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

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DAM.BREAK MODELING

53

EXPLANATION

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4000

4,7 MILES DDWNSTREAM FROM
LAWN LAKE DAM

. OBSERVED LEADING EDGE
ARRIVAL TIME

X OBSERVED PEAK TIME

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01530

0630

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0730 0830 0930
TIME OF DAY IMOUNTAIN DAYLIGHT TIME I. JULY 15. 1982

1030

1130

FIGURE 46.-Selected modeled and observed hydrographs.

of breach width were made for Lawn Lake dam. Hypo-
fuetical breach (bottom) widths of 25 ft and 200 ft were
compared with model results of the actual breach width
of 55 ft. For consistency, these scenarios were compared
to the calibrated model results. This range in breach
widths was suggested by McMahon (1981). Outflow
discharges for hypothetical breach widths for a time of
failure of 10 min for Lawn Lake dam are shown in figure
48. This figure demonstrates how much greater
discharge could have been. Results of these model
simulations are summarized in table 10 for a breach
width of 25 ft, and in table 11 for a breach width of
200 ft.
Data in table 10 indicate that, for a breach width of
25 ft. peak discharges would have been 7,000 ft'/s less
downstream from Lawn Lake dam, to 1,300 ft'/s less
at the downstream end of the study reach at mile 12.5.
Maximum flood depths averaged 0.6 ft lower. Flood

wave traveltime was the same. Model results also in-
dicated that. even with this hypothetical breach width
of 25 ft, the resulting flood would have overtopped
Cascade Lake dam by 4.2 ft and would have resulted
in the failure of the dam,
For a breach width of 200 ft, peak discharges would
have been 22,600 ft'/s greater downstream from Lawn
Lake dam, to 5,400 fts/s greater at the downstream
end of the study at mile 12.5 (table 11~. Maximum flood
depths averaged 2.7 ft higher. The flood wave would
have reached Estes Park 0.4 hour earlier. Model results
(tables 10 and 11) indicate that hydraulic differences as
a result of breach-width size decrease in the doWnstream
direction. and with sufficient distance. would converge
to similar flood hydrographs.
The model also was used to evaluate a worst-case
scenario for the failure of Lawn Lake dam that would
reflect a very abrupt and almost complete breach of the

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