Dam Failure Case Study: Bonasa Breaks Ranch Dam (Washington, 2017)

This small, private dam in southeastern Washington breached on April 13, 2017, resulting in the rapid, uncontrolled release of 29 acre-feet of reservoir. Flood water traveled down a narrow canyon and emptied into the Grande Ronde River, causing destruction to timber and wildlife habitat. The earthen dam, originally used to form a stock pond, existed as early as the 1960s and was raised to a height of 19 feet in 2006-2007. The embankment consisted of a non-zoned section composed of homogeneous earthfill, primarily silty sand and sandy silt. A 3-foot-diameter corrugated metal pipe was embedded in the raised section of the embankment to serve as an outflow conduit. The dam raise triggered State jurisdiction. However, the dam raise was unpermitted and unknown to the State; there was no documentation of engineering design or construction control. Although the dam failed during late winter thaw, the antecedent rainfall was minimal and the highwater mark was below the crest. Based on observations in the breach, the failure appears to have occurred from backward erosion piping. The breach was caused by a combination of (a) the absence of engineering design; (b) improper earthwork; and (c) the lack of engineering oversight, monitoring, and testing during construction.