Erosion and Sediment Pollution Control Program Manual.pdf

Rainfall Intensity (I):
Step 1: Calculate Time of Concentration — travel time for the hydraulically longest watershed
flow path.
Sheet Flow (Overland Flow)
Travel time for sheet flow, up to a maximum of 150 feet, may be estimated by the
use of the formula:
Tc ( sheet flow)
0.
5
0.
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2
3
L ( n)
Where: Tc = Time of concentration (minutes)
L = Length of flow path (ft)
S = Surface slope (ft/ft)
n = Roughness coefficient (See Table 5.3)
S
NOTE: The maximum flow path length (L) for any disturbed area is 50 feet. Do not
use the sheet flow equation for newly graded fills or cut slopes. Runoff from these
areas should be considered shallow concentrated flow.
TABLE 5.3
Roughness Coefficient for Sheet Flow Tc Computations
n Type of Cover
0.02 Smooth pavement
0.1 Bare parched soil
0.3 Poor grass cover
0.4 Average grass cover
0.8 Dense grass cover