Agenda - Council - 01/26/2021 - Laserfiche WebLink

Home

Riverstone South EAW Draft — December 2020 end receiving water for stormwater from the site, with a water level that bounces about 6.6 feet in response to a 1 % frequency storm event. The deep pond will also serve some infiltration functions. Surface water runoff from the project will mostly infiltrate into the ground. Runoff from large storm events may also flow overland to the southeast of the project and the Mississippi River. Proposed discharge flow rates are expected to be equal to or less than the existing flow rates. Requirements for stormwater management and erosion and sediment control 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 95' 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. 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 pollutants, such as phosphorus, was also achievable. Although nutrient removal efficiency varies with site conditions, well -designed wet ponds and constructed wetland treatment systems are effective in removing sediment and associated pollutants, such as trace metals, nutrients and hydrocarbons. Stormwater basins also remove or treat oxygen -demanding substances, bacteria and dissolved nutrients. Stormwater and Erosion Control BMPs Because project construction will disturb more than one acre of land, the project proponent will be required to apply to the MPCA for coverage under the National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS) General Permit prior to initiating earthwork on the site. BMPs will be employed during construction to reduce erosion and sediment loading of stormwater runoff. Inspection of BMPs will be required after each rainfall exceeding 0.5 inches in 24 hours, and on a routine basis every 7 days. The NPDES permit will also require perimeter sediment control maintenance and sediment removal. BMPs to be implemented during construction include: 1. Construction of temporary sediment basins in the locations proposed for stormwater ponding, and development of these basins for permanent use following construction. 2. Installation of silt fence and other perimeter erosion controls prior to initiation of earthwork and maintenance of these controls until viable turf or ground cover is established on exposed areas. 15