(A) Release rates. Post-development discharge rates for new developments and redevelopments shall not exceed the existing condition discharge rates from the property for the two-year, ten-year, 25-year, 50-year, and 100-year critical storm event to discharge rates at or below those which existed prior to development. As a minimum to determine the critical storm, the one-hour, two-hour, three-hour, six-hour, 12-hour, and 24-hour storm events shall be analyzed. Additionally, the discharge from a stormwater detention facility shall not cause an increase in flooding or channel instability downstream when considered in aggregate with other developed properties and downstream drainage capacities.
(B) Design methodologies.
(1) Detention, along with green infrastructure, non-structural and structural BMPs, are recommended for projects involving more than half an acre of new impervious area.
(2) Green infrastructure and non-structural BMPs are required to control stormwater runoff for projects with less than one quarter of an acre of new impervious area.
(C) Detention basin outlet design.
(1) The detention basin outlet control structure shall be designed to account for observed or anticipated downstream tail water elevations. The tailwater elevations used in the detention model shall be for the particular storm frequency being routed through the detention basin.
(2) An emergency spillway or overflow device shall be provided and set at an elevation equivalent to the 100-year design high water.
(a) The maximum high water elevation shall be computed based on a routing of the design storm (100-year critical storm event or 24-hour event, whichever produces the higher pond elevation) assuming the low flow outlet is blocked with high water elevation at the overflow structure's sill.
(b) A minimum freeboard of one foot shall be provided above the design high water elevation. The freeboard should be measured from the design high water elevation to the top of the berm.
(c) The overflow path shall not be located on top of the outlet pipe.
(3) The outflow pipe shall be sized for the developed flow rate.
(4) Control structures and overflow structures are to be reinforced concrete, including precast. Other approved materials may be allowed only for underground detention with the approval of the City Engineer.
(5) 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 12 inches. If this minimum orifice size permits release rates greater than those specified in this section, and regional detention is not a practical alternative, outlets, structures such as perforated risers or multi-stage flow control orifices/weirs shall be used. For example, in smaller basins a smaller rectangular or V-notch weir could be used to control discharge with the approval of the City Engineer. The minimum area for an individual control device in a multi-stage control is 12.56 square inches (equivalent to a four-inch circular pipe).
(D) General detention pond requirements.
(1) Pool elevation for ponds. The limits of maximum ponding in dry basins or ponds and permanent lakes or ponds shall not be closer than 30 feet horizontally to any building, and not less than two feet vertically between the lowest sill elevation of any building.
(2) Pool Elevation for parking lots. The limits of maximum ponding in parking lots shall not be closer than ten feet horizontally from any building and not less than one foot vertically below the lowest sill elevation of any building. Parking lots used for automobiles shall have a maximum depth of six inches of water.
(3) Detention basin elevations:
(a) If the detention basin discharges to a piped sewer system, the low elevation of the detention basin shall be above the 25-year, critical storm hydraulic elevation of the receiving storm systems, as applicable.
(b) If the detention basin discharges to an open channel, or to a piped sewer system affected by flood levels in a nearby downstream open channel, then the low elevation of the basin is desirable to be above the 100-year flood elevation in the open channel as established by the FEMA Flood Insurance Study.
(c) In all cases mentioned above, if the low elevation of the basin is below the receiving system hydraulic grade or channel flood elevation, then the basin shall be sized to store the entire design storm volume, unless otherwise approved by the City Engineer.
(4) The maximum side slopes for dry basins or ponds, and the fluctuating area of permanent ponds or lakes shall be three to one (three feet horizontal, one foot) vertical without fencing.
(5) Dry basins or ponds and the fluctuating areas of permanent ponds or lakes are to be appropriately vegetated to the maximum high water elevation. Areas above that elevation shall be appropriately stabilized and vegetated. Sod and mowing above that elevation may be approved and is required for dam embankment slopes and downstream toe areas for wet basins where riprap is not appropriate.
(6) Permanent erosion control protection must be provided at the ends of discharging pipes or swales in which the storm event velocity exceeds two feet per second. Stone riprap, scour stop, or appropriate vegetation are required, unless otherwise approved.
(7) In basins with concrete walls or riprap covered slopes, provisions should be made for mowing equipment to reach the bottom (ramps or similar).
(8) Railroad tie walls cannot be used where water will be in contact with the railroad tie wall.
(9) Detention basin fencing. A four-foot (minimum) fence shall be provided around the perimeter of any basin where the side slopes exceed three to one (three feet horizontal, one foot vertical). Fencing material shall be approved by the city and shall NOT consist of post and rail or other fencing which prevents easy observation of required detention basin maintenance. For example, privacy fencing should not be used.
(10) Existing depressional storage volume will be maintained and the volume of detention storage provided to meet the requirements of this chapter shall be in addition to existing storage.
(11) Detention on prime farm land. The placement of detention basins shall avoid the utilization of prime farmland. All detention basin construction shall examine potential impacts to adjacent agricultural land and shall address measures that will be implemented to eliminate the impacts.
(12) Basin inlet and outlet orientation. The distance between detention inlets and outlets shall be maximized. Inlets and outlets shall be at opposite ends of the basin, providing that the orientation does not create undue hardship based on topography or other natural constraints. There shall be no low flow bypass between the inlet and outlet, and paved low flow channels shall not be used.
(13) Any development involving the construction, modification, or removal of a dam as defined in 17 Ill. Admin. Code Part 3702 (rules of construction of dams) shall obtain an IDNR/OWR dam safety permit or a letter stating no permit is required prior to the start of such activity.
(14) Infiltration Basins. Infiltration basins may be used as detention facilities subject to the following:
(a) The basin must be designed to dewater within 48 hours following the end of the 100-year critical duration storm event.
(b) The underlying soils must have an infiltration rate of at least 0.5 inches/hour as determined by an engineer.
(c) Pretreatment facilities mush be provided to prevent obstruction.
(d) The basin must be at least 200 feet away from any water supply wells.
(e) Runoff from the areas that have water quality concerns or are subject to frequent winter deicing must not be routed to the infiltration facility.
(f) The bottom of the infiltration basin must be at least four feet above the seasonal high groundwater elevation.
(g) Infiltration BMPs shall be selected and located based on suitability of soil and shall be constructed where a minimum depth of 24 inches between the bottom of the facility and the infiltration horizon exists. Soil tests, taken at the same location that the BMP will exist, shall demonstrate a stabilized infiltration rate.
(h) Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during construction.
(i) Infiltration BMPs shall not receive runoff from disturbed areas until the entire contributory drainage area is stabilized with vegetation.
(j) It is ideal for roof drains and sump pumps to be tributary to infiltration or vegetative BMPs. Catchment facilities for water re-use is also permitted.
(15) As-built survey.
(a) An as-built survey of the detention basin showing the final constructed slopes, storage volumes, and outlet structure configuration shall be submitted to the City Engineer for approval prior to the issuing of any building permits.
(b) As part of the as-built survey, a monument shall be set to reference the predicted detention basin high water elevation. The monument should be labeled “100-year storm high water” and should be graded to the proper elevation and be located on a common ground near the pond outlet pipe.
(16) Pond ownership. Residential detention/retention basins shall be located on outlots maintained by the homeowners association. Commercial or industrial basins shall be located in a regional outlot owned by a corporation or on the developed lot and maintained by the commercial property owner. A special service area may be created for potential maintenance by the city should the homeowners' association or corporation become defunct, unable, or unwilling to provide sufficient maintenance of the detention/retention basin(s).
(E) Wet detention basin design (retention). In addition to the other requirements of this subchapter, 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.
(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 eight feet deep over 25% of the bottom area to prevent winter kill.
(2) Wet basin shoreline slopes. The side slopes of wet basins at the normal pool elevation shall not be steeper than three to one (horizontal to vertical). It is recommended that aquatic vegetation be established around the perimeter to provide protection from shoreline erosion. For basins in excess of five acres, riprap shoreline protection shall be provided.
(3) Permanent pool volume. The permanent pool volume in a wet basin at normal depth shall, at a minimum, be equal to the runoff volume from its watershed for the two-year, 24-hour event (calculated during dry weather conditions).
(4) Permanent pool depth. Permanent retention ponds or lakes are to be designed to minimize fluctuating lake levels. Maximum fluctuation from the permanent pool elevation to the maximum ponding elevation shall be three feet.
(5) Soil borings. To ensure soils are conducive to impound water, if a retention (i.e., wet) pond is proposed, a geotechnical report with soil borings is required.
(F) Dry detention basin design. In addition to the other requirements of this subchapter, 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. Paved low flow channels may be used in a dry basin, provided provisions are made to prevent ponding.
(1) Dry basin drainage. Dry basins shall be designed so that 80% of their bottom area shall have standing water no longer than 72 hours for any runoff event less than the 100-year event. Grading plans shall clearly distinguish the wet portion of the basin bottom. Underdrains directed to the outlet may be used to accomplish this requirement.
(2) Velocity dissipation. Velocity dissipation measures shall be incorporated into dry basin designs to minimize erosion at inlets and outlets and to minimize resuspension of pollutants.
(3) Maximum depths. The maximum depth of water in a dry detention basin or pond shall not exceed four feet. Projects which need a deeper basin to attain the required detention volume due to physical constraints may be evaluated on a case-by-case basis. The design and construction of dams greater than six feet or as directed by the city must be sealed and certified by a professional engineer registered in the state with demonstrated expertise in geotechnical engineering.
(4) Flow through the pond. All ends of pipes discharging into a dry basin or pond shall be conveyed to the low flow pipe or control structure, by means of a concrete swale or permeable swale meeting the following criteria.
(a) Vegetated swales shall be a minimum four to one lateral (25%) slope to the center and a minimum 2% longitudinal slope to ensure positive drainage. Permeable swales shall be a minimum of six inches deep and four feet wide or 1.3 times the diameter of the pipe entering the basin, whichever is greater. The bottom of the detention basin shall be sloped a minimum of 2% towards the edge of the swale.
(b) Reinforced concrete paved swales according to the typical details are required to convey swales with a slope less than 1%.
(G) Underground detention basin design.
(1) Adequate access for basin maintenance and inspection shall be provided. A means of visual inspection from the ground surface of the low flow device, overflow weir and outlet structure is necessary. Access shall also be provided to allow for cleaning of the low flow device from the ground surface.
(2) Underground basins shall be acceptable for non-residential projects only.
(3) Acceptable materials for underground basins are poured-in-place or precast reinforced concrete, RCP, HDPE, and aluminized CMP. The CMP gauge shall be approved by the city prior to installation.
(4) Provide immediate manhole access from ground surface for both sides of the low flow device. Also, provide a manhole at upstream end of underground basins, for access, inspection, to facilitate maintenance and air release.
(5) Adequate flow line spot elevations, sections and profiles including pipe length and slope shall be labeled to define basin and pipe geometry.
(6) Structural pretreatment or pretreatment chambers are required immediately upstream of an underground detention facility to collect debris and grime prior to allowing it to enter the detention chamber.
(H) Detention in floodways. The placement of detention basins within the floodway is prohibited.
(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 stream-flow 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 existing ten-year flood elevation shall be replaced below the existing ten-year elevation. All floodplain storage lost above the existing ten-year flood elevation shall be replaced above the existing ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(2) 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 100-year 24-hour events from the property. If on-stream detention is used in watersheds larger than one square mile, the applicant will use hydrographic modeling to demonstrate that the design will not increase the water level 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 not occur downstream of a wastewater discharge; and
(f) Shall not contribute to the duration or flood frequency of any adjacent land.
(I) Detention in wetlands, rivers, streams, lakes, ponds or depressional storage areas.
(1) Existing wetlands, rivers, lakes, ponds or depressional storage areas shall not be modified for the purposes of stormwater detention unless it is demonstrated that the proposed modifications will maintain or improve its habitat and ability to perform beneficial functions and shall comply with § 156.006.
(2) Existing storage and release rate characteristics of wetlands, rivers, lakes, ponds or depressional storage areas shall be maintained and the volume of detention storage provided to meet the requirements of this chapter shall be in addition to this existing storage.
(3) Alteration of drainage patterns. Site drainage patterns shall not be altered to substantially decrease or increase the existing area tributary to the wetlands, rivers, lakes, ponds or depressional storage areas.
(4) 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, river, lake, pond or depressional storage area. This basin shall be constructed before property grading begins and shall be maintained throughout the construction process.
(J) Street detention, parking lot detention.
(1) Street detention. Streets are not to be used as part of the detention system.
(2) Parking lot detention. The maximum stormwater ponding depth in any parking area shall not exceed six inches for more than four hours.
(K) Buffer Areas. Buffer areas shall be required for all areas defined as Waters of the United States (WOTUS). The buffer areas for all WOTUS shall extend landward from the ordinary high water mark The buffer area for jurisdictional or mitigated wetlands shall extend from the edge of the delineated wetland. A property may contain a buffer area that originates from WOTUS on another property. Buffer area are divided into two types: linear buffers and water body buffers.
(1) Linear buffers. A 30-foot-wide minimum buffer shall be designated along both sides of all channels meeting the definition of WOTUS.
(2) Water body buffers. Water body buffers shall encompass all non-linear bodies of water meeting the definition of WOTUS including wetlands, lakes, and ponds.
(a) For all water bodies with a total surface area less than 2.5 acres, a minimum buffer width of 30 feet shall be established.
(b) For all water bodies with a total surface area greater than 2.5 acres, a minimum buffer width of 40 feet shall be established.
(3) Easements. Buffer areas shall be located within special easements or covenants for use by the managing district, local unit of government, homeowners association, or other entities responsible for the adjacent stormwater facilities, ponds, lakes, or channels.
(4) Development within a buffer area. All buffer areas shall be maintained free from development including disturbance of soil, dumping or filling, erection of structures, and placement of impervious surfaces except as follows:
(a) Passive recreation (e.g., birdwatching, picnicking).
(b) Trails running parallel to the buffer. Trails should be no wider than ten feet and the runoff from such facilities is diverted away from the WOTUS or managed before flowing to the WOTUS.
(c) Minor structures relating to parks and recreation uses less than 300 square feet.
(d) Utility structures including drainage facilities, except private sewer collection systems such as septic systems shall not be constructed within the buffer areas.
(e) Boat docks, ramps, piers, and beaches.
(f) Unimproved access through buffer areas for maintenance purposes.
(L) Infiltration practices.
(1) Generally. To effectively reduce runoff volumes, infiltration practices including basins, trenches, and porous pavement shall be located in hydrologic soil groups “A” and “B” as designated by the USDA Natural Resources Conservation Service. Infiltration basins and trenches designed to recharge groundwater shall not be located within 75 feet of a water supply well or 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 the dry basin’s bottom area for the maximum design event to be exfiltrated. The bottom of infiltration basins or trenches shall be a minimum of four feet above the seasonally high groundwater and bedrock level. Engineering calculations demonstrating infiltration rates shall be included with the application.
(2) Vegetated filter strips and swales. To effectively filter stormwater pollutants and promote infiltration of runoff, sites should be designed to maximize the use of vegetated filter strips and swales. Whenever practicable, runoff from impervious surfaces should be directed onto filter strips and swales comprised of native grasses and forbs before being routed to a storm sewer or detention basin. Vegetated filter strips and swales should be designed such that the time of concentration within the area is at least ten minutes.
(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 the 100-year, 24-hour pool shall be as level as practicable to prevent accidental falls into the basin and for stability and ease of maintenance.
(2) Safety ledge. All wet detention basins shall have a level safety ledge at least four feet in width, two to three feet below the normal water depth, or it must be protected by an enclosed fence at least 48 inches in height
(3) Velocity. Velocities throughout the surface drainage system shall be controlled to safe levels (less than 10 fps) taking into consideration rates and depths of flow.
(N) Accommodating flows from upstream tributary areas. Stormwater runoff from areas tributary to the property shall be considered in the design of the property's drainage system. Flows from upstream areas that are not to be detained should be routed around the basin being provided for the site being developed, unless approved by the City Engineer.
(1) Upstream areas not meeting ordinance requirements. When there are areas not meeting the storage and release rates of this chapter tributary to the applicant's property or there are known downstream flooding issues, regionalized detention on the applicant's property shall be explored by the applicant or the city. When it is deemed beneficial by the city or the applicant to explore such a design, the following steps shall be followed:
(a) The applicant shall compute the storage volume needed for his or her property;
(b) Areas tributary to the applicant's property, not meeting the storage and release rate requirements of this chapter, shall be identified; and
(c) Using the areas determined above plus the applicant's property area, total storage needed for the combined properties shall be computed.
(2) Allowable release rates and storage shall be computed using the combined property areas. If tributary areas are not developed, a reasonable fully developed land cover, based on local zoning, shall be used for the purposes of computing storage.
(3) Once the necessary combined storage is computed, the city may choose to pay for over-sizing the applicant's detention basin to accommodate the regional flows. If regional storage is selected by the city, then the applicant's responsibility will be limited to the storage for his or her property as computed above. If regional storage is rejected by the city, the applicant shall bypass all tributary area flows around the applicant's basin whenever practicable. If the applicant must route upstream flows through his or her basin and the upstream areas exceed one square mile in size, the applicant must meet the provisions of division (H)(2) above for on-stream basins.
(4) Upstream areas meeting ordinance requirements. When there are areas which meet the storage and release rate requirements of this chapter, tributary to the applicant's property, the upstream flows shall be bypassed around the applicant's detention basin if this is the only practicable alternative. However, if the city decides to route tributary area flows through an applicant's basin, the final design stormwater releases shall be based on the combined total of the applicant's property plus tributary areas.
(O) Early completion of detention facilities. Where detention, retention or depressional storage areas are to be used as part of the drainage system for a property, they shall be constructed as the first element of the initial earthwork program. Any eroded sediment captured in these facilities shall be removed by the applicant on a regular basis and before project completion in order to maintain the design volume of the facilities.
(P) Maintenance considerations. The stormwater drainage system shall be designed to minimize and facilitate maintenance. Turfed side slopes 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. Pre-sedimentation basins shall be included, where feasible, for localizing sediment deposition and removal. Site access for heavy equipment shall be provided.
(Ord. 2017-03, passed 3-6-2017)