Agenda - Council Work Session - 06/17/2003

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The Lower Rum River Watershed Management Organization will require storm water runoff from the District to be pre-treated prior to discharging to the Mississippi River. To achieve the water quality goals for the District, storm water will first be routed to an existing storm water pond that is in the Business Park located immediately north of the District as shown on Exhibit B. 4.0 DESIGN CONSIDERATIONS In designing the proposed facilities, the Soil Conservation Service (SCS) method of determining total and peak rates of runoff was utilized. The facilities were designed using the computer model "HydroCADD". The SCS 24-hour Type II rainfall distribution was used in the model. Aerial topography from May 1994 and existing' record drawings were used to estimate drainage areas and existing pond storage volumes. Exhibit C shows the three drainage areas that were delineated for the study. Area 1, which contains the District, and area 2, which includes the south portion of the Business Park, were assumed to contribute to the existing storm water pond located in the southeast comer of the Business Park after the proposed improvements are constructed. Area 3 has a number of ponding areas and it was assumed that this area would contain its owr~ storm water runoff. 5.0 PROPOSED IMPROVEMENTS The proposed storm sewer improvements are shown in Exhibit D. The improvements to the district consist of the following: Construct a Trunk Storm Sewer System along the north property line of the District Connect the Trunk Storm Sewer to the existing storm water pond located in the Business Park Connect the Trunk Storm Sewer to the existing 42-inch storm sewer system located along the west side of Sunfish Lake Boulevard The proposed design is based on containing the volume of runoff from a 10-year 24-hour event within the existing ponding areas. Events greater than the 10-year event will require storage of storm water runoff in tow areas and parking areas. The study briefly reviewed an alternative that would provide an outlet to the Mississippi River, which would protect all Iow areas and parking areas during the 100-year storm event. This alternative was determined not to be economically feasible since the cost would be almost twice that of the proposed alternative. No field survey data was collected for this study. As part of the final design process elevations of the lowest building entry points should be obtained by field survey. It should then be verified that the volume of runoff produced from a 100- year storm event can be stored in existing ponds, Iow areas, and parking areas without flooding building space. Page 2 \\HaOl\Shared Docs\Municipa&ARAMSEY~41 ~ra417FSR.doc