Crossed fingers: Green River Valley, threatened by weakened reservoir, may be past flood risk

El Nino helping Green River Valley avoid floods

SEATTLE — With winter half gone, some weather experts say the Green River Valley south of Seattle may have avoided catastrophic flooding, at least this season.

The El Nino weather pattern affecting the Pacific Coast has helped bring a warm January to the Pacific Northwest, along with drenching rains and snow to California. Here, it’s a huge contrast to a year ago when record rains fell in the Green River watershed in the Cascade foothills southeast of Seattle. That weakened a reservoir wall at the Howard Hanson flood control dam, imperiling the heavily developed valley downstream.

Cliff Mass, professor of atmospheric science at the University of Washington, said the current mild weather could lead to Seattle’s warmest January on record. And in this region, he said, “Once you get past the 20th of February, one month from now, I mean, winter’s over.”

For those living and working below the dam, “I think they’re home free at this point,” he said.

So has the worst passed?

“You can’t say that,” said J. Brent Bower, hydrologic program manager for the National Weather Service in Seattle.

Bower points out that Washington suffered one of its most devastating floods in February 1996, which caused more than $300 million in damage throughout the state. The Green River Valley escaped serious harm, but the 1996 combination of heavy rain and melting mountain snow is precisely what worries emergency officials now.

El Nino is a periodic ocean warming along the equatorial Pacific that affects the climate. Western Washington, meteorologists say, is having a typical El Nino season: normal winter weather until around Jan. 1, then mostly milder, warmer and wetter conditions than usual. Bower and Mass both say there’s no major storm in sight for the Seattle area for the next week or so.

Meteorologist Larry Schick at the Army Corps of Engineers’ Seattle District says that while the bulk of the flood season appears over, there’s no guarantee.

“We’re not quite there yet,” he said. “We’re close, though.”

Bower likewise says he’s getting more optimistic as the days go by, but that’s still tempered with a large dose of caution.

“I think it would be a mistake to say the worst is over,” he said, “because this is the weather business and that’s not a prudent tack to take.”

The series of rainstorms that have soaked Southern California resulted from a weather pattern that pushed the storm track south, Bower said. Without that pattern, “instead of hitting down there, it could have come up here.”

After the rains weakened the Hanson Dam’s right abutment a year ago, the Army corps, which operates the dam, quickly drained the reservoir and drastically restricted the amount of water it could hold. Over the summer, the corps hurriedly made temporary repairs, but by fall, corps officials warned there was still a 1-in-25 chance that heavy rains might force them to release enough water to cause flooding in the valley.

Downstream, millions of dollars have been spent to sandbag the meandering river’s channel and to urge residents to protect their property and possibly evacuate. Besides the suburbs of Auburn, Kent, Renton and Tukwila, the valley has one of the largest warehouse areas on the West Coast and is home to a major Boeing factory and Starbucks’ regional coffee roasting plant.

King County is still passing out free sand and sacks to valley residents and businesses, and valley cities are patrolling levees and the sandbags atop them to make sure they remain sound. Officials say they aren’t slacking off just because the weather’s been good.

“Oh my goodness, no,” said Christine Lange, spokeswoman for County Executive Dow Constantine. “We’re in full force, just making sure people are aware of the risk.”

As it does every winter, the corps has kept the dam’s reservoir essentially empty so it can absorb heavy flows from the watershed, Schick said.

While Seattle had a lot of rain earlier this month, Schick said precipitation at the dam has been close to normal. Rainfall that’s spread over a week or more isn’t really a problem, he said: “It’s the short, intense storms like last year that dump a lot of rain in just a few hours.”

Meteorologists said the downpour in early 2009 was exactly that: a powerful, moisture-laden “Pineapple Express” storm track from the mid-Pacific that directly hit the Green’s watershed.