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

HYDROLOGIC ANALYSIS OF THE FLOOD

11

FIGURE B.-Empty Cascade Lake prior to failure. View is toward upstream face of dam (date unknown). Photo courtesy
of the National Park Service.

of the dam of 12 ft was used for analysis of the flood
hydraulics. Postflood surveys indicated the capacity of
the dam was 12.1 acre-ft, corresponding to the top of
the dam.

CAUSE OF FAILURE

As early as 0700 MDT on July 15, 1982, a greater
than normal inflow into Cascade Lake was observed.
By 0715 MDT, water was flowing over the top of the
dam. The dam was overtopped for nearly 30 min, to
a maximum depth of 4.2 ft of water, before tipping
over and failing at 0742 MDT. Hydrostatic pressure
of the water on the dam and erosion of the abutments
were the probable causes of the dam failure. The top-
ple failure was recorded photographically by residents
of nearby summer cottages (figs. 9A-D). For com-
parison, figure 9E, taken after the flood shows a view
similar to that in figure 9D. At the time of failure,
inflow into the Cascade Lake dam (Site 1) was ap-
proximately 7,210 ft'/s (determined from a slope-area
discharge measurement upstream from the lake), and

the reservoir capacity due to the additional depth of
water (4.2 ft) above the top of the dam was about 25.1
acre-ft. The toppled dam released a peak flow of 16,000
ft'/s (see "Dam-Break Modeling") down the Fall River
into Aspenglen Campground, about 1/3 mi downstream.
Within 5 min after the dam failure, the flood surge had
passed the campground.

HYDROLOGIC ANALYSIS OF THE FLOOD

The flood resulting from failures of two relatively
small dams was much larger than many engineers would
have expected and was catastrophic in its geomorphic
effect, due to three primary factors. First, the time of
breach development was short for both dams, allowing
stored water to be released rapidly. Second, the width
of dam breach was large for both dams, allowing the
stored water to be released rapidly. Third the slope
of the stream (exceeding 20 percent on the Roaring
River) and narrow channel limited attenuation of the
peak discharge. However, as discussed in "Dam-Break