07-08-25 Agenda Packet - Laserfiche WebLink

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Page 2 of 3 Strategy): start pumping when the lake reaches an elevation of 936 feet above sea level (or 935 in the fall) and continue pumping until it drops 2 feet to 934. Implementation challenges of the DNR Pumping Strategy have occurred when the pump has underperformed, failed, and/or suffered vandalism to both the floating pump platform and control panel. Since 2003, the lake has dropped to at least 933 3 in drought conditions and risen to 937 during mechanical difficulties. This lake level bounce of 4 feet since 2003 has contributed to significant shoreline erosion, fish kills, challenges to water quality, and infestation of invasive weeds such as purple loosestrife. City/DNR records indicate that a total of 651 days of pumping have occurred in the 40 years since 1984. The Karth Lake Level Model discussed below implies that this pumping has cumulatively lowered the lake’s level by 36 feet. Since Karth Lake’s normal level of around 935 feet above sea level is only about 10 feet below the lowest point of Amble Drive (946 feet), pumping has been an essential component of flood mitigation within the watershed. The Project ‘s Details and Recommendations: HE developed an initial Karth Lake Level Model to project pump impact on lake level over a one-month period following a single rain event. That model utilized external data on topography, evaporation, pump specs, etc. A KLIDB member expanded the model to accommodate a year’s worth of daily inputs (such as daily precipitation data collected from the nearest measurement stations by the State of Minnesota) and other enhancements. Those enhancements were made under the direction and review by HE and RCWD. An essential facilitator for this effort has been the collection of data: The DNR installed a lake gauge in the spring of 2022 and volunteers have made weekly and sometimes daily readings which are available to the public on the DNR LakeFinder website. The model was calibrated and validated with data collected from 2022 through 2024. Lake levels were then modeled using alternative pumping parameters by applying 60 years of actual daily rain data under the assumption they would fall on Karth Lake’s current watershed. The findings are startling: • Without pumping, Karth Lake would periodically overflow onto Amble Drive, flooding at least half of the houses on the lakeshore and spreading to off-lake houses in the watershed. The historical pumping of a cumulative 37 feet from the lake level supports this finding. 3 When the lake is at 934, the bottom of the original lake level gauge is exposed sand. Before the DNR gauge was installed in May 2022, only a few measurements below 934 exist, including a 2011 Minnesota State LIDAR flight measurement of 933.