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<br />L For a breach wi dth of 25 feet, the peak di scharge waul d have been
<br />7,000 cubic feet per second less downstream from Lawn Lake dam to 1,300 cubic
<br />feet per second less at Estes Park, Maximum flood depths would have averaged
<br />0.6 foot lower; the flood wave would have reached Estes Park at the same time.
<br />For this hypothetical case, Cascade Lake dam still would have failed,
<br />2. For a breach width of 200 feet, peak discharge would have been 22,600
<br />cubic feet per second greater downstream from Lawn Lake dam to 5,400 cubic
<br />feet per second greater at Estes Park, Maximum flood depth would have aver-
<br />aged 2.7 feet higher; the flood wave would have reached Estes Park 0.4 hour
<br />earlier, The model also indicated that the outflow peak discharge from the
<br />worst-case failure of Lawn Lake dam could have been at least 56,000 cubic feet
<br />per second.
<br />
<br />If Cascade Lake dam had not failed (or had not been present), peak
<br />discharges would have been reduced by 11,300 cubic feet per second immediately
<br />downstream from the dam to 500 cubi c feet per second 1 ess at Estes Park.
<br />Maximum flood depths would have averaged 0.6 foot lower, and the flood wave
<br />would have reached Lake Estes 0,3 hour later.
<br />
<br />INTRODUCTION
<br />
<br />Just before sunrise, at about 0530 MDT (Mountain Daylight Time), on the
<br />calm, clear morning of Thursday, July 15, 1982, the privately-owned Lawn Lake
<br />dam, a 26-ft-high earthen structure, located at an elevation of about 11,000
<br />ft in Rocky Mountain National Park, breached due to a piping failure (Office
<br />of the State Engineer, 1983), releasing 674 acre-ft and an estimated peak
<br />discharge of 18,000 ft3/s of water down the Roaring River (fig. 1).
<br />
<br />"I started to hear a sound like an airplane. Also, there were loud
<br />booms, It got louder and louder. I thought it was breaking the
<br />sound barrier. I kept looking for a plane, but couldn't see one.
<br />I got suspicious and started to look upstream, I saw trees crashing
<br />over and a wall of water coming down. I started to run as fast as I
<br />could for high ground. There was a deafening roar. I fell and got
<br />up and kept running. I stood on high ground and watched it wipe
<br />out our campsite. It knocked everything in its path over; Steve
<br />didn't stand a chance,"
<br />
<br />With these words Steven Cashman described his harrowing experience with
<br />the flood that swept his camping companion to his death in the Roaring River,
<br />Other campers along the Roaring River estimated the wall of water to be 25 to
<br />30 ft high, carrying with it large trees and boulders, so that the water
<br />looked like a "wet, brown cloud" and sounded like extremely loud continuous
<br />"thunder" or a "freight train."
<br />
<br />The flood, attenuated by the relatively flat Horseshoe Park (an old
<br />glacial lakebed), continued along the Fall River, where it caused Cascade Lake
<br />dam (an approximately 12-ft-high, 12-acre-ft concrete gravity dam) to fail
<br />because of tipping over from overtopping, Peak discharge upstream from
<br />Cascade Lake dam was 7,210 ft3/s; however, discharge increased to 16,100 ft3/s
<br />or more as a result of the Cascade Lake dam failure, The flood continued to
<br />
<br />3
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