Landslide leads to flood concerns around Stillaguamish River

Snohomish County is encouraging residents who live in the floodplain downstream of milepost 41 to the city limits of Arlington to “find another place to stay tonight.”

Saturday’s mudslide has blocked the north fork of the Stillaguamish River near Oso, leading to concerns about flooding upstream of the landslide and flash flooding downstream, according to the National Weather Service (NWS).

A flood warning is in effect for the north fork area and a flood watch has been issued for Snohomish County. However, the NWS said “catastrophic flash flooding” is unlikely.

Hydrologists estimate the water is rising 10-12 inches per 30 minutes upstream of the landslide dam, Snohomish County said.

Even if all the water backed up behind the landslide were to rapidly drain, the river at Arlington would not come close to flood stage. The area around the slide could see flooding.

A river gauge immediately downstream of the landslide showed the river level drop from 3.1 feet to .9 feet an hour after the landslide occurred, confirming it has blocked the river.

However, during a news briefing Saturday night, Snohomish County Executive John Lovick said officials were still recommending evacuation for those below the mudslide.

As searchers look for the last of the missing in Oso, Washington, where a massive landslide virtually wiped out the small community, it’s becoming more obvious that authorities knew about but failed to fully heed the warnings of scientists that such a disaster was a real threat.

Not only that, they even considered – but then rejected – a suggestion that they buy out home and business owners whose properties lay just across the Stillaguamish River from a steep hill that had fallen away several times before.

The Seattle Times newspaper reported this week that Snohomish County officials analyzed the situation, finding that the costs of a buyout “would be significant, but would remove the risk to human life and structures.”

Instead, they decided to build a wall intended to stabilize the slope, leaving existing structures in place and allowing more to be built. Eight people in those newer homes are dead or missing from the landslide, including four children, the newspaper reported.

Experts studying the most recent slide – and the several that preceded it over the years, particularly one in 2001 – say this was a fatal mistake.

“[T]o my mind this was [a] foreseeable event, and as such the disaster represents a failure of hazard management,” writes Dave Petley, a professor of hazard and risk at Durham University in the United Kingdom and author of The Landslide Blog, which is hosted by the American Geophysical Union.

“The 2001 landslide left material high on the hillside that was sitting above a scar that was far too steep,” Dr. Petley writes. “The LIDAR [Light Detecting and Ranging] data suggests that the runout from such a collapse could be extensive. In that context I find the decision to build new houses at the foot of the landslide to be very surprising.”

In retrospect, some residents in the area are surprised as well … some of them furious that they weren’t warned of the threat.

Davis Hargrave, a retired architect who lost dozens of neighbors and his weekend home, said knowing the county took the threat as seriously as it did would have prompted him to ask many more questions.

“We are not a bunch of stupid people ignoring warnings,” Mr. Hargrave told the Seattle newspaper. “We all make risk assessments every day of our lives. But you cannot make a risk assessment on information you do not have.” (...)