2006-02-23_AgendaPacket - Laserfiche WebLink

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4. TYPE OF PROJECT The City of Shoreview is proposing to excavate approximately 0.96 acres of the 2.9 acre Commons Pond (DNR #63W). 5. PROJECT DESCRIPTION Commons Pond, DNR wetland # 63w, has three inlets and one outlet. On the north side of the pond is a 48 inch RCP that directs storm water to the pond from a large area of residential development. A large sediment plume is located at the end of this pipe. The second inlet enters the pond on the west end via. a 27 inch RCP. The third inlet is a surface ditch, just northeast of the second inlet. The overall watershed draining to this pond is 164 acres, including the Community Center/City Hall buildings. The pond was excavated to a depth of 4 feet with 4:1 side slopes in the late 1970's according to City records. These conditions limit emergent wetland vegetation growth, but encourage submergent vegetation. The pond has gotten shallower over the years as a result of sedimentation from storm sewer inlets and from an accumulation of organic biomass from decaying submergent vegetation. Due to anaerobic decomposition, this pond frequently gives off a rotten egg odor. - The City's Community Center roof drains to this pond. Additionally, the air intake system of these buildings are located on the roof. Therefore, the anaerobic gases generated from the pond are often circulated through the Community Center, as well as noticeable throughout the adjacent neighborhood, generating many complaints. The City has obtained a permit from the DNR to install aerators in the pond to reduce the anaerobic conditions. However, the City has found that the aerators frequently get plugged dine to the think submergent vegetation and organic matter near the surface of the pond: Therefore the City proposes to excavate a portion of the pond to address two issues. First, excavation is needed at the 48 inch to remove accumulated sediment to improve pond function. The second is to remove some substrate and vegetation to allow the aerators to function properly. The proposed excavation will occur on the northeast and west ends of the ponds. The proposed excavation. will be located approximately 10 feet away from the shoreline to minimize impact to any existing emergent vegetation and to eliminate the need to conduct any bank stabilization. All excavated material will be disposed of on City property in existing disturbed upland locations. Excavation would begin in February and would be completed before spring. A small amount of rip-rap (<400 square feet) will be placed at the end of the inlet pipes (48-inch and 27-inch) for improved stabilization. 6. PROJECT ALTERNATIVES As previously described, the City has attempted to solve the problem with aerators. This method has not worked consistently due to the current pond depth and extent of the vegetation. The City has considered chemical applications to dissipate or control the odors, but found that this method to be labor intensive and costly, given. multiple applications possible per year. The excavation options proposed provides the lowest cost alternative and would require minimal annual. - a; maintenance beyond monitoring the aerators, which the City is currently doing. j~ `:.f