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

HISTORY OF THE DAMS

Downstream from the Estes Park powerplant, at an
elevation of about 8,000 ft and about 1 mi downstream
from Cascade Lake, the Fall River valley narrows notice-
ably, marking the probable maximum extent of
Pleistocene glacial advances (Richmond, 1960; Jones
and Quam, 1944). Below 8,000 ft, the Fall River flows
down a relatively steep and narrow valley bordered by
numerous bedrock outcrops and thin, small, discon-
tinuous flood plains. The Fall River joins the Big
Thompson River in Estes Park and then flows 0.7 mi
before entering the west end of Lake Estes. Lake Estes
is a large reservoir, formed behind Olympus dam, com-
pleted as part of the Colorado-Big Thompson project by
the US. Bureau of Reclamation in 1948.

HISlORY OF THE DAMS

Two dams in Rocky Mountain National Park failed on
of July 15, 1982-the Lawn Lake dam and the Cascade
Lake dam. A discussion of their histories follows.

LAWN LAKE DAM

Lawn Lake dam was an earthen dam constructed in
1903 by the Farmers Irrigation Ditch and Reservoir
Company to impound additional water in an existing
mountain lake for irrigation storage (fig. 3). Originally
Lawn Lake was a natural moraine-dammed lake with a
surface area of 16.4 acres, based on the original map and
impoundment plans filed with the Colorado State
Engineer's Office (Office of the State Engineer, 1983).
The irrigation company's plans called for an earthen
dam to be 24 ft high to the spillway crest, which would
create a reservoir of 47.1 acres surface area and 811 acre-
ft of storage. However, based on several capacity surveys
and records of the private irrigation company, the height
of the Lawn Lake dam was 20.35 ft to the spillway crest
with a reservoir capacity of 611.96 acre-ft, in September
of 1907. In September 1931, probably in response to
below-average precipitation and dust-bowl conditions on
the Colorado High Plains, the dam was enlarged to a
height of 24 ft at the spillway and a reservoir capacity
of 817.2 acre-ft. This enlargement was never approved
by the Colorado State Engineer (Office of the State
Engineer, 1983). Postfailure surveys (discussed later) in-
dicated that the dam was 26 ft high (water was 24 ft
deep), had a capacity of 674 acre-ft, and was about 560
ft long at the time of failure (figs. 4A and 4B).
Lawn Lake dam was constructed of heterogeneous
local earthfill from the surrounding ground moraine.
These glacial deposits have a maximum thickness of

5

about 25 ft in the immediate vicinity of the dam. The
underlying Precambrian igneous and metamorphic
bedrock is highly fractured and jointed, but fresh. About
4 to 5 ft below the crest of the dam, a 12-in.layer of dark,
organic material appeared to cover the entire dam
(fig. 5). The origin of this organic horizon is uncertain,
but it probably represents a top layer of humus on the
1903 embankment prior to the height enlargement of
4 to 5 ft in 1931.
E arthfill material for the dam is composed primarily
of silty and poorly graded sands with varying amounts
of fine gravels and considerable amounts of organic
material. Geotechnical testing of 20 samples taken from
the damfill material following the failure was conducted
by the Office of the State Engineer (1983). The tested
samples contained 7 to 32 percent nonplastic to high-
plasticity fines; they were classified primarily SP-SM
and SM in the Unified Soil Classification (US. Army
Corps of Engineers, 1953). Permeability testing on three
samples gave results of 0, 0.7, and 6.6 ft/d. Results of
triaxial shear tests on remolded samples are given in
table 1. The remolded samples are probably not
representative of in situ strength properties.

POSSIBLE CAUSES OF FAILURE

Six possible causes of the failure of the Lawn Lake
dam were investigated by the Office of the State
Engineer (1983): (1) Overtopping; (2) earthquake shak-
ing; (3) rodent damage; (4) frost penetration: (5) embank-
ment stability: and (6) piping.
Overtopping and earthquake shaking could be
dismissed quickly as possible causes of failure. Weather
was not a factor in the failure. Rainfall during July
10-15,1982, in Estes Park totaled 0.11 in., and no large
rainstorms occurred in the area during this period. The
sky was clear the morning of the dam failure. Snowpack
was about normal and runoff slightly below normal in
the surrounding region during the winter of 1981-82.
However, based on postfailure surveys of the dam and
reservoir, the high-water line in the reservoir at the time
of failure was about 2 ft below the crest of the dam. The
National Earthquake Information Center in Golden,
Colo., indicated that no earthquakes were detected at
the time of failure; in fact, no earthquake has ever been
measured in the area of the dam (Office of the State
Engineer, 1983).
The possiblity of failure from rodent damage and frost
penetration was more difficult to dismiss. Marmot bur-
rows were observed along the crest and downstream
slope of the remaining dam. Through excavation, per-
sonnel of the State Engineer's Office determined that
the maximum extent of any burrow was 3 to 4 ft, and