Composite Curve Number Calculator with Unconnected Impervious Area

Composite Curve Number Calculator with Unconnected Impervious Area

Calculate the composite curve number for a watershed that has an unconnected impervious area using:
\[ CN_{comp} = f_{UIA} \times CN_{UIA} + \left(1 – f_{UIA}\right) \times CN_{perv} \]

* Enter the percentage of unconnected impervious area, the curve number for that area, and the curve number for the pervious area.

Step 1: Enter Watershed Data

Example: 30% of the watershed is unconnected impervious

Example: 75 (typical value for unconnected impervious surfaces)

Example: 70 (typical for pervious areas)

Formula: \[ CN_{comp} = f_{UIA} \times CN_{UIA} + \left(1 – f_{UIA}\right) \times CN_{perv}, \quad f_{UIA} = \frac{\text{Unconnected Impervious (\%)}}{100} \]