Nearly completed Anderson Reservoir tunnel will protect South Bay during droughts, earthquakes

SANTA CLARA COUNTY, Calif. (KGO) -- The multi-billion dollar Anderson Dam Seismic retrofit project is approaching a major milestone.

The tunnel that will reduce flood risk and allow for water to be diverted out and around the dam is nearly complete - 80 % overall.

And we got a look at what the tunnel looks like from the inside.

It's part of the bigger upgrade to make Santa Clara County's biggest reservoir safer and more efficient.

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"It's a much larger amount of water that we are able to release, that we would only have to do in an emergency -- such as an earthquake or another flood event or something like that -- where we could quickly lower the reservoir if needed," said Valley Water Project Manager Ryan McCarter.

The tunnel will be 1,750 feet long with a 20-foot diameter when completed. And it's going to make a major difference when trying to move water from the reservoir.

The current pipe is just too small, and during last year's storms, water couldn't release fast enough.

The reservoir hit 60% capacity, dangerously close to the current 68% limit for seismic safety.

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But the new tunnel won't let that happen.

"About 15 times more water can be released," McCarter said. "So, right now, we can do about 400 cubic feet per second -- which is under 3,000 gallons. And we'll be able to do 6,000 cubic feet per second -- or about 45,000 gallons."

The new building outside of the tunnel will slow the energy of all that water as it flows into the Coyote Creek.

This should dramatically reduce flooding like we saw in East San Jose seven years ago.

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"If this project had been in place in 2017, we would not have seen the flooding we did there," Santa Clara Co. Supervisor Cindy Chavez said. "This project is so important and so big, that it reaches all the way into Alviso."

Phase one of the project is almost done.

The next step will be using specialized drilling equipment to install the final 350 feet of pipe -- which will complete this very important tunnel.

"What we're really celebrating here is a milestone that gets us closer to a completed project," Chavez said. "But it also creates a high-level of safety in the interim making sure that communities that are living along this creek aren't negatively impacted as construction is going forward."

By next year, the tunnel will be diverting water around the dam and out of the reservoir, so the seven-year seismic retrofit can officially begin.