Agenda - Public Works Committee - 05/15/2012

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149th Lane Description As shown on Exhibit 5, a low area exists on Lot 3, Block 1 of Ramsey Meadows 4th Addition. There is a drain tile that drains the low area to DNR Wetland 65 8W. During large storm events, DNR Wetland 65 8 W backs up through the drain tile and into the low area. DNR Wetland 658W drains east under Trunk Highway 47 (TH 47) through a small weir structure and 15" pipe to DNR Wetland 659W. The weir structure is at an elevation of 860.9 and drains to the 15" pipe at an elevation of 860.0. The current outlet elevation is approximately two feet above the bottom of Wetland 658W. The existing 100-year high water level (HWL) for DNR Wetland 658W is 862.1 and for DNR Wetland 659W is 860.4. As shown on Exhibit 6, there is an existin drainage and utility easement over a majority of Block 1 of Ramsey Meadows 4 th Addition including the low area in question. Alternatives The following alternatives address the water elevation in DNR Wetland 658W. Alternative 1 Alternative 1 will include removing the existing weir structure and leaving only the 15" culvert as the outlet from DNR Wetland 658W. By removing the weir structure, the wetland will begin to discharge at an elevation of 860.0 as opposed to 860.8. Removing the weir structure, resulting in a lower normal water level in the wetland, will result in a 100-year HWL for DNR Wetland 658W of 862.0. The 100-year HWL for DNR Wetland 659W did not change. The estimated cost to remove the weir structure is $1,265. Table 8 includes the individual costs for this alternative. It is our understanding that any work proposed below an elevation of 860 will need the approval of the Minnesota Department of Natural Resources (DNR). This alternative will not have any effect below an elevation of 860 and is not anticipated to need the approval of the DNR. It appears the weir structure is part of the TH 47 storm sewer system. Removing the weir structure may require Mn!D OT' s approval. Alternative 2 Alternative 2 will include removing the existing weir structure and constructing an additional outlet from DNR Wetland 658W under TH 147. The proposed additional outlet would be at the same elevation as the existing outlet, 860.0. It was assumed the additional pipe would have to be jacked under TH 47. Three different sized additional outlet pipes were analyzed. The following table summarizes the 100-year HWL's for DNR Wetlands 658W and 659W and the estimated costs to construct each outlet pipe: