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Keechelus Dam

Keechelus Dam
 
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Yakima Project
 
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Keechelus Dam was constructed at the lower end of a natural lake and is on the Yakima River, 10 miles northwest of Easton. This earthfill structure, completed in 1917, is 128 feet high and contains 684,000 cubic yards of material. Keechelus Lake has an active capacity of 157,900 acre-feet constructed over a natural lake having unknown dead storage capacity. Total rehabilitation of the outlet works and control tower was started in 1976. The two original cylinder gates were replaced by a single 8.5-foot-square hydraulically operated slide gate, and a new concrete chute and stilling basin that is 156 feet long, 18 feet wide, up to 28 feet deep were constructed. The concrete outlet conduit was partially lined with reinforced concrete, and a 22-inch-diameter pipe was installed in the outlet conduit to bypass minimum flows for fishery and stream enhancement when the outlet gates are closed.

In mid-1998, it was determined that dam safety deficiencies existed at Keechelus Dam due to the potential for dam failure from piping and/or internal erosion of embankment materials. A reservoir operating restriction to elevation 2510 feet was imposed together with increased monitoring and surveillance pending implementation of corrective actions. This operating restriction limited storage to 140,920 acre-feet. Modifications were performed on the dam between April 2002 and November 2003 under Reclamation's Safety of Dams Program. Modifications included removing and rebuilding much of the dam cross-section to include embankment zones and drainage features, construction of a bentonite slurry cutoff wall in the right abutment, and construction of a downstream drain along the entire length of the dam -- all to control seepage and prevent internal erosion of the new structure. The reservoir operating restriction was lifted in early 2004.

Geology

Bedrock in the vicinity of the dam is rhyolite of the Naches Formation, which crops out on the left side of the spillway and provides the foundation for the spillway structure and the left abutment of the dam.  Forming most of the foundation for the dam embankment are two Quaternary-age glacial units that extend across the Yakima River valley floor.  The older and more extensive unit is Quaternary Glacial Drift, deposited in a terminal moraine to unknown depths.  Quaternary Outwash Sediments overlie a portion of the glacial drift and form the shallow foundation of the dam to the right of station 44+00, to a maximum known thickness of 42 feet.  Both units are generally dense, as indicated by the majority of standard penetration test (SPT) blowcounts and crosshole shear wave velocities.

Differentiation between the two units is principally based on percentage of gravel and oversize materials (which is higher in the glacial drift), and on density (indicated by field hand test, blowcounts, and shear wave velocities).  The glacial drift seems to have a very low permeability, leading to the assumption the extensive seepage seen downstream of the dam either passes through the outwash sediments, or along the contacts with either the embankment or the glacial drift.


Last updated: Jul 09, 2012