Wingate University Student Housing Apartments - Sand Filter Design-WU 01.15.18

Sand Filter Design Project: WU‐Student Housing
Section 11 NCDENR BMP Manual Location: Wingate NC
Date: 8/28/2017 Rev 01.06.18
Designer: Clint Lawrence, PE
Flow to Basin Data
Composite CN after development:3.32 ac. Impervious Area CN= 98
Soil Type: 0 ac. Roof area CN=
SCS Soil Group: 0 ac. Gravel CN= 89
B Soils 1.93 ac. Grassed Area CN= 61
C Soils 0.15 ac. Grassed Area CN= 74
5.40 ac. Total Area CN=
Composite CN 84.1093
Volumetric Runoff Cofficient, Rv:
Rv =.05+.009(I) % Impervious Area 61.48%
Rv 0.603333
Water Quality Protection Volume, WQv:
WQv =1.0RvA/12
WQv 0.27 Ac‐Ft
11,827 c.f
WQv ADJ 8,870 c.f.  75% reduction of WQv
WQv 0.4525 in. of runoff
Modified CN
CN =1000/[10+5P+10WQv‐10(WQv^2+1.25WQvP)^.5]
P1
Modified CN 93.04175
Sand Filter Area By Darcy's Equation;
k= 3.5 ft/day
Af =(WQv*df)/[k*(ha+df)*tf] df= 1.5 ft
Af 1351.605 sf ha= 1 ft
tf= 1.5 days
Sand Filter Media Stage‐Storage:
Elev
Qo =Af(K)(hf+df)/(df)(Cf) 533 0.056956019
Use Af 1406 sf 534 0.094926698
k 3.5 ft/day 535 0.132897377
df 1.5 ft 536 0.170868056
537 0.208838735
538 0.246809414
Underdrain Capacity:
D=16((Q*n)/(s^0.5))^(3/8))3.735975
IF D is less than 7.84 use (2) 6' Pipes (2) 6" Undergrounds Proposed
BMP section 5.7
Underdrain Capacity:
2# of 6'' dia. Underdrains
64 ft Length of underdrains
256 # of 3/8 dia. Performations assuming 50% clogged
0.009501 cfs Capacity of one perforation =CA(2gh)^.5
1.216117 cfs Capacity of all performations assuming 50% clogged
0.789 cfs Capacity of all 6'' dia. PVC pipes by Manning Eq
0.394 cfs Capacity assuming 50% clogged
OK underdrain capcity greater than max. media capacity
Anti‐Flotation Base:
Boyant force=Weight of water displace by outlet structure
5.33 ft
5.33 ft
10 ft
9,500 lbs Wt of water displaced=Volume of structure*62.4 lb/cf
19,150 lbs Wt of four 8'' thick walls at 145 lb/cf (Pre Cast Structure)
2 Factor of Saftey
None Req Wt. of base required at 145lb/cf
0 c.f. concrete required at 145 lb/cf
0ft min, square concrete base, 12'' thick