Rain Grove

Technical Standards for Rain Groves

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 grove  shall be 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 grove shall be designed to pond water nine (9) inches deep. Before exiting the basis as surface flow. The surface area required of a rain grove can be found using the following equation:

Rain Grove surface area = Rain grove 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. There are three principal parts to the rain garden cross section that must be designed beginning with the bottom section: 

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. 

7. Plant the micro-detention with a combination of native wetland and facultative flowering and fruiting trees and large shrubs arranged in a naturalized way with taller plants in the middle reducing the height to the outer dimension. The main ground cover shall be mowable turf grass that can take inundation.  Woody shrubs should be planted in the interior within a heavy mulched area not to exceed 20% of the area of the rain grove within an 8” raised planting bed with a curvilinear edge.  Tree plantings on the perimeter of the facility shall not exceed twelve (12) feet in height.   

8. Apply Darcey’s Law to establish draw-down time in soil and Manning’s formula for the pipe drainage system. It should completely drawn-down for any design storm from 2-5 days.

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 grove a minimum of forty-eight (48) 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. A grading plan, planting plan and drainage plan stamped by a landscape architect must be submitted for this comprehensively designed rain grove storm water facilities.