SW6190506_Runoff Volume Match Calc-rev_12/20/2019

RUNOFF VOLUME MATCH CALCULATIONS -Tart Memorial Park
Rv= 0.05 + 0.91a
where Rv = Runoff coefficient (unitless)
la = Impervious fraction (unitless)
DV = 3630 x Rd x Rv x A
where DV = Design volume (cu ft)
Rd = Design storm depth (in) (1.0)
A = Drainage area (ac) (34.87 ac)
Pre -developed conditions
Pre -developed Impervious fraction la= 214883.9 / 34.87/43560= 0.141
Rv = 0.05 + 0.9 x 0.141
Rv = 0.177
DV= 3630 x 1 x 0.177 x 34.87
DV= 22445.2 cu ft
Post developed conditions
Post developed Impervious fraction la= 250586.7 / 34.87/43560= 0.165
Rv = 0.05 + 0.9 x 0.165
Rv = 0.198
DV= 3630 x 1 x 0.198 x 34.87
DV= 25122.91 cu ft
Pre to Post Increase
25122.91 - 22445.2 / 22445.2 x 100 = 11.93 %
Credit for Disconnection of Impervious area draining to fields
Post developed impervious area= 250587 sf
Less impervious area to fields 25421
Post devel Imp not drain to fields 225166 sf
Reduce Volume of Impervious area draining to fields by 65%:
Post develop Imp area after 65% reduction 225166 + (1-0.65) x 25421= 234063.35 sf
Post developed Impervious fraction la= 234063.4 / 34.87/43560= 0.154
Post Rv= 0.189
Post DV= 23883.66 cu ft
Revised Pre to Post Increase after Disconnection credit
23883.66 - 22445.2 / 22445.2 x 100 = 6.41 %
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