Portland's west side could have a water problem if -- or when -- the Big One hits.

The Bull Run watershed 26 miles east of downtown provides drinking water to nearly 1 million customers in the metro area, and that water crosses the Willamette River at six locations.

But the pipelines serving downtown Portland and the Washington Park reservoirs are an average of 77 years old. Two are buried in shallow and unstable soil in the riverbed. Two more 24-inch pipes are attached to the Ross Island Bridge, which is expected to fail during a big earthquake.

All of those pipelines were designed before the city knew Portland could experience a catastrophic earthquake. The city's 130,000 westside customers could be without potable water for six months or longer if and when the Cascadia Subduction Zone ruptures. The last time that occurred, geologists estimate it produced at least a 9.0 magnitude earthquake.

Teresa Elliott, the Water Bureau's chief engineer, said the city projects a 15 percent chance of a Cascadia earthquake during the next 50 years. "We have another 33 percent chance of a 6.0 magnitude earthquake from one of our local faults," she said, "and our soil is extremely liquefiable on both sides of the river."

Portland is preparing for that harsh reality by moving forward with a $57 million project to build a 42-inch pipe 80 feet beneath the Willamette River.

It's not the only costly project to make the water agency's infrastructure more likely to withstand massive earthquakes. Portland is expected to approve a $170 million plan to build a covered reservoir in Washington Park, which sits on a fault line and is historically prone to landslides.

The pipe project has been on water officials' wish list for a long time. It's on the bureau's five-year capital project list, and the cost is already built into utility rates.

Elliott and Mike Stuhr, the Water Bureau's administrator, briefed the City Council on the project Thursday. The bureau plans to find one contractor to design and build the pipe, but Stuhr needs City Council approval first.

The $57 million estimate is likely well below the actual cost. According to city documents, officials have "low confidence" that will be the final figure. Preliminary designs should clarify the cost.

Portland hopes to begin construction in the summer of 2017 and finish by 2019.

-- Andrew Theen

atheen@oregonian.com

503-294-4026

@cityhallwatch