Massive mudslide in Colorado on Sunday, Memorial Day weekend

The Sunday, May 24, Mesa County Mudslide. Dimensions
are uncertain, but at least 0.5 mile wide, 2-3 miles long, and
250 feet deep. Photo by Aaron Ontiveroz, Denver Post
as published here.

UPDATED: May 26 and May 28, 2014

Heavy rain again (after Oso and Afghanistan) appears to have triggered a massive mudslide, this time in the Grand Mesa country of Colorado. The location is 11 miles southeast of Collbran, about 40 miles east of Grand Junction. The area is remote, cell coverage is sparce to nonexistent, and news is just starting to break of this event. More than 3/4" of rain hit the area on Sunday.  Three men, locals who had gone into the area to investigate the possibility of a smaller slide when they noticed problems with their irrigation water, are missing.

Update May 26: Weather.com meteorologist Jon Erdman said that Grand Junction picked up 0.42 inches of rain on Sunday, and that higher totals atop Grand Mesa above the slide were likely Heavy rain is expected toward the end of the week and into the weekend, making the slide area still very dangerous.

Location of Collbran ("A") relative to Grand Junction

This slide (dimensions in figure caption) is significantly larger than the Oso, WA, landslide which measured about 1500 feet wide, 4400 feet long, and 30-70 feet deep. The area is on U.S. Forest Service and private land. No structures or major were involved. The sheriff has reported that the person who reported the slide heard a sound like a freight train, and that "the slide came down with so much force and velocity that it came to a hill and went up and over a hill and then came back down--a significant hill." The area remains unstable as of this writing (2:30 PDT, Monday).

Another photo of the mudslide taken by Aaron Ontiveroz
as found here.

The area where the landslide climbed a hill is at the
upper right of this image. It is at a sharp bend in
the path of the flow. Photo also by Aaron Ontiveroz,
Denver Post.

I couldn't find any references to previous slides in this area, but did find a 2013 paper entitled "Characteristics of Landslides in Western Colorado, USA", focused around the Somerset-McClure Pass area only about 50 km away (as the crow flies, a lot longer by any access roads as they are separated by the Grand Mesa National Forest and some rugged country.) The authors of this paper are N.R. Regmi, J.R. Giardino, and J.D. Vitek, and it is published in Landslides, on-line 05 June 2013. The Colorado Geological Survey also has an extensive website and inventory program that can be viewed here. It is painfully clear that western Colorado has major landslide problems.

Update 5/26: Geologist Jonathan White, with the Colorado Geological Survey, said that another slide seems inevitable because of the buildup of water in a depression created by the big slide. The depression that he's referring to may be (???) at the top of the slide in the lower photo. Water has been flowing into the depression already, but White says that it's impossible to tell when the next slide could occur--perhaps even years from now when people have forgotten the danger. The terrain is too unstable for any work to drain the water, and Jonathan Godt, a geologist with the USGS says that "practical engineering measures for things of this size are pretty limited."

Update, May 28: The last photo in this set shows the mobility of the flow as it climbed up and over the ridge in the upper right where it took a sharp right turn in the flow path. From a topo map of the area, it looks like the elevation of the headwall at its crest is roughly 9700 feet, and that the elevation where the slide began the right-angled turn is about 8000 feet. This drop of 1700 feet elevation occurred over about 1.5 miles. It looks like the topped a ridge that is a couple of hundred feet higher than the valley bottom (where it spilled over the ridge on the right side as viewed in the photo, left side when viewed from the flow direction). Since the slide was reported to be several hundred feet thick, it's not clear how much of this spill-over was due to the energy of the flow versus the thickness of it as it approached the ridge. Note the erosion line of the trees in the extreme upper right of the photo and how it appears to be close to the elevation of the top of the overtopped ridge. This suggests to me that the overtopping had a significant contribution just due to the huge thickness of the flow.

There is also a very unusual change in texture between the material that did not go up the hill (streamwise right) and the material on the proximal side of the hill (rough vs. smooth?), and I'm wondering if bigger particles in the flow got left behind as the more fluid finer-grained material climbed the hill? Did the flow climb the hill on its left side (toward the top of the photo) and then fall back toward the valley on its right side (the smooth area on the right side of the ridge)? Is it possible that the "slide" changed to a muddy flow at this point--certainly there is a change in the texture of the slide downstream from this area, as pointed out today by Dave Petley on his AGU landslide blog. Field observations will be needed to clarify many questions.

After making the first turn to the right, the material then turned to the left, spread out around and over a region of ridges (emanating from the hill on the streamwise right side of the flow in the middle of the photo), and eventually exited from the main channel, narrowly missing a developed site of some sort (natural gas, fracking facility??).  I'm not sure what this is, and the WWW is full of speculation. The elevation of the toe of the slide is about 7300 feet, making the total drop of the order of 2400'.