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

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Pearson Farm Residential Development EAW Draft — March XX, 2017 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. Identib) specific erosion control, sedimentation control or stabilization measures to address soil limitations during and after project construction. Pre -Construction Site Runoff Soils in the project area are quite sandy and most precipitation likely infiltrates into the soils, resulting in little surface runoff. Surface runoff from the project area under existing conditions likely contains some sediment pesticides, fertilizers, and other nutrients. Existing runoff drains overland from the project area to local wetlands. Nearly the entire project area drains east through a culvert under Puma Street NW. From the culvert, runoff drains through an intermittent drainageway and wetlands to a point about 1.4 miles downstream from the project area, where the drainageway ends. Runoff appears to infiltrate into the ground at that location. Although the region contains highly permeable soils, some surface runoff may reach the Mississippi River during extreme runoff events. Post -Construction Site Runoff Compliance with LRRWMO, City of Ramsey, and NPDES requirements will minimize and mitigate potential adverse effects on receiving waters. Project development will change the land use from agricultural to residential use. This land use change is expected to have mixed effects on runoff water volume and quality. Residential development may reduce concentrations of agricultural chemicals, but may increase the loading of suspended solids during construction. Other typical components of impervious surface runoff, such as hydrocarbons and heavy metals, may also increase. Post -development runoff water quality will be typical of suburban developments, and will likely be slightly degraded by pollutants carried in runoff from streets, roofs, and driveways. Residential project construction will add about 14.5 acres of impervious surface, consisting of streets, homes, and driveways. The increased impervious surface area is expected to increase in urban runoff pollutants and the volume of runoff during significant storm events. However, it is anticipated that stormwater rate and volume controls that comply with City of Ramsey and LRRWMO requirements will limit increases in runoff volume and associated pollutant transport to large storm events that occur infrequently. The creation of stormwater ponds and infiltration basins is expected to mitigate potential adverse effects from the increase in impervious surface. The project will include approximately 6.5 acres of stormwater ponds and infiltration areas in compliance City of Ramsey and LRRWMO requirements (Figures 3, 4 and 8). The proposed design also includes excavation of a deep 3-acre pond. The deep pond will connect to the storm sewer system and discharge to the east under Puma Street NW, but it will not be designed as a stormwater pond and is not required to meet stormwater treatment requirements. However, the deep pond will serve some infiltration functions. 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 13