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(A) The area of hydrological disturbance on the site shall be used to calculate the required site runoff storage volume. The tributary area of the site at the point of discharge shall be used to calculate the allowable release rates of the primary restrictor for the site runoff storage facility.
(B) The site runoff storage volume required shall be calculated based on maximum release rates of 0.15 cubic feet per second per acre for the 100-year, 24-hour storm event, and 0.04 cubic feet per second per acre for the two-year, 24-hour storm event, according to the methods identified in division (C) below.
(C) Event hydrograph routing methods, such as HEC-1, HEC-HMS, and TR-20 or TR-55 tabular method, using SCS curve number methodology, shall be used to calculate design runoff volumes for site runoff storage facilities. Event methods shall incorporate the following assumptions:
(1) Antecedent moisture condition equals two;
(2) Appropriate Huff rainfall distribution, except that SCS type II distribution is acceptable with TR-55 tabular method only; and
(3) Twenty-four-hour duration storm, with a 1% probability (100-year) of occurrence in any one year as specified by Bulletin 70, “Northeast Sectional Rainfall Statistics”.
(D) For sites where the undeveloped release rate is less than the maximum release rate of this section, the developed release rate and corresponding site runoff storage volume shall be based on the existing undeveloped release rate for the development site.
(E) Hydraulic computations for the release structure must assume backwater conditions up to the ten-year flood elevation on the adjacent receiving watercourse.
(F) Retention requirement.
(1) The runoff from a three-quarter-inch rainfall event over the hydraulically connected impervious area of a new industrial, commercial or multi-family development shall be stored below the elevation of the primary gravity outlet (retention) of the site runoff storage facility. The facility may be designed to allow for evapotranspiration 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 retention volume may be reduced by the village if the soils are undisturbed in situ or prepared to maximize infiltration and deep-rooted grasses or other plants that assist in the promotion of infiltration and transpiration 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/planting requirement.
(3) Subsurface drainage systems may be designed as a component of the retention portion of the site runoff storage basin to assist in infiltration in accordance with the following criteria:
(a) No such subsurface drainage pipe shall be located within ten feet of drainage pipes directly connected to the site runoff storage basin; and
(b) For purposes of meeting the maximum subsurface drainage requirements, flow control orifices and weirs may be incorporated into the design.
(G) Site runoff storage facilities shall be designed and constructed with the following characteristics.
(1) At least one foot of freeboard above the 100-year design water surface elevation shall be required within the storage facility.
(2) Storage facilities shall be accessible and easily maintained.
(3) All design site runoff storage volume shall be provided above the seasonal high ground water table or the invert elevation of the ground water control system.
(4) Storage facilities shall be designed to control sediment and floating debris. Unless specifically approved by the Administrator, concrete-lined, low-flow ditches shall not be used in site runoff storage basins.
(5) Storage facilities shall minimize the negative impacts of stormwater runoff into the waterquality by incorporating best management practices in the design of the facilities.
(6) Storage facilities shall maximize the distance between site runoff storage inlets and outlets to the extent possible.
(7) Storage facilities shall be designed so as not to exceed the existing pre-development peak runoff rate of the 100-year, critical-duration rainfall, when the primary restrictor is assumed to be blocked.
(8) Storage facilities with single-pipe outlets shall have a minimum inside diameter of 12 inches. If design release rates necessitate a smaller outlet to be constructed, perforated risers or flow control orifices shall be used to prevent smaller openings from clogging.
(H) Storage facilities located within the regulatory floodplain shall:
(1) Comply with this subchapter; and
(2) Store the required amount of site runoff to meet the release rate requirement under all stream flow and backwater conditions, up to the ten-year flood elevation on the adjacent receiving watercourse. The village may approve designs that can be substantiated through hydrologic and hydraulic engineering analysis to provide a new watershed benefit not otherwise realized by strict application of the requirements set forth in this division.
(I) Storage facilities located within the regulatory floodway will be allowed only as a variance and shall:
(1) Meet the requirements for locating storage facilities in the regulatory floodplain; and
(2) Be evaluated by performing hydrologic and hydraulic analysis consistent with the standards and requirements for watershed plans; and
(3) Provide a net watershed benefit.
(J) Site run-off facilities may be located offsite if:
(1) The offsite storage facility meets all of the requirements of this article; and
(2) Adequate storage capacity in the offsite facility is dedicated to the development; and
(3) The development includes a means to convey stormwater within an easement to the offsite storage facility.
(K) Site runoff storage volume provided by enlarging existing regulatory floodplain storage (on stream runoff storage) will be allowed only as a variance. The applicant must demonstrate that flood damage will not be increased, and the development will not increase flood flows for both the two-year and 100-year floods.
(L) Structures built across the channel to impound water to meet site runoff storage 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 to be perennial, unless better data is provided by the developer. In all cases, it must be demonstrated that all such structures will not cause short-term or long-term instability, and that they are permitted under a joint application to the IDNR, COE and IEPA. Where such facilities are approved, the applicant must also comply with division (F) of this section.
(Ord. 2019-03, passed 4-15-2019)