by Philip S. Prince
“Blowout” is definitely an unusual name for a type of landslide, and even the guy who came up with the name (William Eisenlohr, Jr., in 1952; link here) didn’t seem to like it. The purpose of the name was to capture the apparent type of movement associated with these slides, which eyewitnesses described as water and soil “bursting forth” from the ground during a 1942 storm which delivered over 20 inches (>500 cm) of rain within just a few hours. The lidar image below shows one such slide, and the scar (“hole”) and trails of debris leading downslope seem to fit nicely with this suggested style of movement.
So, did water actually gush up out of the ground and blow this material out, as the “blowout” name implies? In the case of the feature above, definitely not. Water would certainly have played a role in this slide’s failure, but only in the tiny spaces between soil and rock particles. There, it reduced interparticle friction and significantly weakened the soil–there was not a “broken water main” inside the slope. The slide shown above would, however, have appeared to burst or erupt out of the hillside, and probably quite quickly, with the sliding material rapidly turning into a fluid and cascading down the slope due to its extreme saturation. A look at some of the details of the slide scar’s shape explains this style of movement.
A specific detail of all of the blowout-style features highlighted in the previous post (link here) is the shape of their failure surface–it is curved, being much steeper on its upslope side and much flatter on the downslope side. The simple cross section sketch shown below illustrates this shape.
The intact, non-sliding hillside is much steeper than the downslope portion of the failure surface, so material moving along the failure surface would actually move outward from the failed area before gravity took over, requiring the slide material to collapse back down to the slope. Many of the blowouts have tall, steep headscarps (upper portion of failure surface), which would provide considerable driving load to push material out of the failed area. Once clear of the failure surface, the sliding material presumably quickly broke apart and fluidized, giving the impression of wet material “blowing out” from the hillside flowing over the slope.
Soil saturation due to the extreme rainfall, along with pore water overpressure once the slope actually failed, could allow the failure to proceed very quickly, potentially making the event dramatic to watch. Little of the failed material appears to remain in the failure scars, suggesting the sliding mass had sufficiently low friction and enough momentum to completely clear the scar. The GIF below narrates the overall process…it may load slowly.
If an observer viewed this process from below, with line of sight generally parallel to the intact slope, the impression would absolutely be of outward, and possibly even upward, movement by the downslope end of the slide mass. I am a big fan of physical models, and some aspects of the blowout movement process can be captured using sandblast beads with a low-friction coating buried within stronger granular material. The beads are designed to flow from a container into a sandblasting device, so they provide a way of hinting at the weak behavior of the saturated soil. The model shown below is photographed straight-on, but the shadow produced at the toe of the soon-to-be blowout shows that it projects upward and outward from the intact slope. This will make more sense when you get to the GIF a couple images down…
The sliding material in the image above is just starting to accelerate out of the failure scar. As it picks up speed, it breaks apart while passing over the downslope lip of the scar, which is visible through the cascading material.
Now, in motion:
The weak material in this little model needs to be much weaker–this is the best that dry materials will do. Even so, the general idea–and visual impression– of an apparent burst of material out of the scar on the slope is communicated. The mass of moving material does not remain intact, and approaches a flow-like movement before rapidly losing momentum. Details like the thin trails of debris leading from the edge of the failure scar are commonly observed on the real failures.
Soil pore water is certainly the key player in the localized occurrence and behavior of the real slides, and a particular aspect of their documented behavior reflects the fluidization of the material that “blew out.” Both Eisenlohr and Hack and Goodlett (link here) noted that blowouts did not significantly damage the slopes below them, over which the material flowed. An intact block of material that fit a failure scar 50 ft x 40 ft x 10 ft (15 m x 12 m x 3 m) would, of course, have “bulldozed” vegetation out of its way, so the blowouts were apparently fluidized during movement owing to the saturation of the soil involved. The YouTube video linked below shows what this fluidization looks like. The video only captures the failure of a small block after the main event, which was probably quite impressive. Note how the small trees near the end of the video are unaffected by the flowing material, keeping in mind that they also survived the main failure (or failures). The shape of the failure surface is also visible, and is highlighted by the block’s movement.
This slide appears to be occurring in a residual soil lacking the rock fragments and boulders of the Appalachian examples. Presumably, the Appalachian examples would have produced a far-traveled sheet of fluidized, finer-grained soil like this New Zealand slide, which would have traveled beyond boulders and larger fragments contained in the colluvial soil. Like the Appalachian examples, the New Zealand slide does not significantly erode the slope below it, though there is some impact. The GIF below shows recovery of the slope over the next few years (might load slowly, etc. etc.), and gives an idea of how some Appalachian blowouts leave only a failure scar with no obvious deposit visible after a few decades.
This GIF might, to some degree, channel Hack and Goodlett’s thinking as they surveyed results of the 1949 Shenandoah Flood 6 years after the event had occurred. The non-destructiveness of the blowouts to the forest soil downslope was a point of considerable interest to them, with their report noting that only sapling trees smaller than 2 inches (5.1 cm) in diameter were knocked down by the blowout debris. Notably, the broken saplings weren’t pulled out the ground, emphasizing just how intact the land surface remained. Most of the blowouts they surveyed were 50 feet (15 m) wide, so a considerable amount of material moved over the slope without causing substantial damage.
While blowout-style slides are clearly less destructive to slopes than erosive debris flows, you still wouldn’t want to be below one when it happened. Many of the Appalachian examples mobilize large boulders within the debris sheet. Finer material, consisting of cobble-size rock fragments and soil, travels well beyond the boulders, as is visible in the Sugar Hollow, Virginia, image below. This sort of event would still be destructive if it occurred upslope of, and within reach of, buildings or infrastructure.
Blowout occurrence also appears to be less predictable than debris flows, in that blowouts develop out of hollows, on planar or convex slopes. Even so, they seem to be rare (only occurring in the most extreme rainfall events) and less mobile than debris flows, so they don’t represent a comparable threat to public safety. These slides are, however, an interesting expression of material movement, and a good example of how saturated soils can do interesting things during disturbance and failure.