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That Sinking Feeling: Corcoran Fears Floods Thanks To Subsidence, Snowmelt

Mar 28, 2017

A new map released by NASA earlier this year shows that large portions of California are sinking. The worst of it is in the San Joaquin Valley. One of the main reasons is the over pumping of groundwater, especially in the last five years of drought.

"The risk here of a catastrophic event is high." - Dustin Fuller

All that sinking and all the snow melting in the Sierra has Central Valley water managers like Dustin Fuller worried.

Dustin Fuller with the Cross Creek Flood Control District is managing the levee raising project.
Credit Ezra David Romero / Valley Public Radio

He's managing an army of earth movers that are scraping top soil off farmland that surrounds the Corcoran State Prison in Kings County. His team is getting ready for what officials feel like could be a big flood, but looking at it now you wouldn’t have a clue that this dry area could become a lake. 

“We’re excavating dirt right now to build a levy,” says Fuller with the Cross Creek Flood Control District that manages water in the Corcoran area. And when I ask him when he needs it built he says “yesterday, yesterday.”

Fuller says the levee used to be tall enough. Due to subsidence it's sunk and now it's not. He’s the water manager that’s overseeing the $14 million levee raising project that’s supposed to protect the 25,000 person City of Corcoran and the prison. Fuller’s worried because the area he manages is part of the historic Tulare Lake Basin which was once filled by the Kings and Tule Rivers. Before it dried due to water diversions it was the largest freshwater lake west of the Mississippi. Fuller says once snow melts there will be flooding and the lake could come back to life.

“The risk here of a catastrophic event is high,” says Fuller. “The easiest way to equate it is you got 20 gallons sitting up in the hills and at some point in time that 20 gallons is going to come down to a five gallon bucket. We’ve got issues.”

"We're using high-tech space techniques to help problems down on the ground and 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 [farmers] pump too much." - Tom Farr, NASA JPL
The 14 mile levee will be 192 feet from sea level in all spots when finished.
Credit Ezra David Romero / Valley Public Radio

Some parts of the 14-mile levee built in the late 60’s only need a few inches of dirt to meet the 192 foot height standard set by the Army Corps of Engineers. Other areas need a couple feet of dirt.  

“The flood is imminent, I would encourage people to buy flood insurance,” says Fuller. “Be prepared with your important documents. Have a plan, don’t wait.”

NASA’s satellite imagery shows a 60 mile area around Corcoran hit hard by subsidence, it sank about two feet in 2015. But to the average person you can’t really tell. Tom Farr with NASA’s Jet Propulsion Laboratory says that’s because the whole region is slowly sinking.

NASA’s research started because of a request from the Department of Water Resources. They mapped the whole state from 2006 to 2015 and saw the worst subsidence in the San Joaquin Valley.

Credit NASA JPL

“Subsidence by really no surprise is continuing there, but it’s actually got a lot faster during the drought period,” says Farr. “But we’re also seeing some slowing down of it during the rainy period especially that started recently.”

A second study from 2015 to 2016 showed a major increase in sinking in the Central Valley. The areas showing the most signs of subsidence are along the California Aqueduct, Corcoran and an area just south of Merced. Farr says NASA will launch a satellite sometime in the next decade with the sole purpose of mapping subsidence.

“We’re using high tech, space techniques to help problems down on the ground,” Farr says. “And 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.”

Water managers in these areas are worried about how much water is going to flow when snow melts. The San Joaquin River runs through the area just south of Merced called El Nido that sunk 16 inches in 2015.

Chris White is with the Central California Irrigation District. He brought me to a place surrounded by farmland between Madera and Merced along the Eastside Bypass of the San Joaquin River. He says you can tell what areas are sinking just by looking at the river.

The area dubbed "El Nido" is also seeing signs of subsidence. Chris White with the Central California Irrigation District says this areas dropped around six feet.
Credit Ezra David Romero / Valley Public Radio

“The water that’s coming through the bridge because of the increased slope of this section of the bypass is a lot of turbulence through the bridge and a lot of velocity as it heads downstream from here,” says White. He says this area has dropped between five and six feet over the last decade. 

White's witnessed the area sink and as it drops it creates a bowl and the river gets deeper and deeper. When flows slow down the water pools and the flow stops. But it’s not all negative here. The NASA research shows that at least for the last two Decembers subsidence momentarily stopped and the ground actually rose a little bit. White says this is probably because farmers don’t pump as much groundwater in the winter. He also says farmers in this area are actively trying to stop subsidence.

Subsidence is seen at deep wells like the one pictured. As the ground sinks the pipes stay in the same place.
Credit Ezra David Romero / Valley Public Radio

“They’re sinking a lot of water in the ground during these flood flows, which is really good to see,” says White. “The idea is to recharge the shallow ground water, drill more shallow wells, and eliminate the deep wells.”

As they slowly fill a shallow aquifer with floodwater using ponding basins, they hope to stop relying on a deep pressurized aquifer that sits underneath a layer of clay. When water is pulled out there the clay compresses causing subsidence.

“Into the future over the next few years they’ve got a nice shallow aquifer to draw off to farm from,” White says. “They’ve been preparing since we talked to them in 2012. Well we didn’t have a wet year until this year.”

White says he won’t be surprised if NASA’s future data from this area show even more recovery from subsidence because of this project. In the meantime he has to figure out a way to correct the sinking on the San Joaquin River so it can continue to flow to the ocean.