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will be necessary. These measures might include a program of public education for the residents of the Twin Lake watershed. The program would include educational .briefings and pamphlets alerting the residents to be mindful of what is allowed to enter the storm sewer systems and the Iake, and would help discourage harmful Iawn care practices. 6.0 MANAGEMENT ALTERNATIVES AND RELATIVE COSTS Management alternatives for Twin Lake are dependent on whether Vadnais Lake is allowed to overflow to Twin Lake. Alternatives without and with. overflow are discussed below. ~ If it can be determined that Twin Lake will not be required to accommodate any flows from the neighboring Vadnais watershed, management considerations are sirnpIified. The lake would still need to be managed to stay below flood levels, and the water quality of the lake would need to be preserved. Lake management would likely take one of two forms. 1. The preferred method would require installing a culvert through the dike currently guarding the entrance to the culvert under I-694. The culvert would allow increased capacity for Twin Lake at lower elevations. To avoid adverse water quality impacts, the culvert should have a flap gate which allows flow only from Twin Lake and to the I-b94 culverts. This alternative would provide additional capacity for outflow during large storm events, but would serve to maintain the lake's isolation from the interstate runoff. Hydraulic and hydrologic considerations for this alternative were discussed above; the arrangement would prevent flooding even if the water Level before the rainfall or snowmelt event were right at the level of the outlet. This system is estimated to cost about $13,000 and would not require pumping to control water levels. Construction could be deferred until a need is demonstrated by the lake level rising to the maximum beginning {action) level of 870.5. 2. If no culvert improvements are made, the lake must be monitored for dangerously high pre-snowmelt or mid-summer water levels. If in the early spring, the lake level were above 870.5 feet, and large accumulations of snow were present, pumping (presumably into Vadnais) could be done to lower the lake and eliminate the possibility of flooding. Similarly, if mid-summer levels exceeded 870.5 feet, pumping could be performed to avoid flooding from large rainfall events. No 23162~264\TWINLAKE.RP'I'~KMIi 10