Technical Standards for Rain Gardens
1. The area (size) of the bio-retention facility should generally vary between five (5) percent and ten (10) percent of it’s drainage area.
2. Size the rain garden large enough to pond runoff from the first one (1) inch of rainfall in the drainage area. To compute this, multiply 0.79 inches by the impervious surface area draining to the bio-retention area. This will yield a ponding volume for standard bio-retention areas.
3. The rain garden shall be designed to pond water nine (9) inches deep before exiting the basin as surface flow. The surface area required of a rain garden can be found using the following equation:
Rain Garden surface area = Rain garden volume ponding ÷ Average depth of water (9 inches typical) In the example given, this equation would be: Surface area = 130 cubic feet ÷ [(9 inches)x(1 foot/12 inches)] =170 square feet. The shape may be designed to best fit the site but should be designed with a simple curvilinear edge with a minimum width is 15 feet for rain gardens and 40 feet for rain groves.
4. Design an overflow to discharge excess water out of the rain garden via a pipe riser or yard inlet type outlet. In cases where a turf or other stable gravel cover exists, water can flow out of the rain garden on one side through a reinforced rock and stone weir. Rip rap or turf reinforcement may be used to line the outlet weir. For weir outlets, adequate drainage down slope of 2% must be present.
5. All bio-retention areas must be designed to include infiltration drainage unless the designer wishes to design the facility using improved soils with permeability rates exceeding 0.5 inch/hour.
6. Plant the micro-detention with a combination of native wetland and facultative plant flowering and fruiting plant of various types including grasses, sedges, rushes and woody shrubs not to exceed a height of 48” Refer to Southern Plants 4th edition, Odenwald, Turner, Claitor’s Publishers, 2006.
7. There are three principal parts to the rain garden cross section that must be designed. They include from the bottom:
a. The sub-surface reservoir and drainage area, which is comprised of a minimum four (4) inch high density black plastic perforated pipe and #57 washed gravel.
b. The sandy loam soil growing medium zone with two (2) to four (4) foot depth with six (6) inch per hour percolation rate. Growing medium shall consist of 1 part clean top soil, 1 part clean sand and 1 part organic matter with a trace of approved herbicide and general 10.10.10 fertilizer introduced at manufacture’s rate of application.
c. The vegetation zone is the highly aesthetic and the visible part of the rain garden facility. Ground cover for this area may be three (3) to four (4) inches of double-shredded hardwood mulch, pine straw, or native grass. Do not use pine chips or other floatable materials. Do not use cypress mulch.
A rock or gravel layer composes of washed no. 57 limestone may be used as a curtain drain and an under drain in heavy clay native soils.
Apply Darcey’s Law to establish draw-down time in soil and Manning’s Formula for the pipe drainage system. It should completely draw-down for any design storm from 2-5 days.
Addition design standards include:
9. Topographic location flat to nearly flat site in an un-floodable area adjacent to an existing drainage swale, or inlet to a storm sewer.
10. Seasonal high water table shall be below the bottom of the garden a minimum of twelve (12) inches. Preferred depth shall be four (4) to six (6) feet.
11. Drainage pipe shall be a minimum four (4) inch perforated black PVC pile able to convey approximately ten (10) times the maximum inflow and be designed into a under ground net work with a clean out at the end of the system for periodic maintenance.
12. Design flow diffuser six (6) to eight (8) feet wide and six (6) inches deep using mixed crushed stone size (2”, 3” 4” and 6”) on the perimeter of the rain grove.