Appendix 305. Location of Detention Facilities.

Detention facilities shall be designed and located to maintain existing drainage patterns to the greatest extent practicable. Detention facilities may not be sited so that they would transfer surface waters from one watershed to another. Separate detention facilities must be provided for each watershed. These facilities may be located on any practicable portion of the development site with the following restrictions.

Detention in Floodplains: Detention in regulated floodplains is strongly discouraged, although detention in flood fringes may be allowed.

Detention in Flood Fringes: The placement of a detention facility in a regulated flood fringe area shall require compensatory storage to the following standard:

A.

Volume: The compensatory storage requirement will be for one and one-half times the volume below the base flood elevation occupied by the detention basin and including any berm.

B.

Location: Excavations for compensatory storage along watercourses shall be opposite or adjacent to the area occupied by detention. All floodplain storage lost below the ten-year flood elevation shall be replaced below the ten-year flood elevation. All floodplain storage lost above the ten-year flood elevation shall be replaced above the ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.

C.

Basin Drainage: The release from the detention storage shall still be controlled consistent with the requirements of this section. The applicant shall demonstrate its operation for all streamflow and floodplain backwater conditions.

Detention on Flood Ways: Detention in regulated flood ways shall not be allowed.

Detention in Wetlands: Existing wetlands shall not be modified for the purposes of detention unless it is demonstrated that the existing wetland is low in quality and the proposed modifications will maintain or improve its habitat and ability to perform beneficial functions. The existing storage characteristics of the wetlands shall be maintained, and the volume of detention storage provided to meet the Storm Water Management Ordinance regulations shall be in addition to the existing storage in the wetland.

On-stream detention: On stream detention basins are discouraged but allowable if they provide regional public benefits. On stream detention basins will be subject to the regulations concerning tributary bypass flows. In addition, impoundment of a stream as part of on stream detention shall:

A.

Not involve channelization of the stream or the filling of wetlands;

B.

Not prevent the migration of indigenous fish species which require access to upstream areas as part of their life-cycle;

C.

Not cause or contribute to the degradation of water quality or stream aquatic habitat;

D.

Not occur downstream of a wastewater discharge;

E.

Include designs calling for gradual bank slopes, appropriate bank stabilization measures, and pre-sedimentation basins;

F.

Comply with 92 Illinois Administrative Code Parts 702 and 708 and the city's floodplain regulations.