§ 55.023  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 § 55.004, 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 this section. Unless exempted in this section, 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.
   (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. 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.
      (2)   When the modified rational method is used, rainfall statistics shall be from the Illinois State Water Survey Updated Bulletin 70 (March 2019) Northeast Sectional Code and the volume determined shall be multiplied by the following factor based on the date of final engineering approval:
         (a)   In the first year after the effective date of the chapter: 110%;
         (b)   In the second year after the effective date of the chapter: 120%; and
         (c)   After the third year following the effective date of the chapter: 130%.
   (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 0.75-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 Administrator 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 that required to empty the calculated extended detention volume within five days of the storm event.
         (b)   No subsurface drainage pipe shall be located within ten feet of drainage pipes directly connected to the detention basin.
         (c)   For purposes of meeting the maximum subsurface drainage discharge requirements, flow control orifices and weirs may be used.
         (d)   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.
         (e)   Farm field tile shall not be considered a subsurface drainage system.
   (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 design 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 Administrator, 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.
      (7)   Storage facilities with single pipe outlets shall have a minimum inside diameter of 12 inches. If design release rates necessitate a smaller outlet, structures such as perforated risers or flow control orifices shall be used.
   (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 §§ 55.060 through 55.066 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;
      (3)   Detention volume provided by enlarging existing regulatory floodplain storage without providing a structure controlling discharge (on-stream detention) will be allowed only as a variance. The applicant must demonstrate that flood damages are not increased and the development will not increase flood flows for both the two-year and 100-year floods on the stream with developed conditions on the site; and
      (4)   The Administrator 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 a through c above.
   (H)   Site runoff storage facility requirements within the regulatory floodway. Storage facilities located within the regulatory floodway shall:
      (1)   Meet the requirements for locating storage facilities in the regulatory floodplain;
      (2)   Be evaluated by performing hydrologic and hydraulic analysis consistent with the standards and requirements for any adopted watershed plans; and
      (3)   Provide a net watershed benefit.
   (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, §§ 55.020 through 55.024.
      (2)   Adequate storage capacity in the off-site facility is dedicated to the development.
      (3)   The development includes means to convey stormwater to the off-site storage facility.
   (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.
(Res. 02-441, passed 10-17-2002; Res. 02-495, passed 11-21-2002; Res. 04-87, passed 3-17-2004; Ord. 19-239, passed 9-19-2019)