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4/90 <br />Historic Flood Even <br />is (Ref. 3) <br />On June 30, 1978, portions of the northern suburbs of St. <br />Paul received heavy rains. The largest observed rainfall of <br />_ 6.7 inches occurred in North Central Ramsey County. The <br />Vadnais Lake rain gauge recorded 6.55 inches for the total <br />rainfall duration of 21 hours (NOAA, 1978). However, 6.00 <br />inches of rainfall fell in the first ten hours of the storm <br />with 2.35 inches falling in the maximum one-hour period. <br />The rain storm had a recurrence interval in excess of 100 <br />years for both the 12-hour and 24-hour durations (U. S. <br />Department of Commerce, 1961). <br />_ Lake level data are available for all of 1978 for Lake <br />Owasso, Lake Wabasso, Snail Lake, and Island Lake. Lake <br />level observations were made generally between one and ten <br />days apart with more frequent observations during <br />appreciable changes in lake levels. Limited lake level <br />observations are also available on Grass Lake for the June <br />30, 1978 event, <br />The initial lake levels prior to the June 30, 1978 event for <br />alI the lakes in the study area were at or above normal <br />design levels because of the 3.0-inch rainfall that occurred <br />on June 25. The normal elevations for Lake Owasso, Lake <br />Wabasso, Grass Lake and Brasil Lake are 886.4, 885.4, 878.9 <br />grad 881.9 feet NGVD, respectively, <br />Th® Peak lake levels and times to peak varied among the four <br />lakes. Lak® Okrasso, Lake Wabasso, and Snail Lake peaked <br />approximately thre® to four days after the rainfall started <br />at maximum elevations of 888.2, 886.1, and 883.6 feet NGVD, <br />respectively. Because of the absence of an overflow or <br />outlet structure and because of the inflow from Lake Wabasso <br />and West Vadnais Lake, Grass Lake peaked approximately 25 <br />days after the June 30 rainfall event at a maximum stage of <br />882.0 feet NGVD. <br />1.2 - 3 <br />