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2 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO depths would have averaged 0.6 foot lower; the flood wave would have reached Estes Park at the same time. For this hypothetical ease, Cascade Lake dam still would have failed. 2. For a breach width of 200 feet. peak discharge would have been 22,600 cubic feet per second greater downstream from Lawn Lake dam to 5,400 cubic feet per second greater at Estes Park. Maximum flood depth would have averaged 2.7 feet higher; the flood wave would have reached Estes Park 0.4 hour earlier. The model also indicated that the outflow peak discharge from the worst-case failure of Lawn Lake dam could have been at least 56,000 cubic feet per second. If Cascade Lake dam had not failed (or had not been present), peak discharges would have been reduced by 11,300 cubic feet per second immediately downstream from the dam to 500 cubic feet per second less at Estes Park. Maximum flood depths would have averaged 0.6 foot lower, and the flood wave would have reached Lake Estes 0.3 hour later. INTRODUCTION Just before sunrise, at about 0530 MDT (Mountain Daylight Time), on the calm, clear morning of Thurs- day, July 15, 1982, the privately-owned Lawn Lake dam, a 26-ft-high earthen structure, located at an eleva. tion of about 11,000 ft in Rocky Mountain National Park, breached due to a piping failure (Office of the State Engineer, 1983), releasing 674 acre-ft and an estimated peak discharge of 18,000 ftals of water down the Roaring River (fig. 1). I started to hear a sound like an airplane. Also, there were loud booms. It got louder and louder. I thought it was breaking the sound bar- rier. I kept looking for a plane. hut couldn't see one. I got suspicious and started to look upstream. I saw trees crashing over and 8 wall of water coming down. I started to run as fast as I could for high ground. There was a deafening roar. I fell and got up and kept runn- ing. I stood on high ground and watched it wipe out our campsite. It knocked everything in its path over; Steve didn't stand a chance. With these words Steven Cashman described his har- rowing experience with the flood that swept his camp- ing companion to his death in the Roaring River. Other campers along the Roaring River estimated the wall of water to be 25 to 30 ft high, carrying with it large trees and boulders. so that the water looked like a "wet, brown cloud" and sounded like extremely loud con- tinuous "thunder" or a "freight train." The flood, attenuated by the relatively flat Horseshoe Park (an old glaciallakebed), continued along the Fall River, where it caused Cascade Lake dam (an approx- imately 17-ft-high, 12-acre-ft capacity concrete gravity dam) to fail because of tipping over from overtopping. Peak discharge upstream from Cascade Lake dam was 7,210 fta/s; however, discharge increased to 16,100 ft'ls or more as a result of the Cascade Lake dam failure. The flood continued to attenuate in its passage down the Fall River and through the town of Estes Park. After Cascade Lake dam failed, two campers were swept to their deaths a short distance downstream from the dam. The flood continued down the Fall River, caus- ing extensive damage to homes, motels, businesses, and bridges, particularly in the town of Estes Park. In Estes Park, the flood entered the Big Thompson River for a short distance before entering Lake Estes, which is formed by the U.S. Bureau of Reclamation's Olympus dam. This reservoir contained all the floodwaters. The peak discharge was 5,500 fta/s entering Lake Estes, which is about 12.5 mi downstream from Lawn Lake. Peak discharge into Lake Estes occurred approximately 3 hours and 40 minutes after the failure of Lawn Lake dam. In that brief time, three people were killed and ap- proximately $31 million in private and public damages, cleanup, and economic loss was reported. The flood resulted in a Presidential Disaster Declaration for Larimer County on July 22, 1982. The purpose of this report is to present the setting, a summary of causes of the dam failures, the hydrologic data on the flood, and to document geomorphic effects of the flood. A secondary purpose is to present data ob- tained from using a dam-break computer model to evaluate that model's capabilities on high-gradient streams, to enhance and provide supplemental hydrologic information, and to evaluate various hypothetical scenarios of dam-breach development and probable impact of the failure of Cascade Lake dam. Documentation and analysis of the flood should provide valuable information on dam-breach floods of small dams on high-gradient streams for future hazard mitiga- tion related to dam failures. ACKNOWLEDGMENTS Data and data interpretation in this report are based on the combined efforts of many private individuals, and upon local county, Federal. and State agencies. This assistance is gratefully acknowledged. Many individuals provided valuable eyewitness infor- mation that helped in the understanding of the flood characteristics. Local residents also granted access to their property to the field personnel working in the flooded area. Appreciation also is extended to the of- ficials of the town of Estes Park, of Larimer County, and to National Park Service personnel who cooperated and contributed data during the data-collection period. Jack Truby, Colorado Division of Disaster Emergen- cy Services, provided estimates of flood damages. Alan Pearson, Colorado Department of Natural Resources, Division of Water Resources, Office of the State Engi- neer, provided information on the history and causes of failures of the dams. William Stanton, Colorado Department of Natural Resources, Colorado Water