In the last few years in some sections of North Carolina, it seems as if the rainfall just won’t end. At DuPont State Recreational Forest in calendar year 2020, one of our rain gages measured 120 inches of rain from January 1 to December 31. The two years before were wet years, and 2021 is on track to be a wetter than normal year.
Likewise, it seems as though hurricanes have lashed the southern United States severely over the last few years, as evidenced by Hurricane Fred in August of this year. One of the most devastating events from a tropical storm dumping many inches of rain, besides the damaging flooding, is that of landslides. In this article, I am going to explain what often triggers landslides, why they occur, and why they can be so dangerous and their impacts on flooding sometimes.
During my career with the Natural Resources Conservation Service, especially here in the mountains of western North Carolina, I saw many slides in different settings, and witnessed their awesome power. I was able to study why slopes slid and the underlying causes of these slides, and their contributing forces related to devastating flooding.
Most landslides occur on slopes, sometimes on slopes deemed flat enough that no one would expect the ground to slide. As such, we need to look at the more common reasons a slope could suddenly collapse.
Gravity, slope geology — that is the rock types, depth to rock, rock strike and dip angles — soil types and consistency, the cohesiveness of the soil particles, soil fill or undisturbed soil, vegetation (or the lack thereof) upon a slope and slope cuts — all are major contributing factors in slope stability.
First, all slopes are under the natural effects of the earth’s gravity. This is a constant. Gravity can play a very major role in whether a slope is susceptible to slide, given the other physical factors mentioned above. The steeper the slope, in general, the greater the effect of gravity upon its stability.
Slope geology is the presence of the type of rock and its depth, consistency, and strike/dip angles relative to the angle or steepness of the slope, be they man made slopes or natural slopes, as on a mountainside. Examples of man-made slopes include earthen dams, roads cut across a natural slope, where part of the road is “cut” material and the other part of the road is fill material. Many roads are thus partially “benched” (cut into the slope) and the other part of the road is the resultant fill material from the bench cut. Basically, a road of this type is built as 50 percent cut and 50 percent fill material, resulting in “two slopes upon a slope.” Here the cut bank generates a slope where the road was “benched into the hillside” and the fill slope — where this cut material was placed to create the other half of the roadway, creating its own slope. So, from top to bottom, you have the natural hillside slope, the cut slope, the road, the fill slope, and the rest of the natural hillside slope below the fill. Thus, you can begin to see a road that’s cut and filled on an existing slope. This is a very common way to build anything from a logging road to a major highway.
Obviously, the soil type present on an existing slope has a major effect on the overall long-term stability of that slope, be it an earthen dam, a road installed across a natural slope, a “cut out area” for a house or other building, and so forth. Sandier soils lack the ability to be as cohesive as clay soil. The stickiness of sand is far less than the stickiness of a clay type soil, thus the stability of the slope can be far less, and especially so under saturated conditions. Undisturbed soil on a slope tends to be more stable than soil on a slope that has been disturbed, such as through any type of construction activity.
Vegetation on any slope tends to make that slope more stable, as the roots from the vegetation tend to bind soils and rocks together. Trees generally provide the greatest stabilizing factors, but grasses or brush also help stabilize soils on a slope.
So, why all of this information just discussed? Because all of these factors will have a bearing on when, where and how a slope may slide under heavy rainfall events.
While I said above that vegetation on a slope usually tends to help stabilize a slope, be it a natural, undisturbed slope, or a vegetated constructed slope of some type, vegetation tends to hold or capture rainfall, causing slower storm runoff and therefore greater infiltration of rainfall into the soil profile. While a soil profile is considered a solid material, if it becomes wet enough during this infiltration process, soil can become a “flowable solid.” Once a soil profile on a slope becomes saturated enough to reach this super-saturation condition, under the effects of gravity, it can begin to move, or flow. If it suddenly moves fast enough, it becomes a landslide, occurring in seconds, and moving tons of slope material and everything on it, rapidly downhill.
Landslides can range from small sloughs (pronounced “sluffs”) to massive mountainsides sliding for thousands of feet downhill, taking everything in their paths with them. Homes, roads, culverts, bridges, vehicles, forests, anything can be destroyed in a matter of seconds. The depth of these slides can vary greatly, from a few feet, to many feet deep, and they can travel many feet per second. Average soil weighs 90 to 120 pounds per cubic foot. Thus, it is not uncommon for hundreds to many thousands of tons to be contained within the slide.
Slides can be from a few feet to many feet down into the ground surface, and the size of the slide can be from a few square feet to many acres in size. The shear volume and speed of a landslide can wipe out everything in its path.
One of the most devastating components of a landslide occurs when the slope gives way, sliding all the way down into a stream or river channel, usually blocking the channel flow temporarily. The slide forms an earthen dam, blocking the water flow down the channel. During the storm and subsequent runoff into the blocked channel, the water forms a lake. At a certain point, the force of the water against this saturated “landslide dam” within the channel overcomes the dam and a catastrophic wall of water is suddenly released down the stream channel. This causes serious flash flooding, creating higher water levels in the channel and floodplain than the runoff floodwater alone would have caused with no landslide dam. Water levels sometimes reach well above normal flood levels, and this is when houses, cars, trucks, factories, infrastructure, buildings, bridges, and so forth are ripped from their foundations and washed down stream. This is also when the sudden loss of life occurs.
The other issue from a landslide reaching a stream or river channel is the tremendous added debris into the fast-moving flood water. Logs, whole trees, houses and other buildings and items now caught in the flood flow tends to block bridges and culverts. So, this already above normal flow now gets even higher, as the water coming down the channel tries to get around blocked bridges and culverts, thus causing even higher and deeper flood flows and doing even more damage. Any human caught in this churning flow of debris and water is certain to die.
So, what is all of this to rescuers? It is very likely you will be searching channels for deceased people, under debris, inside cars, inside destroyed houses or other buildings, people caught in strainers — in short, seeing things like you have never seen before. The magnitude and force of these events will not allow much in the way of rescue. Rescuers must be EXTREMELY CAREFUL around unstable slopes that have slid and in the channels below the slides. Go online and search for landslide videos. I think that you will see that, when the ground moves, you can do nothing to stop it. Look in awe at these violent forces of nature, and then know that your rescue efforts will be just beginning when the ground stops moving!