6.4. SR 04-20-1998 - Laserfiche WebLink

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Life expectancy of the proiect Besides the inlet and outlet sediment sampling done as part of the 1997 lake study, fourteen years of USGS data was available to determine the suspended sediment yield in the Elk River. Based on the above described monitoring, the estimated average bedload delivery rate to Lake Orono is 3,000 C.Y. per year. This is not a high rate of sedimentation compared with other rivers. However, combined with increased erosion control efforts upstream, the removal of 90,000 to 100,000 C.Y. of material from the western lobe of the lake should allow the project to last for 30 to 40 years. Implementation of erosion control measures adjacent to the areas of the lake proposed to be excavated for safety reasons should allow these portions of the project to last indefinitely. The proposed project is not expected to affect the sedimentation rate of the Elk River. The western most lobe of the lake is where sediment from the river is naturally collecting now. The shallow nature of the excavation adjacent to the Elk River (1 to 2 feet gradually tapering up to the existing river bottom) should not upset the equilibrium of sediment upstream. In addition, areas with stabilizing vegetation will not be disturbed. The only vegetation proposed to be remove by the project are 2 or 3 small pockets of scrub willows (less than ½ inch diameter) that have colonized on deposition areas in the immediate mouth of the Elk River within the last four years. Construction methods The city is proposing to draw down the lake by gradually, over a four day period, from its normal level of approximately 872 MSL. The lake will be drawn down for approximately two week prior to excavation activities commencing to allow the sediment time to dry. The material will then be bulldozed into windrows, loaded into large tired off-road trucks and hauled to upland sites in the immediate vicinity of the lake for disposal. Water from the Elk River that passes through the lake during construction will be directed away from the work areas to avoid potential negative impacts. If areas need to be built-up to allow truck traffic, the lake sediment itself will be used. The lake was drawn down twice in 1997. No negative impacts due to erosion or to fisheries or wildlife were observed during these draw downs. Additionally, during these draw downs it was observed that absolutely no vegetation exists on the lake bottom in any of the areas where work is proposed. The city will require that the contractor doing the work has a spill prevention and counter control plan approved to ensure immediate clean-up of potential contaminates. Specific erosion control measures are identified on the construction plans and additional measures will be implemented if needed. Even when allowed to dry, the material proposed to be removed from the lake will have a high enough moisture content that dust will not be a problem. But, river water will be used to control dust, if needed.