(A) The required Stormwater Management Plan shall identify means for controlling the stormwater runoff release rate from the development and providing storage potential for the excess stormwater runoff where required. All computations, plans and specifications related to the implementation of this subchapter must be prepared and sealed by a professional engineer registered in the state.
(B) The Stormwater Management Plan shall contain, but not be limited to, the following information, unless specifically excluded by the city’s Drainage Coordinator:
(1) A topographic map of the project site and adjacent areas, of suitable scale and contour interval, which shall define the location of streams, the extent of floodplains and calculated high water elevations, the shoreline of lakes, ponds, swamps and detention basins including their inflow and outflow structures, if any;
(2) Maps showing pre-development and post-development drainage areas used for computations of the design runoff hydrographs and hydrologic data for each drainage area: runoff curve number, time of concentration, area and the like;
(3) The location and flow line elevation of all existing sanitary, storm or combined sewers;
(4) Detailed determination of runoff anticipated for the entire project site following development indicating design volumes and rates of proposed runoff for each portion of the watershed tributary to the storm drainage system, the calculations used to determine said runoff volumes and rates and restatement of the criteria which have been used by the project engineer throughout his or her calculations;
(5) A layout of the proposed stormwater management system including the location, size and elevation of all drainage structures, storm sewers, channels and channel sections, detention basins and analyses regarding the effect said improvements will have upon the receiving channel and its high water elevation;
(6) The slope, type and size of all existing and proposed storm sewers and other waterways;
(7) For all detention basins, a plot or tabulation of storage volumes with corresponding water surface elevations and of the basin outflow rates for those water surface elevations;
(8) For all detention basins, design hydrographs using the Soil Conservation Service Unit Hydrograph methodology of inflow and outflow for the two year, five year, ten year, 25 year, 50 year and 100 year design storms such that post-development peak flows are equal to or less than pre-development peak flows. The release rate from any detention facility is to be designed to be equivalent to or less than that of the site or drainage area prior to the proposed development for these storms. The next two downstream structures from the site must be capable of passing these storms or they become the limiting factor for the drainage area. A basin emergency overflow must be capable of controlling, at a minimum, the 100 year storm discharge.
(9) For minor drainage system, the design storm return period is ten years. For a major drainage system, the design storm return period is 100 years;
(10) A profile and one or more cross-sections of all existing and proposed channels or other open drainage facilities, showing existing conditions and the proposed changes thereto, together with the highwater elevations expected from stormwater runoff under the controlled conditions called for by these regulations and the relationship of structures, streets and other utilities to such channels;
(11) Embankment design computations including seepage control, slope protection and freeboard calculations (use soil conservation service criteria as a guide) for permanent pool basins; and
(12) Calculations of effects (if any) on established floodplain boundaries. Changes to approved floodplain areas must be calculated in accordance with the city Flood Prevention Ordinance and FEMA requirements.
(Ord. 980.2-9-94, passed 8-24-1994; Ord. 2023-022, passed 6-26-2023)