| Centralia, Washington is located in the southern end of the Puget Trough about 135 kilometers south of Seattle. At about 10:15 AM on October 5, 1991 the hillslope under the southwestern side of a concrete-lined water-supply reservoir used by the city of Centralia located on Seminary Hill suddenly failed. A roily mass of water, vegetation, and sediment flowed down a small, steep tributary into the eastern part of the city, destroying two houses, flooding scores more, and forcing the evacuation of 400 people. The dam failure occurred on a clear sunny morning after a prolonged period of dry weather. Temperatures were well-above normal in August and September, and no measurable rainfall had occurred for 35 days prior to the failure (NOAA, 1991). 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. -- Costa, 1994 |
