Investigating and Addressing Seepage and Slope Stability Issues at Oroville Dam

Oroville Dam with a maximum height of 770 feet is the tallest dam in the United States. It is owned and operated by the California Department of Water Resources (DWR). Many people are familiar with the 2017 Oroville Dam Spillway Incident that damaged the concrete chute of the gated control spillway (Flood Control Outlet) and the resulting 2-year urgent replacement of this structure. The 2017 Spillway Incident did not threaten or involve the embankment dam in any way. What most people are unaware of, however, is that during this same period, there were questions being raised by regulatory agencies and by various individuals in the media about seepage and slope stability issues associated with the embankment dam, issues that were unrelated to the spillway or the 2017 Incident. One of the issues raised was the presence of a green, vegetated area located at about mid-height on the downstream face of the dam, known locally as “The Green Spot.” Other issues raised during this time involved the fact that while Oroville Dam was well-instrumented with many performance monitoring instruments during its original construction in the 1960s, these instruments had failed over time and after 2000, there were no longer any reliable piezometers in the dam. In addition, the accuracy of the seepage measured at the toe of the dam using a relatively sophisticated and elaborate way of collecting and measuring the seepage was called into question. This was due to the very low seepage that was being measured by the system. Some of these concerns were voiced in a 2014 report by the Federal Energy Regulatory Commission (FERC) Ninth Part 12D Independent Consultants. This Independent Consultants report called for major new investigations and evaluations of the materials placed in the dam, seepage through and beneath the dam, and new slope stability analyses. Particular emphasis in these investigations was to be given to “The Green Spot.” (Continued)