State Action to Protect fish habitat May Have Created the Deadly Oso Landslide.

 

Unusual geological conditions under the Whitman Bench above the Steelhead community of Oso, and the state’s action to protect the North Stillaguamish fish habitat, may have created the conditions that caused the unprecedented massive mudslide.

 

Background. Like any major disaster, a number of separate circumstances came together to cause the deadly mudslide on March 22, 2014. During the last glaciations the Whitman Bench was formed in the bottom of a lake formed by the glacier. Over time, out wash allowed sand and gravel layered up on the bottom of the this ancient lake bed, with alternating layers of sand and gravel, silt and clay. The sand and gravel layers are very permeable and allow ground water to percolate downward though the layers until it reaches these silt and clay strata, forming perched water tables at each of these relatively impermeable layers. This can be seen in the pictures of the landslide head scrap. Notice that moisture can be seen still draining from these perched water table days after the landslide. https://www.facebook.com/photo.php?fbid=750455381655085&set=p.750455381655085&type=1

 

There had been a long history of relatively minor slides at this location, with a number of studies by a various state agencies and private contractors. It was known as the Hazel slide, and all of the local residence even called the slope “slide hill”. With the bend in the river undercutting the toe of the slope, relatively minor slides will occur on a regular basis. None of these smaller landslides have ever damaged homes nor even were large enough to cross the river channel to the steelhead community on the far side of the river. These smaller recreational lots were created in 1949, and a number of homes have existing in this area since the 1930’s. Some of these older homes were damaged by high water events from the river, but there is no record of any landslide activity that damaged any homes. The closest homes were about 600 ft from the toe of the slope prior to the larger 2006 landslide. There were only 6 building permits for homes or manufactured homes issued since the 2006 slide, the closest being 800 feet from the toe of the 2006 slide, the rest were 1000 to 2000 ft away. The current Standard of Care, or "best available science", codified in the International Building Code adopted by this state, defines the hazard zone at the toe of a steep slope to be half of the height of the slope. Earlier codes used by this state required only a 15 ft set back, regardless of the size of the hillside. Hazel is only about 600 ft high, so based on the current standard all of these homes should have been well out of the danger zone. Even landslide hazard studies would estimate the natural angle of repose for the loose hillside material, typically 20 to 30 degrees, and recommend any construction be outside this zone. In this instance, the angle of repose was zero since it spread out along the valley floor. So the debris acted more like a liquid rather than slumping soil. The historic landslides in the area, even the massive events, that date back to the last ice age, were not as extensive as the recent event. https://www.facebook.com/photo.php?fbid=750455554988401&set=p.750455554988401&type=1

 

Something was Different So the question is, what changed from ancient times, up through the last century, to now? After the 2006 slide a decision was made by the state to redirect the river channel away from the toe of the slope and to stabilize the old land slide debris from further sliding to protect the fish habitat. If the state geologists saw any hazard to the homes across the river, they never noted it in any of the many reports for this area that they prepared. The normal steps for stabilizing a hillside is to stop the base erosion, and then anchor the toe with some sort of structure, ballast, a retaining wall, or deep driven piles, and the last critical step is to dewater the hillside. Without lowering the water table, the underlying soil is weakened and very large hydraulic pressure is put on underlying soil. I suspect that this site was not dewatered after the state spent millions of dollars to redirect the river and to amour the toe of the 2006 landslide debris. I have not located any detailed plans, or reports of exactly what measures were taken, but there are extensive pictorial records of the stabilizing measures that were installed. There is no evidence in the photos that any dewatering measures were taken. What no one anticipated was that without the ability of the river to carry away the old landslide debris on the slope, the disrupted strata from the 2006 slide, particularly the alternating clay layers previously mentioned, would have collapsed onto the face of the slope, and formed a cap or seal against the undisturbed permeable strata. This was like creating a dam, holding back all of the perched water tables on the hillside, allowing the water table to extend almost to the top of the slope. The hydrostatic pressures could be up to 600 ft deep. This could put as much as 45 tons per square foot of pressure on the underlying soil. Below is the before and after profile. https://www.facebook.com/photo.php?fbid=750454928321797&set=p.750454928321797&type=1

 

The 2006 landslide acted like a dam, the clay layers within the strata would have laid down over the face of the slope capping it off. Normally the river would have washed away the debris, exposing the slope and allowing it to drain off. But with the river redirected and the toe of the old landslide protected from erosion, it would have allowed the impermeable clay to consolidate, with the sandy layers washing out from between the layers, eventually sealing off the face of the slope like a dam. With the extra heavy rainfall, the undisturbed older permeable strata deep within the formation would have filled faster than it could drain out, until there was structural failure deep down within the formation. This is why when the underlying soil failed is was more like a dam bursting, sending saturated soil more than a mile from the head scrape. https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-prn2/t1.0-9/s350x350/1456141_750455668321723_7448594350092962717_n.jpg

 

This massive debris field is much further from a 600 ft high hillside than has been seen before. Who would have anticipated that anyone would be in danger standing up to a mile away from a 600 ft high hillside? This is why the danger was not anticipated. Had the river been allowed to wash away the debris, or if they installed adequate under-drains to dewater the hillside, it may have allowed the formation to drain. The hydraulic pressure would not have caused the slope failure and subsequent deadly debris flow. The state’s attempt to preserve the fish habitat may have not only destroyed it, but also wiped out 49 homes and killed so many innocent people

 

Peter Chopelas PE