The central retention facilities shall meet the following minimum criteria:
(A) A combination of storage and controlled release of stormwater runoff is required for all nonresidential developments exceeding five acres in area, for all residential developments exceeding ten acres in area and for all residential developments between five and ten acres which have an impervious area of 60% or greater. However, a residential development of ten acres or less must have an adequate outlet as signified on the permit 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.
(B) (1) The release rate of storm-water from all developments requiring detention shall not exceed the storm- water runoff rate from the area in its natural undeveloped state. 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 those areas after development, the release rate must be limited to the carrying capacity of these natural channels. 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, 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.
(2) Channels, road crossings, and other conveyance systems that connect new developments with central retention facilities that require improvement must be incorporated into the overall design. Calculations and drawings showing the proposed improvements are to be submitted for approval of the Village Engineer.
(C) Drainage systems shall have adequate capacity to bypass through the development the flow from all upstream areas for a storm of design frequency of ten years assuming that the land is in a fully developed state under present zoning or zoning proposed under the Comprehensive 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 that evidence of its construction can be shown.
(D) The live detention storage to be provided will be calculated on the basis of the 100-year frequency rainfall as published by the U.S. Weather Bureau for this area. The detention volume required will 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.
(E) Construction, filling or other similar physical occupation of the flood plain, may be permitted if adequate compensatory storage volume is provided.
(Ord. 74-0-043, passed 11-18-74) Penalty, see § 51.999