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)


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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)
T t
= 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

Rainfall Distributions

SCS Runoff Curve Number Method
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: q p q p
= q u
A m
QF p
= peak discharge (cfs) q u
A m
= unit peak discharge (csm/in)
= drainage area (mi 2 )
Q = runoff (in)
F p
= pond and swamp adjustment factor

Graphical Peak Discharge Method
1.
2.
3.
4.
5.
Inputs:
Tc (hr)
Drainage Area (mi 2 )
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 q u with Tc, Ia/P, and rainfall distribution type.
Example distribution
There is one distribution for each rainfall type.
T c

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
1.
2.
Required Information:
Subdivision of the watershed into areas of homogenous watershed characteristics
Drainage Area (mi 2 )
3.
4.
5.
6.
7.
8.
9.
Tc (hr)
T t
– 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= q t
A m
Q q= hydrograph coordinate (cfs) at hydrograph time t q t
=tabular unit discharge located in exhibit 5
(csm/in) depends on rainfall distribution
A m
= drainage area of individual subarea (mi
Q = runoff (in)
2 )

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:


 q o
= peak outflow discharge q i
= peak inflow discharge
V r
= runoff volume
Use Worksheet 6b to estimate q o
The worksheets are in the back of your handout.

Storage Volume for Detention
Basins: Manual Method

Storage Volume for Detention
Basins: Manual Method
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Limitations
Less accurate as the q o
/q i 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.