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An Introduction to TR-55PowerPoint Presentation

An Introduction to TR-55

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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

- Tc = Sum ( Time of travel for segments)

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 Limitations

Worksheet available to aid in calculation

- See Worksheet 4 in the back of your slides

- 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)

- q= qtAmQ

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

of your handout.

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

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.

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