4.12. SR 10-25-1993 - Laserfiche WebLink

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" e The Honorable Mayor and City Council October 25, 1993 Page Two 2. Storm Sewer As you are aware, the Comprehensive Drainage Plan calls for a 72-inch diameter storm sewer line from the railroad line through the MnDOT interchange to the Mississippi River. Unfortunately, MnDOT's final design of the TH 10/101/169 interchange does not provide enough elevation difference to install a 72-inch diameter culvert. In order to get comparable capacity in a smaller size culvert, three parallel 42" diameter culverts would need to be installed. The other concern with the MnDOT design is their traffic and construction staging plan does not allow for open cutting of the entire length. The total distance from the Mississippi River to the north end of the interchange project is approximately 850'. 350' of this length would require jacking in order to maintain the integrity of the MnDOT staging and traffic plan. The remaining 500' could be open cut. e Obviously, the installation of 3 parallel pipes is going to be much more costly than the installation of 1 larger diameter pipe. We have estimated that each 42" diameter pipe from the Mississippi River to the north end would cost approximately $175,000 to $200,000. Therefore, the installation of 3 parallel pipes would cost between $525,000 and $600,000. It should be noted that this work does not include crossing under the railroad tracks which would still require a 72" diameter culvert. If the City were to decide to not install 3 42" culverts but only one to save monies on the installation costs, the effect would be the requirement of more ponding to be constructed upstream. We have prepared a brief analysis of the increased ponding relative to the culvert capacity through the MnDOT interchange. These are listed in the following table: 140 CFS 220 CFS 300 CFS Ponding 180 AC-FI' 135 AC-FI' 110 AC-FT Pipes 1 - 42" 2 - 42" 3 - 42" Flow e It is unlikely that the DNR and the Army Corps of Engineers will allow this amount of ponding in wetlands upstream of the railroad tracks. Therefore, it should be assumed that ponding will utilize land which would otherwise be suitable for development. We would recommend at this point in time that the city proceed through MnDOT to include three 42" diameter pipes in the MnDOT project with the option to eliminate 1 or 2 of them depending on the construction costs. This approach would allow us to get an actual construction cost and do a much better cost-effective analysis of the cost of culverts vs. ponding after bids are open.