California agricultural canal loses 60% of capacity

San Joaquin Valley’s Santa Rita Bridge subsidence

The Friant-Kern agricultural irrigation canal in the San Joaquin Valley of California has sunk so much due to subsidence it has lost 60% of its flow. The 152-mile canal delivers water to one million acres of farmland. It has sunk 2-3 feet in some areas. Five years of drought and its accompanying over-pumping of ground water to compensate is the cause.

Subsidence happens when the ground slips or collapses. In this case, pumping water out of underground areas made of clay creates the problem.

There is no way to make to ground go up again. Workarounds can help, however they are expensive and the earliest they could start is 2019. Other canals have similar, sometimes worse, problems. Subsidence has been a problem in the Central Valley for decades. If groundwater continues to be over-pumped, subsidence will continue.

“Subsidence by really no surprise is continuing there, but it’s actually got a lot faster during the drought period,” says Tom Farr with NASA’s Jet Propulsion Laboratory. “In those areas they’re just more susceptible. They have certain kinds of layers of fine grain material that are the type that compact and they pump too much.”

To fix the flow problems DeFlitch says the entirety of the canal needs to be adjusted to make up for the subsidence, which will cost around $400 million. But because of the high price tag he says his team is focusing first on fixing the area most severely impacted. Even though only preliminary design work has been done, he says to raise the canal by three feet will cost tens of millions of dollars and about three to five years to complete.

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