§ 164.023  SITE RUNOFF STORAGE REQUIREMENTS (DETENTION/EXTENDED DETENTION).
   (A)   Release rate.
      (1)   If no release rate is specified in an adopted watershed plan in accordance with § 164.003, then sufficient flood storage shall be provided so that the site will not discharge at a rate greater than 0.15 cfs/acre of development during and after a rainfall event with a 100-year frequency except for sites exempted in § 164.020(C). Unless exempted in § 164.020(C), sites shall not discharge at a rate greater than 0.04 cfs/acre of development during and after a rainfall event with a two-year frequency.
      (2)   This area of hydrologic disturbance on the site shall be used to calculate the required site runoff storage volume. The on-site watershed area tributary to the point of discharge shall be used to calculate the allowable release rate for the site runoff storage facility, which shall be the maximum release rate allowed considering only the on-site watershed area runoff.
      (3)   In the event the downstream creeks, streams, channels, conduits or other drainage facilities are inadequate to receive the release rate herein above provided and there is no adopted watershed plan available, then the allowable release rate shall be reduced to that rate permitted by the receiving downstream creeks, streams, channels, conduits or other drainage facilities; and additional detention volume shall be required to store that portion of the runoff exceeding the capacity of the receiving drainage facilities.
   (B)   Design methods.
      (1)   Event hydrograph routing methods or the modified rational method may be used to calculate design runoff volumes for site runoff facilities. The methods must be HEC-1, (SCS methodology), HEC-HMS, TR-20 or TR-55 tabular method.
      (2)   Event methods shall incorporate the following assumptions:
         (a)   Antecedent moisture condition equals two;
         (b)   Appropriate Huff rainfall distribution; and
         (c)   Twenty-four-hour duration storm with a 1% probability (100-year frequency) of occurrence in any one year as specified by data from Illinois State Water Survey Updated Bulletin 70 (March 2019) Northeast Sectional Code.
   (C)   Existing release rate less than allowable.  For sites where the undeveloped release rate is less than the maximum release rate in division (A) above, the developed release rate and corresponding site runoff storage volume shall be based on the existing undeveloped release rate for the development.
   (D)   Downstream water surface elevations. All hydrologic and hydraulic computations must utilize appropriate assumptions for downstream water surface elevations, from low flow through the base flood elevation, considering the likelihood of concurrent flood events.
   (E)   Extended detention requirement.
      (1)   The requirements of this section will apply only when an existing agricultural land use is downstream of and adjacent to a site runoff storage facility outlet. The runoff from a three-quarters-inch rainfall event over the hydraulically connected impervious area of the new development shall be stored below the elevation of the primary gravity outlet (extended detention) of the site runoff storage facility. The facility may be designed to allow for evapotransporation or infiltration of this volume into a subsurface drainage system and shall not be conveyed through a direct positive connection to downstream areas.
      (2)   The hydraulically connected impervious area used in the calculation of required extended detention volume may be reduced by the Chief Subdivision Engineer if the soils are prepared to maximize infiltration and deep rooted grasses or other plants selected for their ability to promote infiltration or water absorption are planted in areas appropriately dedicated. The reduction in hydraulically connected impervious area used in the calculation shall be equal to the area of the development meeting the above soils/native planting requirement.
      (3)   Subsurface drainage systems may be designed as a component of the extended detention portion of the detention basin to assist in infiltration in accordance with the following criteria.
         (a)   The extended detention volume shall be discharged at a rate no greater than (no earlier than) that required to empty the calculated extended detention volume within five days of the storm event.
         (b)   For purposes of meeting the maximum subsurface drainage discharge requirements, flow control orifices and weirs may be used.
         (c)   All design extended detention volume shall be provided above the seasonal high ground water table or the invert elevation of the ground water control system.
         (d)   Farm field tile shall not be used as a component of the extended detention system, unless evaluated per § 164.022(D).
   (F)   Site runoff storage facility design requirements.  Storage facilities shall be designed and constructed with the following characteristics.
      (1)   The site runoff storage facility shall provide one foot of freeboard above the calculated (blocked restrictor) high water elevation.
      (2)   The storage facilities shall be accessible and easily maintained.
      (3)   Storage facilities shall facilitate sedimentation and catchment of floating material. Unless specifically approved by the Chief Subdivision Engineer, concrete lined low-flow ditches shall not be used in detention basins.
      (4)   Storage facilities shall minimize impacts of stormwater runoff on water quality by incorporating best management practices.
      (5)   Storage facilities shall maximize the normal flow distance between detention inlets and outlets, to the extent possible.
      (6)   Storage facilities shall be designed so that the existing conditions pre-development peak runoff rate from the 100-year, critical duration rainfall will not be exceeded assuming the primary restrictor is blocked. One foot of freeboard shall be provided over the blocked restrictor water level.
      (7)   Storage facilities with single pipe outlets shall have a minimum inside diameter of 12 inches. Where flow control orifices are used, the minimum diameter is four-inch. If design release rates necessitate a smaller outlet, structures such as perforated risers or other self-cleaning restrictors shall be used.
      (8)   Basin side slopes should not be steeper than four to one (horizontal to vertical); five to one side slopes are preferred. For wet-bottom basins, side slopes not steeper than two to one may be used below the safety ledge.
      (9)   Wet-bottom storage facilities must include a safety shelf, minimum six feet width, two and one-half feet to three feet below the normal water line.
      (10)   The permanent pool volume in wet-bottom basins shall be at least equal to the two-year, 24-hour runoff volume from the tributary watershed. The minimum permanent pool depth is three feet, excluding safety shelves. If fish habitat is to be provided, over 25% of the bottom area must be at least ten feet deep.
      (11)   Dry bottom detention basins shall be graded with not less than 1% slope across basin floor.
      (12)   All detention basins shall be provided with an overflow structure capable of passing pre- development peak runoff rate capable of meeting the requirements of division (F)(6) above.
      (13)   All detention basins discharging at grade shall discharge a minimum of 20 feet from any property line unless an agreement allowing discharge closer to the property line is secured from the adjacent property owner or road authority. Vegetative buffers, level spreaders or other appropriate BMPs shall be utilized at the outlet structure to promote spreading and infiltration of the discharge.
      (14)   Riprap or other approved method of dissipating energy shall be utilized at all end sections and point discharge locations.
      (15)   Standards for naturalized detention basins include: flat side slopes (eight to one from two- year water level to one-half foot below normal water level), shallow zones of emergent vegetation at water’s edge, combination of natural vegetation and open water area. Uses of aerators, cascades, water falls and the like are encouraged. A planting/ maintenance shall be submitted and approved by the Chief Subdivision Engineer.
   (G)   Site runoff storage facility requirements within the regulatory floodplain. Storage facilities located within the regulatory floodplain shall:
      (1)   Conform to all applicable requirements specified in §§ 164.060 through 164.067 of this chapter;
      (2)   Store the required amount of site runoff to meet the release rate requirement under all stream flow and backwater conditions in the receiving stream up to the ten-year flood elevation; and
      (3)   The Chief Subdivision Engineer may approve designs which can be shown by detailed hydrologic and hydraulic analysis to provide a net watershed benefit not otherwise realized by strict application of the requirements in division (G)(1) and (G)(2) above.
   (H)   Site runoff storage facility requirements within the regulatory floodway. Site runoff storage facilities shall not be located within the regulatory floodway.
   (I)   Off-site facilities.  Site runoff storage facilities may be located off-site if the following conditions are met.
      (1)   The off-site storage facility meets all of the requirements of this subchapter, §§ 164.020 through 164.025.
      (2)   Adequate storage capacity in the off-site facility is dedicated to the development and placed in an appropriate easement on the off-site property.
      (3)   The development includes means to convey stormwater to the off-site storage facility. The conveyance system shall be placed in an appropriate easement on the off-site property.
   (J)   Cross-stream structures for site runoff storage facilities. Structures constructed across the channel to impound water to meet detention requirements shall be prohibited on any perennial stream unless part of a public flood control project with a net watershed benefit. Those streams appearing as blue on a USGS quadrangle map shall be assumed perennial unless better data is obtained. All cross- stream structures for the purpose of impounding water to provide detention in all cases on perennial and intermittent streams must demonstrate that they will not cause short term or long-term stream channel instability.
(Ord. 10-164, passed 6-17-2010; Ord. 19-238, passed 9-19-2019)