FLOOD02077

• County: Larimer
• Community: Larimer County
• Stream Name: Lawn Lake Dam
• Title: Hydrology, Geomorphology, and Dam-Break Modelling of the July 15, 1982 Lawn Lake Dam and Cascade Lake Dam Failures
• Date: 1/1/1983
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

Big Thompson River and tributaries above Estes Park, that is, above an eleva-
tion of 7,500 ft. This was consistent with the results of Jarrett and Costa
(1983) in whi ch streamflow records of 69 unregul ated streams in the South
Platte River, the Arkansas River, and the Colorado River basins were examined
to separate peak discharges from snowmelt and rainfall runoff during each
water year. Above about 7,500 ft, snowmelt is the major cause of flooding,
not large, intense rainstorms.

The failure of the lawn lake dam resulted in a peak flow of 5,500 ft3/S
at streamflow-gaging station 06733000 Big Thompson River at Estes Park
(Site 6), just upstream from lake Estes. Data presented in figure 13 indicate
that the 1982 dam-break flood was 2.1 times the 500-year flood for this
location. In Estes Park, flood peaks were 2.5 ft above the level of the
500-year flood (Colorado Water Conservation Board, 1983). The previous flood
of record, from a rapidly melted snowpack, was 1,660 ft3/s in June 1965
(tab 1 e 2); thi s fl ood was estimated from the compos ite fl ood- frequency curve
in figure 13 to be about a 15-year flood.

Upstream on the Fall River at its confluence with the Roaring River
(elevation 8,550 ft), the flood peak (water only) was estimated to have been
12,000 ft3/s (see "Dam-Break Modeling" results). This represented a unit dis-
charge of 1,000 ft3/s/mi2, an unprecedented value in historic times for the
Colorado Front Range above an elevation of 7,500 ft. A regionalized flood-
frequency curve for the mouth of the Roari ng Ri ver is shown in fi gure 14,
constructed from regional regression equations for mountain areas in Colorado
(McCain and Jarrett, 1976). Based on historic flow data under the present
climate regime, the estimated 500-year flood for this location on the Roaring
River is 400 ft3/s. The 1982 dam-break flood was 30 times the 500-year flood
for this stream; a flow of 12,000 ft3/s would be very rare,

Along the Roaring River, deposits of Pleistocene ground moraine and
outwash were extensively scoured and eroded. A late Pleistocene end moraine
at an elevation of 10,900 ft (Richmond, 1960) was overtopped and deeply
eroded. In July 1983, extensive field work was conducted in Rocky Mountain
National Park to investigate whether or not any stratigraphic or geomorphic
evidence existed of comparable large postglacial floods in any of the streams
draining into lake Estes, The sediments and landforms produced by the lawn
lake dam failure are so distinctive that the evidence of any comparable
floodflow in the Holocene should be easy to recognize, No unequivocal evi-
dence of a similar large flood was found in any stream valley that drains into
Lake Estes. The dam-break fl ood of 1982 was very 1 i ke ly the 1 argest flood
that has occurred in the Roaring River, Fall River, and Big Thompson River
since at least the last glacial retreat, about 10,000 years ago.

Along the Fall Ri ver between the Roari ng Ri ver confl uence and Cascade
lake dam, slopes are very gentle, and no extensive modifications occurred,
The flood surge of 16,000 ft3/S from the Cascade lake dam fai lure caused a
large amount of scour and erosion along the Fall River between the dam and
just below Aspenglen Campground. Extensive Pleistocene moraine deposits were
scoured and reworked, The flood surge from the Cascade lake dam failure may
have been the largest flow in this stretch of the Fall River since the drain-
ing of the glacial lake in Horseshoe Park. This unquestionably was an ex-
tremely rare discharge. These estimations of the flood frequencies from the

29