FLOOD02077

• County: Larimer
• Community: Larimer County
• Stream Name: Lawn Lake Dam
• Title: Hydrology, Geomorphology, and Dam-Break Modelling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures
• Date: 1/1/1983
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

Earthfil1 in the dam was sufficiently fine-grained to be frost-
suscept i b 1 e, Soil-property changes induced by numerous freeze-thaw cyc 1 es
could have been a possible cause of failure, However, no ice lenses nor
frozen ground were found during investigations at the damsite in July 1983;
any changed soil characteristics would be reflected in the geotechnical
investigations of the fill material.

Results of an undrained embankment-stability analysis by the Office of
the State Engineer (1983) indicated a safety factor of only 0.6 existed in the
dam for the assumed phreatic conditions, Assumption of an undrained condi-
tion, based on the results of permeability tests, is unrealistic. Some type
of steady-state drai nage must have exi sted in the dam whi ch kept pore pres-
sures below critical values and prevented the dam from failing.

During investigations at the damsite, numerous pieces of lead caul king
that was used to form the seal between the outlet pipe and the gate valve
(fig, 6) were found. This lead caulking was corroded and deteriorated to the
point that it would be unlikely to form a water-tight seal. The use of lead
caul ki ng to secure the pi pe and va 1 ve was not in comp 1 i ance with ori gi na 1
plans and specifi cat ions approved by the Co 1 orado State Engi neer in 1902,
which indicated that the valve would be encased in concrete.

Cause of Failure

Data gathered by the Office of the State Engineer (1983) indicated that
the most 1 i ke ly cause of the Lawn Lake dam failure was deteri orat i on of the
lead caulking used for the connection between the outlet pipe and the gate
valve. The resulting leak eroded a pipe-shaped cavity in the earthfil1
(called piping), creating a void along the outlet pipe. The leak continued to
remove easily eroded earthfi11 and may have reduced the strength and stability
of the dam embankment by affecting the pore-pressure distribution. Such a
scenario of leakage, piping, and resulting embankment failure required the
prerequisite conditions of a closed or nearly closed outlet gate and a near-
full reservoir, to supply the head necessary for accelerated leakage and
subsequent progressive piping through the years, Just prior to failure, the
outlet gate was nearly closed and the reservoir was full.

The void created along the outlet pipe would have become enlarged by the
internal erosion of embankment material. Campers in the vicinity of Lawn Lake
the night before the failure reported hearing a noise that sounded like strong
wi nds, Thi s sound i ndi cated that the reservoi r may have been di schargi ng
through the dam for at least 3 to 4 hours before the hole enlarged
sufficiently--or embankment failure occurred--above the outlet pipe, causing a
total breach of the dam where the outlet works were located (figs, 7A,B),

Surveyed breach dimensions are: Depth of 28 ft, top width of 97 ft, and
bottom width of 55 ft, Following the complete failure of the dam embankment
at about 0530 MOT, the resulting outflow of water reached its peak discharge
very quickly (estimated to be within 10 min), The resulting flood peak of
18,000 ft3/S (see "dam-Break Modeling") then proceeded down the Roaring River
toward Horseshoe Park and the Cascade Lake dam,

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