An Introduction to TR-55

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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

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)
• 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
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

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 Method
• Use Inputs to find the Ia/P ratio
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 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
• 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
• 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.pdf

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