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3.0 PRESENT STUDY 3.1 Methods Hydrologic modeling and several sources of data were combined in this hydrologic evaluation of Twin Lake. A topographic map based on 1991 aerial photographs of the lake area was commissioned in order to accurately assess the drainage patterns and surrounding developmern (see Figure 2), Field investigations and surveys "were conducted to locate drainage structures within the lake`s watershed, and determine flow elevations and structure sizes, Previous hydrologic studies of the area were reviewed, and Mn/DOT plans were examined to collect information on existing culverts. State and local agencies - including the City of Vadnais Heights; the City of Little Canada; the Minnesota Department of Natural Resources (MONK); the Minnesota Pollution Control Agency (MPCA); the Ramsey County Public Works Department; the Saint Paul Water Utility (SPWU); and engineers at SEH -were contacted and interviewed to gather historical data on water levels, drainage patterns and drainage plans. Finally, hydrologic modeling was conducted and analyzed to evaluate likely changes in lake levels as a result of storm and snowmelt events. 3.2 Water Level Data Historical water elevation records for Twin Lake are apparently unavailable; neither the MDNR nor the MPCA were able to supply such information. The scarcity of data makes the establishment of "normal" and high water levels difficult. On the United States Geological Survey quadrangle map of the area, the water level for Twin Lake is shown as 869 feet. However, the topographic map commissioned for this study shows the water surface at 866.6 feet, and a 7/13/93 Ban survey reported the water surface elevation as 868.2 feet. The 1989 SEH report reports the normal elevation for Twin Lake is 872.2 feet; this elevation, however, appears to be based on culvert invert elevation data rather than actual lake IeveIs. A wide range of projected high water elevations also. exists. A 1982 MDNR study reported a projected 10-year high water elevation of 870.6 and a 100-year level of 871.5 for Twin Lake. The MDNR, however, was unable to identify the basis of these projections. The Little Canada buildmg inspector reported a "high water mark" of Elevation 872 that is employed by the City of Little Canada in issuing building permits; he was similarly unable to identify the source of the information. A 100-year lake level of 873.2 feet was projected in the SEH report (1989) for the City of Vadnais Heights, but that may be high because of the high normal lake level which was assumed. 23162~264\TWINLAKE.RPTIKMH 4