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Valley Public Radio Staff
Thu November 21, 2013
That Sinking Feeling: Valley Land Subsidence Poses Problems for Water, High Speed Rail
The U.S. Geological Survey released a study today showing that large groundwater withdrawals are causing land in California’s Central Valley to sink. A 1,200 square mile area is sinking up to a foot a year in some places. The situation has become so serious that it’s threatening flood control and water deliveries. The proposed high speed rail system will also have deal with the changing terrain. But Amy Quinton reports from Sacramento, finding a solution won’t be easy.
Chase Hurley is general manager of the San Luis Canal Company in Dos Palos. He points to a small dam near the river in western Madera County. It’s likely the most important structure for the irrigation company because it guides water from the river into its canal system.
Hurley: “That dam, and this canal are sinking roughly six inches a year. So when that happens the dams not going to be high enough to physically gravitational push that down the canal.”
Groundwater withdrawals in the Central Valley are causing the ground to sink. It’s a phenomenon called subsidence, and it’s historically been a problem in the Central Valley. But satellite maps show the problem is much worse. Dane Mathis is an engineering geologist with the California Department of Water Resources.
Mathis: “We’ve got some recent information from NASA and some staff at jet propulsion laboratory and have noticed two areas that are subsiding nearly up to a foot a year for the past four to five years.”
The rate of subsidence varies geographically, but it covers a large area across Western Madera and Merced County. And going right through it, is the San Jose to Merced segment of California’s proposed high speed rail system.
Vacca: “We’re aware of it, we take that into consideration as another engineering element to take into our design elements.”
Frank Vacca is in charge of engineering teams for the California High Speed Rail Authority. He says High Speed Rail will use ballasted track comprised of stone, which he says is flexible enough for subsidence.
Vacca: “If you have to adjust it you can add more stone and quickly bring it back to the level you require it at, so it makes it very economical to adjust the track.”
But other engineers say there are a lot of variables. Bruce Kutter, a civil engineering professor at UC Davis says the integrity of the rail bed will depend on the severity of the subsidence and how rapidly it occurs along the track.
Kutter: “Over a short distance if there’s a significant change in the elevation of the track, the ride will become uncomfortable and bumpy and potentially unsafe.”
If the subsidence is gradual over the length of the track, Kutter says it would be manageable. In some areas, the rate of subsidence is causing alarm. Reggie Hill is with the Lower San Joaquin Levee District, which operates the Eastside Bypass.
Hill: “In the last five years there’s been subsidence of four feet. Our bypass system is right in the middle of it, we convey flood flows around the San Joaquin River, because the San Joaquin River has constraints in its channel, and the Bypass system is designed to carry twice the amount that the river can.”
But when channels sink, river banks erode. The loosened soil moves downstream, reducing capacity and pushing water close to the top of 14 miles of levees
Hill: “It’s an extreme situation we need to address it and now that we’re aware of it we need to jump on it as quickly as we possibly can.”
Subsidence is putting hundreds of acres of farmland, an elementary school, the city of Dos Palos and Highway 152 at risk. Michelle Sneed, a USGS hydrologist says subsidence is causing even more problems.
She unlocks a shed about 50 feet away from the Delta Mendota Canal, just south of a small town called Oro Loma in the Central Valley. She’s measuring groundwater levels near the canal, which carries water to parts of the central valley and southern California.
Sneed: “In this location there are several visual effects that you can see as a result of subsidence, couple of them are buckles in the lining of the canal.
Sneed’s been studying the effects of subsidence on the Delta Mendota Canal for several years.
Sneed: “It’s not a small problem. It’s a problem for not only the Delta Mendota Authority and the Bureau of Reclamation to maintain the flow and level in the Delta Mendota Canal but there are canals that are just snaking through this area.”
California’s Delta: Inside and Out