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TABLE 1 <br />..__._. <br />MODELING RESULTS <br />Starting Lake Level <br />(feet MSL) Source of Starting <br />Lake Level )final Lake Level. <br />(feet MSL) <br />870.7 Com uted see text 875.1 <br />869 _ USGS Quad Sheet 873.7 <br />868.2 Barr Surve 7/13/93 873.0 <br />866.6 MarkHurd Topo Map <br />(based on 1991 photo) 871.7 <br />The outflow from Twin Lake during flood conditions could be increased by removing the earthen <br />dike that currently restricts flow to the culvert under I-694. If this were done, and no additional work <br />was done to enlarge the capacity of the I-694 culvert, modeling indicates that the Lake could start at the <br />existing invert Elevation 872.3 and still not exceed the limit of 875 feet. Under these conditions_ the <br />maximum predicted elevation as a result of the 100-year, 10-day snowmelt event is 874.0 feet. <br />However, this method of increasing flood outflow carries the disadvantage of allowing a route for storm <br />runoff from the I-694 system to find its way into Twin Lake. <br />This water quality concern could be addressed by leaving in place the dike that separates the I-694 <br />stormwater flow from the Twin Lake system. Flood outflow could still be provided by piercing the <br />existing dike with a culvert that connects the Twin Lake system to the I-694 drainage. A simple flap <br />gate could be provided to prevent back flow from the freeway system to Twin Lake. <br />This culvert-and-gate system was evaluated using the hydrologic model. Assuming no upgrading <br />of the conveyances beneath I-694, the system hydraulics were first evaluated. The analysis showed a <br />maximum possible outflow of approximately 12 cfs with lake water Levels at 875 feet. Modeling <br />demonstrates that the above system would be capable of keeping lake water levels below 875 feet, even <br />when the initial water level was set at 873.9 feet. <br />5.0 LAKE MANAGEMENT CONSIDERATIONS <br />5.1 Flood Level <br />The modeling results indicate that maintaining lake water levels below the designated flood stage <br />of 875 feet should not be difficult. Without any changes to the existing structures, there should belittle <br />23\62\264\TWINLAKE.RPT\I~MH 8 <br />