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

DESCRIPTION OF FLOOD-WAVE CHARACTERISTICS

27

amount of debris. The leading edge of the flood wave was
moving at an average of 9.1 mi/h in the Roaring River
(fig. 15). The peak probably was very close to the leading
edge of the flood wave (see "dam.Break Modeling"). The
relatively slow speed of the leading edge was due to
retardance by the vegetation and resulting debris dams.
The leading edge probably was very similar to the
1889 flood that resulted from the failure of South Fork
dam near J obnstown, Pa., which claimed 2,209 lives. In
the 1889 flood, eyewitnesses described the flood as car-
rying extremely large amounts of debris and traveling
much slower than expected.
From time to time as the flood entered narrow places in the valley,
the massed debris acted as a dam and the giant flow seemed to slow
and stop; then the front would boil and seethe and huge trees, ejected
by overwhelming pressures, would shoot into the air as the flood o~ce
more surged ahead. (Clark, 1982, p. 141).
Ample evidence of large debris dams remained in the
Roaring River (fig. 17). The 1982 flood also carried large
amounts of debris, including very large boulders
(fig. 1GB).
For these conditions, the theory and application of
conventional energy and flow-resistance concepts (such
as Manning's n-values) probably are not applicable. The
occurrence of numerous debris dams caused localized
backwater, resulting in predominately subcritical flow.
However, when these debris dams break, flow probably

was supercritical for a short distance until another
debris dam formed. Thrbulence was extremely high, as
observed by Stephen Gillette at Horseshoe Falls, where
he saw boulders and trees being thrown into the air.
Across the large alluvial fan formed at the base of
Horseshoe Falls, water spread. The depth, width, veloci-
ty, or cross-sectional area of flow are unknown. Early
in the flood, the flow path was down the major axis of
the fan. This was the area of the fan where the largest
boulders and thickest sediments were deposited. Soon
after the flood wave arrived, the main flow path became
plugged with sediment and debris; on the falling limb
of the hydrograph, the main flow gradually shifted to
the right side of the fan. This migration of the main flow
effects from the left to right parts of the alluvial fan is
indicated by the distinctly finer-grained deposits on the
right side of the fan, flow paths visible on large-scale
aerial photographs, and the geographic position of the
boulder berms formed during the first few minutes of
the flood.

FALL RIVER, HORSESHOE FALLS
TO CASCADE LAKE DAM

The confluence of the Roaring River with the Fall
River is at the upstream end of Horseshoe Park (fig. 1).

FIGURE 17.-The remains of a large debris dam in the Roaring River valley near river mile 3.21.