Technical data

Using the Rational Method to Calculate Runoff
41
To design a culvert (continued)
Steps
Use
to look up
12 Click OK to close the Culvert Settings dialog box
and return to the Culvert Calculator.
13 Click Over-Top to access the Culvert Weir Editor
dialog box to check the overtop conditions of the
culvert.
Change the Overtop Flow
Values to Use in the
Culvert Calculations
14 Verify that the culvert design meets all of the
relevant design criteria and conditions.
15 Create a Performance Curve graph for the
designed culvert by clicking P-Curve.
Display a Performance
Curve for a Culvert
16 Create a Fit Curve graph for the designed culvert
by clicking Fit-Plot.
Display a Headwater
Versus Flow Curve for a
Culvert
17 Click the Save button to save your culvert design
data to a file.
18 Click OK to exit the Culvert Design calculator.
Using the Rational Method to
Calculate Runoff
Autodesk Civil Design provides several different methods for
calculating peak runoff from a watershed area. One of these methods
is the Rational Method. Despite its many governing limitations, the
Rational Method still remains the most widely used method for
calculating storm water runoff in small urban areas or for highway
drainage. The method is based entirely upon a rational analysis of the
rainfall-runoff process in which a simple formula, Q = CIA, is used to
estimate the peak runoff occurring in the defined watershed area for
the selected storm event. This estimate of peak runoff can then be
used as a design flow for sizing proposed inlets, pipes, culverts and
other hydraulic structures.