A 2-mile plus tunnel that could cost in excess of $25 million is being pursued by the South San Joaquin Irrigation District to avoid a major disruption in water supplies to 52,000 irrigated acres within the district as well as the cities of Manteca, Lathrop, and Tracy.
Should disruption of water supplies occur at the worst possible time it could force water use reductions exceeding 50 percent for the City of Manteca during the hottest months of the year once the state mandate limiting groundwater pumping is fully implemented. That would be a best case scenario in the summer as drought conditions would make the requirement for water cutbacks even more draconian.
The SSJID board last week authorized spending $702,740 for Condor Engineering to do a 30 percent design for the 13,000-foot tunnel.
The tunnel would bypass the joint supply 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 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.
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.
“The only way we can access the canal for maintenance is for workers to drive in the canal (when there isn’t water flowing),” noted SSJID General Manager Peter Rietkerk.
SSJID’s Forrest Killingsworth noted addressing future slides especially during irrigation season will be extremely problematic in areas that are considered more unstable than where the 2013 slide occurred.
That’s because safety would be compromised to a degree with the heavy equipment needed to make repairs that has a tendency to shake the ground in areas where the hillside is not easily accessible.
Worst case scenario would
be landslide right before
heat starts hitting
South County
Should the canal be blocked before or during the irrigation season it would likely have dire consequences not just for farmers but the three cities. Assuming the 36,000 acre foot Woodward Reservoir is full when a slide damages the canal, there would likely only be able to be three irrigation runs without additional water flowing into Woodward that has a high seepage rate due to the type of soil beneath it. During the late fall and winter months, Manteca gets its entire water supply from Woodward Reservoir. The rest of the year more than 50 percent of the city’s water comes from the SSJID system with the rest coming from wells.
A pending state mandate that restricts groundwater pumping so that basins are replenished to the point there is no drop in water levels within a set 12-month period would severely limit Manteca’s options and would trigger at least a need to cut back water use 50 percent until water was able to flow through the supply canal and into Woodward Reservoir. By comparison, Manteca reduced water use by 29 percent at the height of the recent drought.
Oakdale Irrigation District spent 10 years planning and designing a similar tunnel that is now under construction and — at 7,000 feet — is just over half as long as the SSJID project.
That timeline included three years to secure an Army Corps permit to cross their property. SSJID does not face such a requirement. The OID canal will take more than a year to construct at a cost of roughly $16 million.
While design work is being done on the tunnel, the SSJID needs to do some work in the in the immediate future to stabilize some segments of the hillside to assure worker safety and avoid a landslide.
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.
To contact Dennis Wyatt, email dwyatt@mantecabulletin.com
