(A) Release rates.
(1) The drainage system for a property shall be designed to control the peak rate of discharge from the property for the two-year, 24-hour and 100-year, critical duration events to levels which will not cause an increase in flooding or channel instability downstream when considered in aggregate with other developed properties and downstream drainage capacities. The peak discharge from events less than or equal to the two-year event shall not be greater than 0.04 cfs per acre of property drained. The peak 100-year discharge shall not be greater than 0.15 cfs per acre of property drained.
(2) Detention basin outlet design. Backwater on the outlet structure from the downstream drainage system shall be evaluated when designing the outlet.
(B) Detention storage requirements.
(1) The design maximum storage to be provided in a detention basin shall be based on the runoff from the 100-year, critical duration event. The design of detention basins for development sizes larger than five acres shall be by runoff hydrograph methods described below. The design of detention basins for development sizes smaller than five acres shall be by the method described in Section 6 of the "Investigation of Hydrologic Design Methods for Urban Development in Northeastern Illinois," dated December 1991 prepared by the Northeastern Illinois Plan Commission, (See Appendix C following this chapter).
(2) Extended detention requirement: When an existing agricultural land use is downstream of and adjacent to a site with a proposed storm water outlet facility, and the agricultural land is not proposed or planned for development, the runoff from a 0.75 inch rainfall event, over the hydraulically connected impervious area of the new development, shall be managed on site in conformance with the Will County Storm Water Ordinance.
(C) Drainage system design and evaluation. The following criteria are to be used in evaluating and designing the drainage system. The underlying objective is to provide capacity to pass the 10-year peak flow in the minor drainage system and an overland flow path for the 100-year peak flows
(1) Design methodologies. Major and minor conveyance systems for areas up to five acres may be designed using the rational formula. The rational formula may also be used in sizing the minor drainage system for larger sites. Runoff hydrograph methods as described in § 158.015.04 must be used for major drainage system design for all systems with greater than five acres of drainage area. All drainage system design shall be in accordance with Part 708 Rules.
(2) Positive drainage. Whenever practicable, all areas of the property must be provided an overland flow path that will pass the 100-year flow at a stage at least one foot below the lowest foundation grade in the vicinity of the flow path. Overland flow paths designed to handle flows in excess of the minor drainage system capacity shall be provided in drainage easements, which cannot be obstructed. Street ponding of one inch is allowed only in the overflow path.
(D) Methods for generating runoff hydrographs. Runoff hydrographs shall be developed using the soil conservation service computer model TR 20 or Pond Pack as developed by Haestad Methods, Inc. If an existing regulatory model uses the Corps. of Engineers computer model HEC-RAS, then that model can be used. A continuous simulation model is acceptable only for very complex projects. No other models are acceptable unless approved by the Village Engineer.
(1) Rainfall. Unless a continuous simulation approach to drainage system hydrology is used, all design rainfall events shall be based on the Illinois State Water Survey's Bulletin 71. A critical duration analysis for the two and 100-year storms of 1, 2, 3, 6, 12, 18, 24 and 48 hours duration shall be prepared in accordance with generally accepted engineering principles. The first quartile Huff rainfall distribution shall be used for the design and analysis of detention and major conveyance systems with critical durations less than or equal to six hours. Their second quartile Huff rainfall distribution shall be used for the design and analysis of detention basins and major conveyance systems with a critical duration greater than six and less than or equal to 12 hours. The third quartile Huff rainfall distribution shall be used for the design and analysis of detention basins and major conveyance system with critical durations greater than 12 and less than or equal to 24 hours. The fourth quartile Huff distribution shall be used in the design and analysis of detention basins and major conveyance systems with durations greater than 24 hours. The first, second, third, and fourth quartile distributions described by Huff are presented in Table 1. The rainfall data to be used for the critical duration analysis and all other appropriate hydrologic calculations are present as Table 2.
(2) Antecedent moisture. Computations of runoff hydrographs which do not rely on a continuous accounting of antecedent moisture conditions shall assume an antecedent moisture condition of two as a minimum.
(E) The village endorses the utilization of the Best Management Practices (BMP) regarding the design of a Storm Water Management System. The primary objectives of a BMP are minimize adverse water quality impacts; preserve beneficial natural features on-site; avoid downstream erosion and habitat loss; maintain natural base flows and groundwater recharge; prevent increases to downstream flooding; provide multiple uses of drainage and storm water storage facilities; and provide for the economical, safe and aesthetically pleasing drainage system for development. BMP methods have been published by the Northeastern Illinois Planning Commission (NIPC) under the following titles: "Designing Stormwater BMP in Northeastern Illinois," "Urban Stormwater BMP for Northeastern Illinois," "Restoring and Managing Stream Greenways: A Landowners Handbook;" and Chicago Wilderness "Biodiversity Recovery Plan;" These, as well as other sources, are recognized BMP which are to be considered in the design of a stormwater management system.
(F) Wet detention basin design. Wet detention basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing, and as much as feasible to be available for recreational use. A professional shall prepare a detailed landscaping plan.
(1) Wet basin depths. Wet basins shall be at least three feet deep, excluding near shore banks and safety ledges. If fish habitat is to be provided they shall be at least ten feet deep over 25% of the bottom area to prevent winter freeze-out.
(2) Wet basins minimum area. All ponds shall have a minimum area of one acre at the normal water level.
(3) Wet basin aeration. Wet basins shall require mechanical aeration unless waived at the discretion of the Village Engineer.
(4) Wetland detention basin design shall be prepared by a wetland designer/ ecologist approved by the U.S. Army Corps of Engineers.
(5) The planting materials for the pond side slope above the normal water level shall be selected with consideration of survival during periods of inundation. Planting materials will be reviewed and deemed suitable by the village.
(6) Wet basin shoreline slopes. The side slopes of wet basins above the normal pool elevation shall not be steeper than 5 to 1 (horizontal to vertical). The side slopes below the normal pool elevation shall not be steeper than 2 to 1 (horizontal to vertical).
(7) Permanent pool volume. The permanent pool volume in a wet basin at normal depth shall be equal to the runoff volume from its watershed for the two-year event.
(8) Inlet and outlet orientation. To the extent feasible, the distance between detention inlets and outlets shall be maximized. If possible, they are to be at opposite ends of the basin.
(9) Stilling/sedimentation basins. A stilling/sedimentation basin shall be constructed at each major inlet to a wetland. The volume of the basins are to be at least 500 cubic feet per acre of impervious surface in the drainage area. Side slopes of the basins shall be no steeper than three horizontal to one vertical and basin depths shall be at least three feet to minimize resuspension of accumulated sediment.
(G) Dry detention basin design. In addition to the other requirements of this chapter, dry basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing and as much as feasible to be available for multiple uses.
(1) Dry basin design details. In order to prevent soil erosion and weed problems, dry detention basins must be landscaped including the establishment of a ground cover over all unpaved areas through sodding or other means which result in a quality of ground cover comparable to that obtained through sodding. Dry detention facilities shall be usable as active recreational areas during dry weather conditions. Dry detention facilities must be designed so that the cross slope is at least 2%. The bottom of the facility shall be provided with an underdrain or low flow channel that is subject to review and approval by the Village Engineer. The underdrain shall be a minimum six inch diameter perforated drain tile covered on all sides with a minimum of six inches of crushed stone conforming to ASTM C33, Size No. 67. The underdrain shall be installed to drain the basin below grade during periods of low flow and shall connect to a storm sewer outfall pipe. The low flow channel shall be concrete and have a width of four feet.
(2) Dry detention facilities shall be designed with side slopes not steeper than 4 to 1 (horizontal to vertical).
(3) Wetland and dry basin drainage. The bottom slope of all dry bottom ponds shall be a minimum of 2%. Underdrains directed to the outlet may be used to supplement this requirement. Grading plans shall clearly distinguish the wet/wetland portion of the basin bottom from the dry portion.
(4) Velocity dissipation. Velocity dissipation measures shall be incorporated into dry basin designs to minimize erosion at inlets and outlets and to minimize the resuspension of pollutants.
(5) Wetland basin inlet and outlet orientation. To the extent feasible, the distance between detention inlets and outlets shall be maximized. If possible, they are to be at opposite ends of the basin. There shall be no low flow bypass between the inlet and outlet and paved low flow channels shall not be used.
(H) 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 2 ½inches. If this minimum orifice size permits release rates greater than those specified in this section, and regional detention is not a practical alternative, alternative outlet designs shall be utilized which incorporate self cleaning flow restrictor.
(I) Detention in floodplains. The placement of detention basins within the floodplain is strongly discouraged because of questions about their reliable operation during flood events. However, the stormwater detention requirements of this chapter may be fulfilled by providing detention storage within flood fringe areas on the project site provided the following provisions are met.
(1) Detention in flood fringe areas. The placement of a detention basin in a flood fringe area shall require compensatory storage for 1.5 times the volume below the base flood elevation occupied by the detention basin including any berms. The release from the detention storage provided shall still be controlled consistent with the requirements of this section. The applicant shall demonstrate its operation for all streamflow and floodplain backwater conditions. Excavations for compensatory storage along watercourses shall be opposite or adjacent to the area occupied by detention. All floodplain storage lost below the ten-year flood elevation shall be replaced below the ten-year flood elevation. All floodplain storage lost above the existing ten-year flood elevation shall be replaced above the proposed ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(2) Detention in floodways. Detention basins shall be placed in the floodway only in accordance with this § 158.052.
(3) On-stream detention. On-stream detention basins are discouraged but allowable if they provide regional public benefits and if they meet the other provisions of this chapter with respect to water quality and control of the two-year and 100-year events from the property. If on-stream detention is used for watersheds larger than one square mile, it is recommended that the applicant use dynamic modeling to demonstrate that the design will not increase flood elevations for any properties upstream or downstream of the property. Also, impoundment of the stream as part of on-stream detention:
(a) Shall not prevent the migration of indigenous fish species, which require access to upstream areas as part of their life cycle, such as for spawning,
(b) Shall not cause or contribute to the degradation of water quality or stream aquatic habitat,
(c) Shall include a design calling for gradual bank slopes, appropriate bank stabilization measures, and a pre-sedimentation basin,
(d) Shall not involve any stream channelization or the filling of wetlands,
(e) Shall require the implementation of an effective non-point source management program throughout the upstream watershed,
(f) Shall not occur downstream of a wastewater discharge, and
(g) Shall comply with 92 Illinois Administrative Code Parts 702 and 708 and the floodplain ordinance of Village of Peotone.
(h) The applicant must show by a detailed engineering analysis that onstream detention provides for all storms of critical duration water surface elevations and discharges less than or equal to an off-stream detention basin constructed in accordance with this chapter.
(i) An IDNR Dam Safety Permit shall be obtained stating the berm or control structure meets the Class III criteria of the IDNR/DWR. If such a permit is not required, a signoff letter from IDNR shall be provided.
(4) Protection of wetlands and depressional storage areas. Wetlands and other depressional storage areas shall be protected from damaging modifications and adverse changes in runoff quality and quantity associated with land developments. In addition to the other requirements of this chapter, the following requirements shall be met for all developments whose drainage flows into wetlands and depressional storage areas (as appropriate).
(5) Detention in wetlands and depressional storage areas. Existing wetlands shall not be modified for the purposes of stormwater detention unless it is demonstrated that the existing wetland is low in quality and the proposed modifications will maintain or improve its habitat and ability to perform beneficial functions. Existing depressional storage and release rate characteristics of wetlands and other depressional storage areas shall be maintained and the volume of detention storage provided to meet the requirements of this section shall be in addition to this existing storage.
(6) Sediment control. The existing wetland shall be protected during construction by appropriate soil erosion and sediment control measures and shall not be filled.
(7) Alteration of drainage patterns.
(a) Site drainage patterns shall not be altered to substantially decrease or increase the existing area tributary to the wetland.
(b) Detention/sedimentation. All runoff from the development shall be routed through a preliminary detention/sedimentation basin designed to capture the two-year, 24-hour event and hold it for at least 24 hours, before being discharged to the wetland. This basin shall be constructed before property grading begins. In addition, the drainage hierarchy must be followed to minimize runoff volumes and rates being discharged to the wetland.
(8) Vegetated buffer strip. A buffer strip, preferably vegetated with native plant species, shall be maintained or restored around the periphery of the wetland. The width of the buffer strip shall be 25 feet for a tributary length of 250 feet or less. The width of the buffer strip shall be 50 feet for a tributary length greater than 250 feet and less than or equal to 500 feet. The width of the buffer strip shall be 75 feet for a tributary length over 500 feet. The tributary length is the maximum length of the project limits to the wetland area or the length from a drainage divide to the wetland area.
(J) Streets, parking lot, and culvert drainage.
(1) Streets. If streets are to be used as part of the minor or major drainage system, ponding depths shall not exceed curb heights except in areas of 100-year overland routes and shall not remain flooded for more than eight hours for any event less than or equal to the 100-year critical duration event.
(2) Parking lots. No ponding is allowed on any paved surfaces. All paved parking lot areas shall have at a minimum a slope of 1%.
(3) Culvert road and driveway crossings. Sizing of culvert crossings shall consider entrance and exit losses as well as tail water conditions on the culvert. Minimum size shall be 12-inch diameter.
(K) Drainage of open areas. All grassed areas shall have a minimum slope of 1% and a maximum slope of 5:1.
(L) Infiltration practices.
(1) To effectively reduce runoff volumes, infiltration practices including basins, trenches, and porous pavement should be located on soils in hydrologic soil groups "A" or "B" as designated by the U.S. Soil Conservation Service. Infiltration basins and trenches designed to recharge groundwater shall not be located within 75 feet of a water supply well or a building foundation. A sediment-settling basin 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 a dry basin's bottom area for the maximum design event to be ex-filtrated. The bottom of infiltration facilities shall be a minimum of four feet above seasonally high groundwater and bedrock.
(2) Vegetated filter strips and swales. To effectively filter stormwater pollutants and promote infiltration of runoff, sites are to be designed to maximize the use of vegetated filter strips and swales. Wherever practicable, runoff from impervious surfaces shall be directed onto filter strips and swales before being routed to a storm sewer or detention basin.
(M) Safety considerations. The drainage system components, especially all detention basins, shall be designed to protect the safety of any children or adults coming in contact with the system during runoff events.
(1) Side slopes. The side slopes of all detention basins at 100-year capacity shall 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 six to one (horizontal to vertical), except as allowed in § 158.052.
(2) Safety ledge. All wet detention basins shall have a level safety ledge at least 12 feet in width three feet below the normal water depth.
(3) Velocity. Velocities throughout the surface drainage system shall be controlled to safe levels taking into consideration rates and depths of flow.
(4) 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 inflow rate based on a fully developed condition of all upstream areas.
(N) Maintenance considerations. The stormwater drainage system shall be designed to minimize and facilitate maintenance. Turfed sideslopes shall be designed to allow lawn-mowing equipment to easily negotiate them. Wet basins shall be provided with alternate outflows, which can be used to completely drain the pool for sediment removal. (Pumping may be considered if drainage by gravity is not feasible.) Presedimentation basins shall be included, where feasible, for localizing sediment deposition and removal. Access for heavy equipment shall be provided.
(Ord. 04-02, § 158.015, passed 4-5-04)