(A) All developments resulting in significant run-off potential shall comply with the requirements of storm water detention - design method 1, described below.
(B) All developments resulting in moderate run-off potential shall comply with the requirements of storm water detention - design method 2, described below.
(C) Storm water detention is not required for:
(1) Developments resulting in minor run-off potential.
(2) The construction of a single-family residence, or two-family residence, that is not constructed in conjunction with a development resulting in moderate or significant run-off potential.
(D) Immediate use versus future use. The ultimate intended future use of the development shall be used to determine the storm water detention requirements. A residential, or mixed-use, subdivision which is platted and constructed in phases will be allowed to construct the storm water detention facilities in similar phases. All other developments shall be required to construct the storm water detention facilities for full development with the initial construction phase.
(E) All storm water detention facilities shall be designed by a licensed professional engineer, at no expense to the City of Mattoon, and shall be submitted to the city for approval. Approval by the City of Mattoon shall not relieve the responsibility for the developer and/or site owner to design, construct, and maintain a functional storm water detention system.
(G) Storm water run-off shall be calculated by the Rational Method (Q=CIA). The following Run-off Coefficients shall be used for consistency among site plans; grass areas C=0.20, crops C = 0.35, aggregate C = 0.50, pavement C = 0.90, buildings/roofs C = 0.90. Rainfall Intensities Versus Time of Concentration Charts shall be in accordance with the current edition of the Illinois Department of Transportation Drainage Manual. Alternate calculation methods can be approved, on a case by case basis, at the sole discretion of the City of Mattoon.
(H) The minimum size for an underground discharge device shall be a 12-inch pipe, or equivalent. The minimum size for restriction devices shall be six-inch diameter short tube restrictors. Restrictor devices shall be conveniently accessible for cleaning and maintenance.
(I) Sideslopes around the perimeter of detention basins shall not be steeper than 4:1, horizontal to vertical.
(J) Manipulation of these requirements by phased development shall not be allowed. All work on a site since the original adoption of these requirements on December 20, 2011 shall be considered cumulatively when evaluating the storm water requirements for successive phases of development.
(Ord. 2011-5341, passed 12-20-2011; Am. Ord. 2019-5423, passed 12-3-2019)