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Oso “Extreme Event” Prompts New Look at Landslide Hazards Regulation


May 14, 2014 by Sue Enger
Category: Emergency Management

Oso “Extreme Event” Prompts New Look at Landslide Hazards Regulation

March 22, 2014 is a day that will be etched in the minds of many Snohomish County residents. On that day, the massive, muddy Oso landslide sped down a mountainside, demolished or severely damaged 49 homes, claimed the lives of 41 persons (two others still missing), blocked the North Fork of the Stillaguamish River, and buried a 0.8 mile stretch of State Highway 530. (Nationwide there are typically 25 to 50 deaths per year caused by landslides.) In its wake, it left some 6 - 10 million cubic feet of mud and debris, piled 30 to 40 feet high, according to various estimates. The landslide traveled 0.7 mile, which “is about three times longer than expected based on published information regarding previous slides of this height and volume worldwide” according to a United States Geological Service (USGS) webpage. Perhaps the most widely quoted impression of the devastated landscape was a remark by National Guard Capt. Brad Sanders: “So if you could imagine houses, trees and a bunch of mud put in a blender and run through a bit and dumped back on the ground, that’s what it is.”

Richard Iverson, a USGS landslide specialist based in Washington, called the Oso landslide an “extreme event.” It was “off the chart,” even for a slide-prone area. USGS scientists agree that several months of near-record rains triggered slope failure, but are still investigating why the deep-seated landslide was so large and traveled so far.  Even so, what we consider to be extreme events may become more frequent considering the greater frequency and intensity of storm events expected with climate change.

Landslides occur regularly throughout Washington, and have caused significant damage (and often resulted in lawsuits) in the past. Nevertheless, they haven’t received the level of study or regulatory attention that other geohazards have according to DNR. The tragic Oso event has moved landslides back onto the radar screen. State agencies and local governments are now in the midst of sorting out what caused the landslide and what can be done to protect the public in the future.

One thing seems clear – local governments cannot guarantee complete protection from an act of nature. The Oso slide traveled far beyond anyone’s expectations. Tim Walsh, Department of Natural Resources (DNR) Chief Hazards Geologist, notes that while some areas may fall clearly within or outside of a hazard area, there will be an uncomfortably large area that is less easily categorized. Balancing public safety and private property rights is difficult given the uncertainty.

Walsh notes that setbacks and other regulations need to be based on localized conditions. Slope aspect, soil composition, rainfall, water flow, and many other factors vary considerably. A number of jurisdictions require geotechnical analysis before permitting development. A conscientious few have their own geotechnical experts on retainer, to conduct analyses and to provide independent review of work submitted by other geotechnical consultants. Seattle, having learned lessons from past landslides and legal challenges, uses covenants to assure that property owners are aware of the landslide risks (see below). Improved mapping technology now exists. Maps are important in identifying and communicating potential hazard areas, although disclaimers are needed with large area maps. Such maps generally should be supplemented with site-specific investigation.

At ground zero, Snohomish County has been considering emergency measures including an emergency development moratorium in areas within one-half mile of any mapped landslide hazard.  Snohomish County Council Chair Somers, who floated this proposal, withdrew it in favor of a proposed interim controls ordinance when it became clear that the temporary ban would have halted new construction permits in most of the unincorporated county. A council staff memo provides useful background on the proposed interim controls that include an increase in setbacks based on the height of slope and requirements for geotechnical reports. King County Executive Dow Constantine announced that King County will be taking advantage of LIDAR technology to produce more detained hazard maps, and may include landslide hazard information in property titles. Pierce County Executive Pat McCarthy has directed staff to further analyze landslide risk, and the county may be leading the way in incorporating improved information from LIDAR imaging into the permitting system.

Existing Local Geologic Hazardous Areas Codes and Document Examples

These are some existing codes that have more thoroughly addressed landslide hazards; however, probably none of the existing codes would have prevented all of the damage wreaked by the Oso landslide.
Agency Resources

These websites have useful information or links:
Resources to Aid Understanding
The Oso landslide and associated flooding caused an estimated $10 million damage to individual homes, and $32 million in costs for emergency services and debris removal. That doesn’t begin to count additional costs to repair damaged roads and infrastructure. Far more devastating is the incalculable cost of lost and disrupted human lives. In hindsight, time and money spent upfront to better identify and reduce landscape risks, and to alert the community about risks, are a wise investment compared with the costs and anguish that follow disaster.

Aerial view of the Stillaguamish River and SR 530 after the March 22, 2014 landslide. Courtesy of WSDOT.

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