2.0

Using the TR-55 Graphical Method to Calculate the Peak Runoff Flow
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The % of Drainage Area label displays the calculated percentage of the
watershed drainage area comprised of ponds and swamps.
8 In the Rainfall, P box, type the rainfall depth, or click Select to select the rainfall
depth and frequency from the Define Rainfall-Frequency dialog box.
The Frequency label near the top of the dialog box displays the selected storm
frequency (1, 2, 5, 10, 25, 50, or 100 years).
9 Review the calculated results for the drainage area at the bottom of the TR-55
Graphical Peak Discharge dialog box:
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Unit Peak Discharge, qu: Displays the computed unit peak discharge for the
watershed.
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Initial abstraction, Ia: Displays the computed initial abstraction for the
watershed.
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Ia/P: Displays the computed Ia/P (Initial abstraction/Precipitation) ratio.
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Runoff, Q: Displays the computed depth of runoff in inches of rain.
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Pond/Swamp Adjust., Fp: Displays the computed Pond and Swamp Area
adjustment factor. The maximum Fp = 0.72 for 5% or greater pond and
swamp areas.
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Peak Discharge, qp: This label displays the computed peak discharge for this
drainage area.
10 You can save and output the calculation results, and view messages and
warnings:
If you want to} Then click}
save the calculation results Save.
clear the calculator of its contents New.
save the calculation results to a text file Output.
view error messages Messages.
11 Click OK to exit the dialog box.
Specifying a Runoff Curve Number
You can enter the curve number for a watershed from the Runoff Curve Number
Editor dialog box when using the TR-55 graphical method.
To specify a runoff curve number
1 Click the Select button next to the Runoff Curve Number field to display the
Runoff Coefficient dialog box.
2 Click Single RCN or Composite RCN from the Runoff Coefficient dialog box:
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Single RCN: Displays the Runoff Curve Number Editor.
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Composite RCN: Displays the Composite Runoff Curve Number Calculator.
3 Select the value you want to display in the TR-55 Graphical Peak Discharge
dialog box:
To select a value from the Runoff Curve Number Editor dialog box, click in the
field that contains the value you want to use, then click OK to return to the
TR-55 Graphical Peak Discharge dialog box. The value is placed in the Runoff