Agenda - Council - 04/13/2021 - Laserfiche WebLink

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Riverstone South EAW January 2021 The project will include approximately 4.82 acres of stormwater and infiltration basins in compliance with City of Ramsey and LRRWMO requirements (Figure 3, Appendix A). Potential adverse effects of runoff volume and quality will be mitigated by the construction of stormwater basins designed to manage peak runoff rates, runoff volume, and water quality. Impervious surface runoff from storm events will be retained in two stormwater ponds and two infiltration basins. After large storm events, some runoff will discharge overland to the southeast of the project, but proposed discharge rates will be less than under existing conditions for all modeled storm events. Runoff from impervious surfaces will be routed to the smaller, western stormwater pond first (Figure 3, Appendix A). Stormwater will be treated in this basin before flowing to the adjoining infiltration basin and discharging through storm sewer to the second, larger pond. The second pond will be a 3.5-acre `lake' that will be excavated to a depth of about 26 feet. The deep excavation is proposed to balance the earthwork on the construction site. The deep pond will be designed to store runoff, as water levels are expected to bounce about 6.6 feet in response to a 1% frequency storm event. The deep pond will also serve some infiltration functions, and it will discharge through a storm sewer to the infiltration basin in the southeast corner of the site. Overland flow from that infiltration basin will either infiltrate into sandy soils or discharge to the Mississippi River. Although the deep pond may interact with groundwater, the filtration of stormwater through soils and vegetation will help ensure that runoff from the site does not contaminate the Mississippi River. Much of the surface water runoff from pervious parts of the project is expected to infiltrate into the ground. Runoff from large storm events that flows overland to the southeast of the project and the Mississippi River will discharge at flow rates that are less than under existing conditions. Proposed stormwater management and erosion and sediment control practices are expected to minimize cumulative effects of post -development runoff on downstream waters. Project construction will include other water quality BMPs such as temporary sediment basins to comply with the MPCA General Stormwater Permit for Construction Activity. Section 117-392 of the of Ramsey City Code requires permanent stormwater pollution controls, including: (1) infiltrating or retaining the first inch of precipitation over the impervious surface of the site; (2) retaining the post -construction runoff volume onsite for the 95th percentile storm; and (3) no increase in the post -construction rate, volume, and duration of runoff over existing conditions for the one- and two-year storms. LRRWMO Stormwater Standards require stormwater basins designed to store the volume of the 100-year storm; stormwater treatment to NURP standards prior to discharge to a lake, stream, wetland, or offsite; and infiltration of a runoff volume equal to 1 inch from all impervious surfaces onsite. For the 25.62 acres of impervious surface proposed with this project, the required stormwater volume control is equal to 2.14 acre-feet of runoff. The proposed project design meets this requirement. Stormwater ponds designed to NURP criteria are considered effective in removing sediment, pollutants, and nutrients, as discussed in Protecting Water Quality in Urban Areas: Best Management Practices for Dealing with Storm Water Runoff from Urban, Suburban and Developing Areas of Minnesota (MPCA 2000). The NURP research projects conducted by the U.S. EPA concluded that 90% removal of total suspended solids was an attainable goal, and that significant removal of other 15