(A) Release rates. Release rates shall be in accordance with the most current edition of the MWRDGC.
(B) Emergency overflow.
(1) All stormwater infiltration, retention, and detention facilities shall be provided with an emergency overflow structure capable of passing the 100-year peak flow inflow rate without damage to buildings or property.
(2) The overflow shall be established at the 100-year high-water elevation.
(3) The remainder of the basin shall have at least one foot of freeboard.
(C) Outlet design.
(1) Single pipe outlets shall have a minimum inside diameter of 12 inches.
(2) If design release rates call for a smaller outlet, a design that minimizes the possibility of clogging, such as a perforated riser, shall be used.
(3) The minimum outlet restrictor size shall be four inches in diameter provided there is adequate downstream capacity.
(4) All outlets shall be "gravity only" unless otherwise approved by the city engineer.
(5) Backwater on the outlet structure from the downstream drainage system shall be evaluated when designing the outlet.
(D) Dam construction. If the construction of a storage basin involves the construction, modification or removal of a dam or in-stream impound structure, as defined in 17 Ill Adm. Code 3702 (Rules for Construction of Dams), the applicant shall obtain an Illinois Division of Water Resources Dam Safety permit or letter stating no permit is required prior to the start of work.
(E) Off-site flow. Stormwater infiltration, retention, and detention facilities required to meet a development's discharge requirements shall be designed to by-pass off-site tributary flow from streams and channels unless approved by the city engineer.
(F) Depressional storage.
(1) The function of existing on-site depressional storage shall be preserved independently of required detention.
(2) When depressional storage is removed it must be compensated for by an on-site runoff storage facility at a 1:1 ratio. This requirement is in addition to the site runoff storage requirements of this chapter.
(G) Storage basins. Detention and retention basins shall be designed with the following guidelines:
(1) Provisions for all basins:
(a) The facility shall incorporate either open water or wetland features to improve water quality.
(b) The facility shall be designed to remove stormwater pollutants.
(c) To the extent feasible, the distance between detention inlets and outlets shall be maximized. Where possible, they should be at opposite ends of the basin.
(d) The dry portions of the facility may be made available for recreational use.
(2) Provisions for areas with open water:
(a) The water shall be at least three feet deep, excluding nearshore banks and safety ledges.
(b) If fish habitat is to be provided, at least 25% of the bottom area shall be 10 feet deep to prevent winter freeze-out.
(c) The side slopes shall be no steeper than 5:1 (horizontal to vertical).
(d) The permanent pool volume at normal depth shall be equal to the runoff volume from its watershed for the two-year storm. If this cannot be achieved, the bottom shall be underdrained or wetland plantings shall be used.
(e) Shoreline protection shall be provided to prevent erosion. Native vegetation is the preferred landscaping material.
(f) Facilities shall be available, if possible, to allow the pond level to be lowered by gravity flow for cleaning purposes and shoreline maintenance.
(3) Provisions where wetland features are used:
(a) The design criteria, vegetation selection, performance criteria, and maintenance and monitoring guidelines in Naturalized Stormwater Management Facilities (South Suburban Mayors and Managers Association, May 2000) shall be followed.
(b) Sediment basins should be provided at all major inlets to the facility. The volume of the basins should be at least 500 cubic feet per acre of impervious surface in the drainage area. Side slopes below one foot of depth should be no steeper than 3:1 (horizontal to vertical) and basin depth should be at least three feet and designed to allow access for sediment removal equipment.
(c) There should be no low flow bypass between the inlet and the outlet. Paved low flow channels shall not be used.
(4) Provisions for shoreline zones:
(a) The "shoreline zone" is the area between one foot above normal pool stage and one foot below normal pool state.
(b) The side slope in the shoreline zone shall not be steeper than 10:1 (horizontal to vertical).
(c) Appropriate soil conditions shall be provided in the shoreline zone.
1. Compaction of both subsoil and topsoil shall be minimized (i.e. to less than 274 psi).
2. Where subsoil compaction cannot be avoided, it should be disked to a depth of 6-8 inches with a chisel plow before spreading topsoil.
3. A suitable uncompacted topsoil, at a minimum thickness of one foot, shall be spread to provide a suitable growing medium for aquatic plants. Coarse soils with minimal clay content and a high organic content are recommended.
(d) Water tolerant, preferably native, vegetation shall be used to landscape the shoreline zone.
1. The selected plants and planting methods shall conform to the soils, hydrology, and water quality conditions present in such facilities, with plants being tolerant of highly variable hydrologic conditions and degraded water quality (e.g. high turbidity and salinity content).
2. Plant selection should conform to the guidance in the Native Plant Guide for Streams and Stormwater Facilities in Northeastern Illinois (NRCS et al., 1998).
(4) Provisions for areas (other than shoreline zones) that are intended to remain dry:
(a) The minimum bottom slope for turf shall be 2%. If this cannot be achieved, the bottom shall be underdrained or wetland plantings shall be used.
(b) The side slopes shall be no steeper than 4:1 (horizontal to vertical) for all turf areas.
(H) Streets, parking, and underground storage.
(1) No storage basin shall be constructed within a distance of 10 feet plus 1.5 times the depth of any basin to the right-of-way of any public roadway without written permission from the city engineer.
(2) The owner of a parcel being developed adjacent to a state or county road right-of-way shall notify the proper highway authority in writing of the proposed development. The owner shall request that the proper highway authority provide, at the cost of the highway authority or as otherwise provided by law, the amount of additional capacity in any stormwater detention facility to be constructed in the development for the future availability of the highway authority for meeting stormwater detention requirements of any future public construction on the highway.
(3) The maximum depth of stormwater storage for paved areas designated for public parking is eight inches.
(4) Underground storage of stormwater shall be avoided. If used, adequate storm sewer inlets must be provided to pass the 100-year flow into the underground storage.
(I) Storage in floodplains.
(1) Stormwater retention and detention facilities shall not be constructed in the SFHA unless approved by the city engineer.
(2) If a storage facility is constructed in the SFHA, the volume of detention storage required to meet the standards of this section shall be in addition to the floodplain compensatory storage required for the development.
(J) On-stream detention.
(1) "On-stream" detention means an in-stream impoundment or other detention basin that includes construction of a dam or other barrier in an existing channel.
(2) All on-stream detention shall provide a Detention Volume Safety Factor equal to one plus 0.05 times the ratio of off-site tributary drainage area to on-site tributary drainage area. The maximum Detention Volume Safety Factor shall be 1.5.
(3) The application for a permit to construct an on-stream detention facility shall provide the city engineer with either an IDNR dam safety permit or a letter from the IDNR stating that a dam safety permit is not required.
(4) On-stream detention shall not be allowed if:
(a) The off-site to on-site tributary drainage area ratio is greater than 10:1 except for developments providing a watershed benefit; or
(b) The tributary drainage area is greater than 640 acres, except for detention that provides a watershed benefit.
(5) The release rate shall be the total of the following:
(a) Cubic feet per second per acre of the total tributary drainage area (on-site and off-site) at the elevation created by impoundment of the on-site two-year storm volume plus the Detention Volume Safety Factor;
(b) 0.15 cubic feet per second per acre of the total tributary drainage area (on-site and off-site) at the elevation created by impoundment of the on-site 100-year storm volume plus the Detention Volume Safety Factor;
(c) The required compensatory storage; and
(d) The release rate and on-site detention volume shall be calculated using the 24-hour storm event. This release rate calculation shall be used unless other site conditions warrant further analysis and modification from this standard or unless watershed-specific release rates have been adopted.
(6) On-stream detention shall provide water quality treatment. One of the following two methods shall be used:
(a) A wet detention facility shall be constructed with a minimum permanent pool volume equal to the calculated sediment volume accumulated over a one-year period for the entire upstream watershed and an average normal water depth of at least four feet. The facility shall also have a live storage volume that meets the standard storage requirements of this section; or
(b) A separate off-line sediment basin shall be used with a volume appropriate for the tributary drainage area to the sediment basin.
(7) Impoundment of the stream as part of on-stream detention shall not prevent the migration of indigenous fish species, which require access to upstream areas as part of their life cycle, and shall not cause or contribute to the degradation of water quality or stream aquatic habitat. Compensatory storage requirements shall be satisfied and shall be in addition to detention volume requirements.
(8) No on-stream detention shall be allowed in areas designated as a critical wetland.
(Ord. O-14-03, passed 2-18-03)