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Calculating the WQCV and Volume Reduction Chapter 3
The same calculation is then performed using the effective imperviousness for the 100-year event:
V 100 I 100−yr Effective
= −0.00005501 ∙ I 2
100−yr Effective + 0.030148 ∙ I 100−yr Effective − 0.12
12
ft 3
∙ Total Area ∙ 43560
ac ∙ ft
V 100 I 100−yr Effective = (−0.00005501 ∙ 47%2 + 0.030148 ∙ 47% − 0.12)
∙ 1.15 ac ∙ 43560
12
= 4865 ft 3
The reduction in the 100-year storage volume, as a result of the conveyance-based BMPs in Sub-basin A,
is 5083 ft 3 – 4865 ft 3 = 218 ft 3 , a reduction of approximately 4.3%.
Calculations for Sub-basin E include the following:
V 100 I Total = −0.00005501 ∙ I 2
Total + 0.030148 ∙ I Total − 0.12
∙ Total Area ∙ 43560
12
V 100 I Total = (−0.00005501 ∙ 73%2 + 0.030148 ∙ 73% − 0.12)
12
∙ 0.15 ac ∙ 43560
ft 3
ac ∙ ft
ft 3
ac ∙ ft
ft 3
= 977 ft3
ac ∙ ft
The same calculation is then performed using the effective imperviousness for the 100-year event:
V 100 I 100−yr Effective
= −0.00005501 ∙ I 2
100−yr Effective + 0.030148 ∙ I 100−yr Effective − 0.12
12
ft 3
∙ Total Area ∙ 43560
ac ∙ ft
V 100 I 100−yr Effective = (−0.00005501 ∙ 69%2 + 0.030148 ∙ 69% − 0.12)
∙ 0.15 ac ∙ 43560
12
= 927 ft 3
The reduction in the 100-year storage volume as a result of the volume-based BMPs in Sub-basin E is,
therefore, 977ft 3 – 927 ft 3 = 50 ft 3 , a reduction of approximately 5%.
ft 3
ac ∙ ft
3-30 Urban Drainage and Flood Control District August 2011
Urban Storm Drainage Criteria Manual Volume 3