All detention facilities and improvements required by this chapter shall comply with the following regulations:
(A) Storage volumes. Storage may be provided by wet or dry bottom basins or reservoirs or roof top storage facilities.
(B) Maximum depth. The maximum planned depth of storm water stored shall not exceed five feet unless natural ground conditions lend themselves to greater depth.
(C) Outlet control structures. Outlet control structures shall be designed as simply as possible and shall operate automatically. They will be designed to limit discharges into existing or planned downstream channels or conduits so as not to exceed existing flow of the site in its natural condition. Perforated risers of concrete, heavy gauge plastic, or other approved materials shall be incorporated on inlet piping for the discharge from all storm water detention basins to allow more gradual discharge of storm water from detention structures. Stone rip-rap shall be placed at the outlet discharge to break the force and dissipate the flow. Perforated risers and rip-rap flow reducing structures shall be designed in accordance with the Illinois Urban Design Manual.
(D) (1) Spillway. Emergency overflow facilities shall be provided unless inflow is controlled to divert flows when the basin is at capacity.
(2) Buffer space. All storm water detention basins shall be designed to provide area downstream of the outlet and alongside the structure to allow for proper maintenance of the sides and bottom of the berm. There shall be at least 15 feet of space around the basin to allow a vehicle to reach all parts of the structure, including the outlet discharge, without encroaching on the neighboring property.
(E) Dry bottom basins. For basins designed without permanent pools.
(1) Interior drainage. Provisions must be made to facilitate interior drainage, to include the provisions of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, or the installation of subsurface drains.
(2) Multi-purpose features. These may be designed to serve secondary purposes for recreation, open space or other types of use which will not be adversely affected by occasional or intermittent flooding.
(3) Cleaning. The basins shall be designed for periodic cleaning and removal of sediments, which shall be removed from the site or otherwise disposed of in an appropriate manner.
(F) Wet basins. For basins designed with permanent pools.
(1) Depth for fish. If fish are used to help keep the basin clean, at least one quarter of the area of the permanent pool must have a minimum depth of ten feet.
(2) Facilities for emptying. For emergency purposes, cleaning or shoreline maintenance, facilities shall be provided or plans prepared for the use of auxiliary equipment to permit emptying and drainage.
(3) Pollution abatement. Aeration facilities may be required when the quality of the influent and detention time would result in a lowering of dissolved oxygen content in the basin.
(4) Slopes. Approach slopes shall be at least 6:1, but not more than 3:1 and shall be at least four feet to six feet wide and slope gently toward the basin. The side slopes shall be of nonerosive material with a slope of 1:1 or flatter. The ledge shall be four feet to six feet wide and slope gently toward the shore to prevent people or objects from sliding into deep water. There shall be a freeboard of 12 inches to 18 inches above the high water elevation on all retention basins. Alternate designs for side slopes may be considered under special circumstances where good engineering practice is demonstrated.
(5) Cleaning. The basins shall be designed to include sediment traps in all inlets. Sediment traps shall be designed to permit periodic cleaning and maintenance. A basin maintenance plan shall be developed to insure that the design depths of the basin will remain over time.
(G) Building regulations.
(1) Roof top storage. Detention storage requirements may be met either in total or in part by detention on flat roofs. Design specifications of such detention shall be a part of the application for a building permit. These specifications shall include the depth and volume of storage, design of outlet devices and down drains, elevation of over-flow scuppers, design loadings for the roof structure, and emergency over-flow provisions. Roof top storage shall not be permitted to drain directly into sanitary sewers or streets.
(2) Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of storm water on a portion of their surfaces not to exceed 25%. Outlets shall be designed to empty the stored waters slowly and depths of storage must be limited so as to prevent damage to parked vehicles. Storage areas shall be posted with warning signs and shall be designed to fill to maximum depth in not less than two hours.
(3) Detention storage. All or a portion of the detention storage may also be provided in underground detention facilities.
(H) Any development which is adjacent to a ravine or ravine buffer and which has in excess of 3,200 square feet of impervious surface shall provide evaporative storage to control for a one-inch rainfall on those surfaces.
(I) Any development which is adjacent to a ravine or a ravine buffer and which has an impervious surface ratio in excess of one half shall provide evaporative or other storage to control for a two-inch rainfall on those surfaces.
(J) Retention in flood plains shall be permitted only in depressional flood plain areas. Retention will not be permitted in riverine flood plains.
('69 Code, § 5-18(b)(5)) (Ord. O-83-1, passed 5-9-83; Am. Ord. O-03-43, passed 12-8-03)