An Introduction to TR-55

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An Introduction to TR-55

• Kristina Schneider
• February 27, 2001

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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)

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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

4
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

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Rainfall Distributions
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Rainfall Distributions
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SCS Runoff Curve Number Method
Runoff
Rainfall
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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

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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

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Graphical Peak Discharge Method

• Inputs
• Tc (hr)
• Drainage Area (mi2)
• Appropriate Rainfall Distribution (I, IA, II, or
  III)
• 24-hour Rainfall (in)
• CN

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Graphical Peak Discharge Method

• Use Inputs to find the Ia/P ratio

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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
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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)

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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.

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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

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Tabular Hydrograph MethodComposite 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
• qttabular unit discharge located in exhibit 5
  (csm/in) depends on rainfall distribution
• Am drainage area of individual subarea (mi2)
• Q runoff (in)

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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

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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.

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Storage Volume for Detention Basins Manual
Method
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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

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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.

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References

• Soil Conservation Service (SCS), Urban Hydrology
  for Small Watersheds, Tech. Release 55,
  Washington, DC. 1986.
• Available online at http//www.wcc.nrcs.usda.go
  v/water /quality/common/tr55/tr55.pdf
• McCuen, R.H., Hydrologic Analysis and Design,
  Prentice-Hall, Inc., Englewood Cliffs, NJ, 1998.