Friant-kern Canal still sinking
Five inches of drop in five months near Terra Bella
The ground beneath the Friant-kern Canal continues to sink due to excessive groundwater usage during the recent drought, and a comprehensive solution to the problem will likely cost millions of dollars and take years to implement.
When officials with the Friant Water Authority (FWA) did 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 three 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 re-surveyed portions of the canal confirmed those fears. Since April, the canal has dropped another 5 inches in one particularly hard hit location where it crosses under Avenue 96 west of Terra Bella.
“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.”
Deflitch said the FWA took a second look at the problem areas to prepare for some of the mitigation and restoration work the canal will require in the initial stages of its repair.
He described the initial survey in April as a quick glimpse to see how bad the problem was, while the survey in August was a comprehensive assessment of damages along the entire length of the canal.
About subsidence
Land subsidence is the gradual sinking of an area of land, which occurs more dramatically in regions where groundwater is overdrafted — meaning more water is pumped out of the ground than can be replenished through normal sources such as rainfall.
During the drought when surface water supplies were limited or unavailable, regional farmers, cities and others who were relying on surface water supplies were forced to turn to groundwater.
When water is removed from the very fine clay-like pore space in the Valley subsurface, it loses its internal strength, and over time compresses and the surface of the land drops.
The Friant-kern Canal was designed as a gravity-fed facility, and does not rely on pumps to move water. It relies on a gradual slope of six inches per mile to deliver water efficiently through its 152-mile length.
Subsidence has caused parts of the canal to sink in relationship to other parts, disrupting that slope and negatively affecting its ability to convey water.
When the land elevation sinks, water in the canal pools up and rises in the low spots, and in the worst areas it can rise high enough to touch against bridges traversing the canal. When this happens, the canal must be operated at a lower flow-stage to ensure that water doesn’t overflow the banks.
The canal’s carrying capacity has been compromised by various factors, including subsidence, since it began operation in 1951. In the past, water managers could manipulate canal operations to help mitigate some of the lost capacity.
However, the new problem that has emerged this year is driven by rapid and severe land subsidence in the Corcoran/tulare basin areas, which are adjacent to the canal near Deer Creek. During 2015-2016, land elevations dropped by two feet near Corcoran. There is no way to operate the canal to eliminate impacts to water users caused by that amount of subsidence.
Impact of the problem The subsidence issue along the canal means that even in 2017– one of the wettest years on record in the San Joaquin River basin– the FWA cannot physically move the amount of water it should be able to deliver to farms and communities on the eastern side of the San Joaquin Valley.
The area of greatest subsidence is between the Tule River and Lake Woollomes, particularly in the area of Deer Creek. In that section of the canal, the current capacity has been reduced to only 40 percent of designed capacity, with a significant portion of that loss happening in the last 6 years.
All contractors who rely on the canal will be affected by the changes necessary to cope with the problem, as reduced capacity along the canal will likely impact longstanding transfer or exchange partnerships among Friant contractors, which have helped to balance water supply throughout the Friant Division.
The contractors downstream from the subsidence area — which include the irrigation districts for Teapot Dome, Terra Bella, Shafter/ Wasco, and Delano/earlimart — will be most affected, because they may not get the amount of water they want during the time they need it.
This may require farmers to again turn to groundwater to make up for the shortage, which could increase the amount of subsidence that is causing the problem in the first place.
Three-stage repair and
funding Returning the canal to its full capacity will be a multi-year project requiring funding from multiple sources, but the FWA has been busy devising a plan that will use temporary solutions while the long-term repair plans develop.
“We’re phasing it in. For the capacity constraints associated just with the subsidence, we’re looking at three phases — an immediate fix, an interim fix, and a long-term fix,” said Deflitch.
For immediate fixes, FWA is hoping to take steps this year by reinforcing existing bridges so if they needed to run water up on them temporarily they could do so with minimal damage. They also hope to re-seal joints from a previous repair performed by the Bureau of Reclamation in 1979, which also followed several dry years. The bureau spent millions of dollars during that repair to offset current and future subsidence, raising the banks of the canal as much as six feet in some areas.
In addition to short term structural fixes, the FWA also plans to make administrative decisions to minimize the impact for the upcoming rainy season. Deflitch said they will try to deliver higher volumes of water earlier in the season when the canal level is typically low and store it in locations downstream of the problem area of the canal for use by FWA’S southernmost contractors.
Deflitch said that funding for these first stages of repair will come primarily from the canal’s water users, but the FWA is also looking at applying for financial assistance with the Bureau of Reclamation for funding of the immediate and portions of the interim repairs.
The interim repairs will take place over the next 12 to 18 months, and will focus on replacing or modifying the style of bridges used over the canal to increase capacity by creating more clearance than current designs.
Long term repairs will focus on raising the canal lining throughout the entire subsidence area, and is a three- to five-year project. Deflitch said that the FWA has not secured funding for the long-term repairs for the canal, nor do they have a longterm cost estimate at this time.
Deflitch noted there is a water bond that will be on the 2018 ballot that could also have some significant potential to assist in canal restoration.
“Groundwater overdraft is part of a larger problem in the San Joaquin Valley, and we’re out of balance,” said Deflitch. “There’s a lot of things that we need to do in the long term to help with that overdraft. There’s only so much that’s available, but we’ve got to start thinking about it on a region-wide basis.”