FLOOD09052

• County: Routt
• County: Moffat
• Basin: Yampa/White
• Title: Dam Failure Floodplain Information for the Yampa River Downstream of Stagecoach Reservoir
• Date: 12/1/1989
• Prepared For: Moffat County / Routt County
• Prepared By: CWCB
• Floodplain - Doc Type: Flood Documentation Report

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9.0 Summary

Stagecoach dam was assumed to fail "in the dry" with
the water at the elevation of the spillway crest. The
dam failure flood would then travel approximately six
miles downstream into Lake Catamount which is assumed
to be full to the elevation of its two spillways.
These spillways will begin to flow as soon as the flood
wave arrives at the upstream end of Lake Catamount.
When the water level in Lake Catamount reaches the top
of the dam, the embankment was assumed to fail. At the
same time, water will also begin to flow over the top
of the dam.

The above scenario is an extremely remote event and
this analysis is not intended to reflect upon the
integrity of Stagecoach dam or Lake Catamount dam.

The resultant flood was routed through the Yampa
River valley for a distance of approximately 115 miles
through Routt and Moffat Counties. A total of 16
valley cross sections were used in the analysis.

The dam failure floodplain was drawn on USGS quad
maps using flood depths from two dam failure computer
models recognized in Colorado. These are the methods
developed by the Colorado Division of Water Resources
(DWR) , also known as the State Engineer, and the
National Weather Service (NWS).

The results of the two models varied greatly both
in peak discharge, flood depths, and travel times. The
differences in flood depth may be attributed to the
difference in the way of representing the cross
section. The DWR model is based ona trapezoidal
cross section and the NWS model fits a mathematical
curve through hypothetical coordinates. With this
difference in mind, engineering judgement was used to
arrive at a flood depth for mapping. The mapped depth
is the vertical distance above the low flow shown in
the channel on the USGS quad maps.

The travel times shown in Table 8 recommended for
use in emergency action planning are based on the NWS
model. The average speed of the flood downstream from
Steamboat Springs decreases from about 5.4 miles per
hour at Milner to about 4.8 miles per hour at Craig.
The approximate travel times are starting from the time
of failure of Stagecoach Dam.

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