DETENTION BASINS

1. Basin volume shall be calculated utilizing the Modified Chicago Method for a 25 year storm. (See Appendix A)

2. A minimum detention time of 24 hours will be required. The maximum allowable outflow will be 0.15 CFS per acre regardless of the calculated rate.

3. The outlet shall be designed using the orifice equation:

A = Q ________ 0.6 (2gh)0.5 A =Area of Opening (SF) Q = Outflow (CFS) 0.6 = Orifice coefficient for circular openings g = acceleration due to gravity (32.2 ft/s2) h = design height of water above opening (ft)

4. Wet detention basin storage volume is defined as "the volume of detention provided above the invert of the outflow device".

5. Wet detention pond configuration will be as follows:

a. In general, depths of the permanent pool should be varied and range between 3 and 6 feet.

b. The minimum volume of the permanent pool shall be 2.5 times the first flush volume of 0.5 inches over the entire drainage area at the design runoff coefficient.

c. A shelf, a minimum of 6 feet wide at a depth of one foot, will surround the interior of the perimeter to provide suitable conditions for the establishment of aquatic vegetation, and to reduce the potential safety hazard to the public.

d. The permanent pool shall be constructed with either a minimum of 1 foot of compacted clay or a 40 mil HDPE liner or equivalent covered with one foot of sand.

6. Basin configuration shall be irregular with a minimum 3 to 1 length to width ratio.