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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)


Rainfall
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
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.

qu

Tc


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 method2
    Storage Volume for Detention Basins: Manual Method


    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.


    References
    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.


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