(A) Normal detention of stormwater shall be required for all developments and proposed developments which would alter runoff as to flow, velocity, or time of concentration. These basins are required to detain the peak post-developed runoff which exceeds the runoff created by a five-year storm under predeveloped condition. The village reserves the option to require more stringent detention requirements based upon the estimated capacity of the existing storm sewer. All calculations must be submitted to the village for approval. Calculations must include a profile of the existing storm sewer from the proposed connection point to a point 500 feet downstream, or the first outfall structure nearest to, or beyond, the required 500 feet. The calculated full flow capacity of the existing stormwater outfall shall also be provided.
(B) Runoffs and capacities are to be computed using the Rational Method and Manning Formula for areas less than 20 acres.
(C) The release rate shall not be greater than the storm runoff created by the pre-developed site during a five-year frequency storm.
(D) Storage volume shall not be less than the storm runoff created by the post-developed site during a 100-year storm event.
(E) Special detention and consideration may be given by the Village Engineer for high impervious areas that are smaller than two acres in size.
(F) An emergency overflow from the basin to a major storm system must be provided to protect the facility and adjacent properties. The designer should investigate the capacity of the downstream drainage facilities to determine if they will be adequate to handle the design flow from this particular development. If the downstream facilities are inadequate, it may be necessary to provide on-site retention or ponding basins to limit the flow to an amount which the downstream system can accept.
(G) Runoff coefficient, outlet size, and detention storage are addressed in tables A and B.
(Ord. 817, passed 8-2-1999)