08-14-2000 Council Packet - Laserfiche WebLink

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I.Introduction The Minnehaha Creek Watershed District (MCWD) directed that an Engineer ’s Report in conformance with Minnesota Statutes 103D.711, Subd. 2 be prepared to evaluate the cost and feasibility of installing an outlet for Mooney Lake (State Protected Water 27-134P). The MCWD "directive was issued in response to petitions requesting such a study from the cities of Orono and Plymouth. The study was to include the .ollowing elements. The preferred route and configuration for the outlet The estimated cost of the project The impact of outlet operation on the downstream conveyance system The benefits of the project II.Background Mooney Lake is a shallow lake (ma.ximum depth of less than 15 feet) with a surface area of approximately 118 acres. The lake has no natural surface outlet. Portions of the lake- lie within the communities of Plymouth, Medina, and Orono. The lake s current watershed (excluding the lake surface area itself) is approximately 500 acres, with 320 acres (64%) lying within Plymouth, 125 acres (25%) \vithin Medina, and 55 acres (11%) within Orono (Figure 1). Land use in those portions of the watershed within the cities of Medina and Orono is dominated by large lot residential development while Plymouth s portion of the watershed is primarily medium density residential. In 1976, the Minnesota Department of Natural Resources (MnDNR) assigned an ordinary high water level (OHW) of 988.0 to the lake. Water levels in Mooney Lake.fluctuate in . response to the annual balance between water lost through evaporation and infiltration and water gained from watershed runoff and direct precipitation on the lake surface. Records for water levels in the lake indicate the highest recorded level was 989.37 on May 26, 1996, while the lowest recorded level was 983.1 in 1961. Over the past 20-r years, the lake has been experiencing relatively high water levels. These high water elevations have been causing damage to trees, yards, and landscaping. Flood nsk to residential structures adjacent to the lake is low. The City of Pl>-mouth has surveyed the walkout elevations for the lowest residences adjacent to the lake (located on the lake s north side). The elevation of the lowest walkout was 992.85 or 3.48 feet above the highest historical water surface elevation recorded for the lake. III.Evaluation of Options A.Route and Cost Analysis Providing an outlet for Mooney Lake will require a lift station and pipe to convey discharge water to an existing drainage system where the water can flow by ^avitj to Lake Minnetonka. Six possible outlet route options were identified as a result of discussions with city staff from adjacent communities and a review of conditions in the field Tw o of the route options were eliminated after further Held investigations and Mooney Lake Outlet Feasibility Study