Valley Water Project Manager Ryan McCarter describes how a new reservoir diversion tunnel is designed during a media tour of the Morgan Hill construction site on May 10. Photo: Michael Moore
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Crews for Valley Water and various construction contractors later this month will finish digging a new 1,736-foot diversion tunnel for Anderson Reservoir—which has been about 80% complete since February.

In late May, crews will dig out the final 350 feet of the tunnel by opening it up into the bottom of the east Morgan HIll reservoir, through a delicate process using a “micro tunnel boring machine” to push through the final stretch of rock and into the water body, according to Valley Water staff. 

The purpose of the new tunnel—which is part of the broader $2.3 billion Anderson Dam Seismic Retrofit Project—is to enable the water district to move much larger volumes of water out of the reservoir than the current facilities allow. 

Such a massive, controlled release of water—up to 6,000 cubic feet per second when complete—can become necessary if the reservoir threatens to overflow due to weather or other emergencies, Valley Water staff said during a recent tour of the construction site at the base of the existing Anderson Dam. 

“It will be used as an emergency outlet only,” Valley Water Project Manager Ryan McCarter said during the May 10 tour and press conference. “So it’s a much larger amount of water we’ll be able to release (that) we would only have to do in emergencies such as earthquake or a flood event…or a large storm where we can quickly lower the reservoir if needed for safety.”

Anderson Reservoir’s current controlled release outlet can only remove up to 450 cfs from the bottom of the reservoir. 

The upgrades are necessary due to studies dating back to 2009 that show that Anderson Dam is seismically unsound and could fail in the event of a major earthquake along the Calaveras Fault. The reservoir’s capacity is limited by state and federal regulators for safety reasons until the dam is rebuilt, but also by the ongoing tunnel construction activity that requires the water body to be almost completely empty. 

After the new diversion tunnel is complete, Valley Water will begin rebuilding Anderson Dam—another complicated project that is not expected to be complete before 2032. 

Santa Clara County Supervisor Sylvia Arenas speaks at a May 10 event at the base of Anderson Dam, where Valley Water hosted public officials and members of the media for a tour of the reservoir’s diversion tunnel construction site. Photo: Michael Moore

During a May 10 tour of the Anderson Reservoir diversion tunnel construction site, local officials and members of the media were guided on foot through the existing 1,400 feet of excavation. Before entering the tunnel, tour participants were given a short safety briefing on how to use a portable “self rescuer” emergency breathing apparatus that every individual must carry into such a project. 

Currently, the tunnel’s diameter is about 24 feet at the reaches closest to the bottom of the reservoir, but it will be outfitted with more reinforced concrete and a large pipe to transport any future controlled water release, Valley Water staff said. When complete and operational, the tunnel will be about 20 feet wide from the base of the dam to the bottom of Anderson Reservoir. 

Valley Water Director John Varela, a Morgan Hill resident who represents South County on the water district’s board, explained that when complete, the tunnel and dam retrofit “will help ensure public safety, protect our regional water supply and provide environmental benefits for future generations.”

Varela and other officials at the May 10 event noted that progress on the tunnel project thus far is due to close coordination among numerous local, state and federal agencies, as well as labor and environmental organizations. 

Morgan Hill Mayor Mark Turner said, “Valley Water and the City of Morgan Hill have a good working relationship, a collaborative partnership that enables us to achieve milestones and successes that deserve to be recognized and celebrated.” 

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Michael Moore is an award-winning journalist who has worked as a reporter and editor for the Morgan Hill Times, Hollister Free Lance and Gilroy Dispatch since 2008. During that time, he has covered crime, breaking news, local government, education, entertainment and more.