Will a large earthquake do enough damage to Anderson Dam that
Morgan Hill could be under water?
The answer is yes, the Santa Clara Valley Water District says.
But, they’re doing everything to prepare the dam infrastructure
from slumping if a powerful (in magnitude and duration) enough
quake strikes close enough to the dam.
Morgan Hill – Will a large earthquake do enough damage to Anderson Dam that Morgan Hill could be under water?
The answer is yes, the Santa Clara Valley Water District says. But, they’re doing everything to prepare the dam infrastructure from slumping if a powerful (in magnitude and duration) enough quake strikes close enough to the dam.
A crowd of about 60 people filled seats at Jackson Elementary School Wednesday night for a neighborhood meeting about the Anderson Dam seismic study. People questioned the rationale behind conducting a costly study – $3.1 million – and repairs – $50 million – that ratepayers will inevitably pay for an event which the probability is too low, some at the meeting argued.
Preliminary findings from its ongoing seismic stability evaluation by outside experts detail that part of the dam could experience “significant slumping” if a 7.25 magnitude earthquake were to occur on the Calaveras Fault within about a mile of the dam. That magnitude is the measure by which the seismic experts used in its study. Though other combinations of duration and magnitude could cause slumping also.
Frank Maitski, the district’s deputy operating officer, reassured those unconvinced attendees that the study and update to the dam were necessary and furthermore the district, who owns the dam, is legally required to ensure it’s well prepared in case of a natural disaster.
To date, the analysis has found loosely compacted layers of liquefiable materials in the lower portions of the dam. These materials are susceptible to a reduction in strength when subjected to severe earthquake shaking. The consultant will continue the analysis to estimate the potential for damage to the downstream slope. The final report is scheduled to be completed by May.
The district’s dam operators will keep the water at no higher than 57 feet below the dam crest until further analysis is completed. Currently, the water level at Anderson Reservoir is at approximately 60 feet below the dam crest, which amounts to 48,806 acre-feet. The California Division of Safety of Dams and the Federal Energy Regulatory Commission, which also has regulatory authority over Anderson Dam, have stated that an additional safety measure is appropriate, and are reviewing this proposed change in operation.
A few questioned how the district could justify lowering the water level and what effect it could have on the homes nearest to the dam. District officials said human safety is their No. 1 priority and keeping the water level at that height would not affect surrounding homes.
Maitski said the process of studying what needs to be done to construction to completion could take about six years. The district is planning to hold other meetings and hearing to keep the community up-to-date on the progress of the seismic study.
Morgan Hill resident Don Welch, who owns a home close to the dam, was concerned his property value would plummet if the dam needed to be drained or the water level lowered too much; boating, fishing and leisure on Anderson is conducive to assessing his property at a better rate.
“When is the economic study going to be done to make sure our property values aren’t decreasing?” Welch asked aloud. “I really do, we really, need that lake for our property values.”
Maitski responded that the dam was not built for leisure it was constructed for water supply and only with partnership with Santa Clara County parks that it can be used for recreation.
Since January, 2009, Anderson Reservoir has been operating under a water level restriction of 20 feet below the spillway – 40 feet below the dam’s crest. This restriction was set on a review of data that existed at the time, which appeared to indicate the presence of alluvium – gravel and sand from the underlying former creek bed – under the upstream dam shell.
In January, 2009, the district hired a highly qualified geotechnical consulting firm to conduct the evaluation of the dam. Since then, the consultant has drilled borings at various locations on the downstream and upstream faces of the dam and conducted laboratory testing and engineering analysis to determine if the materials used to construct the dam are sufficient to withstand a major quake.
