Even the best laid plans sometimes need a second look.
Like a ready-to-go home improvement project bogged down by an undetected mold infestation and now in need of a design reassessment, the water district’s seismic retrofit of Anderson Dam is now pending evaluation of two alternative designs after an analysis revealed that fault traces along the dam could affect the project’s current design and cause the structure to fail.
Last fall, a geotechnical investigation turned up the fault traces two years after the planning phase was completed and the project design was well under way.
The water district initiated the project to retrofit the seismic stability of the county’s largest reservoir, built in 1950. A 2011 study found that the dam was not capable of withstanding a 7.25 earthquake along the Calaveras fault, or the maximum credible earthquake in that area. Completed in summer of 2013, the project planning phase gave way to a design that proposed enlarging and reinforcing the embankment on both sides of the dam, raising its height to prevent slumping during an earthquake, and modifying the spillway and outlet to prevent a large flooding event. Original project plans and design were based on studies and documents dating back to the creation of the dam.
The two new alternatives focus on further modifying the embankment to fortify the dam’s core. Based on a failure analysis, seismic activity along the recently identified fault traces could cause cracking and erosion of the dam’s core, allowing water to seep through and ultimately cause the embankment to collapse. A failure of Anderson Dam could mean catastrophic flooding as far north as San Francisco Bay and as far south as Monterey Bay, affecting thousands of properties and major roadways.
Due to the potential failure of the dam during an earthquake, Anderson Reservoir has been operating with water level restrictions placed by the Division of Safety of Dams (DSOD) for the last few years. For public safety reasons the reservoir has been limited to about 75 percent of its maximum storage capacity. The maximum water surface level is about 45 feet below the spillway to safely retain water in the event of an earthquake.
Because of the magnitude of risk, the water district wants to secure the public’s safety with the most effective design. During the next few months, staff and consultants will evaluate the two alternatives and develop a proposal to present to the DSOD, the Federal Energy Regulation Commission (FERC) and a board of consultants, composed of third-party geotechnical and dam safety experts. Once the input from the agencies is combined, the project team will prepare a final seismic retrofit plan and present an update to the water district board of directors by the end of the year, including revised project costs and schedule, should there be any changes.
Re-evaluations and adjustments to projects in progress are not a rare occurrence, according to Dave Gutierrez, Chief of the Division of Safety of Dams. When doing a project of this size, there needs to be constant double-checking and cross-referencing before proceeding to the next big step to ensure safety and efficiency.
New obstacles are discovered more often than not, due to the expansive knowledge of earthquake engineering today. California has a complex geological system with a lot of faults, stated Gutierrez in a water district board meeting on June 28. There’s a lot more we know today than we did in the 1950s when the dam was created, he said.
The water district is also carrying out seismic retrofit projects for the Calero, Guadalupe and Almaden dams. Also operating under DSOD storage restrictions, these projects are currently in the planning phase.
Watch the interview below with Dave Gutierrez for more information on the importance of upgrading and re-evaluating our dam structures.
Subscribe for updates on the Anderson Dam Seismic Retrofit Project here.