(A) Stormwater modeling shall be in accordance with United States Soil Conversation Service (SCS) Technical Release 55 or 20, i.e. TR-55 or TR-20.
(B) Permanent best management practices for sites where the sum of new and reconstructed impervious surface is one or more acres shall be designed to meet the requirements for Post-Construction Stormwater Management as described in the MN Small Municipal Separate Storm Sewer Systems General Permit No. MNR040000 in place at the time of submittal. Stormwater detention basins shall be designed to store sufficient excess runoff from the proposed development. Composite post-developed site runoff must be equal to or less than the composite pre-developed site runoff for the 5 and 100 year rainfall events. The SCS 24-hour, 5 year and 100 year rainfall events are 3.7 inches and 6.1 inches, respectively.
(C) Post-developed design site conditions, requiring diversion of stormwater from an existing inlet point to another inlet point on a different outfall sewer, shall meet the following: post-developed flow draining to an existing stormwater inlet point shall be equal to or less than the undisturbed pre-developed flow draining to that point.
(D) The following information shall be submitted to the city for review by the City Engineer:
(1) SCS curve numbers for the pre-developed and post-developed site conditions;
(2) Soil type(s) used in calculations;
(3) Size, elevation and location of proposed stormwater system outfall(s);
(4) Size, elevation, location and detail of proposed pond outlet structure;
(5) Map or plan showing the pre-developed and post-developed design drainage districts;
(6) Pond grading plan showing the proposed contour elevations and location of the pond’s emergency overflow spillway; and
(7) All drainage hydrographs, pond routing and peak elevation/outfall calculations.
(Am. Ord. 153, 4th Series, passed 10-3-2022)