Tuesday, November 25, 2014

Slope Fractures Before the Nile Landslide of 2009

DEM of Sanford Pasture Landslide
A landslide on the Naches River west of Yakima in 2009 diverted the Naches River and closed a State Highway. That slide was a very small part of a much older ancient massive landslide, the Sanford Pasture Landslide. The part that failed was a small section on the northwest end of the larger ancient landslide complex.

Sliding Thought Blog put up a nice post about the Sanford Pasture Landslide HERE. Alas, the posts at Sliding Thought have been discontinued - sensitive higher ups in the agency depriving us landslide wonks of some excellent insights.

When pulling some images together for a talk I decided to include a note on the Sanford Pasture Landslide simply because it was so large. It surpasses the huge Church Mountain Slide in Whatcom County and is much larger than the more famous Bridge of the Gods slide in the Columbia River Gorge. When pulling historic aerials together, I noted that the set of historic images captures the development of the fracture that began to form several years before the slide. The fracture was noticed by the Department of Natural Resources prior to the slope failure.

Pre facture image from 1998 with rock quarry in lower central part of image

Fracture can be seen on the steep slope on upper right in this 2009 image

2010 post slope failure with diverted river
WDOT has a lengthily post slide technical report that compiled lots of information and provides good figures and the geologic interpretations as well as assessment of the current stability of the slide area (NileValleyLandslidegeotechnicalreport.pdf).

No comments: