Bureau of Reclamation to reevaluate seismic hazard at Ochoco Dam in Oregon

PRINEVILLE, ORE. — The Bureau of Reclamation announced that a technical review of Ochoco Dam completed in 2010 concluded that further onsite investigations are warranted in order to evaluate the dam’s ability to withstand the forces of a major earthquake. These investigations were prompted by recent reevaluation of the seismic hazard in the region.

Ochoco Dam is located about six miles east of Prineville, Ore., and located within a seismic area known as the Cascadia Subduction Zone. This area has the potential to experience very large magnitude earthquakes and generate long durations of strong shaking. The dam is operated by the Ochoco Irrigation District.

“Ochoco Dam was modified for seepage concerns in the mid-1990s,” said Larry Wolf, Pacific Northwest Region Safety of Dams program manager. “It was evaluated for earthquake loadings but only local earthquakes were considered. Since that time, we have learned more about the potential of the Cascadia Subduction Zone and our understanding of the earthquakes has expanded since the modifications were completed.”

Although the focus of this analysis is Ochoco Dam, the area of impact from a possible 9.0 earthquake in the Cascadia Subduction Zone would encompass a broad geographical area stretching from Vancouver Island in British Columbia to northern California.

“Such an earthquake may impact a large number of Reclamation dams in the region, as well as other vital infrastructure,” said Wolf.

Drilling to test the strength of the dam’s foundation will begin around May 24, and will likely continue for several months. The operation will employ one drill rig and several support vehicles on the downstream side of the dam. Local traffic will not be impacted.

Officials believe that the dam may have gained some strength due to increased loading from the previous safety modifications in the mid-1990s.

“We will coordinate the seismic safety assessment process with the Ochoco Irrigation District,” Wolf said. “The first step is to gather additional geologic information. Experts will use new data from the recent Japan Subduction Zone earthquake to evaluate the potential effects of similar earthquakes in the Cascadia Subduction Zone. All that information will then be used to reevaluate the dam and adjacent structures.”

Ochoco Dam was constructed between 1918 and 1920 by private interests. The dam is a zoned earthfill constructed by hydraulic fill techniques. “Hydraulic fill dams typically do not perform well during seismic events,” Wolf said. The dam was modified by Reclamation in 1949-1950 and then again in the mid-1990s, including removing and replacing the center portion of the embankment. Much of the right side of the dam is of original construction and sits on an ancient landslide.