Scientists exploring Oso slide zone for answers



Posted on May 16, 2014 at 4:59 AM

NEAR OSO,  Wash. - Scientists with the U.S. Geological Survey are out on the debris field gathering soil and water samples to figure out two things:  first, the flooding threat from the compromised channel of the north fork of the Stilliguamish come next fall and winter when the rains return, and second, how this slide happened to begin with.

So Chris Magirl, a research hydrologist from the USGS's Washington Water Science Center in Tacoma, along with James Forman, a hydrological technician, keep searching for the clues that will aid in analyzing those two main questions.

Right now the news looks good for the river, says Magirl.  The river, with some help from excavators in recent weeks, has been cutting a channel through the debris flow of some 10-million cubic yards. There are a couple of problems.

"Right now, the river isn't wide enough,"  said Magirl.

So samples of clay, sand, silt, gravel and soil are being analyzed for how they have and will continue to deposit themselves on the river bottom downstream.  If the channel isn't wider by this fall, heavy rains will back up in the direction of Darrington.  If a staggering amount of slide debris washed downstream continues to build up along the bottom toward Arlington, it means the lower portion of the river won't be as deep and won't be able to carry as much water volume.

Their research is also expected to help fisheries biologists assess the impact all that sediment will have on returning salmon and its ability to damage eggs and young fish.

The samples are sent to a lab at the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash., near Portland.  There the samples are also studied by landslide experts on how months of heavy rain prior to the March 22 slide may have triggered it.

"Understanding how much clay is in this material, how it behaves when it gets wet, how far it might move with gravity in sheer conditions.  That's critically important to the science of this," said Magirl.

USGS landslide experts estimate that the slide moved in excess of 60-miles per hour, sweeping away homes along Steelhead drive killing 43 people.

USGS scientists have been at the location from the beginning along with geologists from the Washington Department of Natural Resources and Snohomish County.  Then, the mission was primarily about safety and providing equipment and analysis to make sure than a 600-foot high scarp of similar material did not collapse on top of search and rescue personnel and hundreds of volunteers, National Guard soldiers and experts from around the country helping to recover the victims. 

The hill continues to be closely monitored as evaluating the threat from this fall's rains grows along with efforts to find the answers that could prevent a repeat of this disaster here or someplace else.