Environmental Engineering Reference
In-Depth Information
WORKSHEET 4 . CHANGE IN RUNOFF VOLUME FOR 2-YR STORM EVENT
PROJECT:
DA:
2-Year Rainfall:
in
Total Site Area:
acres
Protected Site Area:
acres
Managed Area:
acres
Existing Conditions:
Q
Runoff
1
(in)
Runoff
Volume
2
(ft3)
Soil
Type
Area
(sf)
Area
(ac)
la
(0.2*S)
Cover Type/Condition
CN
S
Woodland
Meadow
Impervious
TOTAL:
Developed Conditions:
Q
Runoff
1
(in)
Runoff
Volume
2
(ft3)
Soil
Type
Area
(sf)
Area
(ac)
la
(0.2*S)
Cover Type/Condition
CN
S
TOTAL:
2-Year Volume Increase (ft3):
2-Year Volume Increase = Developed Conditions Runoff Volume - Existing Conditions Runoff Volume
1. Runoff (in) = Q = (P - 0.2S)
2
/ (P+ 0.8S) where
P = 2-Year Rainfall (in)
S = (1000/ CN)-10
2. Runoff Volume (CF) = Q x Area x 1/12
Q = Runoff (in)
Area = Land use area (sq. ft)
Note: Runoff Volume must be calculated for EACH land use type/condition and HSGl.
The use of a weighted CN value for volume calculations is not acceptable.
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