In addition to the other rules and regulations set forth in this chapter, wet bottom stormwater retention facilities shall comply with the following design criteria:
A. Perimeter Native Grass Plantings: The perimeter area of a retention pond immediately above the normal water line shall be planted with a band of native grass in accordance with the following specifications:
1. Minimum Planting Width: The native grass planting band shall be a minimum width of ten feet (10') for small retention ponds with a surface area of less than fifteen thousand (15,000) square feet measured at the normal water level, fifteen feet (15') for medium retention ponds with a surface area of less than forty thousand (40,000) square feet measured at the normal water level, and twenty feet (20') for large retention ponds with a surface area of more than forty thousand (40,000) square feet measured at the normal water level.
2. Design: A qualified landscape architect and/or ecologist shall be required to either design or certify the design of these plantings.
3. Seed Versus Plugs: The use of plugs shall be encouraged in lieu of seed. If seed is used, the letter of credit "maintenance" period for these improvements shall be three (3) years instead of two (2) years and shall be in the amount of twenty five percent (25%) versus ten percent (10%).
4. Inspection And Certification: In order to verify native planting bands have been established properly and in compliance with the approved design, an inspection shall be performed and a report created by a qualified environmental consultant hired by the pond owner to verify the status of the plantings and to outline a recommended future maintenance program.
B. Side Slopes: The side slopes of retention ponds shall be five to one (5:1) or flatter, from the normal water level to the limits of the native vegetation, and the side slopes shall be four to one (4:1) or flatter from the limits of the native vegetation to one foot (1') above the high water level.
C. Safety Ledge: A ten foot (10') wide safety ledge shall be provided three feet (3') below the normal water level. The safety ledge shall slope back towards the shore at a five percent (5%) grade. The side slopes shall be four to one (4:1) or flatter from the safety ledge to the normal water level and three to one (3:1) or flatter from the bottom of the pond up to the safety ledge.
D. Retaining Walls: Retaining walls shall be permitted on no more than fifty percent (50%) of the pond perimeter. Retaining walls shall be constructed of modular blocks. Retaining walls 3.0 feet or taller are required to be designed by a registered structural engineer or a registered architect and shall include a safety fence. 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 normal water level) on a retention pond shall be four feet (4').
F. Outfall Pipe: The outfall pipe from a retention pond shall have a flared end section and steel grating.
G. Perimeter Berm: For any retention 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: An emergency overflow weir shall be provided at an elevation above the design high water level, but at least 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 poured 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)