8-13-5: DETENTION FACILITY DESIGN CRITERIA:
In addition to the other rules and regulations set forth in this chapter, open stormwater detention facilities shall comply with the following design criteria:
   A.   Bottom Slope: The bottom slope shall be two percent (2%) or steeper in order to prevent wet areas which may kill the intended ground cover or promote the growth of nuisance plant species.
   B.   Low Flow Pipes And/Or Paved Channels: Low flow pipes and/or paved channels shall not be permitted. The ends of storm sewers draining runoff into the detention pond shall be located at least fifty feet (50') away from the pond outlet to ensure sufficient filtering of contaminants.
   C.   Side Slopes: Vegetated side slopes shall be four to one (4:1) or flatter. Seeded slopes shall be protected with an approved erosion control mat designed for that application.
   D.   Retaining Walls: Retaining walls shall not be permitted on more than fifty percent (50%) of the pond perimeter. Retaining walls shall be constructed of modular blocks. Retaining walls 3.0 feet or taller shall be designed by a registered structural engineer or a registered architect. Stepped walls count as one wall system (e.g., a 2 foot lower wall plus a stepped 2 foot upper wall equals a 4 foot wall). Safety fencing shall be provided along the top edge of any retaining wall greater than 3.0 feet in height.
   E.   Bounce: The maximum bounce (high water level minus the outlet invert elevation) on a detention pond shall be four feet (4').
   F.   Outfall Pipe: The outfall pipe from a detention pond shall be provided with a flared end section and steel grating.
   G.   Perimeter Berm: For any detention pond at least partially enclosed by an earthen berm, the top of the berm shall be at least one foot (1') higher than the design high water level and at least two feet (2') wide.
   H.   Emergency Overflow Weir: An emergency overflow weir shall be provided at an elevation above the design high water level, and not less than six inches (6") below the top of berm. The overflow weir shall be armored from the top of berm to the outside toe of slope. If the weir is armored with riprap or other porous material, the subgrade under the riprap shall be equal to or above the design high water level. The weir shall be wide enough to convey one cfs per tributary acre. Weirs facing adjacent single-family residential properties or public streets shall be incorporated into the landscape design for the facility and shall be designed to minimize negative visual impacts by incorporating landscape stone, grasscrete or an approved equal material in lieu of concrete or standard riprap.
Requests for relief from the requirements of this section will be processed by the Oversight Committee. A public hearing or a published legal notice is not required for any such request unless the request also involves relief from the requirements of the DuPage County Countywide Stormwater and Floodplain Ordinance. The applicant will be responsible for payment of any nonrefundable application fees to appear before the Plan Commission/Oversight Committee. (Ord. O2017-38, 6-19-2017)