Runoff
The Kirpirch Method is a developed formula for predicting time of concentration in a watershed. The formula indicates where tc is the time of concentration in minutes, L is the maximum hydraulic flow length in feet, and H is the difference in elevation in feet between the outlet of the watershed and the hydraulically most remote point in the watershed.
With a maximum elevation of 1150 feet and minimum of 977 feet, Hadley Valley’s longest stream channel is 27,926 feet. The average slope is .62 percent (Figure 12). The stream length is calculated using the shape length of National Wetland Inventory (NWI) linear features. Elevation is calculated by locating entry and exit points of the stream overlayed with contours. The total time to concentration is 14.64 minutes according to the Kipirch Method.
tc = 0.0078 L0.77 (L / H)0.385
Most watersheds contain more than one soil type with multiple land uses and slopes. It is necessary to
determine a CN or runoff curve number