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Table 3-6. Recommended Runoff Coefficient Values for Rational Method
Land Use Runoff Coefficient (C) by Hydrologic Soil Group and Ground Slope
A B C D
<2% 2 - 6% >6% <2% 2 - 6% >6% <2% 2 - 6% >6% <2% 2 - 6% >6%
Forest 0.08 0.11 0.14 0.10 0.14 0.18 0.12 0.16 0.20 0.15 0.20 0.25
Meadow 0.14 0.22 0.30 0.20 0.28 0.37 0.26 0.35 0.44 0.30 0.40 0.50
Pasture 0.15 0.25 0.37 0.23 0.34 0.45 0.30 0.42 0.52 0.37 0.50 0.62
Farmland 0.14 0.18 0.22 0.16 0.21 0.28 0.20 0.25 0.34 0.24 0.29 0.41
 
Res. 1 acre 0.22 0.26 0.29 0.24 0.28 0.34 0.28 0.32 0.40 0.31 0.35 0.46
Res. 1/2 acre 0.25 0.29 0.32 0.28 0.32 0.36 0.31 0.35 0.42 0.34 0.38 0.46
Res. 1/3 acre 0.28 0.32 0.35 0.30 0.35 0.39 0.33 0.38 0.45 0.36 0.40 0.50
Res. 1/4 acre 0.30 0.34 0.37 0.33 0.37 0.42 0.36 0.40 0.47 0.38 0.42 0.52
Res. 1/8 acre 0.33 0.37 0.40 0.35 0.39 0.44 0.38 0.42 0.49 0.41 0.45 0.54
 
Industrial 0.85 0.85 0.86 0.85 0.86 0.86 0.86 0.86 0.87 0.86 0.86 0.88
Commercial 0.88 0.88 0.89 0.89 0.89 0.89 0.89 0.89 0.90 0.89 0.89 0.90
Streets: ROW 0.76 0.77 0.79 0.80 0.82 0.84 0.84 0.85 0.89 0.89 0.91 0.95
Parking 0.95 0.96 0.97 0.95 0.96 0.97 0.95 0.96 0.97 0.95 0.96 0.97
Disturbed Area 0.65 0.67 0.69 0.66 0.68 0.70 0.68 0.70 0.72 0.69 0.72 0.75

These Runoff Coefficients are recommended for storms of 2 to 10 yrs reoccurrence in simple drainage areas less than 100 acres. When calculating Runoff Coefficients you are recommended to determine the percentage of your drainage area which fits within each land use category found in Table 3-6 and multiply that percentage value by the corresponding Runoff Coefficient and add to the other calculated coefficients to determine a weighted Runoff Coefficient. See Example below for a drainage area with 80% Residential and 20% Pasture.

1 2 3 4
Land Use Percent of Total Land Area Runoff Coefficient Weighted Runoff Coefficient1
Residential (1/2 acre) 80 0.35 0.280
Pasture 20 0.42 0.084
Total Weighted Runoff Coefficient=0.364
1 Column 4 equals Column 2 multiplied by Column 3.

Knox County Tennessee, Stormwater Management Manual, Section on the Rational Method

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