USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Multiple Flow Processes Accompanying a Dam-break Flood in a Small Upland
Watershed, Centralia, Washington
-- John E. Costa, 1994,
Multiple Flow Processes Accompanying a Dam-break Flood in a Small Upland
Watershed, Centralia, Washington:
U.S. Geological Survey Water-Resources
Investigations Report 94-4026, 24p.
Abstract
On October 5, 1991, following 35 consecutive days of dry weather,
a 105-meter long, 37-meter wide, 5.2-meter deep concrete-lined
watersupply reservoir on a hillside in the eastern edge of Centralia,
Washington, suddenly failed, sending 13,250 cubic meters of water
rushing down a small, steep tributary channel into the city.
Two houses were destroyed, several others damaged, mud and debris
were deposited in streets, on lawns, and in basements over four city
blocks, and 400 people were evacuated. The cause of failure is
believed to have been a sliding failure along a weak seam or joint
in the siltstone bedrock beneath the reservoir, possibly triggered by
increased seepage into the rock foundation through continued
deterioration of concrete panel seams, and a slight rise (0.6 meters)
in the pool elevation. A second adjacent reservoir containing 18,900
cubic meters of water also drained, but far more slowly, when a 41-cm diameter
connecting pipe was broken by the landslide. The maximum discharge
resulting from the dam-failure was about 71 cubic meters per second.
A reconstructed hydrograph based on the known reservoir volume and
calculated peak discharge indicates the flood duration was about 6.2
minutes. Sedimentologic evidence, high-water mark distribution, and
landforms preserved in the valley floor indicate that the dam failure flood
consisted of two flow phases: an initial debris flow that deposited coarse
bouldery sediment along the slope-area reach as it lost volume, followed
soon after by a water-flood that achieved a stage about one-half meter
higher than the debris flow. The Centralia dam failure is one of three
constructed dams destroyed by rapid foundation failure that defines the
upper limits of an envelope curve of peak flood discharge as a function
of potential energy for failed constructed dams worldwide.
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01/17/01, Lyn Topinka