FLOOD06021

• County: Larimer
• Stream Name: Big Thompson
• Basin: South Platte
• Title: Flood History of the Big Thompson River Basin: Flash Floods, Paleofloods, and Dam-Break Floods
• Date: 1/1/1979
• Prepared For: Larimer
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

2

LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES. COLORADO

depthe would have averaged 0.6 foot lower; the flood wave would have
reached Estes Park at the same time. For tbis hypothetical case.
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 sacond greater at Estes Park. Meximum flood
depth would have averaged 2.7 feet higher; the flood wave would have
reached Estes Park 0.4 hour earlier. The model s1so 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 faiJad (or had not been present). peak
discharges would have been reduced by 11.300 cubic feet per second
;mm""etely downstream from the dam to 500 cubic feet per sacond
less at Estes Park. Meximum flood depths would have averaged
0.6 foot Iowar. end the flood wave would bave 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 2&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 ft3/s of water down
the Roaring River (fig. 1).

I started to hear a sound liks ao airplane. Also, there were loud booms.
It got louder aod louder. I thought It was breaking the sound bar-
rier. I kept looking for a plane. but couldn't see ons.l got suspicious
aod started to look upstream. I saw trees crashing over end a wsll
of water coming down. I started to run as fast as I could for high
ground. There was a deafening roar. I fell end got up aod kept runn-
ing. I stood on high ground aod watched it wipe out our campsite.
It knocked everything in its path over; Steve didn't staod a chaoce.

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 ft3/s; however. discharge increased to 16.100 ftSls
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

4-1

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 ft'/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 snmmary 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. AlIUl
Pearson, Colorado Department of Natural Resources,
Division of Water Resources. Office of the State Engi-
neer, provided information on the history and cause~
of failures of the dams. William Stanton, Colorad<
Department of Natural Resources, Colorado Watel