Hyatt Dam, a 96-year old dam in Reclamation's inventory, is a classic example of episodic internal erosion during normal operating conditions occurring over time. The homogeneous earthfill dam and its appurtenant structures were constructed in 1922-1923 by the Talent Irrigation District, and transferred to Reclamation in the 1950s. Excessive seepage, principally located on the left side of the dam, was first observed in the mid-1970s, and those initial observations indicated that seepage flows were clear. However, several instances from 1987-2009 indicate that episodic internal erosion was taking place in the vicinity of the downstream toe of the embankment. In 1987, a milky plume appeared in one of the seepage monitoring weirs and in 2000, large quantities of turbid water flowed out of the toe drain pipe during investigative excavations of the toe drain. A drop in toe drain flows in 2009 suggested that the toe drain was plugged, unable to collect and convey seepage from the embankment and foundation. Also in 2009, new point sources of seepage were observed at the downstream toe of the dam and existing seepage areas appeared to have worsened. Field investigations, engineering analyses and subsequent risk analyses indicated that without modification, Hyatt Dam would continue to experience episodic internal erosion, which could progress to dam failure. The estimated high risks for internal erosion of the embankment and foundation overburden materials were driven by unfiltered seepage exiting at the downstream toe of the embankment with sufficient velocity to initiate internal erosion of embankment or foundation materials, and the underlying fractured basalt foundation that was not treated during original construction. Designs were developed in 2016 to modify the dam to lower the risk of an internal erosion failure, and the modifications were completed in 2017. Upon exposure of the basalt foundation, the rock surface was found to vary significantly from what was anticipated during preparation of the modification designs. The undulating nature of the bedrock and existence of steep bedrock offsets and overhangs necessitated implementing design changes during construction to treat the foundation, adjust the new toe drain system alignment to match the existing bedrock surface, and modify filter material placement to address wet areas exposed on the downstream face of the excavated embankment.
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