FLOOD06904

• County: Larimer
• Basin: South Platte
• Title: Hydrology, Geomorphology, and Dam-Break Modeling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures, Larimer County
• Date: 1/1/1986
• Prepared For: Larimer County
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

SUMMARY AND CONCLUSIONS

61

FIGURE 54.- View upstream from Olympus dam and Lake Estes, containing the floodwaters (aerial).

million. The Colorado State Engineer determined that
the cause of failure was deterioration of lead caulking
used for the connection between the outlet pipe and the
gate valve. The resulting leak eroded the earthfill, and
progressive piping led to failure of the embankment.
Floodwaters from Lawn Lake dam overtopped a sec-
ond dam, Cascade Lake dam, located 6.7 mi down-
stream, which also failed. Cascade Lake dam was a
17.ft-high, 12.1 acre-ft capacity concrete gravity dam,
which failed with 4.2 ft of water flowing over the top,
corresponding to a discharge of 4,500 ft'/s. Full-breach
development was estimated to have taken 1 min. The
resulting surge of water (16,000 ft'/s) swept into
Aspenglen Campground, only 1/3 mi downstream, and
claimed the lives of two of the three victims. The flood,
no longer a wall of water, continued down the Fall River
into the town of Estes Park, which received extensive
damage from the overbank flow. The flood resulting
from the failures of the two relatively small dams was
much larger than expected, and was catastrophic in its
geomorphic effects.
Flood data were obtained at two gaging stations and
five miscellaneous sites downstream from the Lawn
Lake dam. Peak discharges were determined from the
slope-area method, from flow over weirs and through

a flume, and from the critical-depth method. Because
of extensive scour and erosion along the Roaring River,
peak discharges were estimated from a dam-break
model. Calculated peak discharges for the flood were
18,000 ft'/s from Lawn Lake dam (dam.break model),
12,000 ft'/s at Horseshoe Falls where Roaring River
joined the Fall River (dam-break model), 7,210 ft'/s
into Cascade Lake dam at the east end of Horseshoe
Park (slope-area' measurement), 16,000 ft'/s from the
failure of Cascade Lake dam (dam-break model),
13,100 fts/s about 1 mi downstream from Cascade
Lake dam (critical-depth measurement), 8,520 fts/s just
upstream from Estes Park (slope-area measurement),
6,550 ft'/s for gaging station 06732500 Fall River at
Estes Park (estimated), and 5,500 ft'/s for gaging sta-
tion 06733000 Big Thompson River at Estes Park (weir
and flume measurement). Maximum depths ranged
from 6.4 to 23.8 ft; maximum widths ranged from 97 to
1,112 ft; and mean velocities ranged from 3.3 to
12.6 ft/s.
The flood peak at the mouth of the Roaring River
(12,000 fts/s) was 30 times the estimated 500-yr flood
for that site. In Estes Park, flood depths were 2.5 ft
above the level of the estimated 500-yr flood. At the
gaging station on the Big Thompson River at Estes