Agenda - Planning Commission - 05/04/2017 - Special

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Pearson Farm Residential Development EAW Draft — March XX, 2017 one- and two-year storms. Section 117-394 of the City Code protects natural wetlands from the discharge of stormwater without appropriate treatment for water quality and runoff quantity control. The minimum treatment required for discharge to wetlands is to meet NURP Guidelines, which include a dead storage treatment pond volume at least equal to the runoff from a 2.5 inch storm over the entire contributing area assuming full development, an average pool depth of more than 3, and a maximum depth of 10 feet. 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. 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 four stormwater ponds and one or two infiltration basins until it is discharged at or below existing peak runoff rates. Wetlands on the site provide potential for additional runoff storage and infiltration. Treated stormwater runoff from the site will be routed east under Puma Street NW into an existing intermittent drainageway, and eventually into the City's storm sewer system that ultimately reaches the Mississippi River. Proposed discharge flow rates will be equal to or less than the existing flow rates. 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. Other BMPs such as vegetative buffer strips are expected to filter sediment and nutrients from runoff before it reaches wetlands and surface waters, mitigating potential effects on water quality. LRRWMO Wetland Protection Standards require establishment of native vegetation within a 16.5- foot-wide buffer strip around wetland boundaries. Temporary sediment basins will be necessary to comply with the MPCA General Stormwater Permit for Construction Activity and to manage stormwater discharge during construction. Stormwater and Erosion Control BMPs Because project construction will involve disturbance of 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 14