The placement of detention basins within the floodplain is strongly discouraged because of questions about their reliable operation during flood events. However, the stormwater detention requirements of this chapter may be fulfilled by providing detention storage within flood fringe areas on the project site, provided the following provisions are met as well as all required state, federal, and local permits:
(A) Detention in flood fringe areas. The placement of a detention basin in a flood fringe area shall require compensatory storage for one and one-half 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 this subchapter. The applicant shall demonstrate its operation for all stream flow and floodplain backwater conditions. Excavations for compensatory storage along watercourses shall be opposite or adjacent to the area occupied by detention. All floodplain storage lost below the existing ten-year flood elevation shall be replaced below the existing ten-year elevation. All floodplain storage lost above the existing ten-year flood elevation shall be replaced above the existing ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(B) Detention in floodways. Detention basins shall be placed in the floodway only in accordance with division (C) below.
(C) On-stream detention. On-stream detention basins are discouraged but allowable if they provide regional public benefits and if they meet the other provisions of this chapter with respect to water quality and control of the five-year and 100-year, 24-hour events from the property. The volume of detention shall be provided in addition to the existing stream floodway storage. Further criteria are presented in § 152.230. If on stream detention is used in watersheds larger than one square mile, the applicant will use hydrographic modeling to demonstrate that the design will not increase the water level for any properties upstream or downstream of the property. Also, impoundment of the stream as part of on stream detention:
(1) Shall not prevent the migration of indigenous fish species, which require access to upstream areas as part of their life cycle, such as for spawning;
(2) Shall not cause or contribute to the degradation of water quality or stream aquatic habitat;
(3) Shall include a design calling for gradual bank slopes, appropriate bank stabilization measures, and a pre-sedimentation basin;
(4) Shall not involve any stream channelization or the filling of wetlands;
(5) Shall require the implementation of an effective non-point source management program throughout the upstream watershed, which shall include as a minimum, runoff reduction best management practices (BMPs) consistent with division (A) above;
(6) Shall not occur downstream of a wastewater discharge; and
(7) Shall not contribute to the duration or flood frequency of any adjacent land.
(Ord. passed - -)