It is a scenario South San Joaquin Irrigation District wants to avoid.

A major rock or land slide on the western side of the steep Stanislaus River Canyon downstream  from Goodwin Dam occurs between March and October.

It sends debris into the 13-foot deep main supply canal above the Stanislaus River effectively plugging it.

Debris removal and emergency repairs could easily take two to three months.

It is a time frame when the in-district 36,000 acre-foot Woodward Reservoir, especially if he had been drawn down during the winter months supplying urban water when diversions from the Stanislaus River cease, would be woefully inadequate.

More than 230,000 residents in Manteca, Lathrop, and Tracy would face significant water rationing.

Some 50,000 irrigated acres would have water deliveries severely curtailed.

Based on yearly issues that represent a $300,000 annual expense to the SSJID to clear debris and repair the canal, such a scenario is plausible at any time.

That threat should be eliminated three years from now.

The SSJID, in partnership with the Oakdale Irrigation District is expected to go to bid this month on a $70 million to $80 million endeavor to construct a tunnel about 2.3 miles long to replace the mostly open supply canal.

Work is expected to start this summer and is targeted for completion in 2028.

SSJID General Manager Peter Rietkerk noted the goal is to ensure a reliable water supply.

As such, it will drastically reduce annual maintenance costs, improve employee safety, and advance regional water quality for years to come.

Landslides and rockslides of various degrees happen annually.

Large ones are less frequent but still occur.

The timing so far has been in the district’s favor.

The last one in early January of 2013 was cleared and the canal fixed with barely a week to spare before the start of irrigation in March of that year.

The canal supplies 100 percent of SSJID’s water.

Some water for OID also flows through the canal.

The cost sharing arrangement of 72 percent for SSJID and 28 percent for OID reflects each district’s capacity in the conveyance facility.

The new tunnel will be 18 to 20 foot in diameter to allow access for crews to do maintenance work when water diversions are not taking place.

The tunnel will allow a maximum flow of 1,263 cubic feet per second — the equivalent volume 1,263 basketballs would hold if they were filled with water and passing a specific point.

The 12,106-foot long tunnel would start in a gate system behind Goodwin Dam built into the canyon’s wall to protect it from future rockslides.

The water will flow into a tunnel buried 200 to 300 feet below the surface.

The tunnel’s end where it will flow back into an open canal is 1.25 miles from the nearest home and 1.5 miles from Knights Ferry.

To give you an idea of the rugged terrain, this is where a section of the canyon you will find the Class V rapid dubbed Mr. Toad’s Wild Ride that drops 40 feet over 100 yards.

The inaccessibility is requiring a temporary bridge to be placed across the reservoir behind Goodwin Dam in order to work on the start of the upper end of the tunnel and the intake area.

A rock boring machine will be used to work on the tunnel.

The district has already put rate hikes in place as part of a comprehensive plan to cover a portion of the cost.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com