Stormwater detention shall only be allowed in floodplains when all other options have been exhausted. Then detention shall be allowed according to the following criteria:
(A) Detention shall not be permitted in floodways, except for on-stream detention as discussed in subsection (C) of this Section.
(B) The placement of a detention basin in a flood fringe area shall require compensatory storage for 1.1 times the volume below the base flood elevation occupied by the detention basin, including any berms. The release from the detention storage provided shall still be controlled consistent with the requirements of § 151.24. The applicant shall certify its operation for all streamflow and floodplain backwater conditions. This should include both the free flow discharge from the basin (no tailwater), as well maximum tailwater caused by the floodplain or other sources.
Excavations for compensatory storage along streams and watercourses shall be opposite or adjacent to the area occupied by the detention basin. All compensatory storage shall be placed in accordance with § 151.52 of this Chapter.
(C) On-stream detention basins are allowable only if they provide regional public benefits and if they meet the other provisions of this Code with respect to water quality and control of the 2-year and 100-year, 24-hour storm events from the property. If on-stream detention is used for watersheds larger than one square mile, it is recommended that the applicant use dynamic modeling to demonstrate that the design will not increase stage for any properties upstream or downstream of the property. Also, impoundment of the stream as part of on-stream detention:
(1) Shall not cause or contribute to the degradation of water quality or stream aquatic habitat,
(2) Shall include a design calling for gradual back slopes, appropriate bank stabilization measures and a presedimentation basin, and
(3) Shall comply with 92 Illinois Administrative Code Part 708 dated November 29, 1988 and this Code.
(Ord. 05-2528, passed 7-26-05)