Calculate C-Factor

Function

The C-Factor is the C in the Q=CIA (quantity of flow = C * Intensity of Runoff * Area). This is known as the Rational Method of flow calculation, and is often used in smaller, urban areas, as opposed to the SCS Method which involves curve numbers (CN), and which typically applies to agricultural and rural settings. However, both methods are used for flow calculations for all varieties of applications. The C factor is a maximum of 1 if all the water runs off (e.g. from a non-porous surface). C factors are very low for wooded, leafy, flat terrain (water is absorbed into the ground). For a site of mixed use, with roads, houses, driveways, lawns and woods, it is necessary to compute the net C factor as a weighted C factor based on the respective areas of distinct surface types. This routine calculates the weighted C factor by permitting selections of C-Factors and polylines, as shown in the dialog here:

Referring to the subdivision drawing shown here, the fastest way to compute the overall C factor for this site, is to first select a C factor for a category (like the woods), and then click your cursor into the area column, then select all closed polylines for wooded areas (2 in this case), to complete the first line of entries. Repeat the process for the 14 roofs, by selecting the roofs from the C factor table of options (Select C-factor button), then click into the Area column and select all 14 roofs. Repeat for driveways and for the roads. For the remainder portion (lawns), it is advised that you determine ahead of time the overall site area, subtract the area of the special features above, and then hand-enter the area after selecting the appropriate C factor.

If you select the area first, you will be prompted Table/<C-Factor>: at the command line. Click the Calc CF button to calculate the weighted C factor, and click Report to fill out a report for the project, which appears in the text editor as shown here: