Agenda - Council - 02/27/2018 - Laserfiche WebLink

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Storm Water Management Storm water management concepts were developed to maintain existing drainage patterns and preserve the conveyance and flood storage capacity of the existing site. Storm water runoff within the study area is generally planned to flow north from Sunwood Drive and west from Center Street NW to a large future regional pond proposed on the north side of Ramsey Parkway. The outlet for this regional pond will most likely to be extended along the future Yolite Street, where an existing 30-inch arch pipe has been extended into the study area from Sunwood Drive. The existing hydraulic model was reviewed to allow for the analysis of storm water within The COR area. Preliminary storm water management conditions and requirements where then developed to determine the elevations required for building pads in the area. The sub -watersheds were modeled using future land use conditions to generally size the regional storm water basin. The basin was sized to ensure proposed discharge rates were equal to existing flow rates. The pond was also reviewed with respect to potential storm sewer depths and potential aesthetic park function. The pond footprint used in the analysis is as shown in The COR Parks + Public Places, December 2015. The normal water level was determined by using the elevation of the outlet pipe located near Sunwood Drive. Additional design parameters and regional storm water management planning should be further refined as the areas begin to develop. Figure 8 shows the general shape and location of the pond used in the analysis. The COR is located within a Drinking Water Supply Management Area. Because of this, the required infiltration (1 inch over the new impervious area) is provided by the City in regional infiltration basins. Developers then pay a share of the cost for creation of the regional facility. Cost estimates were not developed for the regional pond construction or private site storm sewer conveyance. Site Grading Existing hydraulic modeling was reviewed to analyze storm water conditions for future development scenarios. The regional pond elevation will be used to determine the minimum elevations required for building pads in the area. The results of the modeling indicate the regional pond could reach a high water elevation of 866.5 during a 100-year storm event. This should be considered a preliminary result at this time, as the actual pond size and shape could be revised based upon other studies occurring at the City. The Lower Rum River Watershed Management Organization requires the lowest floor elevation be a minimum of 2 feet above the 100-year high water level. Based on that requirement, the minimum building elevation should be 868.5. Figure 9 depicts the average existing lot elevations, the proposed minimum lot elevations required, and an approximate fill volume required to raise each of the sites to the minimum elevation. Each site requires an average of approximately 2 feet of fill to meet minimum elevations required. A new City park, located directly adjacent to Center Street NW, is in the planning stages at the City. The costs associated with importing fill for this parcel have been included in the alternate cost analysis section Prepared by: Bolton & Menk, Inc. COST ALLOCATION ALTERNATIVES Center Street Area Analysis I R16.114929 Page 7