by Philip S. Prince

Debris flows move fast, making them an exceedingly dangerous type of landslide. “Fast” is always a relative term, but in the case of debris flows, speeds of 30 or 40 miles per hour (~50-60 kilometers per hour) are quite reasonable. Debris flow speed is often sufficient to cause the flow to “run up” the slope on the outside of bends in the channel the flow is following. The result is a scoured track notably higher on the outside of the bend than on the inside. This is “superelevation”–the “sloshing” of a flow up a slope due to its speed and momentum when it encounters a channel bend. The sketch below illustrates the idea.

The sketch actually contains a number of superelevated areas due to the bends in the channel. Each is characterized by asymmetry of the damaged, scoured area–higher on the outside of the bend than the inside. The GIF below puts the process in motion. The key point to look for is the flow running up the slopes as it rounds the bends in the channel.

The GIF exaggerates the process for illustrative purposes, but real-world examples of the superelevation concept can be quite dramatic. The image below shows the results of a Jun 27, 1995, debris flow in Madison County, Virginia, near Graves Mill. The scoured area clearly “swerves” away from the blue stream channel due to significant superelevations around bends. Notably, scour from the flow reached 290 feet from the stream channel in the foreground, where moderate slope offered less resistance to superelevation. At the end of the yellow arrow, the flow superelevated about 52 ft (16 m) up a steeper slope.

The ~52 ft superelevation site was photographed soon after the event. A particularly impressive photo is shown below, and the source report (Eaton et al., 2004) can be found at this link. The small red dot in the image above shows the approximate location from which the photo was taken. Note that the flow scoured trees and soil away, leaving bare bedrock. Obviously, structures in the path of a flow like this would not stand a chance.

As indicated by the caption of the photo, superelevation height is used to estimate debris flow velocity, with the 52 ft run-up suggesting a ~45 mph speed. At this highest superelevation point, the flow had traveled about 3,100 ft (940 m) and descended 885 ft (270 m) in elevation. The image below shows the 52 ft superelevation point (yellow arrow) in relation to the initiation zone of the debris flow. Note the number of flows in the area, a result of the extreme but localized precipitation associated with an atypical thunderstorm (check out this link).

As shown in the earlier GIF, this flow produced a number of superelevations due to slight bends in the stream valley the flow followed. Bracketed yellow lines show them in the image below, with the 52 ft feature near the center of the image.

To the east of this flow, a smaller flow produced another impressive superelevation. The following images show the flow’s track with and without a bracketed yellow line to indicate the superelevated zone. I am not sure exactly how high the flow reached here, but Google Earth suggests the height approached 48 ft (15 m).

A notable aspect of the imagery shown thus far is that it was taken soon after the actual debris flow events, likely in 1995 or 1996, as no vegetation has returned to the scoured flow tracks. The area looks much different today, but 1-meter resolution lidar imagery preserves evidence of the superelevations. Seeing the scoured areas may take a bit of focus, but they are definitely there, and can be nicely matched to the 1995 (?) imagery. The GIF below transitions between the 1995 imagery, current imagery, and a lidar hillshade/slopeshade image.

Traces of the superelevation to the east are visible in lidar imagery as well. The GIF below cycles through the same imagery types. This example is equally subtle but readily identifiable once you know it’s there.

In areas affected by debris flows sufficiently long ago that forest recovery largely obscures scoured flow tracks, lidar imagery can provide useful information about flow paths and superelevation potential. All of mountainous Appalachia can experience destructive and potentially lethal debris flow events, but the details of flow behavior can vary between sub-regions with different rock and soil types and different slope geometries. Studying past flow behavior can help geologists increase understanding about potential slope behavior during extreme rainfall events. This information can, in turn, inform planning and land use decisions, particularly when human safety is a consideration.

The 1995 Madison County debris flows are also an excellent illustration of how localized extreme rainfall and debris flow occurrence can be. The image below shows a larger view of the setting of the two superelevated flows described above, which are indicated by yellow arrows. Notably, debris flows are restricted to the southeast slopes of the mountain, where the superelevated flows occurred. Just across the ridge, no flows or landslides of any kind occurred.

The extreme rainfall (~30 in or 76 cm in 8 hours or less) that produced the debris flows resulted from upslope flow of moist air into a small but very intense thunderstorm. The details of the process are described in the second paper linked above (and again here), but a takeaway is that wind direction, altitude of winds, and the shape of topography “funneled” moist air upslope and into the storm to focus the catastrophic rainfall on a small area. The interplay of topography, weather, and slope geometry in this Madison County, Virginia, example nicely illustrates how meteorologists and geologists can support each other’s work to help people avoid debris flow hazards.

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.

by Philip S. Prince

All parts of the Appalachian Mountains are no stranger to episodes of localized but catastrophically extreme precipitation, with the eastern Kentucky event of July (2022) and its tragic consequences being the most recent reminder. These precipitation events, which typically occur during summer months, can deliver double-digit inches of rain (more than 25 cm) in just a few hours, with event rainfall totals sometimes exceeding 30 inches (~76 cm) in well under 24 hours. Unsurprisingly, such a quantity of rain produces significant flooding and landsliding, often in the form of fast, mobile debris flows, which present extreme risk to human life. While the landslides associated with these storms are well remembered by those who experience them, their visual record is usually eliminated in a couple of decades by the re-growth of thick vegetation. The GIF below shows how June 1995 debris flows that occurred near Graves Mill in Madison County, Virginia, are significantly less visible after 20 years. This storm produced in excess of 30 inches of rain. The GIF may load slowly…

These extreme precipitation events need to remain part of the collective societal conversation, and high-resolution lidar imagery provides an excellent way to keep their landslide record visible, regardless of vegetation. Lidar imagery also reveals unique and more subtle details of how mountain slopes respond to such extreme rainfall, including an unusual type of landslide that appears to occur in only the most exceptional rainfall events: the “blowout.” This style of slope failure was first described Pennsylvania in 1952 in association with a July 1942 storm, and several examples of it are unmistakably present on the Transylvania County, North Carolina, slope shown below. The large scar just left of the center of the image is about 100 ft (30 m) across.

When I started mapping landslides in Transylvania County in early 2022, I had no idea why “blowout” was a choice for landslide type within our mapping software. I did, however, have a sense that the slides shown in the lidar image above were different from any features I had yet encountered in the field. They resembled debris flows, but did not create an erosive track. Scars left by the slides tended to have flat bottoms, suggesting rotational failure, and were round or elliptical in outline with smoothly curved failure surfaces. Many of the slides were particularly interesting to view with lidar because of the streams of debris they produced, which would have traveled as an overland “sheet” of soil, cobbles, and boulders at the time of failure. The view below is looking right-to-left along the slope shown above. This perspective highlights the shape of the failure scars nicely.

ALC Principal Geologist Jennifer Bauer suggested that these Transylvania slides were “blowouts” after examining a few in the field and reflecting on her experiences mapping in Watauga County, North Carolina. Watauga County experienced a catastrophic precipitation event in 1940 that produced slides recorded as blowouts, and a bit of lidar comparison made clear that the unusual Transylvania County slides were definitely 1940 Watauga-style blowout features. Some Watauga examples are shown in the images below.

Significantly, the Transylvania slides were themselves located in an area struck by the region’s “storm of record” in July 1916 (and possibly the 1940 storm as well), so the extreme precipitation context also fit the Watauga analog setting nicely.

The 1916/1940 events appear to have generated no shortage of blowouts of various size in central Transylvania County, though many occurred in areas that were probably sparsely inhabited or uninhabited at the time. The example below is an exception, and the current property owner shared his family’s recollection of the failure during the 1916 storm. An interesting detail of the story is that material from the blowout travelled all the way to the French Broad River, but left no eroded track on the landscape (more on this in an upcoming post). This “trackless debris flow” description seems apt for the blowout landslide style in general.

I was interested in the origin of the actual “blowout” terminology, which was first used by Eisenlohr (1952) to describe landslides that occurred near Port Allegany, Pennsylvania, during a July 1942 storm (click here for link). Eisenlohr explicitly states that the term (originally written as “blow-out”) was selected “for want of a better expression.” Note that the “blow-outs” were specifically associated with areas receiving over 10 inches (25.4 cm) of rainfall, consistent with the events producing blowouts in western North Carolina.

I thought this Pennsylvania “type locality” was worth a lidar look, and, indeed, it revealed characteristic blowouts despite hosting highly distinct bedrock geology and slope geometry. The next three images show examples from Port Allegany’s immediate surroundings.

So, are blowouts a hallmark feature of extreme precipitation events, at least in Appalachia? The 20th century has seen several notable “double-digit” rain events throughout the Appalachian range, and the most extreme ones all produced blowouts that are readily visible with good lidar imagery. Hack and Goodlett (1960) (linked here) used the term to describe features formed on Shenandoah Mountain in Augusta County, Virginia, in a June, 1949, extreme rainfall event, and they are clearly blowouts, as the term was previously–and still is–used. Several are visible in the foreground below, with many more highlighted in the background.

The June 1995 Madison County, Virginia, event (link here) also produced blowouts amongst the numerous debris flows. A notable example is shown below–this failure is located on a surprisingly modest and convex slope, where soil pore water pressure might not be expected to reach extreme highs.

The same system that produced the 1995 Madison County storm also produced localized extreme events further south. The headwaters area of the Moorman’s River above Sugar Hollow Reservoir in Albemarle County, Virginia, was particularly hard-hit. Numerous blowouts are visible in lidar imagery of slopes in this area. The first image below labels several; the second image is a detail of the 75-ft wide feature. The third image shows an interesting cluster of smaller blowouts…note the thin translational slide next to the “8 m” label.

In areas of Blue Ridge Virginia devastated by the remnants of Hurricane Camille (link here) in August 1969, blowouts are visible amongst the almost unbelievably widespread debris flow scars. The significant human toll associated with debris flows from this storm renders mention of the less hazardous blowout-style failures a bit out of context, but blowouts are present (as expected) given the magnitude of precipitation experienced during this landmark event. The image below shows a few blowouts, but the debris flow features visible in the foreground (the longer tracks; not the elliptical blowout) are certainly the most significant aspect of the event.

In September 2004, the remnants of Hurricane Ivan (link here) produced at least one blowout failure near Nickajack Creek in Macon County, North Carolina. As with the Camille event, this tiny failure is overshadowed by the deadly debris flow on nearby Peeks Creek, but its presence likely offers good indication of precipitation intensity associated with Ivan’s remnants.

Blowouts appear to have a definite association with very extreme rainfall, and (in Appalachia at least) they may only occur in precipitation events that exceed totals necessary to cause debris flows. Most of the events referenced here involved extreme rainfall onto already saturated soils. Many of the blowouts observed in the field or with lidar imagery developed on planar or convex slopes, a distinction from typical debris flow initiation in concave hollows on mountainsides. This distribution is visible below, in a slightly-rotated view of the earlier Sugar Hollow image.

All of the blowouts visited in the field occurred in colluvial soils, and blowouts observed with lidar imagery all occur in locations likely underlain by colluvium. Many of the features visited were wet during dry weather, suggesting details of shallow groundwater movement, soil type, bedrock depth, and localized pore pressure increases during extreme rainfall all come together to make blowouts happen in specific locations. Their tendency to cluster or align along bedrock boundaries beneath soil supports this idea, which was suggested by Eisenlohr and Hack and Goodlett based on observation in their generally flat-lying sedimentary rock study areas. The second Sugar Hollow image (4 images up from above) shows a nice example of clustered failures. While Sugar Hollow is underlain by low-grade metamorphic rock, mechanical distinctions and intersecting joint sets are very much present and influence shallow groundwater movement.

A particularly interesting blowout detail is that they tend to look the same, regardless of area geology, elevation, relief, or latitude. A Port Allegany, Pennsylvania, blowout looks just like many Transylvania County, North Carolina, features, despite the areas being over 500 miles (~800 km) apart and being underlain by nearly flat-lying sedimentary rock and intensely deformed metamorphic rock, respectively.

All of the examples in this post differ in terms of bedrock (among other details), but the failures are remarkably similar in appearance. Presumably this detail has been adequately illustrated by this point…

The association of blowouts with extreme rainfall might make older features an interesting source of information about climate (or specific weather event) history in an area, but the features don’t seem to preserve well in the landscape. The only “older,” or at least slightly less-fresh looking blowouts I have seen are near Hiawassee, Georgia, on colluvial slopes along Ramey Mountain. They lack the crisp features seen in the examples already shown, but the distinct failure shape and debris sheet are definitely present.

Do the Hiawassee feature and its neighbors record an older extreme precipitation event? Material entrained in the debris sheet might offer an answer, but using these features to date older events would likely be challenging. They do, however, provide a glimpse into unique slope behavior during the most extreme storms, and might end up occurring more frequently in coming decades…

by Philip S. Prince

The high-quality land surface imagery that ALC currently uses for landslide mapping is generated from a lidar data set produced in 2017. This 2017 data, which has 0.5 meter (~1.6 ft) resolution, is a major improvement over the previous 2005 data, which was 6 meter (20 ft) resolution. For geologists trying to understand landslide processes, this resolution improvement is significant. The GIF below shows what enhanced lidar quality looks like by comparing Google Earth aerial photography, a 2005 lidar overlay, and then a 2017 lidar overlay. The 2017 overlay should look like putting on glasses (with the correct prescription, that is!), as it brings the land surface into crisp detail. Landslides are labeled with yellow arrows, and they are tough or impossible to see in the 2005 overlay.

Just left of the GIF’s center, a line of four arrows indicates four closely spaced slides. The slide on the right end of this group is about 100 ft (30 meters) wide, and is only marginally visible in the 2005 overlay despite its size. The 2017 overlay, of course, makes the slide (and others) clearly visible, but it goes a step farther by revealing fine details about how the sliding material traveled downslope. The image below focuses on the large, 100 ft-wide slide, and points out the “trails” of debris left on the slope by the mass of sliding material. Keep the physical size of this feature in mind–the debris trails look insignificant, but they are actually made of small boulders mixed with soil.

The ability to see these debris trails extending downward from the edges of the slide scar provide valuable information about movement of the slide material when the failure occurred. This was likely a single, rapid event, during which the slide material fluidized and traveled over the land surface as a sheet of boulders, trees, and mud. Material along the edges of this flowing mass lost momentum and ended up as the trails of remnant debris, while the bulk of the flow would have continued quickly to the creek near the bottom of the image. I am a big fan of using models to illustrate geologic process, and the GIF below shows a model landslide that develops similar debris trails. Note that these would not form if the mass of sliding material traveled down the model slope as an intact block. Click on the GIF to open it in a separate window and get a longer view of the final “comparison” frame.

The real slide almost certainly occurred during an extreme precipitation event, so this small model (made of sand mixed with cornmeal, with a dash of abrasive beads) omits plenty of details. The model does, however, show how faint deposits of material on the land surface can help track the direction and style of landslide debris movement. The ability to see these faint deposits with the 2017 lidar provides geologists with the opportunity to understand where slide material can end up on its course downslope. Notably, subtle debris deposits like the “trails” are, in most cases, best viewed using lidar-derived hillshade imagery, which creates a land surface image with natural-looking shadows. “Slopeshade” imagery, which brightens flat areas and darkens steep areas to create recognizable topography, reveals features in areas that hillshade can obscure, but it also reduces detail in fine debris deposits. The following two images allow a comparison of these land surface image styles.

With the new 2017 lidar, both of these imagery styles are useful-and critical-to a thorough evaluation of the landscape. Slopeshade imagery looks metallic due to its unusual distribution of light and shadow, but areas facing the artificial “sun” in hillshade imagery are too bright and washed out to see much of anything, so the two styles must be used alternately or in a transparent overlay combination. My favorite combination is a hillshade base with a 30% transparent “veil” of slopeshade on top of it. I also regularly flip back and forth between the two image styles, depending on what type of landscape detail I need to see.

So, what does all of this good data mean, beyond just being interesting to look at? Throughout western North Carolina, we can now see where slope failures happened and, if the failures produced highly mobile flows that occurred within the last 100-150 years, where the material traveled as it went downhill. Identifying areas that could potentially be reached by highly mobile, fast moving debris flow events (often called mudslides by news outlets) is essential to improving public safety by letting folks know if they may be at risk during extreme rainfall events. The slide shown in the image below provides an example of interesting overland debris flow movement that affects an unexpectedly wide area.

The lumpy texture of the slide debris is quite similar to the first example shown in this post, but in this case the material traveled farther and spread out onto the sloping valley floor. The spread of this material is particularly interesting, as it indicates that flow-type slides can deliver boulders, mud, and logs to broad areas of the landscape, depending on how the land surface is shaped. For a sense of scale, the rectangular slide block visible on the mountainside (it did not progress to a flow event) is about 200 feet (60 meters) across, so the debris covered area seen here is quite wide. Again, the trails of lumpy debris are actually made of boulders, and you wouldn’t want to be in their way as the material travels downslope. The detail image below shows an up-close view of the slide deposit, where its rough texture is actually registered and reproduced by the lidar elevation model. A 40 foot wide lump of bouldery debris is labeled to give a sense of the deposit’s size.

This impressive lidar-derived imagery is critical to a better understanding of what can happen in mountainous western North Carolina during extreme precipitation events. Images like the ones in this post are essential to communicating the possible results of large flow-type landslides, which are particularly dangerous due to their speed and mobility. There are plenty of these failures to be seen in the rugged landscapes of western North Carolina, and sometimes it seems like landslide mapping makes just about everywhere look risky. This isn’t the case, and it’s important to remember that there are two sides to the coin. While these images clearly show where flowing slide deposits went, they also show where the flowing material didn’t go! While these unimpacted parts of the landscape are less attention-grabbing, they represent safer choices for building and living. Being able to identify them is certainly equally valuable to helping people live and work safely in the mountains.