning's roughness coefficient of
0.035 for improved earthen sections and 0.025 for ditches with paved
inverts.
e. Use erosion control methods acceptable to the City of Pearland when
design velocities are expected to be greater than 3 feet per second.
C. Culverts
a. Culverts will be placed at all driveway and roadway crossings and other
locations where appropriate.
b. Culverts may not be extended across property frontage to cover the
roadside ditch except for driveways.
c. Culverts will be designed assuming either inlet control or outlet control,
whichever the situation dictates.
d. Roadside culverts are to be sized based on drainage area. Calculations
are to be provided for each block based on drainage calculations.
Headlosses in culverts shall conform to TxDOT Hydraulics Manual.
e. Cross culvert for open channels with roadside shall be no smaller than
18 inches inside diameter or equivalent. The size of culvert used shall
not create a headloss of more than 0.20 feet greater than the normal
water surface profile without the culvert.
Page 26 of 63 Stormwater Design Criteria
f. Stormwater discharging from a ditch into a storm sewer system must be
received by use of an appropriate structure (i.e., stubs with ring grates
or type "E" inlet manholes).
D. Depth and Size Limitations for Roadside Ditches
The use of roadside ditch drainage systems is stipulated by other City
development codes. When the use of open ditch drainage systems is
approved, the following shall apply to the design of roadside ditches:
a. Roadside ditches shall drain streets and adjacent land areas.
b. Residential Streets - The maximum depth of proposed roadside ditches
will not exceed 4 feet from the centerline of pavement.
c. Commercial and Thoroughfare Areas - The maximum depth of
proposed roadside ditches will not exceed 4 feet from edge of the
roadway.
d. Roadside ditch bottoms should be at least 2 feet wide, unless design
analysis will support a narrower width.
e. Roadside ditch side slopes shall be set at a ratio of 3:1 or flatter.
f. Ditches in adjoining and parallel easements shall have the top of bank
not less than 2 ft from the outside easement line.
g. The minimum street right-of-way for open ditch drainage in residential
developments shall be 70 feet in width. Rights-of-way shall be wider
for deep ditches. The minimum open-ditch section roadway shall be 24
feet pavement with 6 feet shoulders on each side.
5.5.5 Design of Outfall Pipes
Outfall design shall conform to BDD4 rules or HCFCD Criteria, as appropriate,
and as approved by the City Engineer and the Drainage District. Section 5.6 of this
Manual generally incorporates BDD4 and HCFCD’s rules and/or criteria and shall
apply to all channel and detention designs subject to their requirements.
Pipe discharges of stormwater into earthen channels shall not exceed 5 feet per
second. Use erosion control methods acceptable to the City of Pearland, BDD4 or
HCFCD when design velocities are expected to be greater than 3 feet per second.
Page 27 of 63 Stormwater Design Criteria
5.5.6 Stormwater Mitigation Detention Alternatives
Detention basin design shall conform to City of Pearland criteria, BDD4 rules, or
HCFCD criteria on a case-by-case basis as approved by the City Engineer. The
City of Pearland detention design criteria is in Section 5.8 of this Manual.
5.6 HYDROLOGIC ANALYSIS OVERVIEW
The selection of an appropriate hydrologic methodology for all projects shall be carried out
in accordance with Figure 5.6-1. The design engineer should contact the appropriate
reviewing agencies prior to preparing the analysis to obtain approval of the selected
methodology.
HEC-HMS was created at the U.S. Army Corps of Engineers (USACE) Hydrologic
Engineering Center (HEC). Please note that a rainfall-runoff analysis using HEC-HMS
should only be used in cases where it is required for FEMA submittals or where a reviewing
agency has determined that the design engineer must investigate the downstream impacts
of the proposed project. In any case, for projects requiring FEMA approval, design
engineers should contact FEMA representatives before selecting the H&H model and
methodology to be used to satisfy FEMA requirements.
Figure 5.6-1 Hydrologic Method Determination – Mitigation Analysis
5.6.1 Peak Discharge Determination
For areas draining less than 200 acres, the natural, existing, and proposed discharge
rates can be determined by the Engineer using the Rational Method Formula, Q =
Page 28 of 63 Stormwater Design Criteria
CIA, where C is the runoff coefficient, I is the rainfall intensity in inches per hour,
and A is the drainage area in acres. See Section 5.5.1.C of this manual for the
application of this method.
5.6.2 Hydrograph Development for Small Watershed (Small Watershed Method)
The small watershed method referred to is the one developed by H.R. Malcolm and
is described below.
A. Introduction
A technique for hydrograph development which is useful in the design of
detention facilities serving relatively small watersheds has been presented
by H.R. Malcolm. The methodology utilizes a pattern hydrograph which
peaks at the design flow rate and which contains a runoff volume consistent
with the design rainfall. The pattern hydrograph is a two-part function
approximation to the dimensionless hydrograph proposed by the Bureau of
Reclamation and the Soil Conservation Service.
B. Equations
The Small Watershed Hydrograph Method consists of the following
equations:
p
p Q
VT
39.1
= (1)
-=
p
ip
i T
tQ
q pcos1
2
for ti ≤ 1.25Tp (2)
p
i
T
t
pi eQq
30.1
34.4
-
= for ti > 1.25Tp (3)
* Calculator must be in radian mode.
where Tp is the time (in seconds) to Qp, Qp is the peak design flow rate (in
cubic feet per second) for the subject drainage area, V is the total volume of
runoff (in cubic feet) for the design storm, and ti and qi are the respective
time (s) and flow rates (cfs) which determine the shape of the inflow
hydrograph. All variables must be in consistent units.
Page 29 of 63 Stormwater Design Criteria
C. Applications
The peak flow rate, Qp, is obtained from the Rational Method Formula. The
total volume of runoff (V) is the same as the rainfall excess. Table 5.6-1
below gives typical values for the rainfall excess based on percent
impervious cover. The actual values may be interpolated from the table.
See Table 5.6-3 for determination of percent impervious cover.
Table 5.6-1. Typical Rainfall Excess Values
To Be Used with Small Watershed Method
100-Year 250-Year 10-Year 5-Year 2-Year
Impervious
Cover
Rainfall
Excess
(in)
Rainfall
Excess
(in)
Rainfall
Excess
(in)
Rainfall
Excess
(in)
Rainfall
Excess
(in)
0 14.3 9.02 6.4 4.76 3.21
20 14.82 9.52 6.87 5.2 3.6
40 15.34 10.01 7.33 5.64 3.99
60 15.86 10.51 7.8 6.07 4.38
80 16.38 11 8.26 6.51 4.78
The Small Watershed Hydrograph Method should only be used where an
impact analysis is not required for the total drainage system including the
detention facility and outfall channel as indicated in Figure 5.6-1. The
Small Watershed Hydrograph Method cannot be used in conjunction with
the HEC-HMS models of watersheds studied in the Flood Insurance Study.
The time to peak of the Small Watershed Hydrograph Method is computed
strictly to match volumes and has no relationship to the storm durations and
rainfall distributions used in the Flood Insurance Study.
5.6.3 Watershed Modeling
In October of 2001, through a joint effort between FEMA and HCFCD, Harris
County began the Tropical Storm Allison Recovery Project (TSARP). TSARP
models for Clear Creek simulated the existing conditions in Clear Creek Watershed
when released in June 2007. The effective models for Clear Creek watershed can
be downloaded from the HCFCD Model and Map Management (M3) System in
HCFCD website at www.m3models.org.
Special care must be taken to make sure that the correct models and methodologies
are used for projects that require FEMA approval. In any case, for projects
requiring FEMA approval, design engineers should contact FEMA representatives
to get directions before selecting an effective H&H model and methodology.
Page 30 of 63 Stormwater Design Criteria
In the case that FEMA approval is not required for the project, design engineers
should use the methodology presented in this Chapter to design drainage facilities
in the City of Pearland.
A. Rainfall Frequency and Duration
The storm event used to establish regulatory flood plain and floodway limits
in the Flood Insurance Study is the 100-year, 24-hour event. For planning
purposes and establishing flood insurance rate zones the 10-, 50-, and 500-
year events analysis also required. For projects requiring FEMA submittals,
the rainfall depths in the most current effective model should be used. For
all other projects requiring a rainfall runoff analysis, the depths should be
based on Table 5.6-2, which includes the maximum values for each depth,
duration and frequency from the NOAA Atlas 14 information.
Partial rainfall amounts for various durations and frequencies for use in the City
(Brazoria County Region 1) are given in Table 5.6-2.
Table 5.6-2. Point Rainfall Depth (Inches) Duration- Frequency Values1
Duration
Depth (in)
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year
5-min 0.6 0.74 0.86 1.04 1.17 1.31 1.65
10-min 0.95 1.18 1.37 1.65 1.87 2.09 2.59
15-min 1.21 1.49 1.73 2.07 2.34 2.61 3.27
30-min 1.73 2.13 2.47 2.94 3.3 3.68 4.66
60-min 2.31 2.86 3.33 3.99 4.5 5.05 6.56
2-hr 2.9 3.63 4.32 5.36 6.24 7.23 9.91
3-hr 3.25 4.11 4.97 6.29 7.48 8.84 12.5
6-hr 3.88 4.99 6.15 7.97 9.66 11.6 17
12-hr 4.55 5.98 7.43 9.72 11.8 14.3 21.2
24-hr 5.27 7.05 8.83 11.6 14.1 17 25.3
1Brazoria County Hydrologic Region 1 Rainfall (inches)
B. Rainfall Depth-Area Relationship and Temporal Distribution
For projects not requiring FEMA submittals, partial rainfall depths should
be used.
In any case, for projects requiring FEMA approval, design engineers should
contact FEMA representatives to get directions before selecting an effective
H&H model and methodology.
1 Source: Brazoria County Drainage Criteria Manual, May 2022
Page 31 of 63 Stormwater Design Criteria
C. Loss Rates
Rainfall excess and runoff volume are dependent on factors such as rainfall
volume, rainfall intensity, antecedent soil moisture, impervious cover,
depression storage, interception, infiltration, and evaporation. The extent
of impervious cover and depression storage is actually a measure of
development and is discussed in the next section. The other factors are
dependent on soil type, land use, vegetative cover, topography, time of year,
temperature, etc.
For projects requiring FEMA approval, the loss input in the most current
effective model should be used. For all other projects requiring a rainfall
runoff analysis, the Green-Ampt loss function available in HEC-HMS shall
be used. A detailed description of the Green-Ampt loss function can be
found in USACE EM 1110-2-1417. The following parameters should be
used to compute the Green-Ampt losses in Brazoria County Region 1:
Initial Canopy Storage = 0 (%)
Maximum Canopy Storage = 0.1 inches
Crop Cpefficient = 1
Initial Content = 0.075 inches
Saturated Content = 0.46
Wetting Front Suction = 12.45 inches
Hydraulic Conductivity = 0.024 in/hr
Additional development in the watershed is analyzed by increasing the
value of the impervious cover parameter in the runoff model. Table 5.6-3
gives appropriate values of percent impervious based on land use types:
Table 5.6-3. Percent Impervious Cover For Land Use Types
Land Use % Impervious
High Density 85%
Dry Detention Ponds 85%
Undeveloped 0%
Developed Green Areas 15%
Residential Lot
(<1/4 acre) 60%
Residential Lot
(<1/2 and ≥1/4 acre or schools) 40%
Residential Lot
(≥1/2 acre or older neighborhoods with
limited roadside ditch capacity) 20%
Residential Rural Lot
(≥5 acre ranch or farm) 5%
Isolated Transportation 90%
Page 32 of 63 Stormwater Design Criteria
Water 100%
Light Industrial 60%
Airport 50%
5.6.4 Unit Hydrograph Methodology
The model for the Clear Creek Watershed flood insurance study is based on the
Clark unit hydrograph. In cases where FEMA submittals are required, the design
engineer should use the Clark unit hydrograph method. In other cases, where a
downstream impact analysis is required, consult the appropriate reviewing agencies
on the applicability of the Clark unit hydrograph. In some cases, other unit
hydrograph methods may be applicable.
The watershed parameters for the Clark unit hydrograph may be developed using
the Harris County methodology. Design engineers should refer to the current
effective model available on HCFCD Model and Map Management (M3) System
Website (http://www.m3models.org/) and the most recent version of the HCFCD
hydrology manual.
5.6.5 Flood Hydrograph Routing
Flood routing is used to simulate the runoff hydrograph movement through a
channel or reservoir system. Flood routing techniques vary greatly between
hydrologic computer models and caution should be used in selecting a routing
method, which adequately represents the channel storage conditions present in
areas with extremely flat slopes, such as within the City of Pearland.
The HEC-HMS program employs several flood routing methods for characterizing
the transfer of the runoff hydrograph through the drainage system of a watershed.
The models developed for the Flood Insurance Study for the Clear Creek watershed
use the Modified Puls Method of routing. This flood routing method is based on
the continuity equation and a relationship between flow and storage or stage. The
routing is modeled on an independent-reach basis from upstream to downstream.
A detailed discussion of the Modified Puls Method can be found in the user’s
manual for HEC-HMS.
A. Storage –Routing Computations Using HEC-RAS
All of the Flood Insurance Study data submitted for the Clear Creek
Watershed have utilized the HEC-RAS computer program to generate the
storage-discharge relationship required for HEC-HMS to utilize the
Modified Puls flood routing. Listed below is a suggested procedure by
which the HEC-RAS program can best be formatted to provide the most
effective input and output data necessary for hydrologic studies.
Page 33 of 63 Stormwater Design Criteria
a. Determine which routing reaches of the subject channel will need to be
evaluated. Routing reaches that are defined in the Flood Insurance
Study usually represent an area between outfalls of two significant
drainage areas.
b. Review all the available data for the routing reaches of the subject
stream.
c. Run HEC-HMS for the 100-year storm event using preliminary channel
routing data or alternate methods (i.e. Muskingum or Lag).
d. Use the effective model to determine the 100-year flows for the stream
in question. Multiply the preliminary 100-year peak discharges
determined above by 0.20, 0.40, 0.60, 0.80, 1.00, 1.20, and 1.50 to
obtain a series of seven discharges for each storage routing reach.
e. The discharges that have been developed are then input to the HEC-
RAS program. The discharges should be held constant throughout the
subject routing reach. Outflows through a routing reach should not vary.
f. Obtain storage outflow data calculated using HEC-RAS utilizing the
most upstream and downstream cross section of the reach.
g. Determine the average flood wave travel time along the reach. To
calculate the average wave travel time, divide reach travel time by a
flood wave velocity factor of 1.5.
h. Determine the number of subreaches to be used in the HEC-HMS
computations. Determine this number by dividing the average flood
wave travel time along the reach by the HEC-HMS computational time
step for each of the flows entered in the HEC-RAS model.
i. Run the HEC-HMS model.
j. Cycle (or balance) the HEC-HMS and HEC-RAS until the 1%
exceedance probability (100-year) flows at the downstream end of the
routing reach match within 5%.
The HEC-RAS model used in the storage-outflow analysis should be
reviewed to ensure that the analysis is correctly determining the total storage
volume. Make sure that the ineffective flow areas are modeled
appropriately.
5.7 HYDRAULIC CHANNEL DESIGN CRITERIA
5.7.1 Introduction
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The hydraulic design of a channel or structure is of primary importance to ensure
that flooding and erosion problems are not aggravated or created. This section
summarizes methodologies, procedures, and criteria which should be used in the
hydraulic analysis of the most common design problems in City of Pearland and
Brazoria County, Texas. In some instances, methodologies and parameters not
discussed in this section may be required. When an approach not presented herein
is required, it should be reviewed early on by the City Engineer.
A. Design Frequencies
All the City of Pearland open channels will be designed to contain the runoff
from the 100-year frequency storm within the right-of-way, except where
channel improvements are necessary to offset increased flows from a
proposed development. In those cases, the 100-year flood profile under
existing conditions of development should not be increased.
In areas served by closed systems, stormwater runoff should be conveyed
during the 100-year frequency storm without flooding of structures. This is
accomplished through the design of the street system, the storm sewer
system, and other drainage/detention systems.
B. Required Analysis
In designing a facility for flood control purposes, a hydraulic analysis must
be conducted which includes all the factors significantly affecting the water-
surface profile or the hydraulic grade line of the proposed facility. For open
channels, the primary factors are losses due to friction, constrictions,
bridges, culverts, confluences, transitions, and bends. The design of
channels or conduits should generally minimize the energy losses caused by
these factors which impede or disrupt the flow. Factors affecting the
hydraulic grade line in closed conduits are entrance losses, friction losses,
exit losses, and bend losses.
C. Acceptable Methodologies
Several methods exist which can be used to compute water-surface profiles
in open channels. The methodology selected depends on the complexity of
the hydraulic design and the level of accuracy desired. Peak discharges and
discharge hydrographs developed using one of the methodologies described
in Section 5.6 must be incorporated into the existing effective HEC-RAS
model in order to determine the impact of any proposed development flood
control facility on the entire channel system.
For the design of proposed channel with flow confined to uniform cross-
sections, either a hand calculated normal depth or direct step computation
Page 35 of 63 Stormwater Design Criteria
is sufficient. Manning's equation should be used for computing normal
depth. For designing a non-uniform proposed channel with flow in the
overbanks, the use of HEC-RAS is recommended. Any proposed channel
improvements to an existing collector ditch or creek within the jurisdiction
of the City must be modeled using HEC-RAS and incorporated into the
model used in the Flood Protection Plan.
Bridges, culverts, and expansion and contraction losses are taken into
account in the HEC-RAS computer program. If these losses are significant
and the normal depth or direct step method is employed, the losses must be
included in the backwater calculations. Design criteria for bridges, culverts,
transitions, bends and drop structures are presented in the remainder of this
section.
5.7.2 Open Channel Design
A. Location and Alignment
The first step in designing or improving an open channel drainage system
is to specify its location and alignment. Good engineering judgment must
be incorporated to insure the proposed channel location provides maximum
service to an area while minimizing construction and maintenance costs.
General factors and the City of Pearland criteria which should be taken into
account in locating improved channels are as follows:
a. Follow existing channels, ditches, swales, or other low areas in
undeveloped watersheds. This will minimize the cost of the channel
itself and the underground storm sewer system and will allow for
overland flow to follow its natural drainage pattern.
b. For safety reasons, channels and roads must not be located adjacent to
one another. Should such a conflict appear unavoidable, the design must
be approved by the City Engineer.
c. The angle at which two channels intersect must be 90 degrees or less
(angle measured between channel centerlines on upstream side on point
of intersection).
d. The minimum radius of curvature for unlined channel bends is three
times the ultimate channel top width, and the maximum bend angle for
both lined and unlined sections is 90 degrees. Bend losses and erosion
protection must be included in the hydraulic analysis of severe curves.
B. Existing Cross Sections
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For determining existing flood profiles, both the channel section and
overbank areas must be used in the hydraulic calculations. Channel sections
must be based on a recent field survey. In some cases, the City of Pearland
may have recent channel improvements information which can be utilized.
Plans of previous channel improvements should only be used for very
preliminary analysis. Overbank areas are best defined by field surveys, but
this is not always practical or economically justified. Elevations in the flood
plain beyond the limits of the channel can be obtained from the best
topographic information available for the study reach.
When designing a channel improvement, the channel sections used should
extend beyond the City of Pearland right-of-way a reasonable distance. The
purpose of including elevations beyond the right-of-way is to avoid a design
which creates ponding adjacent to the right-of-way a reasonable distance
and depends on the adjacent terrain, but in no case shall it be less than 20
feet.
C. Typical Design Sections
Typical channel sections have been established which should be used in
designing improved channels. Minimum dimensions are based on
experience of constructing and maintaining channels.
For some applications, other cross section configurations may be necessary.
A proposed cross section different from the typical sections presented
should be reviewed by the City Engineer for approval before proceeding
with design or analysis.
a. Earthen Channels
The following are minimum requirements to be used in the design of all
earthen channels:
1. Maximum earthen side slopes should be 4 (horizontal) to 1
(vertical). Slopes flatter than 4:1 may be necessary in some
areas due to local soil conditions. For channels and detention
reservoirs 6 feet deep or greater, side slopes selection shall be
supported by geotechnical investigations and calculations.
2. Minimum bottom width is ten (10) feet unless approved by the
City Engineer or BDD4.
3. A minimum maintenance berm is required on either side of the
channel of between 10 and 30 feet depending on channel size as
depicted in Table 5.7-1A and 5.7-1B.
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These criteria and regulations shall be applicable for all channels that
will be accepted for maintenance by BDD4 or HCFCD. Small channels
on private property, not draining public water, that do not conform to
these criteria shall remain the responsibility of the owner. These private
small channels within the City of Pearland and road-side ditches along
City streets shall be designed in accordance with Section 5.5.
Table 5.7-1A. Key To Easement Requirements
Channel
Depth [feet]
Channel Bottom Width [feet]
6 8 10 12 15 20 30 40
4 A A B B B B C C
6 A A B B B B C C
8 B B B C C C C C
10 C C C C C C C C
12 C C C D D D D D
14 D D D D D D D D
16 D D D D D D D D
18 D D D D D D D D
TABLE 5.7-1B. Ultimate Maintenance Requirement for Channels
Key Value Total Berm Width
[feet]
Berm Width Each
Side [feet]
Berm Width Uneven [feet]
Side 1 Side 2
A 30 15 20 10
B 40 20 25 15
C 50 25 30 20
D 60 30 30 30
Larger maintenance berms may be required due to the future needs of
an ultimate channel. Right-of-way requirements for all main outfall
channels are included in the Brazoria Drainage District No 4 (BDD4)
Flood Protection Plan.
1. Backslope drains or interceptor structures are necessary at a
minimum of 1,000 feet intervals to prevent sheet flow over the
ditch slopes.
2. Channel slopes must be re-vegetated immediately after
construction to minimize bank erosion.
3. Flow from roadside ditches must be conveyed to the channel
through a roadside ditch interceptor and pipe.
b. Concrete-Lined Trapezoidal Channels
In instances where flow velocities are excessive, channel confluences
create a significant erosion potential, or right-of-way is limited, fully or
Page 38 of 63 Stormwater Design Criteria
partially concrete lined channels may be necessary. The degree of
structural analysis required varies significantly depending on the
intended purpose and the steepness of the slope on which paving is
being placed. Slope paving steeper than 3:1 shall be designed based on
a geotechnical analysis that addresses slope stability and groundwater
pressure behind the paving.
The following are minimum requirements for partially or fully concrete
lined trapezoidal channels:
1. All slope paving should include a minimum 24-inch toe wall (9
inches thick) at the top and sides and a 48-inch toe wall (9 inches
thick) across or along the channel bottom for clay soils.
2. Fully lined cross-sections should have a minimum bottom width
of eight (8) feet.
3. Concrete slope protection placed on 3:1 slopes should have a
minimum thickness of 4 inches and be reinforced with #3 bars
on 18-inch centers both ways.
4. Concrete slope protection placed on 2:1 slopes should have a
minimum thickness of 5 inches and be reinforced with #3 bars
on 15-inch centers both ways.
5. Concrete slope protection placed on 1.5:1 slopes should have a
minimum thickness of 6 inches and be reinforced with #4 bars
on 18-inch centers both ways. Poured in place concrete side
slopes should not be steeper than 1.5:1.
6. In instances where the channel is fully lined, no backslope
drainage structures are required. Partially, lined channels will
require backslope drainage structures.
7. Weep holes may be required to relieve hydrostatic head behind
lined channel sections. Check with the geotechnical
investigation report.
8. Where construction is to take place under conditions of mud
and/or standing water, a seal slab of Class C concrete should be
placed in channel bottom prior to placement of concrete slope
paving.
9. For bottom widths of twenty (20) feet and greater, transverse
grade beams shall be installed at twenty (20) feet spacing on
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center. Grade beams shall be one foot wide, one foot-six inches
deep, and run the width of the channel bottom.
c. Rectangular Concrete Pilot Channels (Low Flow Sections)
In limited right-of-way, it is sometimes necessary to have a vertical
walled rectangular section. Presented below are minimum requirements
for rectangular concrete pilot channels:
1. The structural steel design is based on Grade 60 steel. This
should be confirmed by a design check based on local soil
conditions.
2. Minimum bottom width should be eight (8) feet to allow for
maintenance.
3. For bottom widths twelve (12) feet or greater, a center
depression is required.
4. For bottom widths twenty (20) feet or greater, transverse grade
beams shall be installed at twenty (20) feet spacing on center.
Grade beams shall be one foot wide, one foot-six inches deep,
and run the width of the channel bottom.
5. Minimum height of vertical walls should be four (4) feet.
Heights above this shall be in two (2) foot increments.
Exceptions shall be on a case by case basis.
6. Escape stairways shall be constructed. Escape stairways shall
be located at the upstream side of all street crossings, but not to
exceed 1,400 feet intervals.
7. For rectangular concrete pilot channels with earthen side slopes,
the top of the vertical wall should be constructed to allow for
future placement of concrete slope paving.
8. Weep holes should be used to relieve hydrostatic pressure.
9. Where construction is to take place under conditions of mud
and/or standing water, a seal slab of Class C concrete should be
placed in channel bottom prior to placement of concrete slope
paving.
10. Concrete pilot channels may be used in combination with slope
paving or a maintenance shelf. Horizontal paving sections
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should be analyzed as one way paving capable of supporting
maintenance equipment.
11. A geotechnical investigation and report shall be performed. Soil
borings shall be obtained at a minimum of every 1,000 feet to a
depth of 1.5 times the proposed channel depth.
D. Water-Surface Profiles
a. General
For steady, gradually varied flow conditions in natural or improved
open channels, the computational procedure known as the standard step
method is recommended for computing water-surface profiles. The
one-dimensional energy equation is solved by using an iterative
procedure to calculate a water-surface elevation at a cross section.
Manning's equation is used to compute energy losses due to friction
(Section 5.7.2.D.b), while losses due to obstructions and transitions are
calculated using the appropriate equations discussed in this chapter. For
cases where the flow is strictly uniform, as determined by the design
engineer, the standard step method can be reduced to a direct step
method or to a uniform flow computation.
The recommended computer program available for computing water-
surface profiles when using the standard step method is HEC-RAS. As
indicated previously, the City of Pearland prefers this program primarily
because it is widely accepted and the program readily facilitates the
design of channel improvements.
Good judgment must be exercised when determining cross-section
locations for water-surface profile calculations. Cross sections should
divide the channel into reaches, which approximate uniform flow
conditions. For example, closely spaced cross sections are required at
an abrupt transition such as a bridge, while relatively uniform channel
reaches with no significant changes in conveyance require fewer cross
sections. As a general guideline, the spacing should not exceed about
1,000 feet.
b. Manning's Equation
Manning's equation is an empirical formula used to evaluate the effects
of friction and resistance in open channels. For uniform flow conditions
where the channel bottom and energy line are essentially parallel,
Manning's equation should be used to compute the normal depth. For
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gradually varied flow conditions, the slope of the energy line at a given
channel section should be computed using Manning's equation.
The equation is:
=
1.49
× × ×
where, Q = total discharge in cubic feet per second.
n = coefficient of roughness
A = cross-sectional area of channel in square feet
R = hydraulic radius of channel in feet
S = slope of energy line in feet per foot (same as
Channel bottom slope for uniform flow).
Channel and overbank sections may have to be subdivided to represent
differences in roughness across the section. Subdividing may also be
helpful in computing Manning's equation for natural, compound or non-
prismatic sections (Section 5.3 References (7, 11)).
c. Manning's "n" Values
Manning's "n" values for design purposes should conform with Table
5.7-2. An "n" value of 0.04 for unlined channels represents a moderate
vegetal growth. For unlined channels, with a design flow larger than
10,000 cubic feet per second, "n" value of 0.035 may be used. For
existing, unimproved channels and overbank areas, "n" values should
be determined in accordance with Section 5.3 References (11, 12, 13,
14)
Table 5.7-2. Manning’s “n” Values and Allowable 25-year Velocities for Channel
Design
Channel Description
Roughness
Coefficient
or Manning’s
“n” Value
Average
Velocity
(ft/s)
Maximum
Velocity
(ft/s)
Unmaintained Earthen 0.05 3.0 5.0
Grass Lined:
Predominately Clay
Predominately Sand
0.045
0.045
3.0
2.0
5.0
4.0
Concrete Lined 0.015 6.0 10.0
Articulated Block 0.045 5.0 8.0
Overbanks and Existing
Unimproved Channels
See Section 5.3
References (5)
Not Applicable Not Applicable
d. Velocities
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Average and maximum allowable velocities based on 25-year flows are
given in Table 5.7-2. In the portion of Brazoria County where sandy
soils are known to exist, soils information may be needed to determine
the predominant type of soil and the corresponding allowable velocities
for unlined channels. Maximum velocities also apply to bridges,
culverts, transitions, etc. Where velocities exceed the maximum
allowed, erosion protection must be provided.
e. Channel Slope
Maximum channel slopes are generally controlled by the maximum
allowable velocity. Channel slopes shall not be less than 0.05%.
f. Starting Water-Surface Elevations
For design of open channels, starting water-surface elevations at the
channel mouth will generally be based on the normal depth in the design
channel.
In determining actual flood profiles or floodplain delineation, the water-
surface elevation from the outfall channel should be projected
horizontally upstream until it intersects the flood profile on the design
channel. An assumption that the peaks occur at the same time will
generally produce a conservative flood profile. Otherwise, an analysis
of coincident flow may be conducted to determine the flow in the outfall
channel at the time the peak flow occurs on the design channel.
g. Headlosses
Manning’s equation is used to estimate energy or headlosses due to
channel friction and resistance. Other sources of losses in open channels
include confluences, transitions, bends, bridges, culverts, and drop
structures. When computing water surface profiles either by hand or
with the help of a computer program, the engineer must include the
significant sources of headloss.
E. Confluences
The alignment of confluences is critical with regards to channel erosion and
energy losses caused by turbulence and eddies. The primary variables used
in designing channel junctions are angle of intersection, shape and
dimensions of the channel, flow rates, and flow velocities.
The angle of intersection between the main channel and tributary channels
or storm sewers shall be 30 degrees. Outfalls or junctions perpendicular to
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the receiving channel will create severe hydraulic problems, and therefore,
will not be allowed without approval by the City Engineer.
Any protective lining must extend far enough upstream and downstream on
both channels to prevent serious erosion. The slope protection must be
carried up to at least the 10-year flood level in both channels. A good grass
cover must also be established from the edge of the protection to the top of
bank.
If the main channel flowline is lower than the side channel flowline, an
erosion control structure must be used in the side channel.
F. Transitions
a. Design
Transitions in channels should be designed to create a minimum of flow
disturbance and thus minimal energy loss. Transitions generally occur
at bridges or culverts, and where cross-sections change due to hydraulic
reasons or right-of-way restrictions. The transition can consist of either
a change in cross-section size or geometry.
All angles of transition should be less than 12 degrees (20 feet in 100
feet). When connecting trapezoidal and rectangular channels, the
warped or wedge type transition is recommended. If super-critical flow
conditions are encountered, standing waves, superelevation, and
hydraulic jumps must be considered.
b. Analysis
Expansion and contraction losses must be accounted for in backwater
computations. Transition losses are usually computed using the energy
equation and are expressed in terms of the change in velocity head from
downstream to upstream of the transition. The headloss between cross
sections is expressed by:
-=
g
V
g
VChL 22
2
1
2
2
where: hL = headloss (feet),
c = expansion or contraction coefficient
V2 = average channel velocity of downstream section (ft/s),
V1 = average channel velocity of upstream section (ft/s), and
g = acceleration of gravity (32.2 ft/sec2).
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Typical transition loss coefficients to be used in HEC-RAS are given
below:
Coefficient
Transition Type Contraction Expansion
Gradual or Warped 0.10 0.30
Bridge Sections, Wedge, or Straight Lined 0.30 0.50
Abrupt or Squared End 0.60 0.80
When computing the backwater profile through a transition, engineering
judgment must be used in selecting the reach lengths. Smooth
transitions require fewer computation steps than the abrupt transitions.
G. Bends
a. Design
Channel bends or curves should be as gradual as possible to reduce
erosion and deposition tendencies. For channel bends with a radius of
curvature measured from the channel centerline of less than three times
the top width of the ultimate channel, slope protection is required. For
both lined and unlined channels, a 90 degree bend is the maximum curve
allowed. Erosion protection on bends must extend at least along and 20
feet downstream of the curved section on the outside bank. Additional
protection may be required on the channel bottom and inside bank, or
further downstream than 20 feet, if the channel geometry and velocities
indicates a potential erosion problem.
b. Analysis
Headlosses should be incorporated into the backwater computations for
bends with a radius of curvature less than three times the channel top
width. Energy loss due to curve resistance can be expressed as:
hL = cf V2/2g
Where, hL = headloss (feet),
cf = coefficient of resistance,
V = average channel velocity (feet per second), and
g = coefficient of gravity (32.2 ft/sec2).
Guidelines for selecting cf can be found in Section 5.3 Reference (12).
HEC-RAS has the ability to incorporate a bend loss computation in
terms of a minor loss coefficient ranging from 0.0 to 1.0. If HEC-RAS
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is used and bend losses are significant, the loss must be added at the
appropriate point in the minor loss table. Bends with a radius of
curvature greater than three times the top width of the channel generally
have insignificant losses and no computation is required.
5.8. DETENTION SYSTEM DESIGN
5.8.1 Introduction
In situations where on-site storage of stormwater runoff is the most effective way
to allow development of properties without increasing the flood potential
downstream, detention systems will be permitted. This section of the Manual
presents background information on stormwater storage techniques and detailed
guidelines and criteria for the design of stormwater storage facilities.
A. Types of Storage Facilities
The physical features of a particular site, as well as the type of development
proposed, will dictate, in many cases, the type of detention storage facility
that may be utilized. Since detention facilities are often designed to remain
dry, they can provide dual purpose functions such as playground and
recreational areas. In limited instances, on-site detention facilities have
been designed to be buried underground and thus are completely out of
sight.
All of these types of facilities are considered acceptable methods of
stormwater detention and can be designed hydraulically to accomplish the
intended purpose. All stormwater detention facilities are subject to periodic
inspection by the City Engineer to insure proper construction and
maintenance.
Dual use of stormwater detention facilities is encouraged. However, when
a dual use is proposed, such as recreation, a joint use agreement is required
between the owner and the entity sponsoring the secondary use. This
agreement must outline the maintenance responsibilities of the entity and of
the owner and must be submitted to the City of Pearland for approval. For
privately maintained or dual use systems, each stormwater detention facility
will be reviewed and approved by the City only if the following assurances
can be provided:
a. Adequate storage is available to provide necessary peak flow reduction;
b. The facility will perform as designed over the expected life of the
project;
Page 46 of 63 Stormwater Design Criteria
c. Provisions for maintenance, including long-term funding for
maintenance, to ensure that the facility does not detract from the value
of adjacent properties;
d. The stormwater detention pond should be designed to drain the
stormwater within 72 hours so as to provide storage room for the
following storm event.
e. The property owner/owners shall enter into a Detention Pond
Maintenance Agreement with the City as described in the Illicit
Discharge Ordinance no. 1570 and Maintenance of Stormwater Storage
Facilities Ordinance no 1059 prior to the final approval of the
construction plan by the City.
Detention ponds may be either on-site or off-site facilities. An off-site
detention basin is defined as one that is located on a City of Pearland,
BDD4, or HCFCD ditch and is receiving runoff from areas significantly
larger than the development project under design. An on-site basin
generally receives runoff from a small drainage area consisting primarily of
one development project. An offsite detention facility shall be located
within ¼ mile of the proposed site. In the discussions that follow, the design
methods presented are generally oriented to on-site detention facilities.
Specific reference will be made to methods for off-site facilities.
B. Geotechnical Design
Before initiating final design of detention ponds over 6 feet deep and 2 acres
in size, a detailed soils investigation by a geotechnical engineer shall be
undertaken. Geotechnical investigation, at a minimum, should address:
a. The ground water conditions at the proposed site;
b. The type of material to be excavated from the pond site and its suitability
for fill material;
c. If an embankment is to be constructed, adequate investigation of
potential seepage problems through the embankment and attendant
control requirements, the availability of suitable embankment material
and the stability requirements for the embankment itself;
d. Potential for structural movement on areas adjacent to the pond due to
the induced loads from existing or proposed structures and methods of
control that may be required; and,
e. Stability of the pond side slopes.
Page 47 of 63 Stormwater Design Criteria
5.8.2 Hydrologic Design
Detention basin design shall conform to City of Pearland criteria, BDD4 rules, or
HCFCD criteria on a case-by-case basis as approved by the office of the City
Engineer. The hydrologic methods for detention design should be in accordance
with Section 5.6 of this Manual. The hydrologic design criteria for stormwater
mitigation is divided into three design categories based on the size of the
contributing drainage area.
Small Projects: less than 2 acres
Medium Project: 2 acres to 20 acres
Large Project: more than 20 acres
5.8.3 On-Site Facilities
A. Small Projects (Projects 2 Acres or Smaller)
Small Projects are defined as those projects that are 2 acres or smaller. For
all small projects located adjacent to the BDD4 drainage system or projects
of any size directly outfalling into BDD4 drainage system requires approval
from BDD4.
a. Single Family Residential (SFR≤2 acres)
1. Single Family Residential (SFR) in-fill lots of 15,000 Square Feet
(SR15) or less and located outside the FEMA delineated 100-Year
Floodplain are exempt from detention if the proposed impervious
cover is less than or equal to 50% of the lot. A grading and drainage
plan is required to show no impont to the neighboring property due to
the proposed site development. This provision is not applicable for the
subdivision in which bigger lots are divided into smaller lots.
2. SFR in-fill lots (15,000 SF < Area < 2 Acres) located outside the
FEMA delineated 100-Year Floodplain. The first 7,500 square feet per
SR15 is exempt from detention. All coverage above the threshold is
required to provide stormwater detention at a rate of 0.65 ac-ft/ac of
increased impervious cover. Additionally, any dirt placed within the
500-year floodplain up to 500-year water surface elevation will be
required to mitigate onsite.
3. The SFR in-fill lot construction within FEMA mapped floodplain will
not be allowed to bring dirt from offsite and must be pier and beam
construction with the use of flood vent as mentioned in the FEMA
guideline. Slab on grade construction is allowed if the provision of fill
mitigation and detention at the rate of 0.65 ac-ft/ac of increased
impervious cover can be made onsite. Additionally, any dirt placed
Page 48 of 63 Stormwater Design Criteria
within the 500-year floodplain up to 500-year water surface elevation
will be required to mitigate onsite. The finished floor elevation of pier
and beam construction shall be 18” above 100-year base flood
elevation.
b. Commercial Projects≤2 acres
1. Minimum detention rate for small commercial projects shall be 0.7
ac-ft/ac of increased impervious cover. Any dirt placed within the
floodplain up to 500-year water surface elevation will be required to
mitigate onsite.
2. Private parking areas, private streets, and private storm sewers may
be used for detention provided that the maximum depth of ponding
does not exceed 9 inches directly over the inlet, and the parking
areas are provided with signage stating that the area is subject to
flooding during rainfall events
1. Use of orifice equation as described in Section 5.8.3 B is required to
determine the size of restrictor.
2. Regional detention pond has been constructed to serve the drainage
area bounded by FM 518 on the north, Old Alvin Road on the east,
Walnut Street on the South, and SH35 on the west. Please contact
the Engineering department for a pro-rata buy-in fee applicable for
SFR and commercial properties within the service area
B. Medium Projects (2 acres ≤ 20 Acres)
For all medium projects located adjacent to the BDD4 drainage system or
projects of any size directly outfalling into BDD4 drainage system requires
approval from BDD4.
a. Medium Commercial Projects (2 acres ≤ 20 acres)
1. Minimum detention rate for small commercial projects shall be 0.7
ac-ft/ac of increased impervious cover. The design engineer shall
also estimate the detention requirement using small watershed
method as presented in Section 5.6.2. The design engineer shall
compare the detention volume and pick whichever provides greater
volume.
2. Use the section 5.8.3.B.d for the design of outfall orifice.
b. Medium Residential Single-Family Project (2 acres ≤ 20 acres)
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If a proposed project consists of a single-family residential home and
does not involve the construction of infrastructure owned and
maintained by the City, the design criteria mentioned in the Section
5.8.3.A.a will be applicable.
c. Medium Residential Subdivision Project (2 acres≤20 acres)
Medium Residential subdivision projects with infrastructure owned and
maintained by the City of Pearland will have their mitigation detention
volumes calculated using the Small Watershed Method presented in Section
5.6.2. All calculations shall be presented to the office of the City Engineer,
including maps of suitable scale showing the flow paths used to calculate
the existing and developed time of concentration. Hydrograph routing
through the detention basin is recommended. The outflow structures (low
level pipe(s) or opening(s) and high level weir) will be sized as follows:
a. Determine the storage elevation in the basin for 2-year, 10-year, and
100-year storm events.
b. Determine water surface elevation in the receiving system (if reasonably
able to) for the 2-year, 10-year, and 100-year storm events.
c. Determine the minimum flowline elevation for the outflow structure.
d. Use the orifice equation to compute the opening size(s) as follows:
gHCAQ2=,
where, Q = Basin Outflow (cfs),
C = Pipe Coefficient,
A = Restrictor cross-sectional area (ft2),
g = Acceleration due to gravity (32.2 ft/s2), and
H = The elevation difference between the detention
basin water surface elevation for the design
storm and the receiving system for a given storm
or the center of the restrictor pipe (feet).
Round up to the next half-foot diameter for restrictor pipes above 18-inch
diameter. Some additional blockage of the pipe may be necessary to obtain
the correct restrictor area (A). No restrictor pipes shall be less than 6 inches
in diameter. The restrictor shall always be placed at the upstream end of a
pipe open towards the detention pond to enable cleaning. A restrictor shall
never be placed at the manhole or junction box.
e. Use weir equation to compute the size of the weir;
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Q = CLH3/2 for rectangular weir equation
Where, Q = Weir discharge (cfs)
C = Weir coefficient
L = Horizontal length of the weir (ft)
H = Head on the weir (ft)
The value of “C” depends on weir shape. Please see appropriate hydraulic
handbook or other applicable references such as HCFCD manual.
For ponds discharging into creeks or ditches, the outfall structure shall be
designed for the 2, 10, and 100-year storm frequencies. Determine the 2,
10, and 100-year detention volumes and compute the water surface
elevations to determine restrictor size to detain to undeveloped flow rates.
Use a vertical structure or multiple pipes separated vertically with the top
of the structure or flowline of the second pipe set at the 2-year or 10-year
water surface so as to be over topped in greater storms. A weir set below
the 100-year developed water surface elevation shall be used to discharge
during the 100-year design condition. This weir should be sized so that the
peak discharge does not exceed the 100-year pre-developed discharge with
the basin full and the tailwater elevation at or below the top of the discharge
pipe.
Storm events in excess of the 100-year event must be considered in the
design of detention facilities from the standpoint of overtopping. For a
detention facility that is an excavated pond and has no dam associated with
it, the outflow structure must be designed with an overflow structure or
swale. This will allow the passage of extreme events with no adverse
impacts to adjacent structures. For detention facilities with a dam, the
possibility of dam failure must be considered as part of the design. Specific
dam criteria for storm events in excess of the 100-year design storm shall
be established by sthe City Engineer on a case-by-case basis.
Regardless of the results of the methodology selected, the minimum
detention required for all medium projects shall be 0.65 acre feet per
acre of the project (disturbed area) in addition to floodplain fill
mitigation.
Use of detention in storm sewer system and parking lot is not allowed
for medium projects. Use of hydrograph timing as a substitution for
detention is strictly prohibited.
C. Large Projects (Projects larger than 20 acres)
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For projects sized between 20 acres and 200 acres, the design engineer may
elect to use rational method for peak discharge and small hydrograph
method for estimating minimum detention requirement. For a project in
excess of 200 acres, HEC-HMS, HEC-RAS modeling shall be performed.
The HEC-HMS modeling shall include analysis of existing and developed
runoff. This analysis must demonstrate no increase in runoff for the 2-year,
10-year, and 100-year storm events. Similarly, a HEC-RAS model shall
demonstrate no increase in the water surface elevation of the receiving
system for the 2-year, 10-year, and 100-year storm events. If the modelling
is associated with a FEMA submittal, the models to be used must be
acceptable to that agency. See 5.6.3 for specific design requirements.
The design of a detention basin basically consists of the following major
phases:
a. Determination of a 2-year, 10-year and 100-year 24-hour design storm
inflow hydrograph to the proposed detention basin.
b. Determination of the maximum 2-year, 10-year and 100-year 24-hour
design storm allowable outflow rate from the detention basin. Outflow
rates shall be equal to or less than historical rates or rates for pre-project
conditions.
c. Design tailwater elevation assumed to be equal to the top of the outflow
pipe or 10-year water surface elevation of the receiving creek whichever
is higher.
d. Preliminary sizing of basin storage capacity and the outflow structure.
e. Routing of the design inflow hydrograph through the basin, and
adjustment of the storage and outflow structure, if required, to ensure
that the maximum allowable outflow rate is not exceeded. This routing
should be performed in an appropriate computer program such as
Interconnected Pond Routing (ICPR) and XP Stormwater Management
Model (XP-SWMM) (or others as approved by the City Engineer). The
outflow structure shall include a pipe or pipes sized to restrict discharge
to the allowable 2-year and 10-year outflow rates and the allowable 100-
year design flow.
f. Analysis of the hydraulic gradients for storm sewers and inflow
channels entering the basin to ensure that these systems will operate
properly under design water surface conditions in the basin.
g. Analysis of rainfall events in excess of the design frequency for
structural and flood considerations, including provisions for a high-level
overflow structure. This design shall consider the possibility that should
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the normal outlet structure from the basin fail, the stormwater can pass
through, over or around the detention basin without damaging adjacent
structures.
h. Investigation of potential geotechnical and structural problems.
Regardless of the results of the methodology selected, the minimum
detention required for all large projects shall be 0.65 acre feet per acre
of the project (disturbed area) in addition to floodplain fill mitigation.
Use of detention in storm sewer system and parking lot is not allowed
for large projects. Use of hydrograph timing as a substitution for
detention is strictly prohibited.
5.8.4 Off-Site Facilities
Off-site detention facilities will generally be regional in nature. The facility may
be sized for one development but will be designed to serve the entire watershed by
reducing the flood potential of a stream. Most of these facilities are envisioned to
be adjacent to a channel to receive flood water from the main drainage artery
through a system of multistage inlet pipes and high-level weirs.
For the design of an off-site detention basin, the hydraulics of the stream and flood
damage relationship of the watershed must be evaluated. This will be performed
under the direction and advice of the City Engineer. This evaluation will result in
flood frequency/stage-damage estimates of the stream.
Upstream discharge of unmitigated runoff into a stream, on which capacity is
reserved in a regional detention basin, may be allowed if analysis of stormwater
flow demonstrates that flood water surface elevations will not cause flooding
between release point and the detention reservoir.
Sizing of the multistage inlets will be based on a plan that will be most beneficial
to the downstream community. Side flow diversions will also be developed and
evaluated by iterations to evaluate the impact of the diversion on the downstream
hydrographs. The arrangement of pipes/weirs shall be designed to minimize
property damages due to different storms within the entire area served by detention.
The City Engineer will advise the design engineer in regard to specific design
configurations.
Off-site facilities will be analyzed using HEC-HMS modeling techniques as
discussed in Section 5.6. The 2-year, 10-year and 100-year will be performed.
Input from the City Engineer is recommended to determine the most appropriate
level to set diversion structures for watershed-wide flood damage mitigation. These
facilities will generally be located along a FEMA studied stream with adequate
models available for the analysis. Routing of the inflow 2-year, 10-year and 100-
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year hydrographs through the detention basin may also be performed using a
computer model such as ICPR, XP-SWMM or other detention reservoir models
approved by the City Engineer. Set the tailwater level in the receiving stream equal
to the top of the outfall pipe or 10-year water surface elevation of the receiving
creek, whichever is higher. In addition, the use of a time-stage tailwater hydrograph
for the receiving creek is encouraged for setting the tailwater requirement.
For off-site facilities, the existing models will be used to develop a proposed (post
project) condition model(s). For such analysis, the proposed development will not
be isolated as a separate subarea. The existing hydrograph parameters (Tc+R) will
be modified or revised to reflect changes in percent land urbanization (DLU),
percent channel improvement (DCI), percent channel conveyance (DCC), and
percent impervious cover. Watersheds being developed may lose some or most of
the percent ponding (DPP) that may exist in the rural portion of certain watersheds.
Because the project area will not be modeled as a separate subarea, the high inflow
to the main drainage artery will not be evident in the model. Rather, because the
subarea parameters will be revised to reflect the impact of the project, the total
hydrograph along the main artery will increase without detention mitigation. The
diversion of the flood waters near the peak of the hydrograph will be effected
through the use of multilevel pipes and a weir to mitigate the increase flow to
downstream reaches.
Regardless of the results of the methodology selected, the minimum detention
required for all projects shall be 0.65 acre feet per acre of the project
(disturbed area) in addition to floodplain fill mitigation.
Use of hydrograph timing as a substitution for detention is strictly prohibited.
5.8.5 Pumped Detention System
All stormwater detention facilities requiring mechanical pumping systems are
generally prohibited, with the exception of pumping of dead storage (maintenance
or amenity water stored at or below the discharge pipe control level). However,
pumped detention shall be allowed under the following conditions:
A. A combination pump and gravity system shall be constructed.
B. The minimum detention rate shall be 0.70 ac-ft/ac.
C. No more than 75% of the total pond capacity shall be pumped.
D. The detention pond shall be designed to empty the storage volume within
72 hours.
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E. The discharge delivery system shall not have peak discharge and/or peak
stages that exceed the pre-developed peak values at any point in time for the
2-year, 10-year and 100-year design storm events.
F. Two pumps minimum shall be required, each capable of providing the
design discharge rate. If three pumps are used, any two pumps must be
capable of handling the design discharge rate. The total discharge pumping
rate shall not exceed the design discharge rate. A gravity overflow route and
outfall must be submitted to the office of the City Engineer for approval.
Pumping from detention into an existing storm sewer is prohibited unless
the pre-developed land already drains into an inlet and storm sewer system.
G. Pumped detention shall not be allowed for detention basins that collect
public water runoff, except for detention basins owned, operated and
maintained by the City or Brazoria Drainage District #4. Public water
runoff shall be defined as runoff water that originates from the property of
more than one property owner.
H. Fencing of the control panel is provided to prevent unauthorized operation
and vandalism, pursuant to the Texas Commission on Environmental
Quality Standards.
I. Adequate assurance is provided that the system will be operated and
maintained on a continuous basis.
J. Emergency source of power is provided for those cases that loss of power
during a 100-year flood event would cause property damage.
K. Sensors must be placed so that pumps would remain in functions as
designed during and after a rain event. Additionally, sensors must be placed
so that pumping will not occur when the level of water in the receiving
system is at or above 3/4 of its full depth. Verification with BDD4/HCFCD
is required when the receiving system is owned and maintained by
BDD4/HCFCD.
L. The Operator shall provide the office of the City Engineer with a quarterly
operational report that shall indicate the operational times, total hours of
operation, and the amount pumped. This report shall be delivered to the
office of City Engineer at the end of each quarter, no later than the 15th of
the month.
M. The City shall have the right to enter the property and inspect the operation
of the system at any time for any reason.
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N. Failure to maintain the pump station in working order is a violation of these
requirements and the City Ordinance for Maintenance of Stormwater
Storage Facilities.
It is recommended that if a pump system is desired, approval by the City of
Pearland of the preliminary conceptual design be obtained before any detailed
engineering is performed.
5.8.6 Structural and Geometric Parameters for Detention Ponds
A. General
Two types of detention facilities are acceptable in the City of Pearland. The
first is a naturalized basin in which standing shallow pools of water and
muddy areas are allowed to exist along the bottom of the basin and support
natural or wetlands vegetation. This type of basin is only maintained around
the sides and perimeter and involves special design considerations at the
outfall structure. Designing this type of facility must be approved by the
City and must consider the aesthetics of the surrounding area.
The second type of detention facility is a manicured or well-maintained
basin, which is mowed regularly and is designed to stay dry between rainfall
events. This type of facility may be more aesthetically pleasing in heavily
populated areas and is more amenable to multiple uses such as parks or ball
fields. The design considerations for each facility are outlined below.
The following parameters shall apply to all detention facilities:
a. Side slopes shall be 4:1 or greater.
b. Minimum maintenance berms shall be a minimum of 10 feet from the
property line, right of way or structure.
c. When a detention facility is constructed adjacent to a BDD4 channel,
BDD4 requirements prevail.
d. A minimum freeboard of 6” for small/ medium projects and 1’for large
projects shall be provided between 100- year WSE and the top of bank
of the detention pond.
Maintenance berms shall not be encumbered by any other permanent
improvements, easements, fee strips, or right-of-way.
B. Wet Detention Pond (Static Water Level)
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Wet detention pond must be approved by the City prior to the design and
preparation of construction plans. The design requirements such as
maintenance berms, backslope drains, and erosion protection measures are
required for all detention ponds. A maintenance plan to remove trash debris
and excessive siltation must be provided to and approved by the City. The
depth of permanent pool shall not be less than 4 feet. Additional storage
volume may be required by the City to offset predicted siltation based on
experiences with nearby storage facilities. It will be responsibility of the
developer, MUD, Homeowner Association, or Landowner to own and
maintain any wet detention ponds.
C. Bottom Design for Naturalized Detention Facilities
The bottom of a detention facility, which is intentionally meant to support
natural vegetation, should be designed as flat as practical but still maintain
positive drainage to the outfall structure. Side slopes should be designed to
allow for regular maintenance and be grass-lined with a 4:1 slope. The
bottom should be graded toward the outfall structure at a minimum slope of
0.002 feet per foot. The remainder of the pond bottom shall be graded
toward the flowline of the pond at a minimum of 0.01 feet per foot. Selected
vegetation may be introduced to the bottom of the facility to encourage a
particular habitat. Other design requirements for channels should be
followed, including back slope drains and erosion protection measures. A
maintenance plan to remove trash debris and excessive siltation must be
provided to and approved by the City. Additional storage volume may be
required by the City to offset predicted siltation based on experiences with
nearby storage facilities.
D. Bottom Design for Manicured Detention Ponds
The design of the detention basin bottom to remain dry and aesthetically
manicured is very important from the standpoint of long term maintenance.
A pilot channel is required to facilitate complete drainage of the basin
following a runoff event. A lined concrete pilot channel should have a
minimum depth of 6 inches and a minimum flowline slope of 0.002 feet per
foot. An unlined pilot channel should have a minimum depth of two feet, a
minimum flowline slope of 0.005 feet per foot, and maximum side slopes
of 4:1.
Bottom slopes of the detention basins should be graded towards the low-
flow pilot channel or outfall. The transverse slope of the bottom should be
a minimum slope of 1%, with 2% preferred.
Detention basins which make use of a channel section for detention storage
may not be required to have pilot channels but should be built in accordance
with the requirements for channels, including side slopes, maintenance
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berms, back slope drains and erosion protection measures previously
discussed.
E. Outlet Structure
For low tail water conditions, the outlet structure for a detention basin is
subject to higher than normal headwater conditions and possibly erosive
velocities for prolonged periods of time. For this reason, the erosion
protective measures are very important.
Reinforced concrete pipe used in the outlet structure should conform to
ASTM C-76 Class III with compression type rubber gasket joints
conforming to ASTM C- 443. HDPE or aluminized steel pipes may also be
used. Pipes, culverts and conduits used in the outlet structures should be
carefully constructed with sufficient compaction of the backfill material
around the pipe structure as recommended in the geotechnical analysis.
Generally, compaction density should be the same as the rest of the
structure. The use of pressure grouting around the outlet conduit should be
considered where soil types or conditions may prevent satisfactory backfill
compaction. Pressure grouting should also be used where headwater depths
could cause backfill to wash out around the pipe. The use of seepage cutoff
collars is not recommended since such collars are often inadequately
installed and prevent satisfactory backfill compaction. A concrete control
structure with a grate area equal to ten (10) times the outfall pipe area shall
be constructed. Concrete paving extending from the outfall area into the
basin a distance of 10 feet shall be placed on the bottom of the basin for
maintenance of the structure. Adequate steel grating around the outfall pipe
intake must be designed to prevent clogging of the pipe from dead or
displaced vegetation.
The construction of stormwater outfall in BDD4/HCFCD channel shall
meet respective design standard. Concrete slope paving or articulated block
on filter fabric will be allowed for the stormwater outfall into City owned
drainage facilities (roadside ditches) within the ROW.
F. Extreme Event Spillways
The drainage system must be designed to adequately deal with an extreme
rainfall event. The extreme event shall be defined as an event which
includes or exceeds the 100-year flow. A sheet flow analysis shall be
provided to show this extreme event flow path to the receiving drainage
system.
A concrete lined extreme event overflow swale shall be provided where this
event enters and exits the detention pond. The BDD4 or County’s standard
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details shall be used for drainage system outfall into the BDD4 or County’s
receiving channel.
G. Additional Design Considerations
The following items describe additional design criteria associated with
detention basins.
a. Erosion Control
Adequate erosion control and re-vegetation shall be accomplished
during and following construction of the basin. The City of Pearland
will allow articulated concrete block on filter fabric as an acceptable
means of slope protection within privately owned property or City
owned facilities.
b. Safety, Aesthetic Consideration and Multi-Purpose Use
The use of a detention basin facility generally requires the commitment
of a substantial land area for the basin. The City of Pearland recognizes
that such a facility may be used for other purposes which are compatible
with the primary intended purpose of providing flood control. Basins
may be utilized as parks and recreational facilities on a case-by-case
basis. The proposed use and the facilities to be constructed within the
basin area must be specifically approved by the City of Pearland. The
City of Pearland will not assume any maintenance responsibility on or
within private detention facilities.
5.9 MISCELLANEOUS DESIGN CONSIDERATIONS
5.9.1 Storm Sewer Outfalls
All storm sewer outfall structures should be constructed in accordance with the City
standard details, BDD4 details, or Harris County Flood Control District’s details
depending upon the outfall location. Design criteria for outfall structures is as
follows:
A. All storm sewer outfall pipes within the City of Pearland right-of-way must
be reinforced concrete pipe with rubber gasket joints, aluminized steel pipe,
or HDPE with a minimum diameter of 18 inches.
B. All backslope drains shall be 24-inch reinforced concrete pipe, aluminized
steel or HDPE.
C. A standard City of Pearland manhole or junction box must be outside of the
City of Pearland ultimate right-of-way. Where a road or railroad right-of-
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way is located adjacent to the channel, the manhole may be placed within
the City of Pearland right-of-way.
D. The grade of the pipe should be that required to produce a three feet per
second velocity when flowing full.
E. Erosion protection (Concrete slope paving, interlocking concrete block, or
other approved product by the City Engineer) is required for all outfall
pipes.
F. Stormwater outfall pipes entering into BDD4/HCFCD drainage facilities
shall meet BDD4/HCFCD design standard and standard detail.
G. Effluent outfalls from treatment plants shall have a paved invert and
concrete slope paving.
5.9.2 Special Erosion and Velocity Control Structures
A. General
Special erosion and velocity control structures will generally include stilling
basins, baffled aprons, straight drop spillways, sloped drops, and impact
basins. Due to the hydraulic and earth loads encountered through these
structures, the structural as well as the hydraulic design is very critical.
A geotechnical engineering investigation to determine the characteristics of
the supporting soil is required for major hydraulic structures
B. Straight Drop Spillway
Straight drop spillways are usually constructed of steel sheet piling with
concrete aprons. Steel sheet pile drop structures can sometimes be
considered a temporary structure.
The distance erosion protection aprons extend upstream and downstream of
the drop is determined using hydraulic analysis. The City of Pearland
recommends using concrete paving upstream and immediately downstream
of the drop. Because of the additional impact load on the downstream slope
paving, a 6-inch thick pad is recommended. Articulated blocks placed on
geotextile fabric should be used at the ends of the concrete paving to
decrease flow velocities and protect the concrete toe.
The drop structure should be designed for active and passive soil forces.
Design calculations are required for each drop structure along with a copy
of a geotechnical report defining soil characteristics of the site.
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C. Baffle Chutes
See Section 5.3 Reference (23) for hydraulic design of stilling basins and
energy dissipation.
D. Sloped Drop Structures
Sloped drop structures can be made of either monolithic poured-in-place
reinforced concrete or articulated cellular concrete block mats. The same
design principles hold true for sloped drop structures as for straight drop
structures; i.e., the draw down curve and hydraulic jump must be contained
within the structures or stilling basin.
The sloped drop structure should have 24-inch toe walls on the upstream
and downstream ends. The sides of the structure should have 18-inch toe
walls.
If an articulated cellular concrete block drop structure is used, the blocks
should be bedded on a filter fabric. The fabric should be heavy duty and
designed for the specific soil condition. The size and weight of the blocks
should be designed for shear forces.
E. Utility Crossings
Approval must be obtained from the office of the respective authorities (the
City, BDD4, HCFCD, TxDOT, or Counties) for all utility lines which cross
a drainage facility. The utility crossing should be designed to minimize
obstruction of the channel flow and conform with the ultimate channel
cross-section. Contact the offices of the respective authorities for
information regarding the ultimate channel section and ultimate channel
right-of-way at a proposed crossing prior to design.
All utility lines under channels should be constructed with the top portion
of the conduit a minimum of five (5) feet below the projected flow line of
the ultimate channel. When appropriate, facilities may be constructed on
special utility bridges or trestles in accordance with standard bridge design
criteria. Pipes or conduits spanning the channel should be located 2’ above
the base flood elevation for hydraulic and maintenance reasons. For utility
crossings on street bridges, contact the appropriate government body for
approval.
All manholes required for the utility conduit shall be located outside the
City of Pearland ultimate right-of-way. Backfill within the City of Pearland
right-of-way shall be in accordance with the backfill requirements specified
by the respective city, county, or utility company.
5.9.3 Stormwater Management
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Stormwater Management shall always be an integral part of the drainage
improvement. See Chapter 8.0 Stormwater Management for details.
5.10 REPORT REQUIREMENTS
Drainage Report/Drainage Impact Analysis must be approved prior to submittal and
approval of the Construction Plan. The drainage report must include minimum of the
following items listed in the drainage report.
EXECUTIVE SUMARY
Include detention summary table for the projects with detention.
SECTION 1: INTRODUCTION
· Project Name and Purpose
· Project Limits
· Project Objectives
· Assumptions and Constraints
· Prior Studies if any
Exhibits
o Vicinity Map/Project Location/Aerial Map
SECTION 2: EXISTING CONDITIONS
· Location and Topography
· Land Use
· FEMA Floodplains
· Right-of-Way
· Pipelines and Utilities
· Survey Datum
Exhibits
o Typical Roadway Section (Existing and Proposed) for roadway projects
SECTION 3: HYDROLOGY AND HYDRAULICS
· Design Criteria-guidelines used, storm frequency, mitigation requirements etc.
· List of programs and software used: include the purpose of uses
· Hydrologic Methodology
· Hydraulic Methodology
· Pre-Project Conditions
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Exhibits
o Overall Drainage Area Map
o Existing Drainage Area Map with 2, 10, 100 year flows and water surface
elevations at Major Outfall Nodes
o Floodplain Maps with project boundary shown on it
SECTION 4: PROPOSED DRAINAGE PLAN
· Description
· Hydrological Analysis – Rational Method, Small Hydrograph Method, HEC-
HMS, etc.
· Hydraulic Analysis – Storm sewer design, Spreadsheet, ICPR, XPSWMM, HEC
RAS, Applicable Sheet Flow Analysis, etc.
· Channel and/or detention layout
· ROW requirement
· Pipeline and Utility Conflicts
· Geotechnical Requirement
· Environmental Issues
Exhibits
o Overall Drainage Area Map
o Proposed Drainage Area Map with 2, 10, 100 year flows and water surface
elevations at Major Outfall Nodes
o Floodplain Maps with project boundary shown on it
o Detention Pond layout that includes a cross section of the detention pond with
100–year water surface elevation and detention summary
o Plan and Profile with 10-yr and 100-yr WSE in case of Roadway
SECTION 5: CONCLUSION
When a detention facility is part of the proposed project, include the following detention
summary table:
Project Name
Project Area Acres
Detention Pond Service Area Acres
Detention Basin Area Acres
Detention Storage Rate Acre-Feet/Acre
Detention Storage Required Acre-Feet
Floodplain Fill Mitigation Volume Acre-Feet
Total Storage Required Acre-Feet
Total Storage Provided Acre-Feet
Storm Event 50% (2-Year) 10% (10-Year) 1% (100-Year)
Design Water Surface Elevation (ft)
(-----Datum, -----Adjustment)
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Maximum Allowable Outflow (cfs)
Maximum Proposed Outflow (cfs)
APPENDIX
· Detailed Hydrological Calculations
· Detailed Hydraulic Calculations
· Digital Data (Models)
· Other supporting documents
5.11 QUALITY ASSURANCE
Prepare calculations and construction drawings under the supervision of a Professional
Engineer (Civil/Structural/Environmental) licensed in the State of Texas. The final
construction drawings and all design calculations must be sealed, signed, and dated by the
Professional Engineer responsible for the development of the drawing.