Case History of 2002 Swift No. 2 Dam Failure

Swift No. 2 Dam failed suddenly in the early morning of April 21, 2002, following 44 years of operation. The 38 foot tall embankment failed in under 4 hours, releasing flows up to 9,000 cfs and resulting in damage to the adjacent highway, powerhouse, and switchyard but no injuries or fatalities occurred. Following failure of the embankment, an extensive post-failure investigation found that a seepage path and backward erosion piping through erodible sandy soils was able to progress under the dam due to a roof formed by a basalt flow. The void that resulted from the backward erosion piping was ultimately estimated to be approximately 7 acres in size and up to several feet in height, that led to a “blowout” of the toe, collapse of the basalt and overlying embankment, and breaching of the embankment dam. This paper summarizes the history of the project, the causes that led to the failure as determined from the post-failure investigation, the impacts of the failure, and probable failure mode as determined by the post-failure investigation.