October 13, 2009
My apologies for the delay on this landslide. Sunday I was out in the field and on Monday I was working on helping get the word out about the destructiveness of this landslide. Here is our quick report for the landslide:
On October 11, 2009, Lorraine Powell (DGER), Jack Powell (FP), and Isabelle Sarikhan (DGER) investigated a landslide on Highway 410 approximately 10 miles west of Naches. The landslide has damaged approximately 12 structures (residential houses and quarry buildings), buried equipment in a quarry, has blocked and diverted the Naches River, and completely closed Highway 410. No injuries or deaths were reported as a result of this landslide, however, many of the surrounding houses have been evacuated and a significant amount of people have been displaced. As the landslide is moving into the valley, the weight of the landslide mass and movement appears to have deformed part of the soft sediment valley floor, further threatening houses in the valley.
The landslide is still active and will probably continue moving for some time. On the west side of the landslide, movement appears to have slowed during our investigation on Oct. 11, although cracks near the main landslide scarp could indicate future landslide propagation to the west. The east side of the landslide is much more active. A small hill acting as a buttress from the original movement of the landslide is in danger of failing. Sounds of groaning and deformation of the hillside indicated high amounts of stress within the landslide mass. In this section, about 6 houses are threatened or are actively being damaged by the landslide.
The presently moving area of the landslide is more than 80 acres; with the potential for other parts of the slope activating that could increase the area by another 15 acres. The landslide is approximately ½ mile long ¼ mile wide and with depths varying from 50-60 feet covering the highway to several hundred feet higher up. The landslide mass moved in blocks, some translational, others rotational.
As the landslide continues to move into the valley, deformation will probably continue, potentially damaging houses that are not in the direct path of the landslide. Currently, the water is being diverted into an abandoned river channel. The diverted river is flooding one building and blocking the only detour road (Nile Road) around the landslide. Deformation has also buckled and made the detour road (Nile Road) impassable. The material blocking the Naches River will be difficult to remove to restore the channel, and the landslide mass over Highway 410 will also be very difficult to remove due to stability issues. Long term closure of Highway 410 over the original roadway path should be expected.
I’ll try and post more later today, the landslide is part of the Sanford Pasture Landslide, one of the larger deep-seated landslides in Washington State. It has an interesting history. I’ll also post more about the anatomy of this landslide and some of the other interesting aspects that aren’t being reported.