A Brief Introduction to TR-55

An Introduction to TR-55

Kristina SchneiderFebruary 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

Rainfall Distributions

SCS Runoff Curve Number Method

Rainfall

Ru

noff

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

Inputs:1. Tc (hr)2. Drainage Area (mi2)3. Appropriate Rainfall Distribution (I,

IA, II, or III)4. 24-hour Rainfall (in)5. CN



Use Inputs to find the Ia/P ratio


Example distribution

There is one distribution for each rainfall type.

qu

Tc


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:1. Subdivision of the watershed into areas of

homogenous watershed characteristics2. Drainage Area (mi2) 3. Tc (hr) 4. Tt – time of travel for each reach (hr)5. Weighted CN6. Appropriate Rainfall Distribution (I, IA, II,

or III)7. Total Runoff (in)8. Ia for each subarea9. Ia/P ratio of each subarea

Tabular Hydrograph Method:Composite Flood Hydrograph

Use worksheet 5a to summarize the basic watershed dataWorksheet 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


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.


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.pdfMcCuen, R.H., Hydrologic Analysis and Design, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1998.

A Brief Introduction to TR-55

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