No person shall cause any development on any property in the village without providing stormwater detention facilities as herein described and specified.
(A) Detention in floodplains.
(1) Detention basins shall not be permitted in floodways.
(2) (a) The stormwater detention requirements of this code may be fulfilled by providing increased storage capacity within flood fringe areas on the project site. The placement of a detention basin in a flood fringe area shall require compensatory storage for one 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 § 152.19. The applicant shall certify its operations for all streamflow and floodplain backwater conditions. Excavations for compensatory storage along streams and watercourses shall be opposite or adjacent to the area occupied by detention. All floodplain storage lost below the 10-year flood elevation shall be replaced below the 10-year flood elevation. All floodplain storage lost above the existing 10-year flood elevation shall be replaced above the proposed 10-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(b) No compensatory storage requirement shall be established for new or existing residential buildings to be located in a residential district already in existence as of the date of adoption.
(3) On-stream detention basins are allowable 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 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:
(a) 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;
(b) Shall not cause or contribute to the degradation of water quality or stream aquatic habitat;
(c) Shall include a design calling for gradual bank slopes, appropriate bank stabilization measures and a pre-sedimentation basin; and
(d)
Shall comply with 17 Illinois Administrative Code Part 3708 and this code.
(B) Detention in wetlands. Existing wetlands shall not be modified for the purposes of stormwater detention unless it is demonstrated that the proposed modification, including such activities as filling, draining, excavating and vegetation removal, will maintain or improve habitat, and water quality. In addition, fill shall not be placed in the wetland and a long-term monitoring and maintenance agreement shall be established to ensure that the modified wetland/detention facility continues to provide wetland functions.
(C) General detention considerations for water quality enhancement.
(1) Inlet and outlet orientation. As much as feasible, the distance between detention inlets and outlets should be maximized. If possible, they should be at opposite ends of the basin.
(2) Velocity dissipation. Baffles or berms should be placed perpendicular to the direction of flow through a detention basin to reduce velocity and prevent scour.
(3) Maintenance. The stormwater drainage system shall be designed to minimize and facilitate maintenance. Turfed sideslopes should be designed to allow lawn moving equipment to easily negotiate them. Wet basins should be provided with alternate outflows which can be used to completely drain the pool for sediment removal. Pre-sedimentation basins should be considered for localizing sediment deposition and removal. Access for heavy equipment should be provided.
(D) Flows from tributary areas. Stormwater runoff from areas tributary to the property shall be considered in the design of the property's drainage system.
(1) Upstream areas not meeting code requirements. When there are areas not meeting the storage and release rates of this code, tributary to the applicant's property, regionalized detention on the applicant's property shall be explored by the applicant. The following steps shall be followed:
(a) The applicant shall compute the storage volume needed for his or her property using the release rates prescribed herein, the applicant's property area, and the procedures described herein.
(b) Areas tributary to the applicant's property, not meeting the storage and release rate requirements of this code, shall be identified.
(c) Using the areas determined above plus the applicant's property area, total storage needed for the combined properties shall be computed.
(d) Allowable release rates shall be computed using the combined property areas. Storage shall be computed as described herein. If tributary areas are not developed, a reasonable fully developed land cover shall be assumed for the purposes of computing storage.
(e) Once the necessary combined storage is computed, the village may choose to have the applicant construct regional storage and the village shall pay for oversizing the applicant's detention basin to accommodate the regional flows. The applicant's responsibility will be limited to the storage for his or her property as computed in this chapter. If regional storage is selected by the village, the design accommodating the volume determined pursuant to division (C) of this section shall be implemented. If regional storage is rejected by the village, the applicant has the option of bypassing all tributary area flows around the applicant's basin. In the event the applicant chooses to route them through his or her basin and the upstream areas exceed one square mile in size, the applicant must construct the basin to comply with the provisions of on-stream basins.
(2) Upstream areas meeting code requirements. When there are areas which meet the storage and release rate requirements of this code tributary to the applicant's property, the upstream flows shall either be bypassed around the applicant's detention basin or be routed through the applicant's detention basin at the discretion of the village. Storage needed for the applicant's property shall still be computed as described above. However, if the village decides to route tributary area flows through an applicant's basin, the final design stormwater releases shall be based on the combined total of the applicant's property plus tributary areas.
(E) Early completion of detention facilities. Where detention, retention or depressional storage areas are to be used as part of the drainage system for a property, they shall be completed before general property grading begins. This is to ensure that if soil erosion and sediment control practices are not adequately implemented, any eroded sediment will be captured in these areas and will have to be removed by the applicant before project completion.
(F) Fee in lieu of detention.
(1) All developments which are approximately three acres and under in size may be required to pay a fee based on the acre feet which would be required under this code as a percentage of the total capacity of an existing or proposed detention facility multiplied by the cost of engineering, land construction, landscaping and maintenance of the detention facility rather than installing detention facilities on the property, when specifically directed by the Comptroller. The village also shall have the option for larger properties of requiring a fee for each acre-foot of detention needed in lieu of the applicant building a basin on-site provided the property will discharge stormwater to the village's storm sewer system.
(2) In instances where regional benefits and economies of scale can be achieved, it may be permissible, and may be required, for adjacent properties to utilize a common regional detention basin. The applicant may have the option of paying a fee based on the ratio of acre feet required to the total capacity of the detention facility and reflecting the cost of engineering, land, construction, landscaping and maintenance of the detention facility so that the village can build regional facilities or applicants can jointly build the necessary facilities themselves.
(Ord. 7-28, passed 5-9-07; Am. Ord. 8-70, passed 7-16-08; Am. Ord. 11-35, passed 8-17-11) Penalty, see § 152.99