Bioretention Checklist & Calculations

Bio -retention Cells and Rain Gardens
DaVita Dialysis
Requirements
❑ Side slopes stabilized with vegetation shall be no steeper than 3:1.
❑ Either a bypass or an internal overflow is required for bypassing storm flows in excess of the design flow
for bioretention cells.
❑ A vegetated filter strip is not required for overflows, bypass flows, or discharges from a bioretention cell
(except for within '/2 mile of and draining to SA waters or unnamed tributaries of to SA waters).
The design shall be located a minimum of 30 feet from surface waters, and 50 feet from Class SA waters.
The design shall be located a minimum of 100 feet from water supply wells.
❑ Bioretention facilities shall not be used where slopes greater than 20%
❑ Inflow must be sheet flow (1 ft/sec for mulch cells, 3 ft/sec for grass cells) or utilize energy dissipating
devices.
Bioretention shall not be used where the seasonally high water table is less than 2 feet below bottom of the
BMP.
❑ SHWT: > 8.5 to 12' required
❑ Bottom of BMP: 1) 279.8, 2) 280.8

Bio -retention Cells and Rain Gardens
1. Water Quality Treatment Volume Provided: ft3
Volume = 3630 * Rd * Rv * Ac
Where Ac=(Drainage Area in sf)/43,560 sf/ac = 23,301 ft2 /43,560 ft2/ac = 0.53 acres
Rd = Design Storm = 1.8 in (Fort Bragg)
I = Impervious Portion / Drainage Area = 19324/23301=0.83%
Rv = 0.05 + (0.9 * 1) = 0.05 + (0.9 *0.83) = 0.80
Volume North Carolina stormwater = 3630 * 1.0 * 0.80 * 0.53 Acres =
Fort Bragg requirement to meet the Energy Independence and Security Act, Section 438 95th % rainfall event
(1.8 inches)
F=Fem Plano (eGtimate): 25 f4 * 90 f4 *1 ft -2,47-5-ft3
Soil Mix/Media
85-88 percent by volume sand
❑ 8 to 12 percent fines (silt and clay), and
❑ 3 to 5 percent organic matter (such as peat moss)
❑ P -Index between 10 and 30
Permeability between 1 and 6 inches per hour, and 1-2 inches per hour is preferred.
❑ 2 feet minimum media depth for grassed cells, 3 feet for shrub/tree cells or when using IWS.
Under Drains
Where the infiltration rate is less than 2 in/hr
❑ Clean-out pipes must be provided (minimum one per every 1,000 square feet of surface area).
❑ Clean out pipes must be capped.
Drainage Considerations
❑ Ponded water shall completely drain into the soil within 12 hours
Ponded water shall completely drain to a level of 24 inches below the soil surface within 48 hours.
2. Water Quality Treatment Volume Provided: 2215 ft3
Volume = 3630 * Rd * Rv * Ac
Where Ac=(Drainage Area in sf)/43,560 sf/ac = 29,387 ft2 /43,560 ft2/ac = acres
Rd = Design Storm = 1.8 in (Fort Bragg)
I = Impervious Portion / Drainage Area = 22,169/29,387=0.75%
Rv = 0.05 + (0.9 * 1) = 0.05 + (0.9 *0.75) = 0.72
Volume North Carolina stormwater = 3630 * 1.0 * 0.72* 0.67 Acres = ""'' "3
Fort Bragg requirement to meet the Energy Independence and Security Act, Section 438 95th % rainfall event
(1.8 inches)
From Plans (estimate): t- it it W