Agenda - Public Works Committee - 02/21/2023

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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 west of Center Street is generally planned to flow north from Sunwood Drive and to a large future regional pond proposed on the north side of Ramsey Parkway. The outlet for this regional pond will most likely 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 were 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 current pond footprint used in the analysis is derived from sketch designs from LHB Landscape Architects. 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 1 shows the general shape and location of the pond used in the analysis. Along with the final pond configuration, proposed construction methodologies, including the requirement for pretreatment of all stormwater (public and private) prior to discharge to the regional pond, are currently being considered by the City. Cost considerations currently include stripping topsoil and vegetation, excavating the pond, installing a 2' impervious clay liner, and fine grading the site. Much of The COR is located within a Drinking Water Supply Management Area. Because of this, the required infiltration (1 inch over all new impervious area) is provided by the City in a regional infiltration basin. Developers then pay a share of the cost for creation of the regional facility. Site Grading Existing hydraulic modeling was reviewed to analyze storm water conditions for future development scenarios. The regional stormwater retention pond elevation was then used to determine the minimum elevations required for building pads in the area. The results of the modeling indicate the regional stormwater retention 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. Figures 11 and 12 depict 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 3-feet of fill to meet minimum elevations required. Tree Inventory Per the City's tree preservation ordinance, a tree inventory survey was completed to determine various Prepared by: Bolton & Menk, Inc. ADDITIONAL PUBLIC INFRASTRUCTURE ANALYSIS The COR Analysis 1 0R1.129128 Page 10