(A) General. It is recognized that the receiving streams and storm sewers within the area of the village do not have the capacity to receive and convey the increased storm water runoff resulting from rapid urbanization occurring in the village and adjacent areas. These receiving streams and storm sewers are subject to frequent flooding which results in a growing rate of property damage. It is the intent of this specification that detention storage of storm water be provided to eliminate the excessive storm water runoff caused by the reduction or elimination of previous land surfaces through the construction of buildings, parking lots, streets, and other improvements to real estate.
(B) Requirements. A building permit will not be issued unless the applicant provides on-site detention of storm water runoff or participates in the provision of a village approved regional detention facility as set forth in the following criteria:
(1) A combination of storage and controlled release of storm water runoff is required for all residential developments exceeding five acres in the area, for all residential developments of less than five acres which have an impervious area of 50% or greater, for all nonresidential developments exceeding two acres in area and for all nonresidential developments of less than two acres which have an impervious area of 50% or greater. However, a residential development of five acres or less and a nonresidential development of two acres or less must have an adequate outlet as signified by the Village Engineer. If the outlet is not adequate, then detention, as determined by the Village Engineer, will be required to store that portion of the runoff exceeding the outlet capacity.
(2) The release rate of storm water from all developments requiring detention shall not exceed the storm water runoff rate from the area of 0.15 cfs per acre.
(a) Because of the flat conditions of the land in this area, channel configurations cut by nature are generally unable to handle the runoff from high intensity rainfalls and results in flood plain storage or spreading of runoff over the land areas during the larger storm periods. In order not to increase the runoff from such areas after development, the release rate must be limited to carrying capacity of these natural channels.
(b) The village will accept the release rate of not greater than that calculated from a storm of three-year frequency with a runoff rate coefficient of 0.15 three-year frequency storm of two- hour duration or 100-year frequency storm of four-hour duration, unless the applicant can show by his detail calculations, which are acceptable by the village that the discharge rate of the natural outlet channel serving the area is greater. In the event the downstream channel capacities are less than the above specified release rates, the release rates from the proposed area shall be reduced to match that available downstream capacity.
(3) Drainage systems shall have adequate capacity to bypass through the development the flow from all upstream areas for a storm of design frequency assuming that the land is in a fully developed state under present zoning or zoning proposed under the Official Plan. The bypass flow rate shall be computed utilizing a runoff coefficient of not less than 0.35. An allowance will be made for upstream detention when such upstream detention and release rate has previously been approved by the village and evidence of its construction can be shown.
(4) The detention volume required shall be that necessary to handle the runoff of a 100-year rainfall, for any and all durations, from the fully developed drainage area tributary to the reservoir less that volume discharged during the same duration at the approved release rate. The live detention storage to be provided shall be calculated on the basis of the 100-year and three-year frequency rainfalls as published by the U.S. Weather Bureau for the Chicago area.
(5) Detention facilities for developments within the village shall be subject to the approval of the Metropolitan Sanitary District of Greater Chicago, the approval of the Village Engineer and the Village Board of Trustees.
(6) Designs for fencing, embankment slopes, depth of permanent pools, protection of inlets and outlets, screenings, physical shape, and appearance shall be as determined and approved by the Village Engineer.
(Ord. 77-05, passed 4-6-77) Penalty, see § 156.99