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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Using similar triangles, the flow into Lake
Catamount at failure is estimated at about

,

qf = (qp) (tb -tf ) / (tb )
= (474,291)(1.70 0.33) / 1.70
= 382,223 cfs.

The flow through the spillways at failure is
estimated at

q. = 26,000 + 4,900
= 30,900 efs.

The volume of water that flows through the spillways
at Lake Catamount while the freeboard storageuspace is
being filled is estimated at

v. = 1/2 (30,900) (0.33) (3,600/43,560)
= 421 acre feet.

Based on the geometry of the dam and the volume and
the surface area of the reservoir, the peak flow
through the breach was calculated by computer program
HAZARD2.BAS at 63,951 cfs. The sum of the discharge
through the breach and through the spillways is

63,951 + 30,900 = 94,851 cfs.

Since the rate of flow into the reservoir at the
time the flood water reaches the level of the crest of
the dam is greater than the rate of flow out of the
reservoir, the water will continue to rise and start
flowing over the top of the dam. This rise will
continue until equilibrium is reached, or when the rate
of flow into the reservoir equals the rate of flow out
of the reservoir.

The maximum discharge over the top of the dam was
estimated by simultaneously solving three independent
equations. These equations are based on an estimate of
the additional volume in the reservoir during overflow
assuming an average surface area of 1200 acres, the
slope of the inflow hydrograph, and the equation for
flow over a broad crested wier. The peak rate of flow
over the top of the dam is estimated at 32,400 cfs which
will occur at a depth of 9.0 feet at approximately 0.91
hours after water begins to flow over the dam.

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