03-05-2018 Council Work Session Packet

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WS – Item #4 WORK SESSION STAFF REPORT Work Session Item No. 4 Date: March 5, 2018 To: City Council From: Diane Hankee, PE, City Engineer Re: West Shadow Roadway Reconstruction Project Background Culvert between Reshanau Lake and Rice Lake As part of the West Shadow Lake Drive Roadway Reconstruction project, the culvert under West Shadow Lake Drive, connecting Rice Lake and Reshanau Lake will need to be replaced. The culvert is an existing 48-inch arch sitting on top of saturated peat soils. City staff have noted over the years that the culvert is plugged with an unknown blockage. The project proposes to minimize future pipe heaving and roadway settling by excavating the peat soils, replace the culvert, and increase the culvert length in order to accommodate a wider roadway for public safety and mobility. During the preliminary design of the project, the City received a letter on September 1, 2017 from the residents along Reshanau Lake. The letter asked for consideration of alternative methods for controlling the connection between the two lakes with “the goal of allowing the same or more amount of water movement between the lakes, but restricting the elevation that Reshanau Lake should decline,” and proposed a type of damming device or multiple smaller pipes. City staff has met with and coordinated with the Rice Creek Watershed District (RCWD) and the Minnesota Department of Natural Resources regarding this project. Agency recommendations are to replace the culvert to the original design conditions to preserve the existing hydrologic connection between Rice Lake and Reshanau Lake. The City completed a lake level analysis using a HEC-RAS model. The model evaluated both the blocked condition (where three feet of the culvert’s 3.75-ft height is assumed blocked) and the original 1998 design conditions. The 2-, 10-, 100-year 24-hour design storms were evaluated, as well as the 100-year 10-day snowmelt event. Larger events, such as the 100-year flood, overtop West Shadow Lake Drive and floodwaters equalize between Reshanau Lake and Rice Lake. For smaller events, there is a more noticeable impact on lake level elevations for Reshanau Lake, increasing the peak elevation for a 2-year event by more than six inches.