An Introduction to TR-55 Kristina Schneider February 27, 2001
Model Overview • Applies to small urban and urbanizing watersheds but can be used with other watersheds if limitations are met. • Uses simplified methods for estimating: • Storm runoff volume (SCS Method) • Peak rate of discharge (Graphical Method) • Hydrographs (Tabular Method) • Storage Volumes (Quick Manual Method)
Rainfall • Includes four regional 24 hour rainfall distributions • The critical parameter is the time of concentration. • Tc = Sum ( Time of travel for segments) Tt = L/V
Rainfall Distributions • Types I and IA – • Pacific maritime climates with wet winters and dry summers • IA is the least intense rainfall • Type III – • Atlantic coastal areas and the Gulf of Mexico where tropical storms with large 24 hour rainstorms occur. • Type II – • The rest of the country • Most intense short duration rainfall
SCS Runoff Curve Number Method Runoff Rainfall
Runoff Calculation Limitations • CNs are for average conditions • Does not account for rainfall intensity or duration • Less accurate when runoff is less than 0.5 inches
Graphical Peak Discharge Method • Allows you to calculate the peak discharge • Equation: qp = quAmQFp qp = peak discharge(cfs) qu = unit peak discharge (csm/in) Am = drainage area (mi2) Q = runoff (in) Fp = pond and swamp adjustment factor
Graphical Peak Discharge Method • Inputs: • Tc (hr) • Drainage Area (mi2) • Appropriate Rainfall Distribution (I, IA, II, or III) • 24-hour Rainfall (in) • CN
Graphical Peak Discharge Method • Use Inputs to find the Ia/P ratio
Graphical Peak Discharge Method • Determine qu with Tc, Ia/P, and rainfall distribution type. Example distribution There is one distribution for each rainfall type. qu Tc
Graphical Peak Discharge Method Worksheet available to aid in calculation • See Worksheet 4 in the back of your slides • Limitations • Can only be used for peak discharge • Watershed must be homogenous • Only one main stream • Cannot perform valley or reservoir routing • Ia/P must be in range given in the manual (3.000 – 0.041)
Tabular Hydrograph Method • Used to compute peak discharges from rural and urban areas. • Can develop partial composite flood hydrographs for any point in the watershed by dividing up the watershed into homogenous subareas. • Also, can be used for estimating the effects of proposed structures.
Tabular Hydrograph Method • Required Information: • Subdivision of the watershed into areas of homogenous watershed characteristics • Drainage Area (mi2) • Tc (hr) • Tt – time of travel for each reach (hr) • Weighted CN • Appropriate Rainfall Distribution (I, IA, II, or III) • Total Runoff (in) • Ia for each subarea • Ia/P ratio of each subarea
Tabular Hydrograph Method:Composite Flood Hydrograph • Use worksheet 5a to summarize the basic watershed data • Worksheet 5b is used to develop the tabular discharge summary • q= qtAmQ q= hydrograph coordinate (cfs) at hydrograph time t qt=tabular unit discharge located in exhibit 5 (csm/in) depends on rainfall distribution Am = drainage area of individual subarea (mi2) Q = runoff (in)
Storage Volume for Detention Basins: Manual Method • Quick way for planners to examine multiple alternatives. • Good for both single- and multi-stage storage basins • Constraints • Each stage requires a design storm • A storage is calculated for each stage
Storage Volume for Detention Basins: Manual Method • Use Worksheet 6a to calculate the volume of storage (Vs) if the following factors are known: • qo = peak outflow discharge • qi = peak inflow discharge • Vr = runoff volume • Use Worksheet 6b to estimate qo The worksheets are in the back of your handout.
Storage Volume for Detention Basins: Manual Method • Limitations • Less accurate as the qo/qi ratio reaches the limit of the relationship graph • Biased towards overestimating the storage needed • Should be used as an initial design step
General Limitations • Flow is based on open and unconfined flow over land or in channels. • Graphical Method is used only for homogenous watersheds. • Tabular Method can be used for heterogeneous watersheds divided up into homogenous subwatersheds. • Approximate storage-routing curves should not be used if the adjustment for ponding is used.
References • Soil Conservation Service (SCS), Urban Hydrology for Small Watersheds, Tech. Release 55, Washington, DC. 1986. Available online at http://www.wcc.nrcs.usda.gov/water /quality/common/tr55/tr55.pdf • McCuen, R.H., Hydrologic Analysis and Design, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1998.