Storm water runoff from areas tributary to the property shall be considered in the design of the property's drainage system. Whenever practicable, flows from upstream areas that are not to be detained should be routed around the basin being provided for the site being developed.
A. Upstream Areas Not Meeting Chapter Requirements: When there are areas not meeting the storage and release rates of this chapter, 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:
1. The applicant shall compute the storage volume needed for his property using the applicant's property area, and the release rates and procedures described in section 11-2-9 of this chapter.
2. Areas tributary to the applicant's property, not meeting the storage and release rate requirements of this chapter, shall be identified.
3. Using the areas determined in subsection A2 of this section, plus the applicant's property area, total storage needed for the combined properties shall be computed.
Allowable release rates shall be computed using the combined property areas. Storage shall be computed as described in section 11-2-9 of this chapter. If tributary areas are not developed, a reasonable fully developed land cover, based on local zoning, shall be assumed for the purposes of computing storage.
Once the necessary combined storage is computed the village may choose to require oversizing of the applicant's detention basin to accommodate the regional flows. The applicant's responsibility will be limited to the storage for his property as computed in subsection A1 of this section. If regional storage is selected by the village, then the design produced in subsection A3 of this section shall be implemented and the village may enter into a recapture agreement with the applicant to permit recovery of the oversizing expense. If regional storage is rejected by the village, the applicant shall bypass all tributary area flows around the applicant's basin whenever practicable. If the applicant must route upstream flows through his basin and the upstream areas exceed one square mile in size, the applicant must meet the provisions of subsection 11-2-9I3 of this chapter for on-stream basins.
B. Upstream Areas Meeting Chapter Requirements: When there are areas which meet the storage and release rate requirements of this chapter, tributary to the applicant's property, the upstream flows shall be bypassed around the applicant's detention basin, or be routed through the applicant's detention basin if this is the only practicable alternative. Storage needed for the applicant's property shall still be computed as described in subsection A1 of this section. However, if the village decides to route tributary area flows through an applicant's basin, the final design storm water releases shall be based on the combined total of the applicant's property plus tributary areas. It must be shown that at no time will the runoff rate from the applicant's property exceed the allowable release rate for his/her property alone. (Ord. 1630, 4-12-1999)