Could Oso-like slide happen in Thurston?

jdodge@theolympian.comApril 3, 2014 

It was a massive mudslide that roared down the hillside at perhaps 40 miles per hour, plunging 900 feet over a 3-mile distance, toppling, burying and incorporating mature, second-growth timber in its torrent of debris. It splashed mud 50 feet high on adjacent trees before it fanned out over state Route 8 and Kennedy Creek Road near Summit Lake.

Known as the Rock Candy Mountain Landslide, this Jan. 9, 2009, event in the upper reaches of the Kennedy Creek watershed of the state Capitol Forest is the closest thing to an Oso-style mudslide in recent Thurston County history, county geologist Mark Biever told Thurston County commissioners during a landslide hazards briefing Wednesday afternoon at the county courthouse.

Fortunately, no one was in harm’s way that day. The large mudslide was apparently triggered by 10 inches of rain over three days, which saturated the unstable glacial till soils sitting on top of bedrock basalt. The mudslide started in a dense, mature timber stand and did not appear to be influenced by logging activities.

The powerful 2009 mudslide in Thurston County came back to life in the wake of the Stillaguamish River mudslide that buried the tiny Snohomish County village of Oso on March 22. With about 50 people confirmed dead or missing in Oso, one of the state’s worst natural disasters was on all the minds in the meeting room.

The 2009 mudslide was close to the 2007 debris torrent that severely damaged the Ranch House BBQ Restaurant. In their March 2, 2009, report after touring the Rock Candy Mountain Landslide area, Biever and county hydrogeologist Nadine Romero didn’t mince their words.

“The awesome display of the January 2009 event illustrates a truly violent type of slide that we rarely get a chance to examine and say: ‘No one was injured or killed,’” they wrote.

Romero described the rainfall leading up to the January 2009 event as an atmospheric river, which is a term used to describe highly concentrated bands of water vapor transported in fall and winter storms from the tropical Western Pacific Ocean to the West Coast.

What’s important here is the fact that current climate-change models suggest these intense pulses of precipitation will grow more frequent in the decades ahead. More rainfall means more saturated soils and higher groundwater levels, which will likely lead to more mudslide activity.

The deadly Oso mudslide is serving as a wake-up call for county land-use planners and engineers here in Thurston County and probably all around Western Washington.

County planning director Mike Kain said 2012 changes to the portion of Thurston County Code that deals with geologic hazards has made it harder for people to build in mudslide-prone areas. But the building setbacks required in Thurston County at the top and bottom of a steep slope wouldn’t have been enough to keep many of the homes in Oso from being buried by that infamous slide.

However, Kain said Thurston County prohibits the type of building in a river floodplain that occurred in the Oso area.

In the aftermath of the Oso slide, Thurston County is ramping up its review of landslide risks in populated areas, using a handy tool called LIDAR, which is a remote sensing instrument that produces revealing images of geologic features beneath built and natural environments, including age-old landslides.

“We’re not going to be able to find all of them,” said Scott Clark, county director of planning and resource stewardship, adding the county will seek state funds to update landslide risk maps and conduct mitigation projects where feasible. “You can’t stop these things from happening — all you can do is get out of the way.”

The existing landslide hazard map for Thurston County shows thousands of acres of vulnerable geology, much of it in the hilly terrain of southeast and northwest Thurston County.

But narrow bands of steep slopes along the more populated shorelines of South Sound show up on the map, including the Carolyn Beach area where a deep-seated landslide near the end of Steamboat Island Road damaged and led to the condemnation of nearly 40 homes in February 1999.

In a nutshell, Thurston County is doing more than ever to keep people from building in harm’s way. But thousands of people already lived in geologically hazardous and environmentally sensitive places before land-use planning evolved.

Commissioner Karen Valenzuela punctuated that point with this observation.

“Given our current codes, the county courthouse wouldn’t have been built on Courthouse Hill.”

John Dodge: 360-754-5444

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