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07-10-2019 Workshop & Council Packet
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07-10-2019 Workshop & Council Packet
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Potential Short-Term Options: <br />We will reach the 30-day mark since we initiated pumping at Twin Lake on Thursday, July 11. Since <br />the pumping operation has been effective at lowering the level and has not caused additional problems <br />downstream, the City Council is asked to consider setting an appropriate level to pump to at this time. <br />The current permitted level of 873.5 was initially chosen so pumping could begin as soon as possible <br />to protect the sanitary sewer system and other public infrastructure as well as the immediate threat to <br />the lowest entry point to homes on the west side of the lake. <br />The DNR is going to perform an Ordinary High Water (OHW) level survey to determine what that <br />level should be. OHW surveys take into account decades of records and markers on the landscape to <br />determine what that level should be. Once established, pumping to a level below the OHW would not <br />be allowed by the DNR. It is not known when that survey will be completed. Both the DNR and <br />MnDOT said they would support pumping to a lower level at this time and left the decision up to the <br />City Council with limited guidance. With that in mind, the following are some potential options that <br />should be considered. It should be noted that in all cases care would be taken to make sure that there <br />were no negative impacts downstream with any pumping operation. <br />1.Continue to maintain the current level at 873.5. The first month of pumping ends on July 11. <br />Based on information obtained during the first 30-days, this option would allow us to downsize <br />to a six-inch pump and continue to maintain the current lake level. According to modeling <br />performed by Barr Engineering, if the West Vadnais water is minimized and there is a four-day <br />100-year rain event, the low home on the lake would technically be safe at anything under 874, <br />but no freeboard (cushion) would be provided for the low home. The cost to maintain the level <br />at 873.5 should be lower on the front end but may add up to more over the long-term as the <br />pump may have to be operated more frequently to maintain this level. <br />2.Bring the level down to the elevation of the pipe under the railroad tracks (approximately 872). <br />This level should provide access to the final sanitary sewer structure that is currently <br />underwater. City of Little Canada public works staff utilized GIS survey equipment to obtain a <br />reading of 872.221 for this structure. In order to do this, the pump intake will have to be <br />moved to the small pond on the south side of the railroad tracks that the overflow pipe dumps <br />into. Further discussions with Northern Dewatering are needed to make sure this is feasible. <br />There would be some additional costs on the front end to set up a new intake and run the pump <br />to get the lake down another 1.5 feet but there would now be two feet of freeboard between the <br />low house and a potential 100-year event. The pipe under the railroad tracks has been in place <br />at its current elevation for many years and is considered the unofficial overflow for the lake. <br />All habitable private structures on the lake are safe at this level. Without additional flow from <br />West Vadnais and normal precipitation, Twin Lake may remain at or near 872 with very little <br />pumping. There is also a chance that the lake would continue to rise even with no rain or other <br />water getting into the lake due to high groundwater levels. If this is the case, it will not be
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