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

HYDROLOGY, GEOMORPHOLOGY, AND DAM-BREAK MODELING OF THE
JULY 15,1982, LAWN LAKE DAM AND CASCADE LAKE DAM
FAILURES, LARIMER COUNTY, COLORADO

By ROBERT D. JARRETT and JOHN E. COSTA

ABSTRACT

At approximately 0530 Mountain Daylight Time on the morning
of July 15. 1932. Lawn Lake dam, a 26-foot-high earthen dam located
in Rocky Mountain National Park. Colorado. failed. The dam released
674 .......feet of water end ao estimated peak discharge of 18.000 cubic
feet per second down the Roaring River vaHey. Three people were killed
end damagee totaled $31 million. The Colorado State "'njpn_ deter.
mined that the probable cause of failure was deterinration of lead caul...
ing used for the COlIDOClion between the outlet pipe end the gete valve.
The resultingJeak eroded the earthfill. end progressive piping led to
failure of the embankment.
Floodwaters from Lawn Lake dam ovartopped a second dam.
Cascade Lake dam, located 6.7 miles downstream. which also failed.
Cascade Lake dam. a INoot high concrete gravity dam. 12.1 acr..
foot capacity dam, faiJad by toppliug with 4.2 feet of water flowing
over Its crest. Ths flood contiuuad down the Fsll River into the town
of Estes Park. which received extensive damage from the overbank
flow.
This report presents the settiug. a summary of the causes of the
dam failures. the hydrologic data, end geomorphic effects of the flood.
Data on dam-breach floods on high-gradient streams are limited. A
dam-break computer model was used to evalute the model's capebil-
ities on high-gradient streams. to enhance end provide supp1ementel
hydrologic information. end to evaluate various hypothetical soenarloe
of dam-breach development end probable impact of the failure of
Cascade Lake dam.
Flood data were obtained at two gaging stations (06732500 Fsll
River at Estes Park end 06733000 Big Thompson River at Estes ParkI
end five miscellaoeons sites downstream from the Lawn Lake dam.
Peak diec:herges were determined using a variety of indirect methods.
Because of extensive scour end erosion along the Roaring River. peak
discharges were eetimated from the dam-break modeL Celeulated peak
discharges for the flood were 18.000 cubic feet per second from Lown
Lake dam, 12.000 cubic feet per second at Honesboe Falls where R0ar-
ing River joins the Fsll River. 7.210 cubic feet per second into Cascede
Lake dam at the east end of Horseshoe Park. 16.000 cubic feet per
eecond from the failure of Cascade Lake dam. 13.100 cubic feet per
second about 1 mile downstream from Cascade Lake dam, 8.520 cubic
feet per second just upstream from Estes Park. 6.550 cubic feet per
second for gaging statioo 06732500 Fsll River et Estes Park. end 5,500
cubic feet per second for gaging etatioo 06733000 Big Thompson River
at Estes Park. Meximum depthe raoged from 6.4 to 23.8 feet; mex-
imum widths raoged from 97 to 1.112 feet; end meao velocities renged
from 3.3 to 12.6 feet per second. Traveltimes of the flood were deter-
mined from eyewitness accounts. The leading flood wave took 3.28
hours to travel 12.5 miles lavarage 3.8 miles per hour). Flood peake
were 2.1 to 30 timea the 5OQ.year flood for selacted locations elong
'the flood psth. Geomorphic end sedimentologic evidenea suggest that

It probably was the largest flood in these basins. at least mea the
retreat of the glaciers several thousaods of years ago.
Geomorphic effacts of the flood resulting from the dam failures were
profound. Chaonels were widened tens of feat end scoured from 5 to
50 feet locslly. In the Roaring River vsllay. alternate river reaches
were Bither scoured or filled. depending on vsllay slope. Generslly.
reaches steeper thao 7 percent were scoured. aod reaches less thao
7 percent were filled. In the Roaring River. 56 percent of the chanuel
was scoured. some by as much as 50 feet. end 44 percent was filled
with coarse sediments. 2 to 8 feet thick.
An slluvial fao of 42.3 acres. containing 364.600 cubic yards of
material, was deposited at the mouth of the Roaring River. The fao
has a meximum thickness of 44 feet aod ao average tbickoess of 5.3
feet. The largest boulder thought to have moved in the flood.
14X17.5X21 feet end weighing ao estimated 452 tons. was located
on the slluvial fao. Down the flow lllds. average particle size chaoges
from 7.S-foot boulders to fine saod end silt in a d1staoce of 1.900 feet.
The slluvial fao dammed the Fall River. forming a lake of 17 acres
upstream from the fao.
Satisfactory results were obtained from the dam-break model. but
not without significant dlffu:uItie. in proper operation of the model.
To calibrate the model, Maoning n-values between 0.1 aod 0.2. or ao
average of 78 percant greater thao field-selacted values. were required;
subcritical flow was verified. The occurrenca of numerous debris dams
caused localized backwater. resulting in predominaotly subcritical
flow. However. when these debris dams broke. flow probably was
eupercritical for a short d1staoea until another debris dam formed.
Without the extensive calibration of the model aod the assumption
of subcritical flow. results would have been signiflcaotly different.
Peak discharges from dam-break modeliog reflact water-only
discharges; totel discharge may have been considerably higher on the
Roaring River end on the Fsll River immediately downstream from
Cascade Lake dam from sediment end debris. At Horseshoe Falls aod
for a obort reach downstream from Cascade Lake dam, geomorphic
aod sedimentologic evidence indicates the flow tamporsrily became
a turbulent, high-concentration. coheslonless sedi",...t-gravity flow.
The sediment end debris may have bulked the peak water flow by 50
to 60 percent. The range of difference of observed aod modeled peak
discharges varied from -3.200 cubic feet per eecond to 600 cubic feet
per second. The range of difference of observed and modeled rr'lAThnurn
flood depth was -1.3 to 2.6 feet end averaged 1.0 foot. The raoge of
difference of observed aod modeled lesdlng edge of traveltime was
-0.4 aod 0.15 hour.
Comparisons were made for hypothetical breach widths of 11) 25
feet end (2) 200 feet. Thay were compared with model resuIts of the
actual breach width of 55 feat:
1. For a breach width of 25 feet, the peak discharge would have been
7.000 cubic feet per second less downstream from Lawn Lake dam
to 1.300 cubic feet per eecond less at Estes Park. Meximum flood
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