Hernandez Reservoir is 100% full but considered safe by agencies

It has been many years since water was released from the Hernandez Reservoir, in South County, and it would only be natural for those who live below the dam to be concerned when they saw water coming over the spillway, said Jason Ince, a spokesperson for the California Department of Water Resources Division of Safety of Dams (DSOD). 

Shawn Novack, San Benito County Water Conservation Manager viewed the full reservoir in early March. “It is an amazing site! I have been here twenty years and I’ve never seen that.”  Novack said that 2016 was the last year the reservoir was at a similar level.

Department of Water Resources Division regulates the Hernandez Dam, which is an earthen structure built in 1962. According to the National Inventory of Dams it is considered at high-risk, meaning should it fail, there could be a loss of human life.

According to FEMA there are 15,000 high-hazard potential (HHP) dams in the US. “Dams can fail for a number of reasons, including overtopping caused by floods, acts of sabotage, or structural failure of materials used during construction,” a FEMA document states. 

“The downstream hazard is based solely on potential downstream impacts to life and property should the dam fail when operating with a full reservoir,” Ince said. “This hazard is not related to the condition of the dam or its appurtenant structures (meaning structures that are part of the dam).” 

Of the seven dams located in San Benito County, four are considered an HHP, including Hernandez Dam. The Hernandez Dam is located in the Diablo Mountain Range. The 124-foot dam was built for irrigation and flood control. It can store 18,000-acre feet of water.  On March 23, Steve Wittry, general manager of SBCWD, told BenitoLink that Hernandez Dam is 100% full.

The other three dams considered an HHP are Bear Gulch, San Justo, and Paicines. A fifth dam, Hawkins Dam is considered at a “significant risk,” meaning there could be an economic rather than human loss.

Even though Hernandez Dam is officially considered HHP, local water authorities say it is operating as designed and is monitored regularly.

“The reservoir’s spillway has been engaged and approximately half a foot of water is spilling through the spillway structure, as designed,” Ince told BenitoLink. “DSOD has been in contact with the owner [San Benito County Water District (SBCWD) ] who reported minor flooding downstream along some access roads, which is reportedly common when this dam’s spillway is engaged. Even though this reservoir has not spilled in many years, it is a common occurrence during high rain years, such as this one.”

Ince said DSOD has no maintenance or operations concerns with Hernandez Dam. Steve Wittry, general manager of SBCWD, confirmed the reservoir is filled to capacity and water is flowing over the spillway. He said SBCWD personnel inspect the dam every other week and the Bureau of Reclamation inspected it two years ago.

“Every rain storm we had since the end of January, the folks in those areas are seeing the creeks and the [San Benito] river rise up as a result of the rainfall,” he said. “The dam slows the flow down but it eventually all goes over the spillway.”

Wittry said if there were a potential breach in the dam, SBCWD has phone numbers for the nearby ranchers and homeowners to warn them. He said there would be an instant surge and a rise in water level in San Benito Creek of about two to three feet. He said by the time the water reached Tres Pinos it would have spread out in the wider riverbed.

As for other county responses to a breach, Sheriff Eric Taylor told BenitoLink, “We have very limited patrols in south county. That is due to staffing and communication issues. That leaves our staff isolated while there. OES [Office of Emergency Services] is in charge of tracking and messaging any issues that have to do with the dams.”

Kris Mangano, emergency services manager, said OES monitors the dam and continues to coordinate closely with SBCWD. 

Wittry said after each significant rain, it takes 12 to 16 hours for the water to work its way from Clear Creek Management Area, which is controlled by the Bureau of Land Management and where the headwaters of the San Benito River is located, to the Hernandez Reservoir.

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