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# An Introduction to TR-55 - PowerPoint PPT Presentation

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

Kristina Schneider

February 27, 2001

• 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

• 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

Rainfall

• CNs are for average conditions

• Does not account for rainfall intensity or duration

• Less accurate when runoff is less than 0.5 inches

• 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:

• Tc (hr)

• Drainage Area (mi2)

• Appropriate Rainfall Distribution (I, IA, II, or III)

• 24-hour Rainfall (in)

• CN

• Use Inputs to find the Ia/P ratio

• Determine qu with Tc, Ia/P, and rainfall distribution type.

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)

• 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.

• 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

Storage Volume for Detention Basins: Manual Method

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

• 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 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.