My cousin, Scott McLean, is an engineer recently retired from Seattle City Light and makes this report.
The big mudslide near Oso, WA that you may have been hearing about in the news strikes close to me in more than one way: that was the way I drove back and forth from Seattle to the SCL (Seattle City Light) Skagit hydroelectric project for 20 years, so I know that road so well that I could probably drive it in my sleep. Because of traveling this way so much over the past years, and because I worked on a big rockslide project (the Ross Slide) for my last three years at City Light, I feel this news story more personally than most.
Regarding the Oso Mudslide, it occurred on 3/22/2014 on Hwy 530 about 10 miles east of Arlington, WA. If you look on a map, follow I-5 12 miles north of Everett then turn east on Hwy 530 at Exit 208, follow Hwy 530 about 15 miles through Arlington and on east to Oso. It is a beautiful highway that passes through the foothills of the Cascades along the valley of the Stillaguamish River, and takes you to the heart of the spectacular scenery of the North Cascades National Park. This whole valley is very prone to landslides. In fact there was another landslide 8 years ago only a mile west of the Oso Slide that threatened two of City Light's transmission towers -- and coming up with ways to protect those towers from the advancing slide was one of my projects at the time. We thought that slide was big: it dropped about a 200-foot stretch of Hwy 530 down about 20 feet as the whole hillside slid into the Stillaguamish River below. But that was small potatoes -- the Oso Slide is something else. In none of the other landslides I am talking about was there any loss of life. The fatality count in the Oso Slide is what makes this one a heartbreaker: 16 as of this morning with over 100 people missing. The slide came through at about 11 o'clock on a Saturday morning -- a time when many families would be at home.
See the attached photos from the Seattle Times. Photo 1 shows an overview of the range of the slide looking downstream to the west. At the very left edge of the photo, what looks like a linear clearcut is SCL's transmission line that carries about 25% of Seattle's power supply from the Skagit River hydro project to the city. You can see Hwy 530 at the upper left corner of the photo meandering into the transmission right-of-way, then it angles down to the right and disappears into the mudflow. What is striking to me is how far the slide debris flowed horizontally. When the mud reached the bottom of the hillside, it went into the river, filled up the river and kept on going another couple of thousand feet right through the middle of a community of about 25 homes, through some woods, and across Hwy 530 into some fields across the highway. What is concerning about this scene is that the slide "scarp" (i.e. cliff at the upper margin of the slide) is severely over-steepened above its "angle of repose" ... so pieces of that slope will calve off like the face of a glacier until the slope is finally reduced to a flatter slope near its angle of repose. It's just a matter of time until more of that hillside comes down. No one knows when. And because saturated soil loses its ability to stand up, the rain which started yesterday and which is forecast to last through this week makes that area a dangerous place for rescue workers to work. Hats off to those brave folks.
Photo 2 is a map showing the outline of the landslide in purple. It appears to me that the areas outlined in red are lots on which homes were located – I count 30 homes including ones south of Hwy 530.
Photo 3 is a screen shot from Google Earth of the slide area; north is at the top. The community of 25 homes between the highway and the river lies on the inside of a curve in the river. To the north across the river from the community of houses, you can see the outline of previous slides at this same location ... the slide area bounded on the west with bare soil that looks almost white in the photo, and the vegetation has a different texture in the slide zone, due to being younger. Hwy 530 clearly shows going generally in the east-west direction, as does the SCL transmission line right-of-way. Near the bottom left edge of the photo, you can see what looks a green meadow sandwiched between the river and Hwy 530 on the south side of the river. That green area is a massive reconstructed hillside placed by WSDOT. Just below (south of the highway) that green patch, you can barely make out the two SCL transmission towers that we were working to protect. Fortunately, the green hillside buttress plus a huge retaining wall built by the State put an end to our worries about those towers.
Photo 4 gives an on-the-ground perspective: mudslide debris and a house in the middle of the highway.
Photo 5 shows the rockslide I worked on for City Light near Ross Dam on the upper reaches of the Skagit River. Our task was to bring down the unstable rock remaining on the cliffs after the slide via blasting, then re-stabilize the rock cliff via rock bolts, then reconstruct the road and docks destroyed by the slide. The blasting, rock bolting and construction of temporary roads and docks was done over the course of 3 years on my watch, and the permanent facilities are slated for construction in the summer of 2014.
--Scott McLean lives in Edmonds.
This book is a version of a PNW myth about the formation of Columbia Gorge which in the story involves the love affair of two volcanoes: Mt. Hood and Mt. St. Helens. It was performed as an opera in Portland, Oregon in the Fifties.
The Pacific NW is always unstable, formed by volcanoes at the overthrust of tectonic plates, so that the constant pressure pushes land up into rumbles and sounds, but the rain coming off the ocean is always hosing is down into the constant run-off. When humans build on a slope, they risk a sudden, devastating slide. Love becomes grieving. The angle of repose, which refers to the geological slant at which the ground comes to rest, becomes eternal because it is where bodies rest. The bridge between lovers falls. We try to find comfort by making the story beautiful.
But it would do more good to listen to soils engineers when they say, "Don't build there. You will awaken mud dragons."