06-09-2004 Council Agenda - Laserfiche WebLink

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The proposed West Vadnais outlet project involves water flow between three water management organizations: Vadnais Lake Area WMO, Grass Lake WMO, and Ramsey - Washington Metro Watershed District. Grass Lake and West Vadnais Lake may exchange water during high water periods. Grass Lake elevation has been markedly elevated since 2001 (extended periods above 882.7, 1988 datum), most obviously encroaching on the I -694 Right -of -Way fence and flooding the bike pathway under Gramsie Road in Shoreview. The highest point controlling flow between Grass Lake and West Vadnais Lake is thought to be a culvert under a Metropolitan Council interceptor sewer access road on the east side of Grass Lake, with bottom -of -pipe elevation of about 881.2 +/- (1988 datum, extrapolated from Profile 4, Grass Lake WMO Level Control Plan, April, 1990). Grass Lake's watershed is approximately 1,389 acres and includes outflow from Bennett, Wabasso, Owasso and Snail Lakes in Roseville and Shoreview. Grass Lake is a shallow lake, which has been largely vegetated until recent high -water conditions created a much larger open -water area. The Grass Lake WMO Water Management Plan discusses the important role of infiltration or seepage from Grass Lake into the groundwater. Evapotranspiration by plants is also an important process, which consumes water in Grass Lake during periods when vegetation is abundant in the .lake. According to long - term.lake elevation data collected by Ramsey County, Grass :Lake is well below the high point elevation connecting it to West Vadnais Lake (881.2 + / -, 1988 datum) for the vast majority of time. An extended wet period (precipitation exceeding the annual average) may reduce the rate of infiltration from Grass Lake and result in an increase in lake level. It is at such tiriies that the Grass Lake elevation may be high enough to' move water. to West Vadnais Lake. The immediate watershed to West Vadnais Lake is small and water level in the lake corresponds to local ground water levels and the water flowing into it from Grass Lake on an •intermittent basis. The water levels of Grass Lake and West Vadnais Lake are thought to be similar during high water periods, although there are limited West Vadnais Lake elevation data to verify this belief. The proposed West Vadnais Lake outlet will provide an outlet for both Grass Lake and West Vadnais Lake, on an infrequent basis, into the Ramsey Washington Metro Watershed District and Gervais Lake. Prior to construction of I -694, West Vadnais Lake's outlet was provided by surface flow to the south, through Black Tern Lake and into Gervais Creek, flowing to Gervais Lake. This flow path was eliminated by construction of the highway in the 1960's. A secondary potential outlet from West Vadnais was then constructed incorporating a short section of pipe and ditch (actually a deep ravine) to Twin Lake. Over time, the ravine was filled in with demolition rubble and the east end of the pipe section was blocked and eventually covered by a storm shelter building. The proposed West Vadnais outlet pipe alignment ends at Jiggs Pond rather than Black Tem Lake because the water level of Black Tern Lake has been manipulated (raised) over the years by development grading and construction of an outlet pipe. - 1 4 -