FLOOD06021

• County: Larimer
• Stream Name: Big Thompson
• Basin: South Platte
• Title: Flood History of the Big Thompson River Basin: Flash Floods, Paleofloods, and Dam-Break Floods
• Date: 1/1/1979
• Prepared For: Larimer
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

DESCRIPTION OF FWOD.WAVE CHARACTERISTICS

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DISTANCE DOWNSTREAM FROM LAWN LAKE DAM, IN MILES

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. ESTIMATED ARRIVAL TIME
I RANGE OF ESTIMATED ARRIVAL
TIME
x GREATER LEVEL OF ACCURACY
ASSOCIATED WITH ARRIVAL
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EXPLANATION

o CASCADE DAM FAILS
9.1 TRAVELTIME OF LEADING EDGE
OF FLOOOWAVe. IN MilES
PER HOUR

FIGURE 15.-ArrIval time, peak time, end traveltime of leadiog edge of flood wave of July 15.1932. (Modified from Graham end B.owo,I983).

ROARING RIVER: LAWN LAKE DAM
TO HORSESHOE FALLS

As discussed in the early section of this report, "The
Setting." the Roaring River is a small. high-gradient
stream (fig. 2). A photograph of the mouth of the Roar-
ing River prior to the flood taken in 1978, showing the
small. heavily vegetated, high-gradient channel, is
shown in figure 16A. The Roaring River streamflow

prior to the flood probably ranged from 25 to 50 fts/s.
A photograph taken after the flood at the same loca-
tion is shown in figure 16B. The sudden breach of Lawn
Lake dam resulted in a flood wave with a characteristic
"wall of water;" in this case, that "wall" was estimated
to be 25 to 30 ft high by a number of eyewitnesses. The
leading edge of the wave front. although very steep.
probably was not a vertical wall of water. This steep
wave front probably was accentuated by the large

4'1-