(A) Erosion control. Temporary and permanent erosion control shall be required for all detention basins in accordance with this chapter.
(B) Verification and final approval.
(1) Erosion protection shall be inspected throughout the project duration.
(2) Detention basin storage volume shall be verified to the satisfaction of the Zoning and Building Director, or his or her designee, through as built surveys or other means.
(3) Inflow, outflow, and emergency overflow elevations shall be verified through as built surveys.
(4) Final vegetative cover and permanent erosion control shall be inspected for completeness of cover.
(5) The basin will receive final approval upon fulfillment of divisions (B)(2), (B)(3), and (B)(4) above, and the anniversary date of maintenance and repair reporting will be recorded as such.
(6) All basins must receive final approval within 90 days of the substantial completion as determined by the Zoning and Building Director, or his or her designee, of any of the following:
(a) The first phase (as shown on approved plans) of construction of public utilities and roadways in any approved subdivision project. Detention structures for the ultimate development area must be constructed during the first phase of the project, and approved at its completion. The detention structures must then be maintained and repaired in conformance with this chapter, during future construction phases.
(b) Parking areas, floor slabs, or other impervious areas (as shown on approved plans, and not including sidewalks) for work on an individual lot requiring an individual permit under this chapter. Phased construction will be treated as in the above case.
(c) Mass earthwork or rough grading if no other phased construction is scheduled to be started within 180 days.
(7) Failure to receive final approval as required will be considered a violation of this chapter.
(C) Infiltration practices. To effectively reduce runoff volumes, infiltration practices, including basins, trenches, and porous pavement, shall follow criteria in the Illinois Urban Manual and other relevant permitting. An appropriate sediment control device shall be provided to remove coarse sediment from stormwater flows before they reach infiltration basins or trenches. Stormwater shall not be allowed to stand more than 72 hours over 80% of the dry basin’s bottom area for the maximum design event to be exfiltrated. The bottom of infiltration basins or trenches shall be a minimum of three feet above the seasonally high groundwater and bedrock level if practicable. Engineering calculations demonstrating infiltration rates shall be included with the application.
(D) Side slopes. The side slopes of all detention basins at 100-year, 24-hour capacity should be as level as practicable to prevent accidental falls into the basin and for stability and ease of maintenance. Side slopes of detention basins and open channels shall not be steeper than three to one, horizontal to vertical; certain types of basins have different requirements as defined by this chapter. Detention basin side slopes above normal pool elevation shall be designed with permanent erosion protection consisting of grass, non-grass vegetation, or other permanent finish. At least six inches of topsoil must be provided on side slopes above normal pool elevation whenever nonstructural, permanent erosion control is not being used. Permanent erosion protection shall be aesthetically suitable to the development or existing surrounding land use.
(E) Overflow structures. All stormwater detention basins shall be provided with an overflow structure capable of safely passing excess flows at a stage at least one foot below the lowest foundation grade in the vicinity of the detention basin. The design flow rate of the overflow structure shall be equivalent to the 100-year, 24-hour inflow rate. Weirs, dams, and specialized outflows shall be designed by a professional engineer registered in the state.
(F) Detention basin outlet design.
(1) Backwater. Backwater on the outlet structure from the downstream drainage system shall be addressed when designing the outlet.
(2) Minimum detention outlet size. Where a single pipe outlet or orifice plate is to be used to control discharge, it shall have a minimum diameter of 12 inches. If design release rates call for smaller outlets, a design that minimizes the possibility of clogging shall be used.
(G) Other design requirements.
(1) “Bubble up” outlets are prohibited.
(2) Pumped outlets and other active control structures are discouraged and must be preapproved on a case-by-case basis by the Zoning and Building Director, or his or her designee.
(3) Temporary erosion techniques shall be used as required to ensure a full stand of cover vegetation in minimum time.
(H) Location requirements.
(1) In subdivisions, detention basins and their 100-year design high water shall be contained within platted lots dedicated for drainage purposes. In redevelopments, detention basins and their 100-year design high water shall be contained within a drainage easement.
(2) Detention basin lots shall have a minimum of 20 feet of frontage on a right-of-way for the purpose of providing unrestricted access for maintenance. Exceptions may be made for infill development.
(3) A 20-foot minimum setback shall be required from all property lines to the normal pool elevation, which is considered to be the elevation of the water level at the permanent depth of the wet basin pool rather than the temporary depth during drainage events.
(4) Detention basins shall be provided with a minimum of one foot of freeboard above the 100-year design water elevation.
(5) There shall be at least two feet of freeboard between the 100-year design water elevation and all boundaries of the parcel or easement containing the basin.
(I) Accommodating flows from upstream tributary areas. Stormwater runoff from areas tributary to the property shall be considered in the design of the property’s drainage system. Flows from upstream areas that are not to be detained should be routed around the basin being provided for the site being developed.
(J) 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, the following steps shall be followed:
(1) The applicant shall compute the storage volume needed for his or her property using the release rates and procedures described in this chapter;
(2) The 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 above plus the applicant’s property area, total storage and release rates needed for the combined properties shall be computed using the release rates and procedures described in this chapter. If tributary areas are not developed, a reasonable fully developed land cover, based on local zoning, shall be used for the purposes of computing storage;
(4) Once the necessary combined storage is computed the county, or its designee, may choose to 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 above. If regional storage is selected by the county, or its designee, the Zoning and Building Director, or his or her designee, will work with the applicant to implement the requirements of this chapter. If regional storage is rejected by the county, or its designee, the applicant shall bypass all tributary area flows around the applicant’s basin whenever practicable as determined by the Zoning and Building Director, or his or her designee. If the applicant must route upstream flows through his or her basin and the upstream areas exceed one square mile in size, the applicant must meet the provisions of § 152.229(C).
(K) Upstream areas meeting chapter requirements. When there are areas which meet the storage and release rate requirements of this chapter, and tributary to the applicant’s property, the upstream flows shall be bypassed around the applicant’s detention basin if this is the only practicable alternative as determined by the Zoning and Building Director, or his or her designee. Storage needed for the applicant’s property shall be computed as described in this chapter. However, if the county, or its designee, 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. It must be shown that at no time will the runoff rate from the applicant’s property exceed the allowable release rate for his or her property alone.
(L) Early completion of detention facilities. Where detention or retention are to be used as part of the drainage system for a property, they shall be constructed as the first element of the initial earthwork program. Any eroded sediment captured in these facilities shall be removed by the applicant on a regular basis and before project completion in order to maintain the design volume of the facilities.
(Ord. passed - -)