06-28-1999 Council Packet - Laserfiche WebLink

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b. Hydraulic Considerations Water discharged from Mooney Lake will travel south through existing conveyance systems in the City of Wayzata before it is eventually discharged to Lake Minnetonka. One concern with the discharge of this pumped water is overburdening downstream conveyance systems. This potential is Wghest if the discharge occurs when the downstream s^'stem is already being used to carry runoff from local drainages. To minimize the potential for overburdening downstream conveyance systems with the Mooney Lake discharge, pumping from the lake could be confined to off-peak conditions. For example, pumping could take place in early spring after snowmelt or in late fall when downstream facilities are at normal water level. In addition, a low pumping rate of I cubic foot per second (cfs) could be used to minimize short term fluctuations in downstream water bodies as well as minimize the cost of the project. Water elevation records collected for Mooney Lake since 1995, observations of shoreline condition around the lake, and discussions with watershed district and city staff suggest that the water level in the lake could be set to fluctuate freely between elevation 987 and 988. We estimate that if pumping is conducted beginning at elevation 988, pumping would be necessary every 3-5 years. The infrequent rate of pumping is in part a function of the relatively small watershed of the lake compared to its surface area and the role that infiltration and evaporation appear to play in limiting lake level rises. A hydraulic modeling analysis was performed using HydroCAD (Version 5.01). The main objective of the analysis was to estimate lake level response to different pumping scenarios for a critical runoff event. The results of that analysis are as follows: • The runoff generated by a critical 10-day 100-year precipitation event (10.8 inches of rainfall over a 10-day period) will increase storage in the lake by about 374 acre-feet. This increase in storage will raise the elevation of the lake by about three feet to an elevation of 990.9 from a starting elevation of 988. • At a pumping rate of 1 cfs, 60-70 days of pump operation would be required to lower the lake elevation by 1 foot. • Pumping from the lake at a rate of 1 cfs during the 10-day 100-year critical event reduces the peak high water level of Mooney Lake by just 0.1 feet. As a result of the above information, we conclude the following with regard to operation of a lift station: 1 The water level in Mooney Lake can be set to fluctuate freely between the elevations of 887 and 888. Pumping from the lake should be considered when the elevation of the lake reaches an elevation of 888. A pumping rate of 1 cfs is recommended to minimize impacts to downstream water bodies as well as to minimize the cost of the project.