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Table Of Contents
Using the TR-55 Graphical Method to Calculate the Peak Runoff Flow
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2 In the Description box, type a description for the calculation.
3 From the Rainfall Dist. list, select the appropriate rainfall distribution for the
project location. The available values are Types I, IA, II, and III.
NOTE
See the map on page B-2 of the SCS TR-55 manual, or contact your local
SCS (now NRCS) to determine the appropriate rainfall distribution for your
project area.
4 Select the Ia/P Interpolation check box to interpolate the Ia/P ratio. Autodesk
Civil Design uses lookup tables provided in the TR-55 manual to calculate this
ratio. These tables are based on specific Ia values, such as .10 and .30. If the
calculated Ia value is .21, and the Ia/P Interpolation check box is not selected,
then Civil Design rounds the value up to .30. By selecting the Ia/P Interpolation
check box, the Ia/P value is interpolated instead of rounded.
5 Select the Auto Calc check box to automatically calculate the Peak Discharge
and Peak Time values after each change you make to the data.
Clear this check box to calculate these values only when you click Compute.
6 In the Subarea Name column, enter a short description for each watershed
subarea, using numbers or letters.
7 In the Drainage Area column, type the area of the watershed subarea. You can
also click Area at the bottom of the dialog box to calculate the drainage area by
drawing a closed polyline or selecting a closed polyline in the drawing.
8 In the Time of Concentration column, enter a Tc. You can also click Tc at the
bottom of the dialog box to calculate the areas Tc.
9 In the Travel Time column, enter a Tt. You can also click Tt at the bottom of the
dialog box to calculate the areas Tt.
10 In the Downstream Subareas column, type the list of the names of the
downstream subareas (from the subarea name column). Separate each name
with a comma. For example, if subarea 1 is upstream of subareas 2 and 4, then
type 2,4 in the Downstream Subareas column for subarea 1.
NOTE
When you type the downstream subarea names into the Downstream
Subarea columns, the Sum Tt column displays the computed Tt for these
downstream subareas. The computed Tt is the time for the runoff to travel
from a watershed subarea to the composite watershed outflow point. For
example, if subareas 2 and 4 are downstream of subarea 1, and the Tt of
subarea 2 is 4.0 and the Tt of subarea 4 is 3.0, then the sum Tt for subarea
1 is 7.0. You cannot edit the value in the Sum Tt field.
11 In the 24-hrs Rainfall column, enter the 24-hour rainfall depth for each
watershed subarea.
You can also click Rainfall at the bottom of the dialog box to enter rainfall
depth.
NOTE
The 24-hour rainfall depth should be the same for all watershed subareas.
12 In the Runoff Curve Number column, enter a RCN. You can also click RCN at
the bottom of the dialog box to display the Runoff Coefficient dialog box, then
click Single RCN or Composite RCN to select or compute the RCN for each
watershed subarea.