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Subsidence on Friant-Kern Canal reduces conveyance capacity by 60% near Ave. 96 between Terra Bella and Pixley

Subsidence on Friant-Kern Canal reduces conveyance capacity by 60% near Ave. 96 between Terra Bella and Pixley

@TheSunGazette

LINDSAY – California’s historic drought officially came to an end last year, but it has left significant problems in its wake. The most concerning of which is subsidence.

Land subsidence is the gradual sinking of an area of land which occurs more dramatically in the region when groundwater is over drafted. Water that is removed from very fine clay-like pore space in the subsurface no longer has its internal strength and over time compresses and the surface of the land drops. During the drought when surface water supplies were limited or unavailable, regional farmers, cities and others turned to groundwater. The resulting subsidence forced cuts in water deliveries to parts of the canal system during critical times this past summer.

Now subsidence is starting to affect the conveyance of water through the Friank-Kern Canal (FKC). According to a press release from the Friant Water Authority (FWA) the canal has dropped five inches in five months near Avenue 96 between Terra Bella and Pixley. The results came from an initial survey in April 2017 to measure subsidence along the Friant-Kern Canal, they expected to see impacts from the recent drought. What they measured in some places was a nearly 3-foot drop in the canal elevation.

Even with last winter’s record breaking rain, snowfall and runoff, they assumed the discouraging trend would continue. What they found in early August when they resurveyed portions of the canal confirmed their fears.

“These findings are not entirely unexpected,” commented Doug DeFlitch, Chief Operating Officer for FWA. “Subsidence is a long-term challenge for the Friant Division, and will not be remedied after one year of good rain.”

Jason Phillips, Chief Executive Officer for FWA added, “The continuing subsidence issue is why FWA is exploring possible funding mechanisms for bringing the Friant-Kern Canal back to its designed operational potential. A fully functioning canal will help achieve the groundwater/surface water balance the Friant-Kern Canal was designed to maintain, and lessen the impacts of subsidence.”

Deflitch says that max capacity for the FKC is 9,000 cubic feet per second. Because of subsidence that has slowed down to 1,700 cubic feet per second. The restriction for customers who get their water from the FKC is that their allotment cannot be fulfilled without additional capacity. Deflitch says that there are three phases to address the issue.

The immediate phase is to look at the bridges that cross the canal. Because of subsidence water pools at the portions that have sunk into the ground which raises the water level. The FWA is assessing the soundness of the bridges to determine if water can go over them, which would allow for additional conveyance and for customers to receive their water.

Deflitch says the median phase is to raise the bridges over the canal. The long term phase is similar to what the Bureau of Reclamation did after a drought in 1979 which is raise the liner of the canal which raises the embankment. The FWA chief operating officer admitted that replacing the canal liner is a project that would cost hundreds of millions of dollars.

The Friant Division was designed to bring stability to the San Joaquin Valley’s groundwater supply, which was threatened at the beginning of the 1920s by decades of groundwater pumping. The Friant Division’s two canals – the Friant-Kern and the Madera – source high-quality surface water from the San Joaquin River that supports crops, cities, and groundwater recharge. This investment to establish the Friant Division has paid off by providing stable surface and groundwater supplies that created and sustain a world-class agricultural sector that in turn supports numerous communities and businesses.

The Friant-Kern Canal is a gravity-fed facility and currently does not rely on pumps to move water. Subsidence disrupts the natural grade line, which negatively affects the canal’s ability to convey water.

FWA is a joint-powers authority formed in 2004 by a majority of the water agencies receiving water from the Friant Division of the Central Valley Project. Its primary purposes are to operate and maintain the Friant-Kern Canal and to serve the information and representation needs of its member agencies.

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