The following regulations shall apply to all developments for which a preliminary plat has been applied for prior to November 1, 1995, or for which a preliminary plat has been approved prior to November 1, 1995, and remains valid.
(A) A combination of storage and controlled release of stormwater runoff shall be required for all nonresidential developments of two or more acres in area, for all multiple-family developments of two or more acres in area, and all single-family developments of five acres or more in area, where the minimum lot for individual lots contained therein area is less than one acre.
(B) The release rate of stormwater from all developments requiring detention shall not exceed the stormwater runoff rate from the area in its natural, undeveloped state.
(1) A release rate of not greater than that calculated from a ten-year rainfall with a runoff rate coefficient of 0.15 will be accepted, unless the applicant can show detailed calculations that are acceptable to the Village Board after a recommendation by the Village Engineer and Plan Commission.
(2) Applicants seeking a modification of the aforesaid release rate must first appear before the Plan Commission, which shall recommend to the Village Board whether or not a modification is appropriate.
(3) After review and recommendation by the Village Engineer and upon competent proof that the discharge rate of the natural or manmade outlet, channel or sewer serving the area is greater, the Village Board may permit such greater release rate as may be reasonable.
(C) Storm sewer systems, streams and channels shall have an adequate bypass capacity through the development for the flow from all upstream areas for a storm design frequency of ten years, assuming that the land is in a fully developed state under present zoning.
(1) The bypass flow rate shall be computed utilizing a runoff coefficient of not less than 0.35, unless otherwise approved by the Village Engineer.
(2) An allowance, equivalent to the reduction in flow rate provided, will be made for upstream detention when such upstream detention and release rate has previously been approved by the village and has been constructed.
(D) (1) The live detention storage to be provided shall be calculated on the basis of the 100-year flood rainfall, as published by the U.S. Weather Bureau for this area.
(2) The detention volume required shall be that necessary to temporarily store the runoff of the 100-year rainfall from the fully developed drainage area tributary to the reservoir, less the volume discharged during the same duration at the approved release rate.
(E) In the event the downstream creeks or channels are inadequate to receive the release rate herein above provided, then the allowable release rate shall be reduced to that rate permitted by the receiving downstream sewers, streams and channels; and additional detention, as determined by the Village Engineer, shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers, streams and channels.
(F) Plans, specifications, and calculations for stormwater detention facilities, as required hereunder, shall be submitted to the Village Engineer for approval prior to the commencement of construction.
(G) The formula to be used for calculating the volumes of water to be stored and or released shall be the same as that used by the Metropolitan Water Reclamation District of Greater Chicago, Drainage and Flood Control Ordinance.
(H) The Village Board reserves the right to waive any or all conditions of this chapter for parcels that will discharge directly into the DuPage or Des Plaines River.
(Ord. 869, passed 11-20-95)