California reservoir needs to be drained due to risk of failure

California officials decided to drain a reservoir as its dam could fail under a strong seismic event.

The Anderson reservoir, an artificial lake in Californian Santa Clara County, was created in 1950 by the construction of the Anderson Dam, an earth dam that reaches a height of 73 meters. 

The volume of the lake is large in comparison to neighboring reservoirs. Its total volume is 111 million cubic meters accounting for 53% of the total district's capacity distributed in 10 reservoirs.

11 years ago, officials announced that the reservoir should not exceed 74% of its total capacity, as it is constructed along the Calaveras Fault, a major branch of the San Andreas Fault System, and, hence, a powerful earthquake (6.6 with the dam at the epicenter, or 7.2  within 2 kilometers of the dam) could potentially result in a dam collapse. In that scenario, a 10-meter height wave would strike Morgan Hill, a city in Santa Clara County, and another 2,4-meter height wave would hit San Jose.

Today, the reservoir is below 30% of its total capacity due to a drought period that has struck California. It has been deliberately kept below 45% since Autumn, 2019.

According to the Santa Clara Valley Water District, a plan to upgrade the Anderson Dam that would cost $563 million, was introduced in 2012. Reconstruction works would initiate in 2022, but, a lack of permits has delayed the procedures. “Valley Water has been working with several state and federal agencies through the years to secure the proper permits as the scope of the project has increased due to additional safety and design requirements,” Norma Camacho, Santa Clara Valley Water Chief Executive Officer, stated.

Given the delays, it was decided to drain the lake before the implementation of the retrofit project. The reservoir must be drained by October 1, 2020, according to the U.S. Federal Energy Regulatory Commission.

This decision can cause significant ramifications. According to Christopher Hakes, a Valley Water dam safety official, the water level in the reservoir provides a supporting effect for the dam's body. "Lowering reservoir levels beyond the current level would decrease the structural reliability of the intake structure and its protection against earthquakes," Hakes had mentioned back in December 2019.

Camacho also added that the drainage would impact the wildlife and severely reduce the water reserve which is significant given that drought periods in California have become very common.

A video showing what would happen if Anderson Dam failed is presented below.