R2004-033 02-23-04 RESOLUTION NO. R2004-33
A RESOLUTION OF THE CITY COUNCIL OF THE
PEARLAND, TEXAS, ADOPTING A REVISED STORM
CRITERIA MANUAL.
CITY OF
DRAINAGE
WHEREAS, the City Council realizes the necessity of revising its storm drainage
criteria to address changes in the design methodology used for projects located in the
various watersheds of the City and its extraterritorial jurisdiction; and
WHEREAS, the Dannenbaum Engineering Corporation has revised the City's
storm drainage criteria for the design of storm drainage improvements; now, therefore,
BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF PEARLAND, TEXAS:
Section 1. That the City Council hereby adopts the revised Storm Drainage
Criteria Manual as prepared by Dannenbaum Engineering Corporation.
PASSED, APPROVED, AND ADOPTED this 23rd
February , A.D., 2004.
day of
TOM REID
MAYOR
ATTEST:
APPROVED AS TO FORM:
DARRIN M. COKER
CITY ATTORNEY
.,:)is
a
w",11 '.:1 . • .:-"•-•;.
w
111 F.r■q7 1111 1 I 1 I
A� A
»s,
a
T E . A
Via. uw -'.
'Op'
h'
Y am., bs: ,,,:;;
STORM DRAINAGE DESIGN CRITERIA
FOR THE
CITY OF PEARLAND, TEXAS
STORM DRAINAGE DESIGN REQUIRMENTS
Prepared By: Revised By:
EAR TN
@ T E C H JJ4
Attica m?ERNATIONALLm COMPANY LJA Engineering& Surveying,Inc.
March 1999 September 2000
Revised B :
DANNENBAUM
ENGINEERING CORPORATION
November 2003
Revised January 2004
City of Pearland, Texas
Storm Drainage Design Criteria Manual
TABLE OF CONTENTS
Section
Page
1.0 INTRODUCTION 1-1
1.1 DESCRIPTION 1-1
1.2 BACKGROUND 1-1
1.3 PREVIOUS DESIGN REQUIREMENTS 1-2
2.0 DRAINAGE POLICY 2-1
2.1 DESIGN REQUIREMENTS 2-1
2.2 STREET DRAINAGE 2-1
2.3 FLOOD CONTROL 2-1
2.4 RELATIONSHIP TO THE PERMITTING AND PLATTING PROCESS 2-1
2.5 FINAL DRAINAGE PLAN AND PLAT 2-2
3.0 REFERENCES 3-1
4.0 DEFINITIONS 4-1
5.0 STORM SEWER AND ROAD-SIDE DITCH DESIGN REQUIREMENTS 5-1
5.1 DETERMINATION OF RUNOFF 5-1
5.1.1 Application of Runoff Calculation Models 5-1
5.1.1.1 Acceptable Methodology for Areas Less than 200 Acres 5-1
5.1.1.2 Acceptable Methodology for Areas Greater than 200 Acres 5-1
5.1.2 Rainfall Durations for Hydrologic Modeling 5-1
5.1.3 Application of the Rational Method 5-2
5.1.3.1 Calculation of Runoff Coefficient 5-2
5.1.3.2 Determination of Time of Concentration 5-3
5.1.3.3 Intensity-Duration Curves 5-3
5.1.3.4 Sample Calculation Forms 5-3
5.2 DESIGN OF STORM SEWERS 5-7
5.2.1 Design Frequency 5-7
5.2.1.1 Newly Developed Areas 5-7
5.2.1.2 Redevelopment or In-fill Development 5-7
5.2.1.3 City of Pearland Projects 5-8
5.2.1.4 Private Drainage Systems 5-8
5.2.2 Velocity Considerations 5-8
5.2.3 Pipe Sizes and Placement 5-8
5.2.4 Starting Water Surface and Hydraulic Gradient 5-9
5.2.5 Manhole Locations 5-9
5.2.6 Inlets 5-10
5.2.7 Outfalls 5-10
i
Oct-03
City of Pearland, Texas
Storm Drainage Design Criteria Manual
TABLE OF CONTENTS
(continued)
Section P ge
5.3 CONSIDERATION OF OVERLAND FLOW 5-11
5.3.1 Design Frequency 5-11
5.3.2 Relationship of Structures to Street 5-11
5.3.3 Calculation of Flow 5-11
5.4 STORM WATER POLLUTION PREVENTION PLANS 5-11
5.5 DESIGN OF ROADSIDE DITCHES 5-12
5.5.1 Design Frequency 5-12
5.5.2 Velocity Consideration 5-12
5.5.3 Culverts 5-13
5.5.4 Depth and Size Limitations for Roadside Ditches 5-13
5.6 DESIGN OF OUTFALL PIPES 5-14
5.7 STORM WATER MITIGATION DETENTION ALTERNATIVES 5-14
6.0 HYDROLOGIC ANALYSIS OVERVIEW 6-1
6.1 PEAK DISCHARGE DETERMINATION 6-1
6.2 SMALL WATERSHED METHOD HYDROGRAPH METHODOLOGY 6-1
6.2.1 Introduction 6-1
6.2.2 Equations 6-2
6.2.3 Applications 6-2
6.3 WATERSHED MODELING 6-3
6.3.1 Rainfall Frequency and Duration 6-3
6.3.2 Rainfall Depth-Area Relationship and Temporal Distribution 6-4
6.3.3 Loss Rates 6-5
6.4 UNIT HYDROGRAPH METHODOLOGY 6-6
6.5 FLOOD HYDROGRAPH ROUTING 6-6
6.5.1 Procedure for Developing Storage-Outflow Using
HEC-2 or HEC-RAS 6-6
7.0 HYDRAULIC CHANNEL DESIGN CRITERIA 7-1
7.1 INTRODUCTION 7-1
7.1.1 Design Frequencies 7-1
7.1.2 Required Analysis 7-1
7.1.3 Acceptable Methodologies 7-1
ii
Oct-03
City of Pearland, Texas
Storm Drainage Design Criteria Manual
TABLE OF CONTENTS
(continued)
Section Page
7.2 OPEN CHANNEL DESIGN 7-2
7.2.1 Location and Alignment 7-2
7.2.2 Existing Cross Sections 7-2
7.2.3 Typical Design Section 7-3
7.2.3.1 Earthen Channels 7-3
7.2.3.2 Concrete-Lined Trapezoidal Channels 7-5
7.2.3.3 Rectangular Concrete Pilot Channels (Low Flow Sections) 7-6
7.2.4 Water-Surface Profiles 7-7
7.2.4.1 General 7-7
7.2.4.2 Manning's Equation 7-7
7.2.4.3 Manning's "n" Values 7-8
7.2.4.4 Velocities 7-8
7.2.4.5 Flowline Slope 7-9
7.2.4.6 Starting Water-Surface Elevations 7-9
7.2.4.7 Headlosses 7-9
7.2.5 Confluences 7-9
7.2.6 Transitions 7-10
7.2.6.1 Design 7-10
7.2.6.2 Analysis 7-10
7.2.7 Bends 7-11
7.2.7.1 Design 7-11
7.2.7.2 Analysis 7-11
8.0 DETENTION SYSTEM DESIGN 8-1
8.1 INTRODUCTION 8-1
8.1.1 Types of Storage Facilities 8-1
8.1.2 Geotechnical Design 8-2
8.2 HYDROLOGIC DESIGN 8-2
8.3 ON-SITE FACILITIES 8-2
8.3.1 Small Projects 8-2
8.3.2 Medium Projects 8-3
8.3.3 Large Projects 8-4
8.4 OFF-SITE FACILITIES 8-5
8.5 PUMP DETENTION SYSTEMS 8-6
Oct-03
City of Pearland, Texas
Storm Drainage Design Criteria Manual
TABLE OF CONTENTS
(continued)
Section Page
8.6 STRUCTURAL AND GEOMETRIC PARAMETERS 8-6
8.6.1 General 8-6
8.6.2 Bottom Design for Natural And Permanent Pool Basin 8-7
8.6.3 Bottom Design for Manicured Basins 8-7
8.6.4 Outlet Structure 8-8
8.6.5 Additional Design Considerations 8-8
8.6.5.1 Erosion Control 8-9
8.6.5.2 Safety, Aesthetic Consideration and Multi-Purpose Use 8-9
8.6.5.3 Storm Water Quality(NPDES) Requirements 8-9
9.0 MISCELLANEOUS DESIGN CONSIDERATIONS 9-1
9.1 STORM SEWER OUTFALLS 9-1
9.2 GENERAL CONTROL STRUCTURES 9-1
9.3 STRAIGHT DROP SPILLWAY 9-1
9.4 BAFFLE CHUTES 9-2
9.5 SLOPED DROP STRUCTURES 9-2
9.6 UTILITY CROSSINGS 9-2
10.0 EASEMENTS AND RIGHTS-OF-WAY 10-1
11.0 SUBMITTAL 11-1
11.1 PRELIMINARY SUBMITTAL 11-1
11.2 FINAL DESIGN 11-1
11.3 SIGNATURE PAGE 11-2
12.0 QUALITY ASSURANCE 12-1
13.0 DESIGN ANALYSIS 13-1
APPENDIX A DETENTION STORAGE VOLUME CALCULATIONS FOR
SMALL AND MEDIUM PROJECTS
APPENDIX B EXHIBITS
iv
Oct-03
City of Pearland, Texas
Storm Drainage Design Criteria Manual
TABLE OF CONTENTS
(continued)
LIST OF TABLES
Table Page
6-1 Typical Rainfall Excess Values To Be Used with Small Watershed Method 6-2
6-2 Point Rainfall Depth(Inches)Duration—Frequency Values 6-4
6-3 Percent Impervious Cover for Land Use Types 6-5
7-1 A Key to Easement Requirements 7-4
7-1B Ultimate Maintenance Requirements for Channels 7-4
7-2 Manning's "n"Values and Allowable 25-Year Velocities for Channel Design 7-8
LIST OF FIGURES
Figure Page
5-1 Average Velocities for Estimating Travel Time for Overland Flow 5-4
5-2 City of Pearland IDF Curve 5-5
5-3 Storm Sewer Calculation Form 5-6
6-1 Hydrologic Method Determination—Mitigation Analysis 6-8
A-1 Precipitation Rainfall Depth-Duration-Frequency Curves Northern Brazoria County,
Texas ..A-1
LIST OF EXHIBITS
Exhibit
5-1 Storm Sewer Manhole, Type"C" for 42"Diameter RCP and Smaller
5-2 Storm Sewer Manhole, Type"C" for 48"to 72"RCP
5-3 Storm Sewer Manhole, Type"C" for 78"Diameter RCP and Greater
5-4 Storm Sewer Manhole for Proposed Concrete Box Sewer
5-5 Storm Sewer Junction Box with Lid for a Maximum 24"Diameter Storm Sewer
5-6 Storm Sewer Manhole Frame and Cover
5-7 Storm Sewer Grate Inlet
5-8 Storm Sewer Type "BB"Inlet Plate, Frame and I Beam
5-9 Storm Sewer Ring Grate for Open end of 18"to 72"RCP Stubs to Ditch
5-10 Storm Sewer Type "D"Inlet Grate and Frame
5-11 Storm Sewer Type "D-1" Inlet Grate and Frame
5-12 Storm Sewer Type "A"Inlet Grate and Frame
5-13 Storm Sewer Type "C" and"H-2" Inlet Frame and Cover
5-14 Storm Sewer Bedding and Backfill
5-15 Storm Sewer Bedding and Backfill For RCP Where Unsatisfactory Soil Conditions Exist
V
Oct-03
City of Pearland, Texas
Storm Drainage Design Criteria Manual
TABLE OF CONTENTS
(continued)
LIST OF EXHIBITS
Exhibit
5-16 Storm Sewer Type"A" Grate Inlet
5-17 Storm Sewer Type`B"Inlet Relocation
5-18 Storm Sewer Type"BB"Inlet
5-19 Storm Sewer Type"BB"Inlet Relocation
5-20 Storm Sewer Type"C"Inlet
5-21 Storm Sewer Type"D"Inlet
5-22 Storm Sewer Type"D-1" Inlet
5-23 Storm Sewer Type"E"Inlet
5-24 Storm Sewer Side Ditch Reception
5-25 Storm Sewer Reinforced Monolithic Concrete
5-26 Storm Sewer Manhole Type`B" for 54"to 78"Monolithic Reinforced Concrete Sewers
5-27 Storm Sewer Manhole Type"D" for Monolithic Reinforced Concrete Sewer 84"
Diameter and Greater
5-28 Typical Storm Sewer Timber Support for Galvanized Corrugated Steel Pipe 48"
Diameter Outfall and Larger
6-1 100-Yr Frequency 24-Hour Hyetograph, 15-Minute Increments
7-1 Berm Drain
7-2 Typical Roadside Ditch Interceptor Structure
7-3 Typical Section—Concrete Trapezoidal
7-4 Typical Weep Hole Detail
7-5 Typical Structure Details—Concrete Low Flow Section
7-6 Concrete Low Flow Section—Typical Sections
7-7 Typical Channel Access Stairway for Concrete Low Flow Structures
7-8 Confluence—Minimum Requirement Chart
8-1 Typical Section—Detention Basin Pilot Channel
9-1 Typical Drawing, Outfall Pipe to Ditches with Paving
9-2 Typical Storm Sewer Outfall Structure
9-3 Steel Sheet Piling Drop Structure
9-4 Typical Section—Utility Crossing
vi
Oct-03
City of Pearland, Texas
Storm Drainage Design Criteria Manual
1.0 INTRODUCTION
1.1 DESCRIPTION
The following chapters include 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.
1.2 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 and Conservation Society synthetic
unit hydrograph analysis using existing stream gaging records and computer programs developed
by the 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 discharge versus drainage area relationships and unitgraph 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). At the time of the
publication of this manual, the TSARP is not complete. However, it is known that the effective
flood insurance models for Clear Creek would be completely revised and as such would entail
certain changes in the design methodology to be used for future projects in the Clear Creek
Watershed. The 2003 revisions to this manual reflect those changes.
It is anticipated that there will be a transitional period between the date of publication of this
manual and the date at which the TSARP models are made effective by FEMA. For this reason,
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 use the most current effective model of the study stream.
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.
1-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
1.3 PREVIOUS DESIGN REQUIREMENTS
The criteria of this Manual supercedes the previous document of the same name dated September
2000 as well as Resolution R2003 — 49, which was approved by minute order of the City of
Pearland Council on March 10, 2003. All items listed herein are intended to supercede those
documents, so that all designs of drainage facilities within the City of Pearland, including all
subdivisions within its extra-territorial jurisdiction, shall be based on the criteria of this Manual
from this time forward, until such time as it may be revised or replaced.
1-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
2.0 DRAINAGE POLICY
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 that the
objective for drainage standards is maintained and that such modifications are made with the
approval of the City Engineer.
2.2 STREET DRAINAGE
Street ponding of short duration is anticipated and designed to contribute to the overall drainage
capability 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 to habitable
structures.
2.3 FLOOD CONTROL
The City of Pearland is a participant in the National Flood Insurance Program. The intent of the
flood insurance program is to make insurance available at low cost by providing for measures
that reduce the likelihood of habitable structural flooding.
Fill placed in the 100-year flood plain as designated on the Flood Insurance Rate Map below the
100-year base flood elevation shall be mitigated by removal of like amount of compensating cut
in the vicinity of the fill. All runoff impacts created by development shall be mitigated equal to
or less than equivalent pre-project runoff rates and flooding levels.
2.4 RELATIONSHIP TO THE PERMITTING AND PLATTING PROCESS
Approval of storm drainage is a part of the review process for planning and platting of new
development. The review of storm drainage is conducted by the City of Pearland Engineering
Department.
2-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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, impact data and those notes to be included as applicable, on the Final
Drainage Plan as specified in Sections 5 through 8.
Plan Specifications:
Scale: Use standard engineering scales, properly identified on
each drawing. Sheet size 24" x 36" or 22' x 34" is required.
The following items shall be shown on a plan of the development as a minimum:
1. Name, address,phone number, and contact person of engineer that prepared the plans.
2. Scale of drawing with a minimum scale of 1"= 100'.
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 0.2 foot intervals or greater with 2 contours minimum covering the entire
development and extended beyond the development boundaries at least 100 feet on all
sides for developments over 5 acres and 50 feet on developments under 5 acres.
7. Lot grading plan,which provides for the passage of sheet flow from adjacent property.
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 run-off 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 and earthworks.
2-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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 specifying 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.
17. Copy of TxDOT permit application 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 T.C.E.Q. must be presented
with all plans. Copies of all submittals to the T.C.E.Q. shall be delivered to the City.
20. Seal of a Registered Professional Engineer on all plans and Registered Public Surveyor or
State Licensed Land Surveyor on the plat.
2-3
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
3.0 REFERENCES
3.1 Brazoria Drainage District No. 4, "Rules, Regulations, & Guidelines", November 5,
1997 (BDD#4 Regulations), amended July 2000.
3.2 Harris County Flood Control District Design Criteria Manual(HCFCD Criteria).
3.3 Frederick, R. H., V. A. Meyers, and E. P. Auciello, NOAA Technical Memorandum
NWS HYDRO-35; 5-to-60-Minute Precipitation Duration for the Eastern and Central
United States.
3.4 Applicable Portions of the City of Houston Design Manual, Chapter 9, "Storm Sewer
Design Requirements", September 1996.
3.5 Ordinances of the City of Pearland(as currently amended).
3.6 Turner Collie & Braden, Inc. Comprehensive Study of Drainage for Metropolitan
Houston.
3.7 Harris County Flood Control District. Criteria Manual for the Design of Flood Control
and Drainage Facilities in Harris County,Texas, February 1984.
3.8 Hydrology for Harris County, 1988, Seminar presented by ASCE and Harris County
Flood Control District.
3.9 Johnson, S. L., and D. M. Sayre. "Effects of Urbanization on Flood in the Houston
Metropolitan Area,"U. S. Geological Survey, April 1973.
3.10 U.S. Army Corps of Engineers. HEC-2 Water Surface Profiles Users Manual, The
Hydrologic Engineering Center,Davis, California, September 1990.
3.11 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.
3.12 U.S. Army Corps of Engineers. Civil Works Bulletin 52-8.
3.13 U.S. Army Corps of Engineers. HEC-1 Flood Hydrograph Package Users Manual, The
Hydrologic Engineering Center, Davis, California,Revised May 1991.
3.14 Joint Venture of Turner Collie & Braden, Inc. and Pate Engineers, Inc. Flood Hazard
Study for Harris County, Final Report, Harris County Flood Control District, September
1984.
3-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
3.15 Chow, V. T., Open-Channel Hydraulics, McGraw-Hill, 1959.
3.16 Ramser, C. E., Flow of Water in Drainage Channels, U.S. Department of Agriculture,
Technical Bulletin No. 129,November 1929.
3.17 Engineer Handbook, Hydraulics, Section 5, U. S. Department of Agriculture, Soil
Conservation Service, 1955.
3.18 Barnes, Harry H., 1967, Roughness Characteristics of Natural Channels, U. S.
Geological Survey Water Supply Paper, 1849.
3.19 King, H. W. and E. F. Brater,Handbook of Hydraulics, 6th Edition, McGraw-Hill, 1976.
3.20 Research Studies on Stilling Basins, Energy Dissipators, and Associated Appurtenances,
Bureau of Reclamation, Hydraulic Laboratory Report No. Hyd-399, June 1, 1955.
3.21 Texas State Department of Highways and Public Transportation standard specifications
for Construction of Highways, Streets, and Bridges latest edition.
3.22 ASTM A796 Structural Design of Corrugated Steel Pipe, Pipe Arches, and Arches for
Storm and Sanitary Sewers.
3.23 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.
3.24 Federal Emergency Management Agency. Flood Insurance Study - Brazoria County,
Texas and Incorporated Areas, June 5, 1989.
3.25 Federal Emergency Management Agency. National Flood Insurance Program and
Related Regulations, Index 44 CFR, Revised October 1, 1986, Amended June 30, 1987.
3.26 Hare, G. "Effects of Urban Development on Storm Runoff Rates," U. S. Army Corps of
Engineers, Galveston District, September 1970.
3.27 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.
3.28 Rogers, Jerry R. "Updated 15-100 Year Rainfall Frequency Analysis for Harris County,"
University of Houston, Harris County Flood Control District, May 1986.
3.29 Texas Water Development Board. "Study of Some Effects of Urbanization on Storm
Runoff from a Small Watershed,"Report 23, August 1966.
3-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
. 3.30 U.S. Army Corps of Engineers. "Introduction and Application of Kinematic Wave
Routing Techniques Using HEC-1," Training Document No. 10, The Hydrologic
Engineering Center, Davis, California, May 1979.
3.31 Hydraulic Design of Stilling Basins and Energy Dissipators, Engineering Monograph No.
25,U. S. Department of the Interior, Bureau of Reclamation, 1964.
3.32 Viessman, Jr., Warren; John W. Knapp; Gary L. Lewis and Terence Harbaugh,
Introduction to Hydrology, Harper&Row, 1977.
3.33 Coenco, Inc. Master Drainage Plan, April 11, 1980,Revised 1988.
3.34 Rust Lichliter/Jameson. Flood Protection Plan for Brazoria Drainage District No. 4,
Brazoria County, Texas,November 5, 1997.
3.35 U.S. Geological Survey, Water Resources Investigations Report 98-4044, Depth-
Duration Frequency of Precipitation for Texas, Austin, Texas 1998
3.36 USACE EM 1110-2-1417
3.37 McCuen,Richard H.,Prentice Hall,Hydrologic Analysis and Design, 1989
3.38 City of Pearland City Ordinance No. 421.
3.39 City of Pearland City Ordinance No. 532-2.
3.40 City of Pearland City Ordinance No. 817.
3.41 City of Pearland City Ordinance No. 817-1.
3.42 City of Pearland City Ordinance No. 669-3.
3.43 City of Pearland City Ordinance No. 741-3.
3.44 City of Pearland City Ordinance No. 881.
3.45 City of Pearland Resolution No. R2003-49.
3-3
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
4.0 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
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".
BDD#4: Brazoria Drainage District No. 4.
cfs: Abbreviation for Cubic Feet Per Second, used as a
measurement of rate of discharge of water.
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.
4-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
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 stormwater
collects and is held temporarily. The collected stormwater
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.
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 storm water 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.
4-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
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. The Assistant City
Engineer is the designated Flood Plain Administrator for
the City of Pearland.
HCFCD: Harris County Flood Control District.
HDPE: High Density Polyethylene
HEC-1: "Flood Hydrograph Package" computer program written
by the U.S. Army Corps of Engineers Hydrologic
Engineering Center.
HEC-2: "Water Surface Profiles" computer program written by the
U.S. Army Corps of Engineers Hydrologic Engineering
Center.
HEC-HMS: "Hydrologic Modeling System" computer program written
by the U.S. Army Corps of Engineers similar to HEC-1.
Intended to replace HEC-1.
HEC-RAS:
"River Analysis System" computer program written by
U.S. Army Corps of Engineers similar to HEC-2.
Intended to replace HEC-2.
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.
4-3
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
HydroPac: A series of engineering computer programs designed by a
private enterprise for computations and modeling of
hydrology and hydraulics concerning storm water runoff
and facilities.
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.
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,
4-4
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
0.011 for HDPE pipes,
0.028 for CMP
varies 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.
Mylar: A copy of a plat or plan made on mylar, which is a
polyester film resistant to tear,warp, curl, crack and peel.
NGVD National Geodetic 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, slope paving, riprap, 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.
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:
4-5
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
3-year frequency - a rainfall intensity having a 33%
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.
100-year frequency - a rainfall intensity having a 1%
probability of being equaled or exceeded in any given year.
Rainfall Runoff: The portion of the precipitation on the land that ultimately
reaches the drainage system.
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.
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
4-6
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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 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.
Technical Paper No. 40: Publication of the U.S. Weather Service.
TSARP: Tropical Storm Allison Recovery Project. Federally
funded flood study managed by Harris County Flood
Control District begun in October of 2001. Study under
way as of publication of this manual
U.S.G.S WRI Report 98-4044: 1998 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, by a City Council
resolution.
Watershed: A region or area bounded peripherally by a ridge of higher
elevation and draining ultimately to a particular
watercourse or body of water.
4-7
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
5.0 STORM SEWER AND ROAD-SIDE DITCH DESIGN REQUIREMENTS
Unless otherwise noted, the City of Pearland adopts the hardware requirements of the City of
Houston Standard Specifications and Standard Drawings. However, casting (manhole covers,
grates, etc.)will be generic or include the City of Pearland designation.
Furthermore, all outfall pipes, ditches, and structures that enter District Channels shall also be
designed in accordance with BDD#4 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 which receive
runoff flows from any other outfall drainage sources other than direct overland runoff flows.
Design of channels that do receive outfall system drainage can be found in Section 7.0 of this
Manual.
5.1 DETERMINATION OF RUNOFF
The quantity of storm water 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.
5.1.1 Application of Runoff Calculation Models
5.1.1.1 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.
5.1.1.2 Acceptable Methodology For Areas Greater Than 200 Acres
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.
5.1.2 Rainfall Durations for Hydrologic Modeling
For design using the HEC-1 model, the 24-hour design storm isohyetograph will be used for
rainfall data for drainage areas larger than 200 acres.
5-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
5.1.3 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 = watershed coefficient
A = area in acres
I = rainfall intensity(inches per hour)
5.1.3.1 Calculation of Runoff Coefficient
The runoff coefficient "C" values in the Rational Method 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 0.95
Residential Districts
Lots more than 1/2 acre 0.40
Lots 1/4 - 1/2 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 0.90
Industrial Districts 0.85
Railroad Yard Areas 0.30
Parks/Open Areas/Rice Fields/Pastures 0.20
*When calculating "C" values for proposed developed areas, multiply listed values
by 1.05 to reflect saturated conditions. Detention areas and lakes shall have a"C"
value of 1.0.
5-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
5.1.3.2 Determination of Time of Concentration
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. For
undeveloped land, the time of concentration of the first analysis point shall consist of
inlet time plus a 15 minute initial concentration time. For developed land, the time of
concentration of the first analysis point shall consist of inlet time plus a 10 minute initial
concentration time. The recommended flow velocities for overland flow are presented in
Figure 5-1.
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 minute or 15 minutes, as applicable, may be used as
the basis of design.
5.1.3.3 Intensity-Duration Curves
The time of concentration of the runoff will be used to determine the rainfall intensity
component of the Rational Method Formula. Figure 5-2 depicts the intensity-duration
curves to be used for this method of storm sewer and roadside ditch design in the City of
Pearland and extraterritorial jurisdiction. These curves were derived from the National
Weather Service publications HYDRO-35 and Technical Paper No. 40 and from the
U.S.G.S WRI Report 98-4044.
5.1.3.4 Sample Calculation Forms
Figure 5-3 represents a sample calculation form for storm sewer systems.
5-3
Jan-04
.I .2 3 .5 I 2 3 5 10 20
sa
50 _!•111111111111!•.t•11/\/111011 MI •
�M�a�iniii1111111� Moran flIP IIIIIIi m
momommRi111111111 moW RTIIP/1111111 AM
30 ■ miif/111111111 ■II■i%I/'/111111 _ 3a
F- ■_a ��mo/I111111111 . ��as dIImmitI�n�■
w111111 AINI11111,tkice■
w 20 1iiiiIIi flII
a
S r
`a =..�v z7,ms1111
i••••••■n•�••�
O ���omem���� J...■■■. A •s- �I•f•••IUIS
Cl) =------- �//11111 J • o ....•..IUII
��.•.r�•��••�� o a �`i• /Il..t.t■/Il�tt� 5
i■ttt�/t�l■t•..t1tl.. ...■■■ �� 3 �it;�lliTi�11�
W 5 tl•t♦tt�!•. ��..■t/ J— � Q I � ■ t ■tt
� m�a�� w71IIIII -.
CC ��■�� %vII �11 ��A s�n�/�11111111 ■
0 3 ■■ }� 1 =111 7 Q,Ae_. Ann1111111llll
■ 2 �I 4 111 Ali 141
IIAIIIii1llLI111 /
■
.r ,
I
.5. . . r Pirlit L i 5
30
III I
.20
.10
nrApie../
..........■tea.
.05 /AINIVIIIIIII,IIIItttl/.tA
.2 .3 .5 t 2 3 5 IO 20
VELOCITY , V ( FT / SEC) .
Figure 5-1 Average Velocities for Estimating Travel Time for Overland Flow
From TxDoT Hydraulics Manual extended below slope
of 0.5 % for use in City of Peariand
:»
Figure 5-2: City of Pearland IDF Curve
Rainfall Intensity Vs. Time of Concentration
100.0o T
1 L L L 1___ _ ___-L__L___J_L J 1 L L L 1_
1 I I I I 1 I
i _F_-fi___- __�-__}__i_-I--I_4--1 i I--I- -I-i
I 1 1 I I I
4 I 4 F-f -t t
- I I F t I
1 1
1 L L L 1 L 1 J J L J 1 L L L 1_
I 1 1 I I I I 1
I I I I 1 I 1 I
1 I--1---t 1. t--t--1--I-t-I 1 1--r--1-1-
I I I 1 I
1 I 1 I I 1
1 L L L 1_ L. L 1 J J L J 1 L L L 1_
1 1 I
I 1 1
I I 1
1 L L 1 L -
1
1 1
1 1
1 I
1 1
-_ _1 L L L1 L 1 J I I L L L1_
10.00 - - , I 1 1 I I 1 1 I I I 1 1
4I I I I I I I I I I I 1 1
Y r r r -- --' ----T--r--1-1-r 1 T r r r r-
I I I I I 1 I 1 I 1 I 1 I
l r r r T-- -- -I-- T--T_1-1-T 1 1 r r r T-
1 I I I I 1 I I I I 1
4 1 IF-F -F-11 I I 1- 4I
1 I I I I I I I I
-. -1 r r r1 - - -I-T -r r r r7
1 11 1 1 I I 1
LI I I I 1 1 1 I
C 1 t-r--t 1 - -1 -I-I- 1 I--r--I-t
I I I I I 1 I
11 I I 1 I
._ I I I I I 1 1 I
__} L L L 1 L. L L J -- 1 L L L1
1 1 1 1 1
C 1 1 1 1 1
41 1 I 1 I
C I 1 1 1
1 I I I
i I- L L} 1 1 J 4 1-I- I- L L1
1
I
100rYq
1
25-YR 1
1
1.00 - , , I I 1 1 10-YIR I
Y r r rY r r 1 1 I - Y - rYTFi-r--rY-
1 1 1
1 1 1 1 I 1 I atYR -
I 1 1 1
r r r T r T T 1 7 T 1 1 r r r T-
I I 1
I I I I I -
I I I I I
, , 1 1 I
I 1 I 1 I
-r r r r I T T 1 1 T 1 T r r r-I
,
1 1 I I 1
1 I I 1
1 t-1---h1 1----1---♦--i--I-1--1 T 1 1--r--I"-I
1 I 1
I t I 1
1 1 I I
1 I I I
1 I I I
Y r-r--r Y Y----t--r1IrI Y r-r-I-r Y-
0.10
1 10 100 1000
Time of Concentration,TC(min)
FIGURE 5-3: STORM SEWER CALCULATION FORM
CITY OF PEARLAND
t
Project:
Job No.:
System:
By: Date:
Checked By: Date:
1 -_-----
FLOWLINE
I THEOR. ACTUAL UPST. UPST. UPST.
TOTAL Intensity Flow LINE DESIGN SLOPE OTHER UP- DOWN- ACTUAL H.G.L. H.G.L. UPST. DNST.
T.C. GUTTER N.G.
(FPS) (%)O (FT) (0) �FT) (FT) TYPE (FT) (FT) (FT)
FROM TO AREA AREA C Tc I Q REACH SIZE MANN. SLOPE Q ' FPS FALL LOSSES STREAM STREAM
M V SLOPE H ISLOPE H.G.L. H.G.L. FLOW ELEV. ELEV. 1 ELEV.
M.H. M.H. (AC) (AC) FACTOR (MIN) (INlHR) (CFS) (FT) (IN) CONST. (/) (CFS) (FPS) (FT) l
, H
. , 'MI _____
. .
. ,
, ,
. . .
. ,
-1, .
___ .. ______ , .
. ,
. •
I , ... _____
. , , .
. , „ . 1
______ , _
, .. , i1 A
i
. I _
! I
54,
City of Pearland, Texas
Storm Drainage Design Criteria Manual
5.2 DESIGN OF STORM SEWERS
5.2.1 Design Frequency
5.2.1.1 Newly Developed Areas
The design storm event for sizing storm sewers will be a 3-year rainfall. 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, the design storm event will be a 5-year rainfall.
5.2.1.2 Redevelopment or In-fill Development
The existing storm drainage system will be evaluated using a 3-year storm, 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:
(a) If the proposed redevelopment has a lower or equal runoff potential, no
modifications to the existing storm drainage system are required.
(b) 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.
(c) If the hydraulic gradient is above the gutter grade (or the edge of road in 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 below one foot below the
floor levels of adjacent existing habitable structures, and exceeds the top of curb
(or the 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 improved development (development which increases the runoff potential
of the site), mitigation in the form of onsite or offsite detention must be either purchased
or provided. Alternatives for mitigation detention are referred to elsewhere in these
Criteria.
5-7
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
5.2.1.3 City of Pearland Projects (Capital Improvement Programs)
Proposed City of Pearland Capital Improvements Program may indicate a larger diameter
storm sewer is planned in the area proposed for drainage improvements. The City
Engineer will provide information on planned capital improvements and should be
consulted as to its impact on new development.
5.2.1.4 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 water and is
located in a drainage easement. The connection of any storm sewer, inlet, or culvert to a
public drainage facility will be approved by the City of Pearland. Storm water shall not
be discharged or flow over any public sidewalk or adjoining property except to existing
creeks, ditches, streets, or storm sewers in public rights 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 by TxDOT
to the plan.
5.2.2 Velocity Considerations
(a) Minimum velocities should not be less than 2 feet per second with the pipe flowing
full, under the design conditions.
(b) Maximum velocities should not exceed 5 feet per second without use of energy
dissipation before release to natural or cultivated grass channels.
5.2.3 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 at least 2' x 2'. Closed conduits (circular,
elliptical, or box) shall be selected based on hydraulic principals 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, or 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 severe depth constraints prohibit.
(d) Locate storm sewers in public street rights-of-way or in approved easements of
adequate width. Back lot and side lot easements are discouraged and must be
justified.
5-8
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
(e) Follow the alignment of the right-of-way or easement when designing cast in place
concrete storm sewers.
(f) A straight line shall be used for inlet leads and storm sewers.
(g) Center all culverts <48"within storm sewer easements wherever possible.
(h) Culverts 48" or larger shall be installed in the center of the street unless they are
installed in a drainage reserve. Culverts 48" or larger that are installed in a drainage
reserve shall be installed 4 feet offset from the centerline of the drainage reserve.
5.2.4 Starting Water Surface and Hydraulic Gradient
(a) The hydraulic gradient shall be calculated assuming the top of the outfall pipe as the
starting water surface elevation when the total time of concentration for the project
drainage system is less than 30% of the time of concentration of its outfall waterway.
When the total time of concentration for the project drainage system is greater than
30% of the time of concentration for its outfall waterway, a comparison between the
3-year water surface elevation of the receiving stream and the soffit of the outfall pipe
must be made. In that instance,whichever value is higher shall be used as the starting
tailwater condition.
(b) At drops in pipe invert, should the upstream pipe be higher than the hydraulic grade
line, then the hydraulic grade line shall be recalculated at a value of 80% of the
upstream pipe diameter above the downstream flowline of the upstream pipe.
(c) For the 3-year design storm, the water surface elevation shall at all times be below the
gutter line for all newly developed areas. In major thoroughfares, the 5-year storm
water surface elevation shall be below the gutter line.
5.2.5 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.
(b) Use manholes for monolithic-concrete storm sewers at the same locations as above
with the permitted exception at intersections of inlet leads unless needed to provide
maintenance access.
5-9
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
5.2.6 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 and contributing drainage area. Inlet
spacing should be designed to conform with 5.3.3 b and c. For minimum gutter
slopes, the maximum spacing of inlets shall result in a gutter run of 350 feet from
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 Appendix B are as follows:
City of
Pearland
Inlet Application Capacity Exhibit.Nos.
Type A Parking Lots/Small Areas 2.5 cfs 5-16
Type B-B * Residential/Commercial 5.0 cfs 5-18
Type C Residential/Commercial 5.0 cfs 5-20
Type D Parking Lots 2.0 cfs 5-21
Type D-1 Small Areas 2.5 cfs 5-22
Type E Roadside Ditches 20.0 cfs 5-23
* Not preferred in Pearland
(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.
5.2.7 Outfalls
Storm sewer and open street ditch outfalls to Brazoria Drainage District No 4 or Harris County
Flood Control District ditches shall be per BDD#4 or HCFCD criteria, as approved by the
District and City Engineer.
5-10
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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, or failure. A representation of the overland
flow scheme must be submitted with the system design.
5.3.1 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 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.
5.3.2 Relationship of Structures to Street
All structures will be higher than the highest level of ponding anticipated resulting from the
extreme event analysis. All finish floor levels of habitable structures shall be in accordance with
the City's Flood Damage Prevention Ordinance.
5.3.3 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) The maximum depth of ponding of the 100-year event allowed will be 9-inches above
the top of curb.
(c) Sheet flow between lots will be provided only through a defined drainage easement,
through a separate instrument, or on the plat.
(d) 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.
(e) 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 and remain in
compliance with the other terms of this paragraph must be provided.
5.4 STORM WATER POLLUTION PREVENTION PLANS
The U.S. Environmental Protection Agency (EPA) and the Texas Commission on Environmental
Quality (TCEQ) require that a Storm Water Pollution Prevention Plan (SW3P) be prepared for
construction activities. Construction plans shall show proposed SW3P measures to control soil
erosion and sediment pollution in storm water discharges during construction. A notice of Intent
5-11
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
(NOI) Form (EPA 3510-6) shall be completed and signed by the Contractor and Owner and
submitted two (2) days prior to start of construction to the TCEQ.
The SW3P shall not be submitted for EPA review; however, the SW3P shall be kept at the job
site for assessment by EPA inspectors. The EPA requires that regular weekly inspections and
inspections after each storm be made of the storm water pollution measures. A record of all
inspections shall be kept. The SW3P shall be maintained throughout the entire length (time) of
the project. Should the pollution protections not be working, the Contractor shall make
adjustments in the measures to correct the problems.
5.5 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
5.5.1 Design Frequency
(a) The design storm event for the roadside ditches shall be a 3-year rainfall.
(b) The 3-year storm design capacity water surface elevations for a roadside ditch shall
be no higher than six inches below the edge of shoulder or the natural ground at the
right of way line,whichever is lower.
(c) The design must include an extreme event analysis to indicate that habitable
structures will not be flooded.
5.5.2 Velocity Considerations
(a) For grass lined sections, the maximum design velocity shall be 3.0 feet per second
during the design event.
(b) A grass lined or unimproved roadside ditch shall have side slopes no steeper than
three horizontal to one vertical.
(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.
5-12
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
5.5.3 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.
(e) Cross open channels with roadside culverts no smaller than 18 inches inside diameter
or equivalent. The size of culvert used shall not create a head loss of more than 0.20
feet greater than the normal water surface profile without the culvert.
(f) Storm water 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).
5.5.4 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 centerline of pavement.
(c) Commercial and Thoroughfare Areas - The maximum depth of proposed roadside
ditches will not exceed 4 feet.
(d) Roadside ditch bottoms should be at least 2 feet wide, unless design analysis will
support a narrower width.
(e) Roadside ditch in 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 feet from the outside easement line.
(g) The minimum street right-of-way for open ditch drainage in residential
developments shall be 80 feet in width. Rights-of-way shall be wider for deep
5-13
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
ditches. The minimum open-ditch section roadway shall be 24 ft. pavement with
6 ft. shoulders on each side.
5.6 DESIGN OF OUTFALL PIPES
Outfall design shall conform to BDD#4 rules or HCFCD Criteria, as appropriate, and as
approved by the City Engineer and the Drainage District. Section 6.0 of this Manual generally
incorporates these two counties' 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. The
bottom and/or opposite bank of the channel shall be protected with slope paving or articulated
blocks on filter fabric as detailed on Exhibit 9-2.
5.7 STORM WATER MITIGATION DETENTION ALTERNATIVES
Detention basin design shall conform to City of Pearland criteria, BDD#4 rules, or HCFCD
criteria on a case-by-case basis as approved by the City Engineer. The City of Pearland
detention design criteria appear in Section 8.0 of this Manual.
5-14
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
6.0 HYDROLOGIC ANALYSIS OVERVIEW
The selection of an appropriate hydrologic methodology for all projects shall be carried out in
accordance with Figure 6-1. The design engineer should contact the appropriate reviewing
agencies prior to preparing his analysis to obtain approval of the selected methodology.
HEC-1 and its successor, HEC-HMS was created at the U.S. Army Corps of Engineers (USACE)
Hydrologic Engineering Center(HEC). HEC-HMS is expected to replace HEC-1 as the standard
rainfall-runoff model and eventually HEC-1 will no longer be supported by the HEC. Please
note that a rainfall runoff analysis using HEC-1 or 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 use the most current effective
model of the study stream.
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 =CIA, where C is the runoff
coefficient, I is the rainfall intensity, and A is the drainage area. See Section 5.1.3 of this manual
for the application of this method.
6.2 SMALL WATERSHED METHOD HYDROGRAPH METHODOLOGY
The small watershed method referred to in Figure 6-1 is the one developed by H.R. Malcolm and
is described below.
6.2.1 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.
6-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
6.2.2 Equations
The Small Watershed Hydrograph Method consists of the following equations:
V
Tp _ 1.39Qp (1)
q, = Qp 1 — cos t` for t1< 1.25Tp (2)
2 Tp
-1.30y
q, = 4.34Qp e TP for t, > 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 q, are the respective time (s) and flow rates (cfs) which determine the shape of the
inflow hydrograph. All variables must be in consistent units.
6.2.3 Applications
The peak flow rate, Qp, is obtained from the Rational Method Formula. For detention mitigation
analyses the Rational Method should be applied in accordance with Section 5.1.3 of this manual,
with the exception that all proposed developed runoff coefficients (C) given in that section
should be inflated by 5%. The total volume of runoff (V) is the same as the rainfall excess.
Table 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 6-3, Section 6.3.3, for
determination of percent impervious cover.
Table 6-1. Typical Rainfall Excess Values
To Be Used with Small Watershed Method
100-Year 10-Year 3-Year
Impervious Cover Rainfall Excess (in.) Rainfall Excess (in) Rainfall Excess (in)
50% 12.2 7.0 4.8
45% 12.0 6.9 4.7
40% 11.9 6.8 4.6
35% 11.8 6.7 4.5
30% 11.6 6.6 4.4
6-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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 6-1). The Small Watershed Hydrograph Method cannot be used in
conjunction with the HEC-1 computer 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.
6.3 WATERSHED MODELING
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). At the time of the
publication of this manual, the TSARP is not complete. However, it is known that the effective
flood insurance models for Clear Creek would be completely revised and as such would entail
certain changes in the design methodology to be used for future projects in the Clear Creek
Watershed. The 2003 revisions to this manual reflect those changes.
It is anticipated that there will be a transitional period between the date of publication of this
manual and the date at which the TSARP models are made effective by FEMA. For this reason,
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 use the most current effective model of the study stream.
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.
6.3.1 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 also require analysis. 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 6-2, which includes the maximum values for each depth, duration and frequency from the
TSARP, TP40 and Hydro 35 information.
Point rainfall amounts for various durations and frequencies for use in the City are given in Table
6-2.
6-3
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
Table 6-2. Point Rainfall Depth (Inches) Duration-
Frequency Values'
Depth (in)
100- 25- 10- 5- 3-
Duration Year Year Year Year Year
5 min. 1.20 1.00 0.90 0.80 0.70
30 min. 3.00 2.40 2.10 1.90 1.60
1 hr. 4.30 3.40 2.90 2.50 2.20
2 hr. 5.70 4.40 3.70 3.10 2.60
3 hr. 6.80 5.10 4.20 3.50 2.80
6 hr. 9.10 6.60 5.30 4.40 3.30
12 hr. 11.10 8.00 6.40 5.30 4.00
24 hr. 13.50 9.80 7.80 6.40 4.80
6.3.2 Rainfall Depth-Area Relationship and Temporal Distribution
In the initial stages of the TSARP it was necessary to address issues having to do with the use of
the new USACE runoff model called HEC-HMS. HEC-HMS is expected to replace HEC-1 as
the standard software for hydrologic analyses. Two important differences in between HEC-HMS
and HEC-1 have to do with the use of depth-area indices to account for point rainfall depths on
large areas and the temporal distribution of rainfall (the rainfall hyetograph).
The version of HEC-HMS that was available at the time of the TSARP does not have an option
for depth-area indices (JD records in HEC-1) for watersheds larger than 10 square miles.
Therefore, it was decided to use point rainfall depths to specify the hypothetical rain events used
in the hydrologic analyses. For projects requiring FEMA approval, the rainfall input of the most
current effective model should be used. For projects not requiring FEMA submittals, point
rainfall depths should be used.
The version of HEC-HMS that was available at the time of the TSARP allows the user to shift
the peak of the storm from 50% of the storm duration (center-peaking event similar to HEC-1)to
25%, 33%, 67%, or 75% of the storm duration. This is in lieu of using the PI records to specify
the temporal distribution of rainfall as is currently done for HEC-1 analyses in Harris County and
the City of Pearland. For projects requiring FEMA approval, the rainfall input of the most
current effective model should be used. For projects not requiring FEMA submittals, the 67%
duration peaking temporal rainfall distribution should be used see Exhibit 6-1.
Source: TP-40,Hydro-35 and U.S.G.S.
6-4
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
6.3.3 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 or HEC-1 shall be used. A detailed description of the 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:
Initial Loss = 0.1 inches
Volume Moisture Deficit = 0.385
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 6-3 gives appropriate values of percent impervious
based on land use types:
Table 6-3. Percent Impervious Cover For Land Use Types
Land Use % Impervious
High Density 85%
Undeveloped 0%
Developed Green Areas 15%
Residential Small Lot
(<1/4 acre or schools) 40%
Residential Large Lot
(>1/4 acre or older neighborhoods with
limited roadside ditch capacity) 20%
Residential Rural Lot
(>5 acre ranch or farm) 5%
Isolated Transportation 90%
Water 100%
Light Industrial 60%
Unknown 0%
Airport 50%
6-5
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
6.4 UNIT HYDROGRAPH METHODOLOGY
The model that 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 most current effective model and the
most recent version of the HCFCD hydrology manual.
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-1 and HEC-HMS programs employ 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 either HEC-1 or HEC-HMS.
6.5.1 Storage—Routing Computations Using HEC-2 or HEC-RAS
All of the Flood Insurance Study data submitted for the Clear Creek Watershed have utilized the
HEC-2 or HEC-RAS computer program to generate the storage-discharge relationship required
for HEC-1 or HEC-HMS to utilize the Modified Puls flood routing. Listed below is a suggested
procedure by which the HEC-2 or HEC-RAS program can best be formatted to provide the most
effective input and output data necessary for hydrologic studies.
(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-1 or HEC-HMS for the 100-year storm event using preliminary channel
routing data or alternate methods (i.e. Muskingum or Lag).
6-6
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
(d) Multiply the preliminary 100-year peak discharges determined above by 0.20, 0.40, 0.60,
0.80, 1.00, and 1.20 to obtain a series of six discharges for each storage routing reach.
(e) The discharges that have been developed are then input to the HEC-2 or HEC-RAS
program. The discharges should be held constant throughout the subject routing reach.
Outflows through a routing reach should not vary.
The HEC-2 or 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. Also, if using HEC-2 make sure that any ET or
X5 cards are removed from the input prior to running the storage-outflow multiple profile
analysis.
6-7
Jan-04
City of Poolland
Figure 6-1 . Hydrologic Method Determination - Mitigation Analysis
Hydrologic Method I -i I 1
Determination - Mitigation Analysis 1 1 1 I
I
I 1 ir
1
Inquire About I
FEMA Submittals Methodology Change in C x A YES q —Project Smaller YES Use Appendix A
Must be Acceptable
g ■■ ■ Eligibility for I ■ ■ ■ Pp
Required? ,,/ to FEMA < 0.7? Purchasing Regional I than 30 Acres? Methodology
I Detention 1
NO I NO I NO
Use Small
Downstream Impact 1 Project Smaller than YES Use 0.65 ac-ft
< Analysis Required byS Methodolo ■ ■ Ill * I Watershed Method
Rewmg ,yo ,7Must be Acce 9 ae 1 2 Acres? Per Ac. Size Outletp Per Section 832to ReviewingA I
Agency 1
NO I
NO 1
I
City of Pearland, Texas
Storm Drainage Design Criteria Manual
7.0 HYDRAULIC CHANNEL''DESIGN CRITERIA
7.1 INTRODUCTION
The hydraulic design of a channel or structure is of primary importance to insure 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 Brazoria County, Texas. In some instances, methodologies and parameters
not.discussed in this manual may be required. When an approach not presented herein is
required, it should be reviewed early on with the City Engineer.
This section does not apply to roadside ditches,which are covered in Section 5.0.
7.1.1 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, storm water runoff should be removed 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. .
7.1.2 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.
7.1.3 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 6.0 must be incorporated into the existing effective HEC-2
or HEC-RAS model in order to determine the impact of any proposed development flood control
facility on the entire channel system.
7-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
For the design of proposed channel with flow confined to uniform cross-sections, either a hand
calculated normal depth or direct step computation 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 or HEC-2 is recommended. Any proposed channel
improvements to an existing collector ditch or creek within the jurisdiction of the City must be
modeled using HEC-2 or 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-2 and
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.
7.2 OPEN CHANNEL DESIGN
7.2.1 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.
7.2.2 Existing Cross Sections
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 survey information which can be utilized.
7-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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 depends on the adjacent terrain,but in no case shall it be less than 20 feet.
7.2.3 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 with the City of
Pearland for approval before proceeding with design or analysis.
7.2.3.1 Earthen Channels
The most common flood control channel is a totally earthen channel. This is generally
the most economical design except in the already developed areas where land costs are
extremely high. The initial construction cost for a concrete lined channel is generally
three to four times that of an earthen channel.
In the design of an earthen channel, consideration of long-term maintenance has a very
strong influence on design parameters. The following are minimum requirements to be
used in the design of all earthen channels:
(a) Maximum earthen side slopes should be 4 (horizontal) to 1 (vertical). Slopes
flatter than 4 to 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.
(b) Minimum bottom width is ten (10) feet unless approved by City Engineer or
BDD#4.
(c)-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 7-lA and
7-1B.
These criteria and regulations shall be applicable for all channels that will be accepted for
maintenance by BDD#4 or HCFCD. Small channels on private property, not draining
public water, that do not conform to these criteria shall remain the responsibility of the
7-3
Jan-04
City of Pearland, Texas •
Storm Drainage Design Criteria Manual
owner. These private small channels within the City of Pearland and road-side ditches
along City streets shall be designed in accordance with Section 5.0.
TABLE 7-lA
KEY TO EASEMENT REQUIREMENTS
CHANNEL CHANNEL BOTTOM WIDTH
DEPTH 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 7-1B
ULTIMATE MAINTANENCE REQUIREMENTS FOR CHANNELS
KEY TOTAL EACH UNEVEN
VALUE SIDE 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 channels are included in the Brazoria Drainage
District No 4 Flood Protection Plan.
(a) Backslope drains or interceptor structures are necessary at a minimum of 1,000
feet intervals to prevent sheet flow over the ditch slopes. A standard detail is
presented in Exhibit 7-1.
(b) Channel slopes must be re-vegetated immediately after construction to minimize
bank erosion.
(c) Flow from roadside ditches must be conveyed to the channel through a roadside
ditch interceptor and pipe (Exhibit 7-2).
7-4
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
7.2.3.2 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 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.
Following are minimum requirements for partially or fully concrete lined trapezoidal
channels (Exhibit 7-3):
(a) All slope paving should include a minimum 18-inch toe wall at the top and sides
and a 24-inch toe wall across or along the channel bottom for clay soils. In sandy
soils, a 36-inch toe wall is recommended across the channel bottom.
(b) Fully lined cross-sections should have a minimum bottom width of eight (8) feet.
(c) 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.
(d) 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.
(e) 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.
(f) In instances where the channel is fully lined, no backslope drainage structures are
required. Partially, lined channels will require backslope drainage structures.
(g) Weep holes may be required to relieve hydrostatic head behind lined channel
sections. Check with the City Engineer. Refer to Exhibit 7-4.
(h) 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. Refer to Exhibit 7-3.
(i) For bottom widths of twenty(20)feet and 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. Refer to
Exhibit 7-3.
7-5
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
7.2.3.3 Rectangular Concrete Pilot Channels (Low Flow Sections)
In limited right-of-way, it is sometimes necessary to have a vertical walled rectangular
section. A standard section was developed some years ago in Harris County which
consists of 4-foot vertical walls and variable bottom widths. Above the vertical walled
section, a trapezoidal section is used varying from earthen to concrete lined depending on
the design requirements. Most contractors in the area have had significant experience in
the construction of this section.
Presented below are minimum requirements for rectangular concrete pilot channels:
(a) Typical structural requirements are shown in Exhibit 7-5. The structural steel
design is based on Grade 60 steel. This should be confirmed by a design check
based on local soil conditions.
(b) Minimum bottom width should be eight(8) feet to allow for maintenance.
(c) For bottom widths twelve (12) feet or greater, a center depression is required.
Refer to Exhibit 7-6.
(d) 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. Refer to
Exhibit 7-5.
(e) 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.
(f) Escape stairways shall be constructed in accordance with Exhibit 7-7. Escape
stairways shall be located at the upstream side of all street crossings, but not to
exceed 1,400-foot intervals.
(g) For rectangular concrete pilot channels with earthen side slopes, the top of the
vertical wall should be constructed in accordance with Exhibit 7-5 to allow for
future placement of concrete slope paving.
(h) Weep holes should be used to relieve hydrostatic pressure as shown on Exhibits
7-4 and 7-5.
(i) 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. Refer to Exhibit 7-5.
(j) Concrete pilot channels may be used in combination with slope paving or a
maintenance shelf. Horizontal paving sections should be analyzed as one way
paving capable of supporting maintenance equipment.
7-6
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
(k) A geotechnical.investigation and report shall be performed. Soil boring shall be
obtained at a minimum of every 1,000 feet to a depth of 1.5 times the proposed
channel depth.
7.2.4 Water-Surface Profiles
7.2.4.1 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 7.2.4.2),
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 City of Pearland, 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 the HEC-2 or HEC-RAS, Water Surface Profiles.
As indicated previously the City of Pearland prefers these programs primarily because
they are 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.
7.2.4.2 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 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:
Q —
1.49 A R2/3 S1/2
n
7-7
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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 (References 3.10 and 3.15).
7.2.4.3 Manning's "n" Values
Manning's "n" values for design purposes should conform with Table 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 References 3.15, 3.17, and 3.18.
TABLE 7-2
MANNING'S "n" VALUES AND ALLOWABLE 25-YEAR
VELOCITIES FOR CHANNEL DESIGN
( Roughness Average Maximum
Coefficient or Velocity Velocity
Manning's (Feet per (Feet per
Channel Description "n" Value Second) Second)
Unmaintained Earthen 0.05 3.0 5.0
Grass Lined: ---__ - -
Predominately Clay 0.04 3.0 5.0
Predominately Sand 0.04 2.0 4.0
Concrete Lined 0.015 6.0 10.0
Articulated Block 0.04 5.0 8.0
Overbanks and Existing See References Not Not
Unimproved Channels 3.15 and 3.18 Applicable Applicable
7.2.4.4 Velocities
Average and maximum allowable velocities based on 25-year flows are given in Table 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.
7-8
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
7.2.4.5 Flowline Slope
Maximum slopes are generally controlled by the maximum allowable velocity. Channel
slopes shall not be less than 0.05%.
7.2.4.6 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 (slope-area method in HEC-2 or HEC-RAS) in
the design channel.
In determining actual flood profiles or flood plain 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. Guidance for determining the timing
of drainage system is presented in Section 5.1.3.2. 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.
7.2.4.7 Headlosses
Manning's equation is used to estimate energy or head losses 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.
7.2.5 Confluences
The alignment of confluences is critical with regards to channel erosion and energy losses caused
by turbulence and eddies. The principle variables used in designing channel junctions are angle
of intersection, shape and dimensions of the channel, flow rates, and flow velocities. Definitions
of the variables are given in Exhibit 7-8.
The angle of intersection between the main channel and tributary channels or storm sewers shall
be 30 degrees as shown in Exhibit 7-8. Outfalls or junctions perpendicular to 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.
7-9
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
7.2.6 Transitions
7.2.6.1 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.
7.2.6.2 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
head loss between cross sections is expressed by:
z 2
hL C z .
2g g
where: hL = head loss (feet,
c = expansion or contraction coefficient
V2 = average channel velocity of downstream section
(feet per second),
V1 = average channel velocity of upstream section
(feet per second), and
g = acceleration of gravity (32.2 ft/sec2).
Typical transition loss coefficients to be used in HEC-2 or HEC-RAS are given below:
Coefficient
Transition Type Contraction Expansion
Gradual or Warped 0.10 0.30
Bridge Sections, Wedge, or 0.30 0.50
Straight Lined
Abrupt or Squared End 0.60 0.80
7-10
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
7.2.7 Bends
7.2.7.1 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.
7.2.7.2 Analysis
Head losses 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 =cfV2/2g
where hL = head loss (feet),
cf. = coefficient of resistance,
V = average channel velocity (feet per second), and
g = coefficient of gravity(32.3 feet/second).
Guidelines for selecting cf can be found in Reference 3.15.
The HEC-2 computer program does not incorporate a bend loss computation. If HEC-2
is used and bend losses are significant, the loss must be added at the appropriate point in
the computation. 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.
•
7-11
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
8.0 DETENTION SYSTEM DESIGN
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.
8.1.1 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 more often than not designed to remain dry,they can provide dual purpose functions
such as parking lots 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;
(c) Provisions for maintenance, including long-term funding for maintenance, are adequate
to insure the facility does not detract from the value of adjacent properties; and,
(d) The facility will be maintained to operate long term and continue to function as designed.
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, BDD#4, 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
8-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
project. 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.
Projects located in the uppermost reach of a drainage basin may use the volume of stormwater
stored in pipes, ditches, or streets as credit for the part of the detention requirement. Storage will
be credited for street volume up to the maximum allowable ponding depth per these criteria.
Sheet flow analysis must be performed to insure that for extreme events, the ponding level in the
streets will not exceed the maximum ponding level.
8.1.2 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,the study 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.
(e) Stability of the pond side slopes.
8.2 HYDROLOGIC DESIGN
(a) Detention basin design shall conform to City of Pearland criteria, BDD#4 rules, or
HCFCD criteria on a case-by-case basis as approved by the City Engineer. The
hydrologic methods for detention design should be in accordance with Section 6.0 of this
manual. The City of Pearland criteria for design of stormwater mitigation detention is
categorized as Small Project (Projects 2 acres or smaller), Medium. Project (Projects
larger than 2 acres, but less than 30 acres), or Large Project (Projects larger than 30
acres).
8.3 ON-SITE FACILITIES
8.3.1 Small Projects (Projects 2 Acres or Smaller)
Small Projects are defined as those projects that are 2 acres or smaller. If a project causes
change in runoff coefficient (existing vs. developed) times the area of the development
equal to or less than 0.7, the project may be eligible for purchasing regional detention.
Mitigation of such facilities will be incorporated within the City of Pearland regional
8-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
detention facilities, provided capacity is available and the development is within the
detention facility service area. If regional capacity is not possible, on-site detention will
be required based 0.65 ac-ft. per acre and the outlet will be sized based on the procedure
presented in Section 8.3.2 (below). In this case the volume that is calculated using 0.65
ac-ft. per acre will be considered to be the 100-year volume. The 10-year and 3-year
volumes will be considered to be 57% and 39% of the 100-year volume, respectively.
The generation of runoff hydrographs and the routing of flood flows are not required for
Small Projects.
8.3.2 Medium Projects (Projects Larger Than 2 Acres, But Less Than 30 Acres)
Medium Projects in the City of Pearland method will have their mitigation detention
volumes calculated using the methodology presented in Appendix A. All calculations
shall be presented to the City Engineer, including maps of suitable scale showing the
flow paths used to calculate the existing and developed time of concentration. See the
example at the end of Appendix A in this manual. Hydrograph routing through the
detention basin is not required. The outflow structures (low level pipe(s) or opening(s)
and high level weir)will be sized as follows:
(a) Determine the 100-year storage elevation in the basin.
(b) Determine the minimum flow line elevation for the outflow structure.
(c) Use the orifice equation to compute the opening size(s) as follows: •
Q=CA.12gH ,
Where: Q = Basin Outflow (cfs),
C = Pipe Coefficient,
A = Restrictor cross-sectional area,
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 top 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.
For ponds discharging into creeks or ditches, the outfall structure shall be designed for
the 3, 10 and 100-year storm frequencies. Determine the 3, 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 3-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
8-3
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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 the City
Engineer on a case-by-case basis.
8.3.3 Large Projects (Projects larger than 30 acres)
Large projects may under certain conditions have on-site detention facilities. Unless
FEMA submittals are required, or a downstream impact analysis is required, large on-site
projects will be analyzed using the Malcolm Method, as discussed in Section 6.0 of this
Manual.
The design of a detention basin basically consists of the following major phases:
(a) Determination of a 3-year, 10-year and 100-year 24-hour design storm inflow
hydrograph to the proposed detention basin.
(b) Determination of the maximum 3-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 will be assumed to be equal to the top of the outflow pipe,
unless special tailwater conditions prevail (see Sections 5.1.3.2 and 5.2.4).
(d) Preliminary sizing of basin storage capacity and the outflow structure.
(e) Routing of design inflow hydrograph through the basin, and adjustment of storage
and outflow structure, if required, to insure that the maximum allowable outflow rate
is not exceeded. This routing should be performed in an appropriate computer
program such as ICPR (or others as approved by the City Engineer) or a spreadsheet
with proper documentation. The outflow structure shall include a pipe or pipes sized
to restrict discharge to both the allowable 3-year and 10-year outflow rate and the
allowable 100-year design flow.
(f) Analysis of the hydraulic gradients for storm sewers and inflow channels entering the
basin to insure that these systems will operate properly under design water surface
conditions in the basin.
8-4
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
(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 the normal outlet structure from the basin
fail, the storm water can pass through, over or around the detention basin without
damaging adjacent structures.
(h) Investigation of potential geotechnical and structural problems.
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 regards to
specific design configurations.
Off-site facilities will be analyzed using HEC-1 or HEC-HMS modeling techniques as discussed
in Section 6.0. The 3-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 3-
year, 10-year and 100-year hydrographs through the detention basin may also be performed
using a computer model such as ICPR 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,
unless special tailwater conditions prevail(see Sections 5.1.3.2 and 5.2.4).
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
8-5
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
8.5 PUMP DETENTION SYSTEMS
All storm water 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) 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 3-year, 10-year and
100-year design storm events.
(b) 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.
(c) A gravity overflow route and outfall must be submitted to 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.
(d) 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 run off water that
originates from the property of more than one property owner.
(e) For detention basins collecting non-public (originates from a single property owner)
runoff water that utilize mechanical pumping systems, a cash amount equal to the fair
market value cost of the pumps and their installation shall be deposited with the City and
placed into escrow prior to approval of the final plat, or prior to the issuance of a building
permit if platting is not required. This deposit shall be placed into a permanent interest
bearing account for the use of the City to maintain the pump system in the event the
owner fails to maintain the pump system in accordance with the requirements of the City.
8-6
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
(f) Fencing of the control panel is provided to prevent unauthorized operation and
vandalism.
(g) Adequate assurance is provided that the system will be operated and maintained on a
continuous basis.
(h) Emergency source of power is provided for those cases that loss of power during a 100-
year flood event would cause property damage.
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.
8.6 STRUCTURAL AND GEOMETRIC PARAMETERS
8.6.1 General
The structural design of detention basins is very similar in many ways to wide bottom channels.
Therefore, the design requirements concerning side slopes and berms are as outlined in Section
7.0 for channels. Design considerations addressed specifically in this section deal with the basin
bottom and outfall structure.
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.
8.6.2 Bottom Design for Natural and Permanent Pool Basins
The bottom of a detention basin, which is intentionally meant to support natural vegetation,
should be designed as flat as practical to still maintain positive drainage to the outfall structure.
Side-slopes should be designed to allow for regular maintenance and be grass-lined with a
preferred 6 to 1 side slope but no steeper than 4 to 1 maximum. The bottom should be graded
toward the outfall structure at a minimum transverse slope of 0.001 feet per foot. Selected
vegetation may be introduced to the bottom of the basin to encourage a particular habitat. Other
design requirements for channels should be followed, including maintenance berms, backslope
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.
8-7
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
8.6.3 Bottom Design for Manicured Basins
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 channel should have a
minimum depth of 6 inches and a minimum flowline slope of .002 feet per foot (Exhibit 8-1).
An unlined pilot channel should have a minimum depth of two feet, a minimum flowline slope of
.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 berms, back slope drains and erosion protection measures
previously discussed.
8.6.4 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 concrete or articulated block on filter fabric spillway for the 100-year discharge or greater
flows shall extend down the bank to the bottom of the channel and up the far side.
8-8
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
8.6.5 Additional Design Considerations
r
The following items describe additional design criteria associated with detention basins.
8.6.5.1 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 block on filter
fabric as an acceptable means of slope protection.
8.6.5.2 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. Also, a parking area may be used for a portion of the storage as long as the
100-year water depth is no greater than 9 inches where cars are parked. 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.
8-9
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
9.0 MISCELLANEOUS DESIGN CONSIDERATIONS
9.1 STORM SEWER OUTFALLS
All storm sewer outfall structures should be constructed in accordance with Exhibit 9-1 and 9-2.
Design criteria for outfall structures is as follows:
(a) All storm sewer outfall pipes within the City of Pearland right-of-way must be galvanized
steel with a minimum diameter of 18 inches.
(b) All backslope drains shall be 24 inch galvanized steel as shown on Exhibits 7-1 and 7-2.
(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-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 is required for all outfall pipes.
(f) Effluent outfalls from treatment plants shall have a paved invert, and riprap in accordance
with Exhibit 9-2.
9.2 GENERAL CONTROL STRUCTURES
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. For example, a 2-foot sloped drop would not require a
soils investigation, whereas a 5-foot straight drop structure would.
9.3 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. Exhibit 9-3
defines the design features of a straight drop spillway.
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
9-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
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.
9.4 BAFFLE CHUTES
See Reference 3.31 for hydraulic and structural criteria regarding baffle block chutes.
9.5 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 do 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.
9.6 UTILITY CROSSINGS
Approval must be obtained from the City of Pearland for all utility lines which cross a flood
control facility. The utility crossing should be designed to minimize obstruction of the channel —
flow and conform with the ultimate channel cross-section. Contact the City of Pearland 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 as shown in
Exhibit 9-4. 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 above the top of banks 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.
9-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
10.0 EASEMENT AND RIGHTS-OF-WAY
To the extent practical, storm sewers shall be placed in reserves, public road rights-of-way or
permanent access strips with drainage easements.
Storm sewers shall have a minimum 20-foot easement. In the event of extreme depth (over 10
feet) the width shall be the diameter or width of the sewer plus twice depth to the center of the
sewer.
Pipes shall be centered within the limits of the reserve and easement.
10-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
11.0 SUBMITTAL
11.1 PRELIMINARY SUBMITTAL
Submit for Review and Comment:
One line drawings are recommended and are required as part of the platting process. One line
drawings should include:
(a) Approximate definition of lots and street patterns.
(b) The approximate drainage areas and design flows for each system.
(c) A definition of the proposed drainage system and stormwater mitigation by single line.
(d) The proposed pipe diameters.
(e) Any proposed drainage easements.
(f) Floodplain boundary, if any.
(g) Proposed management of perimeter drainage.
11.2 FINAL DESIGN
Submit the Following for Approval:
(a) Preliminary mark-ups and copies of any documents which show approval of exceptions
to these design requirements.
(b) Design calculations for storm line sizes and grades, and for detention facilities, if any.
(c) Design calculations for the hydraulic grade line of each storm sewer line or ditch, and for
detention facilities, if any.
(d) Contour map and drainage area map of the project.
(e) Plan and profile sheets showing storm water design(public facilities only).
(f) Projects located within a floodplain boundary shall:
1) Show the floodplain boundary or floodplain area, as appropriate, on the one-line
drawing or drainage area map.
ti
11-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
2) Comply with all applicable floodplain development ordinances of the City of
Pearland.
(g) Soil boring logs with construction drawings.
11.3 SIGNATURE STAGE
Submit the Following for Approval:
(a) Review prints.
(b) Original drawings.
(c) Stormwater detention maintenance agreement letters.
(d) Dry utility approval letter.
•
11-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
12.0 QUALITY ASSURANCE
Prepare calculations and construction drawings under the supervision of a Professional Engineer.
The final construction drawings and all design calculations must be sealed, signed, and dated by
the Professional Engineer responsible for the development of the drawings.
t
12-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
13.0 DESIGN ANALYSIS
All projects shall be tied to National Geodetic Vertical Datum (NGVD), 1987 adjustment, or
latest City of Pearland update. The City of Pearland vertical control (benchmarks) used to
prepare the 2-foot City of Pearland topographic maps shall be used where available. For areas in
or adjacent to flood plains, equations shall be used to calculate the base flood elevation (from
Flood insurance Rate Maps or subsequent locally adopted flood elevation) to the 1973, or 1978
adjustment. In the event GPS surveying is used to establish benchmarks, at least two references
to benchmarks must be identified.
Plan sets will include a drainage area map, which will contain calculations of runoff flow rates.
All drainage systems for curb and gutter pavements shall be underground closed conduits;
individual residential lot drainage is exempt. Drainage systems for pavements without curb and
gutter shall be roadside open-ditch sections.
Soil boring with logs shall be made along the alignment of all storm sewers having a cross
section equal to or greater than 42 inches in diameter or equivalent cross section area. Boring
should be taken at intervals not to exceed 500 linear feet and to a depth not less than 3 feet below
the flow line of the sewer. The required bedding will be determined from the soil boring and
shall be shown in the profile of each respective storm sewer.
13-1
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
APPENDIX A
DETENTION STORAGE VOLUME CALCULATIONS
FOR
SMALL AND MEDIUM PROJECTS
The small and medium size projects are most sensitive for short duration, high intensity storms.
Medium projects (between 2 and 30 acres) will be based on SCS concepts using triangle
hydrograph approximations. The runoff volumes are based on a triangle hydrograph with a
runoff duration equal to twice the time of concentration (Tc) of the existing, pre-project
condition. This concept is illustrated in Figure A-1.
Figure A-1.
Tce
Tcd
a) < Qd
cc
Qe
o _
Time
The variables in Figure A-1 are defined as follows and should be in consistent units when
making calculations:
Tce = Time of concentration for the existing(predeveloped)watershed
or subarea
Tcd = Time of concentration for the proposed or developed watershed
or subarea
Qe Existing condition peak discharge (calculated using the rational
method)
A-1
Jan-04
City of Pearland, Texas
. Storm Drainage Design Criteria Manual
Qd = Developed condition peak discharge (calculated using the rational
method)
Q; = Flow corresponding to the point in time where the flows of the
developed and existing conditions hydrographs are equal
In order to determine the required storage volume Vs for onsite detention, it is necessary to
determine the incremental volume V,of runoff due to development. In Figure A-1 V,is the
difference between the areas of the developed runoff hydrograph and the existing runoff
hydrograph up to the time period equal to twice the developed time of concentration(Tcd). This
is expressed mathematically as:
= Vd —Ve
Where Vd and Ve are the developed and existing conditions runoff volumes up to the time period
equal to two times Tcd, respectively.
Determining Vd is simple and can be calculated by multiplying the developed peak discharge by
the developed time of concentration. Determining Ve is slightly more tedious and requires first
determining the point Q, shown in Figure A-1. It can be shown that doing so will result in the
equations given below,which should be used for design purposes.
The design engineer must first determine whether the point representing the existing conditions
peak discharge Qe lies inside or outside the region bounded by the developed runoff hydrograph.
To do this, first calculate the following ratios:
TCe
Y Tcd
and
aQe
Qd
If a >—2-y, Qe lies outside the region bounded by the developed,runoff hydrograph Otherwise, Qe
lies inside this region. Next, the incremental increase in runoff V is calculated using one of the
equations below:
a�
V; = QdTcd y fora >—2-y (1)
oy +a
or
y +a +ay(y +a -4)
= QdTCd for a < 2-y (2)
y —a
A-2
Jan-04
City of Pearland, Texas
Storm Drainage Design Criteria Manual
After determining V,, the required volume of storage VS should be calculated using the equation
below:
V _ 13.5V,. (3)
s D @ 2Tcd
Where
D@2Tcd = Depth at time equal to 2 Ted, or the maximum rainfall depth (in), from
depth-duration-frequency curves at the time equal to twice the developed
conditions time of concentration(min).
A-3
Jan-04
0
0
- o
_I--l-4-I•----1 -I L J I- 4 11J 1 1 1 0 .
I I I I I T
I I I I I
1-1-r+-r---+ I-I--r--y -r-- ++1--+-+ +
I I I I
-rrr?-r T r , r 1 771 7 T T
I I I I I
-rrr-r-r T r - r 7 7T1 T T T
I I I I
_LLL1_L 1 JJ L J L J 11J 1 1 1
re rt
0- N 1r ul M .
I
,A 1
I O
O
1 1 I 1 1 I I 1 I 1 1 1 1 1 1 1 I O
L -1-F-I-+-I----+ 1 1 -I--I 1----I ++y--+-+ + r
M _LLL1_L 1 I I L I_J L J 11J 1 1 1
V I
rrrl I I 1
-r T 1-1 r 1 TT1 T T
1 I I
I I I 1 I
I 1 I I 1
CI 1 I I • I
CI) I
O u -F-,V-4-I----+ - -I--I----I 4I#I-I--4-+ +
I I I
M ^ I • •1 I I I I
_LLL1_L 1 L I L J 11_I 1 1 1
I I 1 I I I I
I I I I
L ^ I I I
I I I 1
-rrr?-r 7 r 1 777 T T 7
I C I I 1 In1 1
•
O 0 I I I
•_ I I C
V I O g
• ��yy I
L {y 1 1 I I I I I I I I I I I I I I r. c
• •i -rrr-r-r 7 r 1 1 - 1 771 T T T
Q O -NI-L J-~--_; -1-I--L I- - -J -! 4--4-1 +
I 1 1 1 1 I 1 1
1 I I I I I
1 co _LLL1_L 1 J L J 11J 1 1 1 F
I I I I I I I I
.0 L _ L I L I J 11J 1 1
— /M I I I I I I
O.
W 'I _ I \ I I I I I
i•a) c -I-rr-t-1____, -I--r--7 -1 77-I Y 7 7
W I I I I I
Cs /�/��� 1 I I
W --N,-J_F--__-L 1- J_ H -1 44--4-1 +
NC \ I I• I I
CO L 1 11 I I
CO I I
--rrT-r T r 1 r --- TT" Z T T
ceO I
r `I- .
O
Q I I I r
I I I I I I
C) -rl-r+-1-- -+ y--r__y__ry_ +y__+_+ +
L I I I I 1
I I I
}� _L.L LJ-L 1 -I L J I- -I J__1_ I 1
V/ 1 11 I I I
L L_ L 1_L 1 L J L J J. I.
I 1 I I I
UM 1 I I I I
-rrr-r-r 7 r 1 r 1 771 T T T
I I 1
1 I I
-hl-r+-I----+ r---I--r---� ++-I--+-+ t
11 1 I I I I 1 I 1 T
0 0 O 0
O 0 r O
0
T
seyaui ui 6y;daa poJuiea
Example of Detention Storage Volume Calculations
For Small and Medium Projects (Appendix A)
Step 1: Create an existing conditions drainage area map at a suitable scale showing the
following features:
• Existing drainage boundaries in project area
• Existing topography using suitable contour intervals
• Existing flow patterns
• Flow path used to calculate the existing time of concentration
The figure below is a pictorial representation of the map for Step 1. The proposed
drainage area is approximately 18.7 acres.
�� J I x
i
1 ,I -
il
1,, /41/.. -),'
c i .
Y M
Step 2. Create a proposed conditions drainage area map at a suitable scale showing the
following features:
• Proposed drainage boundaries in project area
• Proposed detention basin
• Proposed flow patterns
• Flow path used to calculate the proposed time of concentration
The figure below is a pictorial representation of the map for Step 2.
'\ el
4,
��1t` °' 1 ' __ PROP.DETENTION BASIN '-•
7
lr
r 1
,r...j
L ,_,...„
1,-;.) .
`rY
e } -- s —
x
_ �` E� ii ``T� r
Step 3. Calculate the existing and proposed time of concentration, The and TCd,
respectively. These calculations should be based on Section 5.1.3.2 of the drainage
criteria manual. For the example presented here The was determined to be 110 minutes
and Tcd was determined to be 18 minutes. These calculations are as follows:
Tc =T .n• +T =15 minutes+1400 feet 1 minute 110 minutes
e Intal overland 0.25 fps 60 seconds
TCe = initial +Toverland l swale +T pipe = •
10 minutes+150 feet 1 minute + 800 feet 1 minute 18 minutes
0.5 fps 60 seconds 4.5 fps 60 seconds
Step 4. Determine the existing and proposed runoff coefficients. This should be based
on the values presented in Section 5.1.3.1 of the drainage criteria manual. For the
example presented here the existing runoff coefficient is 0.2 (undeveloped property) and
the composite proposed runoff coefficient (including the area of the detention basin and
the extra 5% as specified in Section 5.1.3.1) is about 0.95.
Step 5. Calculate the existing and proposed 100-year peak discharges using the rational
method. The rainfall intensities should be based on Figure 5-2 of the drainage criteria
manual. For the example presented here the existing 100-year intensity is 3.2 inches per
hour and the proposed 100-year intensity is 8.0 inches per hour. Therefore:
Qe=0.2 x 3.2 x 18.7 = 12.0 cfs
Qd=0.95x8.0x 18.7 = 142cfs
Step 6. Calculate the ratios y and a. Sample calculations are presented below:
Tce 110
6.1
Tcd 18
12.0
a= e = 0.085
Qd 142
Step 7. Calculate 2-y. Then calculate the incremental increase in runoff due to
development, V,using the appropriate equation, (1) or(2), from Appendix A. For the
example presented here 2-y is equal to -4.1, so the appropriate equation to use is equation
(1)from Appendix A. The sample calculation using equation (1) is presented below:
=QdTcd y—a = (142)(18)(60 seconds) 6.1-0.085 149145 cubic feet 3.4 acre feet
y+a 6.1+0.085
Step 8. Using figure_determine the depth of a 100-year rainfall at a duration equal to
two times Tcd . For the sample here,this depth is approximately 3.7 inches.
Step 9. Calculate the required detention storage volume VS based on equation (3)in
Appendix A. The sample calculation using equation (3) is presented below:
13.5V.` (13.5)(3.4)
V = _ 12.4 acre feet
s D @ 2Tcd (3.7)