(A) Applicability. This code shall apply to all development within the limits of the city. Residential developments having a total area of less than five acres, and commercial or industrial developments having a total area of less than two acres, may be given a waiver by the city in accordance with § 154.043, subject to the following conditions.
(1) The city retains the right to require detention storage in all cases in which the proposed development will generate excess runoff that adversely affects the carrying capacity of the receiving watercourse.
(2) Developments less than two acres with less than 30% of the area paved and developments generating less than one cubic foot per second (CFS)/acre increased runoff shall not be required to provide detention storage, unless conditions in division (A)(1) above is applicable.
(3) This code shall apply for all newly platted areas and new developments proposed after the date of passage of this code. All development that have an approved preliminary plan by the Plan Commission at the time of the approval of this code will not have to conform to this code.
(1999 Code, § 34-5-52)
(B) Affidavit of disclosure of property interest. The effective acreage for a site is not limited to a fractional part of the total. If a project is developed in phases or small plats, the total acreage of the project site must be considered. At the time the owner of any development submits a preliminary plat or preliminary plan, he or she shall also identify to the city all contiguous property or property in the watershed that he or she has interest in.
(1999 Code, § 34-5-53)
(C) Method of evaluation. The storage capacity and discharge rate shall be based upon the calculated volume and peak flow of the storm water runoff, respectively. The calculations for sites having an area of 100 acres or less shall be made using either the Illinois Manual for Soil Erosion and Sedimentation Control Method or the Rational Method. If the site is larger than 100 acres, then the Engineer shall use the Illinois Manual for Soil Erosion and Sedimentation Control Method or if another method is desired to be used, the Engineer shall submit a proposed method of evaluation for the calculations for review and approval. The permitted discharge rate of storm water runoff shall be determined by calculating the rate of runoff for the site’s pre- and post-development conditions. The Engineer shall determine the most critical storm looking at three different time periods: the time of concentration; a one-hour storm; and a 24-hour storm.
(1999 Code, § 34-5-54)
(D) Detention of differential runoff. All new developments shall provide a storm water system that ensures that the rate of flow of storm water runoff discharged from the site after development does not exceed the rate of flow of storm water runoff discharged from the site before development of a 25-year storm, unless given a waiver by the city in accordance with § 154.043. Data shall be submitted for the 15-, 25- and 100-year frequency storm.
(1999 Code, § 34-5-55)
(E) Flows from upstream areas. Flows from upstream areas outside the site should be based upon the assumption that those areas are fully developed under forecast land use patterns. The required storage volume will be based upon the site only, with flows from upstream areas being by-passed or discharged via overflow spillways or other devices for the 100-year storm.
(1999 Code, § 34-5-56)
(F) Facilities in floodplains. If detention storage is provided within a floodplain, only the net increase in storage volume above that which naturally existed on the floodplain shall be credited to the development. No credit will be granted for volumes below the elevation of the base flood at that location unless compensatory storage is also provided. Where encroachments in the existing floodplain fill the valley storage areas, an equal amount of detention volume shall be provided.
(1999 Code, § 34-5-57)
(G) Land credit for detention facilities. The number of units/lots shall be based on the total area of the tract to be developed. All areas to be used as detention facilities shall be included in this total area.
(1999 Code, § 34-5-58)