Weathered and weakened portions of concrete contributed to the Oroville Dam's spillway failing last February, causing panic and mass evacuations in Northern California. This was part of the findings by an Independent Forensic Team (IFT), appointed by the California Department of Water Resources (DWR).
The IFT's September 5 preliminary report said that the service spillway chute failure was most likely "initiated by the uplift and removal of a section of the slab in the chute downstream." The team found that once the initial section of the chute slab was removed, the "underlying moderately to highly weathered rock and soil-like material beneath the slab in this location was directly exposed to high-velocity spillway flow."
The report detailed that the high-velocity flow rapidly eroded the foundation materials at this location, removed additional chute slab sections in both upstream and downstream directions, and quickly created the erosion hole that was observed on February 7 at the dam, located on the Feather River east of the city of Oroville, CA. The IFT says its findings are based on eyewitness accounts as well as photo and video records.
Assigned by the DWR last May, the IFT is comprised of a panel of experts in dam safety and construction, engineering geology, hydraulics, hydraulic structures and all things related to dam building. The team was put together with help from the Association of State Dam Safety Officials (ASDSO) and the U.S. Society on Dams (USSD).
The DWR released a statement last week, saying they are committed to applying lessons learned from Oroville and the IFT preliminary report. "The IFT’s findings and recommendations are relevant not just for Oroville or DWR but for all dam owners," reads the DWR statement. "The IFT’s research will push the entire dam safety community forward to make lasting changes and improvements internationally."
As DWR awaits the IFT's final report sometime this fall, it is continuing its approximately $500 million Lake Oroville Spillways Emergency Recovery Project. Led by Kiewit and DWR engineers, the project is currently focused on repairing and reconstructing the gated flood control spillway, also known as the main spillway. This part of the project is slated to be complete by November 1 of this year.
Placement of reinforced, structural concrete is 20 % complete and by November1 there will be 1,220 ft of spillway chute with structural concrete – 870 ft on the upper chute of the spillway and 350 on the lower chute of the spillway. The 1,050-ft middle section of the spillway chute, including filling in two scour holes, is now 33 % complete, with approximately 100,000 cu-yards of roller compacted concrete placed. This middle section will be completed to final design with structural concrete in 2018.
The 730 ft of main spillway leading to the radial gates will be patched, reinforced and left in place this year. It will be removed and reconstructed with structural concrete in 2018, says DWR. The agency also continues to make progress at the emergency spillway, and says it is on schedule to complete construction of the secant pile wall, or cut-off wall, in late December 2017 or early January 2018. Currently, crews have completed 15 percent of the secant pile wall.