A joint-supply canal put in place 110 years ago is at risk of cutting off water supplies to the cities of Manteca, Lathrop, and Tracy as well as to 52,000 acres in the South San Joaquin Irrigation District.
Landslides over the years have blocked the movement of water in the canal that runs along the northern wall of the Stanislaus River canyon from Goodwin Dam where water is diverted to a point east of Knights Ferry where the canal departs from the canyon and water ultimately flows toward the in-district Woodward Reservoir.
A large landslide in 2013 knocked the canal out of commission several months before the start of the irrigation season. The district was able to make repairs in time before water was needed by farmers. The district at the end of the irrigation season in late September fills Woodward Reservoir — the in-district storage facility — to have ample water to serve the cities before water is diverted from Tri-Dam Project reservoirs via the Stanislaus River sometime after the first of each year. It also provides for an in-district supply to start the irrigation season.
Had that landslide occurred in the summer at the height of water demand, a two-month long disruption in irrigation water would have been catastrophic for many growers. It would also trigger significant mandatory water cutbacks in Manteca and Lathrop as ground water wells would not be able to meet historic water demands on their own.
The SSJID board has been pursuing a replacement tunnel after sorting through options to substantially increase the reliability of water flows as well as reducing costly annual maintenance work that puts crews at risk. Condor Engineering was commissioned to do a 30 percent design for the 2-mile plus tunnel.
The board has now opted to move forward with a 60 percent design that will employ the latest sound wave technology to better predict rock matter and density between drill core samples to help determine a modified alignment that takes all variables into account.
“A tunnel assures built in resiliency and limits the potential for hazards,” noted SSJID General Manager Peter Rietkerk.
The 13,000-foot tunnel is now projected to cost more than $37 million. SSJID would cover 72 percent of the cost and Oakdale Irrigation District 28 percent based on historic water use.
Large swaths of the hillside have been determined to be unstable and are in areas that are extremely difficult to access should a rockslide occur. Engineers have noted the biggest problem with a future landslide is not clearing and repairing the canal but stabilizing the hillside.
Such work to stabilize the canyon walls, however, can be compromised to a degree by heavy equipment needed to make repairs that has a tendency to shake the ground in areas where the hillside is not easily accessible.
The only way SSJID can access the canal for maintenance is for workers to drive in the canal when there isn’t water flowing. The district also has done work above the canyon walls to shore up rocks and soil to try and reduce the potential for landslides.
The canyon tunnel is expected to go 100 years or so without the need for major work. In doing so that would reduce maintenance costs, significantly improve employee safety, and enhance reliability.