Work progressing on Anderson Dam retrofit

Morgan Hill, CA - Rarely is great grazing land for cattle found in a spot like the bottom of a reservoir…

At Anderson Dam in Morgan Hill, Tuesday, the low water level – just 3% of capacity -- is a sign something is happening below.

"We’re at Stage 1 of the retrofit project. And that stage is to put a tunnel through the bottom of the damn," said Ryan McCarver, a manager in the Anderson Dam Safety Division.

For over a month, a team of engineers and excavators have been digging a 1,750-foot outlet tunnel beneath the water. Their progress is slow and steady – 10 feet a day through bedrock, which is on schedule. The opening is 20 feet in diameter but will narrow to 13 feet deep inside.

When completed, the opening will allow for the emptying of 6,000 cubic feet of water per second. Currently, that rate is about 400 cubic feet per second.

"Being able to get a larger amount of water out of the reservoir helps us lower the reservoir in case there’s an emergency event, such as an earthquake or large storm," said McCarver.

First built in 1950, Anderson is the largest South Bay reservoir, holding 90-thousand acre-feet of water. It provides drinking water for businesses and two million people in Santa Clara County. But a seismic flaw could compromise the dam, putting South Bay residents and their homes at risk.

"A lot of times the soil within the dam itself…basically acts like a liquid. Which means it doesn’t have any strength to resist the force from the water," said Dr. Laura Sullivan-Green, chairwoman of the Civil & Environmental Engineering Dept. at San Jose State University.

The retrofit work at Anderson is moving slowly, with safety at top of mind.

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Hard hats, yellow vests, and work boots are mandatory.

"It’s of critical importance from a water supply, and a health and safety for the citizens of Santa Clara County," said John Varela, chairman of the Valley Water Distribution Board of Directors.

The tunneling work will take about two years to complete. Once that’s done, the actual dam will be taken down and rebuilt using modern construction technology. That work won’t be complete for another 10 years.

"We’ll also have the ability to move imported water to store in Anderson Dam. Which we do now and have in the past. But we’ll have a higher capacity to move water in and out of Anderson," said McCarter.

For now, each stint inside the tunnel, and each load of dirt removed, is another step toward seismic safety for this key South Bay resource.

Jesse Gary is a reporter based in the station's South Bay bureau. Follow him on Twitter @JesseKTVU and Instagram @jessegontv