The reservoir that failed was named "Reservoir Number 3." This reservoir is 105 m long, 37 m wide, 5.2 m deep, contained 13,250 cubic meters of water, and was constructed in 1914. It was one of two adjacent reservoirs constructed of unreinforced concrete panels to store water from well fields for the water supply of the City of Centralia. The second reservoir ("Reservoir Number 4") is 121 m long, 39 m wide, 6.1 m deep, stored 18,900 cubic meters of water and was constructed in 1926. Both reservoirs have 1:1 interior sideslopes. The reservoirs were excavated into bedrock below original ground level, and some of the excavated material was used as fill on the west side of the hillslope. The embankment failure under Reservoir Number 3 caused the service and drain pipes connected to the larger second reservoir (Reservoir Number 4) to break, allowing uncontrolled release of an additional 18,900 cubic meters of water through a 41-centimeter-diameter pipe over the next several hours.
This report presents documentation of the failure mechanism, peak discharge, geomorphology, and sedimentology of the failure of a constructed dam in a small upland watershed. Such floods are poorly documented compared with rainfall-runoff or snowmelt floods, and present unique hazards because dam failures and resultant flash flooding can occur at any time without warning, even during sunny weather.
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