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.
Conclusions
The failure of Reservoir Number 3 on October 5, 1991 in Centralia,
Washington, from a deep-seated bedrock foundation slide is more
than a curiosity. Rapid release of 13,250 cubic meters of water
eroded hillslope deposits, fill, and bedrock. The flood quickly bulked
into a debris flow with an estimated volume of 1,800 cubic meters that
swept into the eastern edge of the city of Centralia. Geomorphic
and sedimentologic evidence can be used to document that the
dam-break flood had at least two phases - initially a debris flow
that was quickly followed by a water flood whose maximum stage was
about one-half meter higher than the debris flow. Flood peak discharge
is calculated to have been 71 cubic meters per second using the
slope-area method in which roughness coeficients were field-selected
after being bracketed by calculations that determine only grain
roughness (a minimum value), and total roughness in steep, fixed-bed
channels (a maximum value, because the stream-bed material here
was entirely mobile). The resulting discharge of 71 cubic meters
per second is consistent with estimates derived by considering the
rate of emptying of the reservoir, and a simplified slope-area
relation that substitutes slope for flow resistance. The flood
peak was in the supercritical flow regime for at least 200 m through
the slope-area reach. A reconstructed hydrograph of the flood
indicates the duration of flow past the slope-area site located
275 m downstream of the reservoir was 6.2 minutes and the entire
flood was over within about 7.3 minutes.
The foundation failure of Reservoir Number 3 resulted in a rapid
draining of water. The resulting flood a short distance downvalley
was large considering the potential energy of the water prior to
the failure when compared with other historic constructed dam
failures. Plotted in this manner, the Centralia flood, along with
two other rapid-foundation failure dam break floods, defines
the empirical limit for flood peak discharge associated with the
failure of constructed dams. These results reaffirm that dam
failure floods, while rare, are important hydrologic events
that need to be carefully documented because such floods are
relatively rare compared with rainfall-runoff or snowmelt floods,
and can occur during sunny, pleasant weather without any
precursory indications. Floods from the failure of dams in small
upland basins present unique challenges and considerations for
public safety.
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01/17/01, Lyn Topinka