Agenda - Planning Commission - 06/06/2013

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Wetland E: At 8.13 acres, Wetland E is the largest wetland in the RTC project area. This wetland is located within a shallow, linear, drainage swale that bisects the west central portion of the RTC project area. This entire wetland has been row cropped to soybeans or corn during recent years. A total of 6.22 acres of Type 1 wetland will be directly impacted though a combination of fill and conversion to stormwater ponds. The remaining 1.91 acres will be retained within a proposed water way corridor and 6.22 acres of Wetland E will require mitigation. Lake Itasca: Lake Itasca and its direct drainage area are located approximately 1.2 miles northwest of the RTC (Figure 12.1). The 1998 LRRWMO Watershed Management Plan includes Itasca Lake as part of the drainage North Rum River Watershed. However, based on the surrounding topography and a survey that was conducted by EOR, the natural drainage path was determined to be to the southeast, into the Mississippi River Watershed. A lake overflow elevation of 871 was determined from a field survey of the area and two -foot contour information from development plans. An analysis of lake elevations for the 100-year, 24-hour rainfall and 100-year, 10-day snowmelt events, show that Lake Itasca does not outlet from the low point along the southeast side of the lake. This assessment held true for both existing and future land uses. Lake levels do, however; rise to within a few one hundredths of a foot for the 100-year, 10-day snowmelt event and within'/2 foot for the 100-year, 24-hour rainfall events. For this reason, the City of Ramsey has proposed installation of an outlet for the lake. Since a DNR permit would be required for an outlet below the DNR OHW, it has been assumed that any outlet would be above the OHW and above the 100-year flood elevation of approximately 871.0 feet. This outlet would provide assurances that existing and future homes will not be impacted by high water. No impacts to Lake Itasca or adjacent wetlands are anticipated. Mississippi River: The Mississippi River is located approximately 1/2 mile south of the RTC. There currently is no outlet from the RTC to the River; all flow leaving the site crosses Highway 10, flows to the southeast in a ditch, and eventually infiltrates. As shown in Figure 17.2c, an overland waterway system is proposed to convey stormwater south from the RTC to the Mississippi River. This waterway system would consist of a series of water quality treatment ponds, infiltration ponds and constructed wetlands. As discussed in Item 17 of this AUAR, a peak flow rate of 25.3 cfs is predicted under post - development conditions for the 100-year storm event. For small events (1-year and less), discharge ranges from 14.2 cfs, assuming no infiltration in the ponds, to 2.3 cfs when infiltration is included. The outfall to the Mississippi River is proposed to follow a County owned linear piece of land that extends from Highway 10 to the River (see Figure 12.5). The use of this property has been agreed upon in concept by the County, and discussions continue among the City, County and developer on its implementation details. The outlet will consist of a 21-inch pipe, enlarged near the Mississippi River to reduce velocities. Potential impacts include disturbance to the river bluff line where the outfall pipe is installed and in -stream scour and erosion where the pipe meets the river. If the County alternative outlet is not ultimately approved, an alternative alignment along Highway 10 to the southeast, with a connection to the River will be pursued. 12-3