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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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an introduction to tr 55

An Introduction to TR-55

Kristina Schneider

February 27, 2001

model overview
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)
  • 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
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
runoff calculation limitations
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
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 method1
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 method2
Graphical Peak Discharge Method
  • Use Inputs to find the Ia/P ratio
graphical peak discharge method3
Graphical Peak Discharge Method
  • Determine qu with Tc, Ia/P, and rainfall distribution type.

Example distribution

There is one distribution for each rainfall type.



graphical peak discharge method4
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
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 method1
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
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
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 method1
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 method3
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
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.
  • Soil Conservation Service (SCS), Urban Hydrology for Small Watersheds, Tech. Release 55, Washington, DC. 1986.

Available online at /quality/common/tr55/tr55.pdf

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