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Chapter 5 Working with Hydrology Commands
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view and plot the idf curve click View.
insert a row of idf values place your cursor in the row above which you
want to insert a row, and click Insert Row.
delete a row of idf values place your cursor in the row to delete and
click Delete Row.
define a new frequency curve click Insert Frequency, then type the
frequency; follow by defining the intensity-
duration values.
For more information, see Specifying the Rainfall Intensity in this chapter.
8 Click OK to return to the Rational Method dialog box. Frequencies now display
in the Rainfall Frequency list.
9 Select the appropriate rainfall frequency.
10 In the Area Description column, enter a short description of the area.
11 In the Drainage Area column, enter the drainage area value. You can also click
Area at the bottom of the dialog box to calculate the drainage area by drawing
or selecting a closed polyline in the drawing.
12 In the Coef. column, enter the runoff coefficient that represents the ratio of
runoff to rainfall.
You can also click Coef at the bottom of the dialog box to select a value. After
you click Coef, you can choose either Land Uses or Soil Types from which to
select the runoff coefficient. After you select Land Uses or Soil Types, a dialog
box is displayed from which you can select a value. To select a value, pick the
field that contains the value you want, then click OK. The Rational Method
dialog box is displayed, and the value is entered in the Coef. field.
NOTE
If the watershed area has multiple land uses or soil types, then click
CmpCoef to define a composite coefficient number. For more information,
see Specifying a Composite Runoff Coefficient in this chapter.
Next, add an adjustment factor to the equation, if required. The adjustment
factor is used to calculate peak flow from major storms.
NOTE
The adjustment factor is not used in all areas. If you are not using this
factor, then leave this field blank.
13 Click Factor at the bottom of the dialog box to display the Frequency Factor
Editor dialog box.
14 Select the Use Frequency Factor check box to apply the frequency factor to the
rational equation.
15 Enter the applicable adjustment factors into the Frequency (Years) column and
the Factor column.
16 Click OK to return to the Rational Method dialog box. An adjustment factor is
placed in the Adjustment Factor column. AutoCAD automatically calculates the
correct adjustment factor based on the Rainfall Frequency you selected and the
frequency factors you entered in the Frequency Factor Editor dialog box.