SW1200502_North Carolina Arboretum Stormwater Calculations3_20200713

North Carolina Arboretum Stormwater Calculations 6/15/2020
Given an existing parking lot to be expanded and proposed parking lot, named PL1 and PL2 respectively.
Discreet NRCS Curve Number Method for Runoff Depth
PL1:
Pre-construction characteristics:
134,929 sf paved; CN = 98; S= (1000/98)-10 = 0.20
153,927 sf pervious; CN = 61; S= (1000/61)-10 = 6.39
Post-construction characteristics:
151,614 sf paved; CN = 98; S= (1000/98)-10 = 0.20
137,242 sf paved; CN = 61; S= (1000/61)-10 = 6.39
Runoff Depth
Q*= (P-0.2S) 2/ (P+0.8S) = (1-(0.2*6.39))2/(1+(0.8*6.39)) =
0.0773 / (6.112) =0.0126 in (Perv. Runoff Depth)
Q*= (P-0.2S) 2/ (P+0.8S) = (1-(0.2*0.2))2/(1+(0.8*0.02)) =
0.9216 / (1.16) = 0.709 in (Imperv. Runoff Depth)
(.0126” (153,927 sf)) + 0.7909” (134,929 sf))/12 = 161.62 cf +8892.9 cf = 9,054.6 cf for PL1 pre con.
(.0126” (137,242sf)) + 0.7909” (151,614 sf))/12 = 144.1 cf +9,992.6 cf = 10,136.7 cf for PL1 post con.
Difference = 1,082.1 cf
PL2:
Pre-construction characteristics:
1,250 sf gravel; CN = 85; S= (1000/85)-10 = 1.76
92,415 sf pervious; CN = 61; S= (1000/61)-10 = 6.39
Composite CN = 61.3, therefore there is no Runoff by the NC Storm Water Manual
Post-construction characteristics:
38,881 sf pervious; CN = 61; S= (1000/61)-10 = 6.39
54,784 sf paved; CN = 98; S= (1000/98)-10=0.204
Runoff Depth
Q*= (P-0.2S) 2/ (P+0.8S) = (1-(0.2*6.39))2/(1+(0.8*6.39)) = 0.077284 / (6.112) =0.0126 in. Perv. Runoff
Depth
Q*= (P-0.2S) 2/ (P+0.8S) = (1-(0.2*.2041))2/(1+(0.8*.2041)) = 0.7909 in. imp. Runoff Depth
(.0126” (38,881 sf)) + 0.7909” (54,784 sf)) = 40.8 cf +3,625 cf = 3,610.7 cf for PL2

Therefore the difference in runoff from PL1 and the runoff from PL2 = 4,692.8 CF
Stage Storage Relationship for Bio Retention Cell
Swale that surrounds the parking lot has the following geometry:
18” of depth; 12” bottom width; 3:1 side slopes;
When the swale is full it provides 8.25 cf of storage per linear foot.
The needed storage is 4,692.8 cf. The swale is 708’ in length, therefore it must provide 6.63 cf of
storage per foot of length.
0.5’depth – 1.25 CF / FT - 885 total CF
1’ depth - 4.0 CF/FT - 2,832 total CF
1.5’depth - 8.25 CF/FT - 5,841 total CF
Therefore the swale provides more than the needed capacity.
The Internal Water Storage volume has not been included in the volume calculations.
Under Drain Sizing:
From Hydrocad, the flows from the parking lot are 0.78 cfs and 0.45 cfs
The underdrain in each swale is specified to be 6” in diameter.
D = 16 ((Q*n)/(S0.5))3/8
D for swale 1 = 16 ((0.78*.011)/(.0707)).375 = 7.25” ;
Therefore a 6” and 4” drain will be required at 0.5% slope for swale 1
16 ((0.45*.011)/(.0707)).375 = 5.9”; Therefore a 6” drain is adequate for swale 2.