FLOOD07351

• County: Larimer
• Community: Estes Park
• Stream Name: Lawn Lake
• Basin: South Platte
• Title: Lawn Lake/Estes Park Flood 1982
• Date: 7/15/1982
• Prepared For: State of Colorado
• Floodplain - Doc Type: Flood Documentation Report

..

. Lawn Lake, OC-3'J
July 15, 1982

Paye S

(Between SO anc 100 cfs was flowing lhroU'Jh the breach in the dam at the
time of the lnspection. Assuming that the outlet conduit was closed at the
time the dam falled, all outflows would have had to have ueen passed through
the emergency spillway. L'sing a splllway bottom width of 25 feet and the
formula:

Q = CLH3/2 = (2.67) (25) (H3/2) - YIelds the following results,

H (feet) Q (cfs)

1 66
2 188

Apparently flow depths in the emergency splllway of at least 12 inches would be
required just to pass normal flows even wlthout the existence of driftwood debr'ls
in the channel).

A hand-level was used to measure spillway freeboard from the control
section to the left abutment of the dam. Spillway freeboard was mE';asured
as four feet.

It appears that the water surface elevation was approximately three feet
below the crest of the dam at the time the dam failed.

2. Phvsical dimensions of the breach.

A 100-foot tape and hand-level was used to survey the dam and locate the breach.
The start of survey was a steel pin driven into a log at the right side of the emergency
spillway channel. The survey station 0+00 lS shown in photo 10. End point for the
survey was a focked pine tree with cutoff limb at the right abutment of the dam (See
photo 13.)

The results of the survey work are shown on the attached figures dated 7-16-82.
Fiqure one shows the elevation view survey looking downstream and locates the
bceach. Precise breach dimensions are shown in fiqure two and the eroded channel cross-
section, downstream of the dam is shown~n fiqure three. Fiqure four shows plan view
I
of the breach.

In general, the breach was located near the outlet section and had a depth of 27 feet.
The top width of the breach was 91.5 feet and the bottom width was 58 feet. The
left side of the bceach was essentially vertical.

3. Geoloqic and Geotechnical Findinqs.

The right side of the breach was carefully examlned for signs of weakness which may
have existed within the embankment. The followlng specific items were noted;

a. An organic day/slit seam, 12 inches in thickness, was found running
through the dam approximately 5 feet below crest elevation. This seam