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1 United StatesDepartment ofAgricultureNaturalResourcesConservatio nServiceConservationEngineeringDivisionT echnicalRelease 55 June 1986 Urban Hydrologyfor SmallWatershedsTR-55To show bookmarks which navigate through the the show/hide navigation pane button , and thenclick the bookmarks tab. It will navigate you to the contents,chapters, rainfall maps, and printable StatesDepartment ofAgricultureNaturalResourcesConservatio nServiceConservationEngineeringDivisionT echnicalRelease 55 June 1986 Urban Hydrologyfor SmallWatershedsTR-55 The U. S. Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color,national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Notall prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communica-tion of program information (Braille, large print, audiotape, etc.) should contact USDA s TARGET Center at (202) 720-2600 (voice and TDD).

2 To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326W, Whitten Building, 14thand Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964 (voice or TDD). USDA is an equalopportunityprovider and (210-VI-TR-55, Second Ed., June 1986)PrefaceTechnical Release 55 (TR-55) presents simplifiedprocedures to calculate storm runoff volume, peakrate of discharge, hydrographs, and storage volumesrequired for floodwater reservoirs. These proceduresare applicable in Small Watersheds , especially urbaniz-ing Watersheds , in the United States. First issued bythe Soil conservation Service (SCS) in January 1975,TR-55 incorporates current SCS procedures. Thisrevision includes results of recent research and otherchanges based on experience with use of the major revisions and additions are: A flow chart for selecting the appropriate proce-dure; Three additional rain distributions; Expansion of the chapter on runoff curve numbers; A procedure for calculating travel times of sheetflow; Deletion of a chapter on peak discharges; Modifications to the Graphical Peak Dischargemethod and Tabular Hydrograph method; A new storage routing procedure; Features of the TR-55 computer program; and revision was prepared by Roger Cronshey,hydraulic engineer, Hydrology Unit, SCS,Washington, DC; Dr.

3 Richard H. McCuen, professorof Civil Engineering, University of Maryland, CollegePark, MD; Norman Miller, head, Hydrology Unit,SCS, Washington, DC; Rawls,hydrologist, Agricultural Research Service,Beltsville, MD; Sam Robbins (deceased), formerlyhydraulic engineer, SCS, South National TechnicalCenter (NTC), Fort Worth, TX; and Don Woodward,hydraulic engineer, SCS, Northeast NTC, Chester,PA. Valuable contributions were made by JohnChenoweth, Stan Hamilton, William Merkel, RobertRallison (ret.), Harvey Richardson, Wendell Styner,other SCS hydraulic engineers, and Teresa June 1986 Update of Appendix A January 1999ii(210-VI-TR-55, Second Ed., June 1986)Chapter 1: Introduction .. 1-1 Effects of Urban development ..1-1 Rainfall ..1-1 Runoff ..1-2 Time parameters ..1-2 Peak discharge and hydrographs ..1-2 Storage effects ..1-2 Selecting the appropriate procedures ..1-2 Limitations ..1-4 Chapter 2: Estimating Runoff .. 2-1 SCS Runoff Curve Number method ..2-1 Factors considered in determiningrunoff curve numbers.

4 2-1 Runoff ..2-11 Limitations ..2-11 Examples ..2-11 Chapter 3: Time of Concentration and Travel Time .. 3-1 Factors affecting time of concentrationand travel time ..3-1 Computation of travel time and time ofconcentration ..3-1 Limitations ..3-4 Example 3-1 ..3-4 Chapter 4: Graphical PeakDischarge 4-1 Peak discharge computation ..4-1 Limitations ..4-2 Example 4-1 ..4-2 Chapter 5: Tabular Hydrograph 5-1 Tabular Hydrograph method exhibits ..5-1 Information required for TabularHydrograph method ..5-1 Development of composite flood .5-2 Limitations ..5-3 Examples ..5-4 Chapter 6: Storage Volume for DetentionBasins .. 6-1 Estimating the effect of storage ..6-1 Input requirements and procedures ..6-2 Limitations ..6-3 Examples ..6-4 ContentsAppendix A: Hydrologic Soil Groups .. A-1 Disturbed soil profiles ..A-1 Drainage and group D soils ..A-1 Appendix B: Synthetic Rainfall Distributions and Rainfall Data B-1 Synthetic rainfall distributions.

5 B-1 Rainfall data sources ..B-2 Appendix C: Computer Program .. C-1 Appendix D: Worksheets .. D-1 Appendix E: References .. E-1 Appendix F: Equations for Figures andExhibits .. F-1 List of figures ..iiiList of of exhibits ..iiiList of worksheets .. ivMetric ivDefinitions of symbols .. ivFigures1-1 Flow chart for selecting the appropriateprocedures in TR-55 ..1-32-1 Solution of runoff equation ..2-22-2 Flow chart for selecting the appropriatefigure or table for determiningrunoff curve numbers ..2-42-3 Composite CN with connectedimpervious area .. 2-102-4 Composite CN with unconnectedimpervious areas and totalimpervious area less than 30% .. 2-102-5 Worksheet 2 for example 2-1 .. 2-132-6 Worksheet 2 for example 2-2 .. 2-142-7 Worksheet 2 for example 2-3 .. 2-152-8 Worksheet 2 for example 2-4 .. 2-163-1 Average velocities for estimatingtravel time for shallowconcentrated flow ..3-23-2 Worksheet 3 for example 3-1 ..3-54-1 Variation of Ia/ P for P and CN.

6 4-14-2 Worksheet 4 for example 4-1 ..4-35-1 Worksheet 5a for example 5-1 ..5-5iii(210-VI-TR-55, Second Ed., June 1986)5-2 Worksheet 5b for example 5-1 ..5-65-3 Worksheet 5a for example 5-2 ..5-75-4 Worksheet 5b for example 5-2 ..5-86-1 Approximate detention basin routing forrainfall types I, IA, II, and III ..6-26-2 Worksheet 6a for example 6-1 ..6-56-3 Worksheet 6a for example 6-2 ..6-76-4 Worksheet 6b for example 6-3 ..6-96-5 Worksheet 6a for example 6-4 .. 6-11B-1 SCS 24-hour rainfall distributions ..B-1B-2 Approximate geographic boundariesfor SCS rainfall distributions ..B-2B-3 2-year, 24-hour rainfall .. B-3B-4 5-year, 24-hour rainfall .. B-3B-5 10-year, 24-hour rainfall .. B-4B-6 25-year, 24-hour rainfall .. B-4B-7 50-year, 24-hour rainfall .. B-5B-8 100-year, 24-hour rainfall .. B-5 Tables2-1 Runoff depth for selected CN s and rainfall amounts .. 2-32-2a Runoff curve numbers for Urban areas.

7 2-52-2b Runoff curve numbers for cultivatedagricultural lands .. 2-62-2c Runoff curve numbers for otheragricultural lands .. 2-72-2d Runoff curve numbers for arid and semi-arid rangelands ..2-83-1 Roughness coefficients (Manning s n) forsheet flow ..3-34-1 Ia values for runoff curve numbers ..4-14-2 Adjustment factor ( Fp) for pond andswamp areas that are spreadthroughout the watershed ..4-25-1 Ia values for runoff curve numbers ..5-2F-1 Coefficients for the equation used togenerate exhibits 4-I through 4-Ill ..F-2F-2 Coefficients for the equation used togenerate figure 6-1 ..F-2 Exhibits4-IUnit peak discharge (qu) for SCStype I rainfall distribution ..4-44-IAUnit peak discharge (qu) for SCStype IA rainfall distribution ..4-54-IIUnit peak discharge (qu) for SCStype II rainfall distribution ..4-64-IIIUnit peak discharge (qu) for SCStype III rainfall distribution ..4-75-ITabular hydrograph unit discharges(csm/in) for type I rainfalldistribution.

8 5-95-IATabular hydrograph unit discharges(csm/in) for type IA rainfalldistribution .. 5-195-IITabular hydrograph unit discharges(csm/in) for type II rainfalldistribution .. 5-295-IIIT abular hydrograph unit discharges(csm/in) type III rainfalldistribution .. 5-39A-1 Hydrologic soil groups for United Statessoils ..A-3 Worksheets2 Runoff curve number and runoff .. D-23 Time of concentration (Tc) ortravel time (Tt) ..D-34 Graphical peak discharge method .. D-45a Basic watershed data .. D-55b Tabular hydrograph dischargesummary ..D-66a Detention basin storage, peak outflowdischarge (qo) known ..D-76b Detention basin peak outflow, storagevolume (Vs) known ..D-8iv(210-VI-TR-55, Second Ed., June 1986)Metric conversionsThe English system of units is used in this TR. Toconvert to the International System of units (metric),use the following factors:From English unitTo metric unitMultiply mileSquare feet per secondCubic meters per per secondMeters per footCubic rounding operations as appropriate to indi-cate the same level of precision as that of the originalmeasurement.

9 For example:1. A stream discharge is recorded in cubic feet persecond with three significant Convert stream discharge to cubic meters persecond by multiplying by Round to enough significant digits so that, whenconverting back to cubic feet per second, youobtain the original value (step 1) with three signifi-cant of symbolsSymbol UnitDefinitionaft2 Cross sectional flow areaAmmi2 Drainage areaCNRunoff curve numberCNeComposite runoff curvenumberCNpPervious runoff curve numberEmaxMaximum stageFpPond and swamp adjustmentfactorHwftHead over weir crestIainInitial abstractionLftFlow lengthLwftWeir crest lengthmNumber of flow segmentsnManning s roughness coefficientPinRainfallPimpPercent imperviousnessP2inTwo-year frequency, 24-hourrainfallpwftWetted perimeterqft3/s (cfs)Hydrograph coordinateqift3/s (cfs)Peak inflow dischargeqoft3/s (cfs)Peak outflow dischargeqpft3/s (cfs)Peak dischargeqtcsm/inTabular hydrograph unitdischargequcsm/inUnit peak dischargeQinRunoffrftHydraulic radiusRRatio of unconnectedimpervious area to totalimpervious areasft/ftSlope of hydraulic grade lineSinPotential maximum retentionafter runoff beginsthrHydrograph timeTchrTime of concentrationTphrTime to peakTthrTravel timeVft/sAverage velocityVracre-ft, ft3 Runoff volumeor water-shed-inchVsacre-ft, ft3 Storage volumeor water-shed-inchChapter 11 1(210-VI-TR-55, Second Ed.)

10 , June 1986)Technical Release 55 Urban Hydrology for Small WatershedsIntroductionChapter 1 IntroductionThe conversion of rural land to Urban land usuallyincreases erosion and the discharge and volume ofstorm runoff in a watershed . It also causes otherproblems that affect soil and water. As part of pro-grams established to alleviate these problems, engi-neers increasingly must assess the probable effects ofurban development, as well as design and implementmeasures that will minimize its adverse Release 55 (TR-55) presents simplifiedprocedures for estimating runoff and peak dischargesin Small Watersheds . In selecting the appropriateprocedure, consider the scope and complexity of theproblem, the available data, and the acceptable level oferror. While this TR gives special emphasis to urbanand urbanizing Watersheds , the procedures apply toany Small watershed in which certain limitations of Urban developmentAn Urban or urbanizing watershed is one in whichimpervious surfaces cover or will soon cover a consid-erable area.