Turlock Irrigation District directors unanimously approved a project which will blast away dirt and rock covering a 360-foot long tunnel on TID’s upper main canal, “daylighting” the canal and removing a mountaintop in hopes of preventing a potentially disastrous tunnel collapse.
The work will cost $975,000 – a small price to pay, considering the potential costs should the tunnel collapse and shut down the upper main canal, TID said.
“The cost associated with that is something we see as significantly greater than the costs of this project,” said TID spokesman Herb Smart.
Per a 1997 TID report, a month-long outage in the heart of irrigation season could cost TID $850,000 – not counting the costs of repairing the canal system. To farmers, a 30 day irrigation outage would result in nearly $14 million in lost revenue.
The 360-foot long, 20-foot wide TID Tunnel 2 was originally built in 1891, and then widened to 30 feet in 1914. The concrete base and concrete lined walls withstood the test of time, but the unsupported rock ceiling has begun to collapse. TID Tunnels 1 and 3 were constructed using different materials, and show no signs of pending collapse.
A 40-foot long by 25-foot wide cavity has developed roughly mid-Tunnel 2, stretching 17 feet above the former tunnel crown. In 2003 a further 15-foot by 15-foot by 4-foot slab fell from the roof into the canal, mid-summer, at the height of canal flows.
Should the entire tunnel collapse, the Turlock Irrigation District’s upper main canal system could be shut down indefinitely. Water already in the system at the time of collapse could overflow, wiping out nearby homes and washing out large sections of canal.
Assuming usage similar to normal conditions, a collapse would leave TID with enough water for about seven days of irrigation – just that stored within Turlock Lake. A collapse resulting in a washout of the nearby Delaney Field canal section could result in a two to three month shutdown of the irrigation system.
The district had initially planned the daylighting for fall 2011, but the California Department of Fish and Game lodged a few complaints during the environmental impact review process. TID altered the plans slightly to ensure threatened native species like the California Tiger Salamander would not be harmed, and Fish and Game now approves of the plan.
The month-plus long project is set to begin as soon as irrigation season ends on Oct. 10. Workers will spend 14 to 16 straight days just of moving dirt and rock – nearly 80,000 square yards – finishing all work by December so as not to interfere with the 2013 irrigation season.
The district will contract with an outside company to conduct the work, but that contractor has yet to be selected through a competitive bidding process.