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The AAPG/Datapages Combined Publications Database

Utah Geological Association

Abstract


Engineering and Environmental Geology of Southwestern Utah, 1992
Pages 1-7

1983 Flood Failures and Repairs of DMAD and Gunnison Bend Dams

W. James Palmer

Abstract

On June 23, 1983 the spillway on the DMAD dam collapsed. The collapse resulted in the breaching of the reservoir and extensive flooding downstream. Millard County emergency officials and Gunnison Bend Reservoir Company officials decided to intentionally breach the Gunnison Bend Reservoir located approximately 20 miles (34 km) down river from the DMAD reservoir. The consensus was, a controlled breach would do less damage than an uncontrolled breach when the floodwaters of the DMAD failure reached the Gunnison Bend dam. Several small farming communities were impacted by the flood as well as highways, businesses and farms.

The Sevier River had been flowing at flood stage for several weeks prior to the collapse and the spillways of both dams controlled the overflows. Just prior to the collapse, flows over the spillway had started to recede slightly. An old outlet structure from an abandoned dam located about one mile (1.7 km) downstream of the DMAD dam failed ten to 12 days earlier. The DMAD spillway collapsed due to severe headcutting of the riverbed below the spillway.

Local forces tried to stop the riverbed headcutting by placing old car bodies weighted with concrete in the river. Such efforts may have slowed the progress of the headcutting, but only slightly. Headcutting of the riverbed was approximately ten to 15 feet (3 to 4.5 m) deep.

The day after the collapse, officials and staff from the Division of Water Resources conducted an on-site inspection of the damage to the DMAD and Gunnison Bend Dams. The first priority was to construct temporary pumping stations along the river to provide irrigation water to 10,000 acres (4,050 ha) of farmland. The second priority was, to initiate repairs to the two dams. These repairs consisted of breach repairs and the construction of a new spillway for each dam. The new Gunnison Bend spillway was a modified chute with a Type II stilling basin. Due to severe headcutting of the riverbed and the uncertainty of the stability of the new riverbed below the DMAD dam, a baffle-apron spillway, which does not have a stilling basin, was chosen for the DMAD dam. Construction of the new spillway at DMAD was completed in January 1984 and within 13 hours water flowed through it and continued for several months.


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