Wet detention basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing, and as much as feasible to be available for recreational use.
(A) Wet basin depths. Wet basins shall be at least three feet deep, excluding near-shore banks and safety ledges. If fish habitat is to be provided they shall be at least ten feet deep over 25% of the bottom area to prevent winter kill.
(B) Wet basin shoreline slopes. The side slopes of wet basins at the normal pool elevation shall not be steeper than five to one horizontal to vertical. It is recommended that native aquatic vegetation be established around the perimeter to provide protection from shoreline erosion. Slopes below a depth of eight feet are permitted to be two to one, in accordance with IDOT Standard Specifications, § 204.
(C) Permanent pool volume. The permanent pool volume in a wet basin at normal depth shall be equal to the runoff volume from its watershed for the two-year, 24-hour event as a minimum.
(D) Wet basin inlet and outlet orientation.
(1) The distance between detention inlets and outlets shall be maximized. Inlets and outlets shall be at opposite ends of the basin, providing that the orientation does not create undue hardship based on topography or other natural constraints. Designers are encouraged to use baffles or berms in the basin bottom to prevent short circuiting.
(2) There shall be no low flow bypass between the inlet and outlet. The minimum flow length shall be ten feet with a recommended minimum ratio of two to one for width.
(E) Safety ledge. All wet detention basins shall have a level safety ledge at least four feet in width two and one-half to three feet below the normal water depth.
(F) Aeration. Wet bottom basins shall have a natural or artificial means of aeration.
(G) Dewatering. An outlet structure shall be provided to allow dewatering of the pond for maintenance. Gravity dewatering is strongly preferred.
(H) Soil permeability. Wet bottom basin design shall include an evaluation of soil permeability. A basin liner shall be included in the design if needed to ensure water retention to normal pool elevation.
(I) Detention/sedimentation. It is encouraged that consideration of routing runoff from the development through a stilling basin be considered.
(Ord. passed - -)