Agenda - Planning Commission - 08/02/2018

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North Dayton Development AUAR ii. Stormwater - Describe the quantity and quality of stormwater runoff at the site prior to and post construction. Include the routes and receiving water bodies for runoff from the site (major downstream water bodies as well as the immediate receiving waters). Discuss any environmental effects from stormwater discharges. Describe stormwater pollution prevention plans including temporary and permanent runoff controls and potential BMP site locations to manage or treat stormwater runoff. Identify specific erosion control, sedimentation control or stabilization measures to address soil limitations during and after project construction. The existing site consists primarily of previously farmed land, wetlands and some brushy wooded areas. The majority of existing site runoff would be from the previously farmed areas. The type of runoff is typical pasture and previously farmed land (i.e. residual fertilizer, herbicide, pesticide, sediment from tilling and planting, etc.). Ultimately, the area drains to the Mississippi River. The proposed development will include significant impervious surfaces, but such surfaces would be accompanied by water quality treatment and quantity discharge controls in accordance with current day regulations. Specifically, stormwater would be treated prior to discharge according to the MPCA requirements for Permanent Stormwater Management System in the NPDES Construction Stormwater General Permit and the City of Dayton requirements for stormwater treatment and rate control. Elm Creek Watershed Management Organization (ECWMO) rules will also guide final design. ECWMO requires rate control, water quality, and water quantity conditions to be met onsite. Post development discharge rates are not to exceed existing rates for the 2-, 10-, and 100-year, 24 hour NOAA Atlas 14 rainfall events. Water quality on site must be such that there is no net increase in total phosphorus or total suspended solids from pre -development to post -development conditions. Abstraction must be provided on site equal to the equivalent of 1.1 inches of runoff from all impervious surfaces. The project will be required to provide both temporary and permanent erosion and sediment control as required by the City of Dayton, Elm Creek Watershed Management Commission, and the MPCA NPDES Construction Stormwater Permit. Temporary erosion control measures may include rock entrances, silt fences, silt curtain, biorolls, erosion control mats, inlet protection, rock check dams, temporary seeding and mulching, and temporary sediment basins, as needed. Due to the Mississippi River being an impaired waterbody and a Scenic or Recreational River segment, stabilization of exposed soils must occur within 7 days after construction activity has ceased, rather than the typical 14 days. Additionally, temporary sedimentation basins will be required for all drainage locations receiving runoff from 5 or more acres of disturbed area, rather than the typical 10 days. In compliance with the Permanent Stormwater Management System in the NPDES Construction Permit, permanent BMPs will be constructed onsite. Prior to discharging stormwater runoff into infiltration and filtration facilities, runoff will be pretreated to remove solids in order to maintain the facilities capacities. It is anticipated that permanent stormwater treatment may include the construction of detentions ponds to remove 22