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<br />~""'i <br />',..'1\--'."''', <br />",.";.",<,;,,.--- <br />; '-' :,\':~:;:-:- <br /> <br /> <br />Figure 8.--Empty Cascade Lake prior to failure. View is <br />toward upstream face of dam (date unknown). Photo <br />courtesy of the National Park Service. <br /> <br />Cause of Failure <br /> <br />As early as 0700 MDT on July 15, 1982, a greater than normal inflow into <br />Cascade Lake was observed, By 0715 MDT, water was flowing over the top of the <br />dam, The dam was overtopped for nearly 30 min, to a maximum depth of 4.2 ft <br />of water, before tipping over and fail ing at 0742 MOL Hydrostatic pressure <br />of the water on the dam and erosion of the abutments were the probable causes <br />of the dam failure. The topple failure was recorded photographically by <br />residents of nearby summer cottages (figs, 9A-D). For comparison, figure 9E <br />taken after the flood shows a view similar to that in figure 9D, At the time <br />of failure, inflow into the Cascade Lake dam (Site 1) was approximately 7,210 <br />ft3/s (determined from a slope-area discharge measurement upstream from the <br />lake), and the reservoir capacity due to the additional depth of water <br />(4.2 ft) above the top of the dam was about 25.1 acre-ft. The toppled dam <br />released a peak flow of 16,000 ft3/s (see "dam-Break Modeling") down the Fall <br />River into Aspenglen Campground, about 1/3 mi downstream. Within 5 min after <br />the dam failure, the flood surge had passed the campground. <br /> <br />HYDROLOGIC ANALYSIS OF THE FLOOD <br /> <br />The flood resulting from failures of two relatively small dams was much <br />1 arger than many engi neers woul d have expected and was catastrophi c in its <br />geomorphi c effect, due to three pri mary factors, Fi rst, the time of breach <br />deve 1 opment was short for both dams, all owi ng stored water to be re 1 eased <br />rapidly, Second, the width of dam breach was large for both dams, allowing <br /> <br />15 <br /> <br />