2003-08-14_AgendaPacket - Laserfiche WebLink

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a EXECUTIVE SUMIVIARY Twin Lake was studied to formulate management guidelines to reduce flood potential and protect water quality for the lake. The effect of possible flows from Vadnais Lake was also evaluated. Although water quality and water level data is scarce for Twin Lake, several conclusions resulted from this study. Water quality in Twin Lake is above average for the Twin Cities area, and the lake should remain isolated to preserve its trophic status. Overflow from Vadnais Lake into Twin Lake is normally prevented by the Saint Paul Water Utility's management of water levels in Vadnais Lake. If this situation should change, the water quality of Twin Lake would likely suffer. Likewise, any outlet to ~ the County Ditch 16 system should protect against backflow to Twin Lake. A 1975 report suggested that 63 cfs flood flow may need to be directed from Vadnais Lake through Twin Lake; present I-694 culverts downstream of Twin Lake could not accommodate a flow greater than about 14 cfs. If an outflow of 63 cfs from Vadnais Lake is necessary, the flow should be diverted around Twin Lake. This would protect Twin Lake's water quality, and would also reduce the potential for r'looding. A route for such flow is suggested, through wetlands west of Twin. Lake, under Interstate 694, and into the County Ditch 16 system. Further study of this route is necessary. Based on apparent low house floor elevations, a reasonable flood elevation for Twin Lake is 875 feat. Assuming that outflow from Vadnais Lake does not flow through Twin Lake, Twin Lake should rarely, if ever, reach. this flood stage. Historic levels for Twin Lake range from Elevation 866.6 to 869. Hydrologic modeling of the watershed estimates the flood level to be 875.1 feet after the critical runoff event (the 100-year, 30-day snowmelt), even assuming a maximum allowable beginning water level of 870.7 feet. Without any improvements, lake levels should be monitored and kept at or below Elevation 870.5. If a 36-inch culvert with a flap gate at Elevation 872.3 is installed upstream of I-694, the maximum allowable beginning water level could be as high as Elevation 873.9. The one-way flap gate would serve the purpose of protecting the water quality of Twin Lake from the freeway runoff. The proposed culvert through the dike at I-694 should be planned, but actual construction could he deferred until Twin Lake rose to its action level of 870.5.. At that time, pumping or culvert construction could be selected as a management technique. 23\6212641TWIl~ILAICE.RPT\KMH 1