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Types of Mass Wasting
FALLS (rock fall and rock avalanche)
SLIDES (rock slide and slump)
FLOWS (rock avalanche, debris flow, earth flow, and creep)
Discussion Mass-wasting events come in many shapes, sizes and speeds. Typically, the steeper the angle of a slope, the faster will be the down-slope movement of rock and sediment. Also, water can play a significant role in mass wasting, sometimes acting as the key component to a mass-wasting event, or serving as a lubricant within a mass of sediment and rock, enabling it to travel faster and further than it would otherwise. To learn more about the causes of mass wasting click here. It is important to understand that one type of mass wasting can evolve into another type of mass wasting as the body of sediment/rock moves down a slope. This can make it difficult to classify a single event as being one type of mass wasting or another. Below is a simple classification of the different types of mass wasting, with each type having sub types.
Falls This type of mass wasting can involve a single rock or thousands of rocks. For a mass-wasting event to be classified as a fall, it must travel at a high rate of speed down a very steep slope. If the slope is vertical or overhung, then the rock(s) will drop straight downward, fragmenting when they hit the base of the slope. Over time, this forms a body of angular rubble called talus, a distinctive transition from the steep slope to flatter ground. Below are descriptions, diagrams and photographs of the two basic types of falls.
1) rock fall A rock fall consists of one or maybe a few rocks that detach from the high part of a steep slope, dropping and perhaps bouncing a few times as they move very rapidly down slope. Rock falls are very dangerous because they can occur without warning, and because the rocks are traveling at high velocity. You can usually tell where rock falls are common by identifying talus at the base of steep slopes. If you are out hiking or camping in mountains or canyons, avoid talus slopes and the rocks that fall onto them!
Diagram 1 illustrates a very steep slope with the potential for a rock-fall event. The actual cause of such an event might be an earthquake, the movement or weight of an animal, or the freezing and thawing of water. Obviously, hanging out at the base of such a slope is not a good idea.
Diagram 2 shows the rock in the process of falling. This usually occurs without warning, and is rarely witnessed. Sometimes a hiker may hear the fall off in the distance, but upon closer inspection will see only a pile of rock rubble at the base of the slope.
Diagram 3 illustrates the rock debris, or talus, that forms at the base of a steep slope as rock fall and break apart on contact with the base of a slope. The more rocks that fall, the greater will be the buildup of talus.
Below are photographs of slopes prone to rock falls, and the talus that gathers at the bases of such slopes.
The pictures below show where rock falls have occurred from the faces of very steep slopes. The first three are from the Escalante region of southern Utah, where each rock fall travelled only a short distance downward, and the buildup of talus is only minor. The fourth photograph was taken outside of Las Vegas, Nevada. Here, the rock fall shattered on impact, with large boulders rolling up to 100 feet from the base of the slope. The fifth photograph was taken in the Sierra Nevada Mountains of California. Here the scale is much grander than the preceding pictures, with the vertical cliff face that generates hundreds of rock falls each year rising 2000 feet straight up! Needless to say, this is a very dangerous location to stand!
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Both of these photographs are from the Goosenecks area of the San Juan River in southern Utah. The first shows the layer of limestone that forms the rim rock along the canyon, with large blocks that have separated from the rock outcrop. The second picture shows one of these blocks that fell and bounced all the way down to the bottom of the canyon, into the river, a journey of over 800 vertical feet!
The first two pictures below were taken in Colorado. The first is from Colorado National Monument, near the Utah border. The second is from the Front Range region, near Pikes Peak. Both show the classic buildup of talus at the base of very steep slopes. Picture 3 shows talus that is gradually covering a roadway in the San Bernardino Mountains of southern California.
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These photographs were taken along a mountain highway in central Colorado. Here, steep slopes composed of fractured sedimentary rocks are prone to rock falls which can shut down the highway for days at a time. The retaining wall next to the highway provides only minimal protection to mass wasting here.
These two pictures both show steep slopes that have been undercut by wave action along the Pacific Ocean coast of southern California. So, both slopes are extremely prone to rock fall!
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2) rock avalanche This type of fall usually forms when a massive rock fall explodes apart on contact with a slope. As this occurs, thousands of rocks continue their flying trajectories down slope, colliding with each other and the slope itself, overwhelming anything in their paths. A rock avalanche is a transitional sort of mass wasting event, changing from a pure rock fall to something more like a rapid flow of material as the material moves further from the base of a slope. Therefore, some geologists classify rock avalanches as flows. Whatever the classification, rock avalanches are extremely dangerous, and you should be wary of locations where they occur with frequency in mountainous regions. The diagrams and photographs below illustrate some of the nature and danger of rock avalanches.
Diagram 1 shows a small town a short distance from a tall, steep-faced mountain. Such a setting is fairly common in mountain ranges on all continents, with the mountain providing a spectacular backdrop for the town. Unfortunately, this situation can also be a recipe for a natural disaster.
Diagram 2 shows what can happen if an earthquake vigorously shakes a tall, steep-faced mountain. Here, several massive blocks of the mountainside have peeled and fallen away, traveling at high speed toward the base of the mountain. Anything or anyone directly in the path of such a huge rock fall will be obliterated.
Diagram 3 paints a very bleak picture for the town. As the massive rock fall contacts the base of the mountain, it breaks into thousands of fragments that continue tumbling down slope at high velocity. The great energy of such a large mass can enable the rock avalanche to travel much further from the base of the mountain than one would expect, in this case destroying and burying the town. Fortunately, such an extreme rock avalanche is a rare event.
Below are photographs of features associated with rock avalanches.
The photograph to the left shows the highest peak in the Peruvian Andes, Nevado Huascaran.
In 1970, and earthquake triggered a massive rock fall event that originated from the barren area near the top of the peak.
It quickly became a rock avalanche that flowed across a broad valley, filling it with rock and debris to depths of 300 feet.
Some of the rock and debris from this high-energy mass-wasting event continued moving away from the mountain, becoming a debris flow that traveled through a narrow stream valley as well as up and over a 1000 foot-high ridge (crossing the middle of the photograph). As the flow spread through the lowlands, it buried two villages in its path, killing more than 20,000 people! The villages were in the wide, muddy area in the foreground of the photograph.
This mountain ridge in the San Gabriel Mountains of California shows the pathways for rock avalanches, both past and future. For mountain climbers these straight, barren areas provide quick access to higher elevation, but they are exceptionally dangerous during an earthquake, or when water is freezing or melting within rock fractures.
Below are photographs that further illustrate aspects of rock avalanches. Photographs 1 through 3 show the deposits from previous rock avalanches.
1 A geology class inspects a small avalanche deposit in southern California.
2 Dan Hooker atop an avalanche deposit near Las Vegas, Nevada.
3 A large rock avalanche pathway and deposit in southern Alaska.
4 Rock avalanche pathways along a ridge in the San Gabriel Mountains of southern California.
5 A view down one of the avalanche pathways from the previous picture.
6 A small rock fall and rock avalanche site, where the avalanche moved out onto a tennis court in Palos Verdes Peninsula, California.
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Slides There are two versions of slides, but what all slides have in common is that the mass of sediment/rock sticks together as a coherent block as it travels down slope along a tilted plane or surface of weakness. Typically, this surface of weakness coincides with the tilt angle of the slope that mass wastes. Ultimately, as the moving slide mass comes to a sudden stop, it may break apart and continue down slope as a type of flow. Below are the two basic types of slides.
1) rock slide This type of slide occurs where there is a tilted, pre-existing plane of weakness within a slope which serves as a slide surface for overlying sediment/rock to move downward. Such planes of weakness are either flat sedimentary surfaces (usually where one layer of sediment or sedimentary rock is in contact with another layer), planes of cleavage (determined by mineral foliation) within metamorphic rocks, or a fracture (fault or joint) within a body of rock. Rock slides can be massive, occasionally involving an entire mountainside, making them a real hazard in areas where a surface of weakness tilts in the same direction as the surface of the slope. Rock slides can be triggered by earthquakes or by the saturation of a slope with water. The addition of water to a slope increases its mass, and therefore increases the pull of gravity on the slope. In addition, water can lubricate a layer of clay or shale within a slope, which then serves as a slide surface for the rock above it.
Diagram 1 shows layers of rock tilted downward to the right. The topmost rock layer is prone to sliding because it lacks support at the base of the slope.
Here, in diagram 2, gravity finally overcame the friction between the topmost rock layer and the rock beneath it. Once this occurs, the topmost rock layer slides downward as a coherent block. As it comes to a sudden stop the slide block may break apart and continue moving for some distance as a rock avalanche or debris flow.
The two pictures below were taken in the Sierra Nevada Mountains of California, near Mt. Whitney. Over a period of millions of years, the Sierras have been uplifted along the Sierra Nevada Fault. As a result, the sedimentary and metamorphic rocks that used to overly the igneous rocks of the Sierras have been stripped away. As a result, the igneous rocks have expanded, forming cracks (called joints) along which huge slabs of rock peel away (exfoliate) and slide down slope. The first picture shows numerous joints tilting downward to the left, and freshly exposed surfaces along which rocks have slid. In the second picture, a large slab of rock that was part of a recent rock slide event rests on top of talus generated by past rock fall events.
In addition to the joints formed due to the unloading of igneous (and other) rocks, rock slides also occur along pre-existing weaknesses in metamorphic rocks where flat minerals are aligned parallel to each other. Such planes of foliation are responsible for the small rock slides that have occurred along the Alaskan road in this picture, where Diane Kawahata provides a scale for the viewer.
Rock slides also result from slippage of tilted sedimentary layers along the contacts between layers, called bedding planes. This is especially true if a clay-rich layer becomes wet and slippery. The picture to the left shows where tilted sedimentary rocks have slid into the Pacific Ocean near Gaviota, California, exposing the planes along which they slid.
Pictured and described below are two examples of major rock slide events.
This is a photograph of the Vaiont River Valley in northern Italy, taken by Ed Bromhead. In 1963, a major rock slide resulted in the deaths of approximately 2600 people. The slide block, labeled on the photograph, moved suddenly into the newly filled Vaiont Reservoir, flushing lake water up and over the dam. The wall of water was over 200 feet high as it swept into nearby villages, wiping out everything in its path.
The rock slide and the ensuing flood could have been readily forseen if better geological consulting had been done before construction of the dam and reservoir. The sedimentary rocks of the Vaoint River Valley include layers of shale, a clay-rich rock. And, the rocks comprising Mt. Toc (pictured), tilt steeply toward the reservoir. After the dam was finished in 1960, filling of the reservoir introduced groundwater into the shale layers, causing them to swell and become slippery. At first, the mountainside began slowly creeping down slope at a rate of half an inch per week. As filling continued and more groundwater seeped into the mountain, the rate of slippage increased to eight inches per day, and ultimately to 30 inches per day just before the 1963 disaster.
Lituya Bay was the site of a massive rock slide in 1958. This slide was generated by a powerful earthquake along the Fairweather Fault which cuts through the St. Elias Mountains just north of the bay. Although this is an uninhabited region of southern Alaska' Pacific coast, sailors do occasionally take advantage of the bay's protected waters for anchoring overnight. The night of the earthquake and rock slide, three different vessels were harbored in Lituya Bay. As the rock slide moved into the bay, it displaced millions of gallons of water, forming a wave estimated at 160 feet high. The wave lifted one boat up and over the right side of the island in the bay, causing little damage to the boat and crew. A boat near the left side of the island was carried out into the ocean by the wave, but the crew survived despite losing their vessel. No trace of the third boat was ever seen. (Note that the source of this photograph is uncertain; probably the USGS.)
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2) slump Slumps are fairly small when compared to rock slides. Slumps form where the base of a slope is removed by natural processes (stream or wave erosion) or by human efforts (road or building construction). Removal of the lower part of a slope effectively removes physical support for the upper part of a slope, causing the formation of a new fracture in the sediment/rock comprising the slope. Soon thereafter, the slope will begin sliding downward, often rotating along the curved surface of rupture. The development of a slump is illustrated in the three diagrams below.
In diagram 1, the residents of the house have a fine ocean view, with stable rocks below them.
In diagram 2, ocean waves have removed the base of the slope beneath the house. Once this support is gone, a fracture will form, angling from the new base of the slope to the cliff top above. Residents of the house will see a widening crack cutting across their lawn, a warning of bad things to come. Usually, people will abandon their homes at this stage, removing as many belongings as possible.
Diagram 3 shows that the slump block has slid downward along the surface of rupture, what was originally the new fracture formed due to the erosion of the base of the slope. Since the fracture's geometry was curved, so too is the surface of rupture, which causes the slump block to rotate outward as it moves downward. Buildings tend to collapse as this occurs. Note that the end of the slump block often breaks apart forming an earthflow which continues to move slowly outward and away from the block.
Below are a series of photographs that illustrate some of the variety and features of slumps. Although most of these pictures are from locations in California, slumps can be found most anywhere there is a slope.
This picture, taken in El Moro Canyon near Laguna Beach, California, shows a new slump. The key features of this slump are labeled, and the outline of the slump block is highlighted with dashed lines. Slumping, along with the natural processes of weathering and erosion by water, causes mountains to become flattened over a period of time.
Point Sal, California The three pictures below were taken at Point Sal, along the coast of central California. Here, a small slump is narrowing the roadway. Picture 1 shows the surface of rupture and slight tilting of the top of the slump block. Pictures 2 and 3 are reverse angle views of the same slump, and the small fissure that can develop as the slump block separates and slides downward along the surface of rupture.
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Portuguese Bend, Palos Verdes Peninsula, California The following pictures were taken at Palos Verdes Peninsula, southern California, at the end of the Portuguese Bend Landslide. This complex area of mass wasting is characterized by earth flow movement within the main body of the mass, and slumping from the point of origin all the way to the end of the mass where it meets the Pacific Ocean. Picture 1 shows the upper end of the slide complex. Pictures 2 through 5 present different views of the end of Portuguese Bend Landslide, showing the relationship of this large landslide to the Pacific Ocean waves which continue to remove the end of the slide mass. As a result, this unstable portion of the California coastline crumbles and slumps into the ocean almost continuously. You should be able to recognize slumps in all four of these photographs. Picture 6 show where a new slump is forming next to the main road in this area, Palos Verdes Drive North.
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Point Fermin, Palos Verdes Peninsula, California Just down the coast from Portuguese Bend Landslide is Point Fermin, famous for its slumps. Until 1929, this area was an active, desirable neighborhood. Unfortunately, ocean waves eroded the base of the cliffs of Point Fermin, causing slumping of the cliffs, forcing abandonment of the neighborhood. What is left, called Sunken City by locals, are broken streets and sidewalks as well as jade plants and palm trees from people's front yards in different stages of movement downward toward the ocean on the tops of several large slump blocks. Pictures 1 and 2 below are views of the Sunken City from the ocean. Picture 1 shows that many people continue to live on the hillside just up from the Sunken City, and one wonders how long it will be until they have to abandon their homes. Pictures 3 and 4 give closer views of the slump blocks and fissures in this area, and pictures 5 and 6 show some Geology students experiencing the Sunken City for themselves.
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La Conchita, California La Conchita is a tiny, picturesque coastal community separated form the nearby Pacific Ocean by Pacific Coast Highway. Directly inland (east) of La Conchita is a 600 foot high bluff composed of poorly cemented sedimentary rocks including sandstone, siltstone, and shale. Very rapid tectonic uplift of this portion of California's coast combined with the weak rock make this stretch of the coastline extremely vulnerable to mass wasting. There are accounts of slides and flows in this area that date back to the mid 1800's, wreaking havoc with rail lines and roads up to the present. As in many areas plagued by mass wasting, slides and flows at La Conchita are related to the introduction of water into the slope material. Water adds mass to a slope, dissolves natural cements that hold sediment grains together, floats sediment grains apart (reduces grain-to-grain friction), and lubricates clay-rich layers of rock or sediment.
In March of 1995, residents of La Conchita witnessed a combination of mass-wasting phenomena associated with an unusually wet rainy season. The event began with a large slump, the toe of which became an earthflow, shown in picture 1 (credit to R.L. Schuster, USGS). As impressive as this slide-flow combination is from the air, it is even more imposing when viewed from the ground, in picture 2. From this perspective, it appeared that the entire mountain was moving into La Conchita. The perspective of picture 3 is from the slump's surface of rupture, or scarp, looking downward into town and the Pacific Ocean. The slump and related earth flow were relatively slow-moving, traveling about 50 feet in a matter of a few minutes time. No one was killed by this event, although nine homes were destroyed. The earth flow was stopped by a strong retaining wall (pictures 4 and 5), giving La Conchita residents a false sense of security.
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Between October, 2004 and January 1, 2005 the La Conchita area received three times the normal rainfall for that period of time. Over the next nine days a series of storms soaked the bluffs above La Conchita, adding more mass to the unstable slopes and causing the level of groundwater to rise higher than usual. On January 10th some emergency officials and a television crew had gathered in La Conchita due to minor mudflows associated with the rain. Out of sight, within part of the 1995 slide mass, weak rocks on top of the saturated zone suddenly ruptured, instantly crumbling into a fast-moving debris flow that roared into the eastern part of La Conchita. As a result of the debris flow removing support from the base of the slope above, some slumping also ensued, adding to the mass and vigor of the debris flow. This dramatic and deadly event was captured by the television crew's camera, which clearly showed chunks of rock and sediment flowing rapidly down slope at a speed of 20 to 30 miles per hour. Geologists estimate that a total of about 400,000 tons of earth moved during this event, which destroyed 13 homes and killed 10 people. The future of the entire community of La Conchita is now very much in doubt because of the ongoing threat of further mass-wasting events, and the prohibitive cost, estimated at $100 million dollars, to stablize the slope above La Conchita.
To better understand past mass-wasting events and the hazards at La Conchita, visit the USGS open-file report entitled "Landslide Hazards at La Conchita, California" by Randall Jibson. It was published soon after the 2005 event, and it is well-written and informative.
Below are series of photos taken soon after the 2005 La Conchita disaster. Pictures 1 through 4 are aerial photos of La Conchita, taken by Mr. Allen Krivanek, showing the path of the debris flow and the resulting destruction. Pictures 3 and especially 4 enable you to see what's left of the retaining wall constructed after the 1995 slump-earth flow event. It may have helped to shield some of La Conchita from the 2005 debris flow, most of which was deflected toward the right (east). A careful look at picture 2 suggests that the road that cuts a diagonal across the slope from left to right is the likely culprit for recent slumps above La Conchita: cutting a road into a slope over-steepens the slope and removes support from the slope mass above the road. See the enlarged "Features of a Slump" picture above for clarification of this simple but important concept.
Pictures 5 through 7 below are Associated Press photographs which provide close-up, ground views of the La Conchita disaster. Picture 8, also an AP photograph, gives a different aerial view than the aerial photographs above, showing the perspective of the debris flow from the source.
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The pictures below reinforce the idea that mass wasting can be dangerous and unpredictable in mountainous environments. In each photograph, slumping has eliminated part of a highway in the western United States. In both cases, traffic was interrupted for over a week while expensive repairs were made. And, in each situation, there is a strong possibility that slumping will occur again in the same area.
McClure Pass, Colorado. (Photograph by Terry Taylor, Colorado State Patrol.)
Zion National Park, Utah. (Photograph by R.L. Schuster, USGS.)
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Flows Of the three basic types of mass wasting, flows are the most complex, both in terms of how they originate and how they move. Unlike slides, in which the material sticks together as a coherent mass as it moves down slope, flows are characterized by internal movements of individual grains (tiny like silt or sand up to large boulders and small blocks of crust) within the flow itself. The internal flow movements of individual grains can be fast and chaotic if the flow originates from a steep slope, or if it contains a lot of water. Or, grain movements can be very slow and somewhat predictable if the slope surface is very gradual in its angle.
Discussed and illustrated below are the four basic types of flows.
1) rock avalanche This type of mass wasting is transitional, usually originating as a massive rock fall which breaks apart upon contact with the ground at the base of a steep slope. Initially, the rocks continue to bounce and fly down slope, still behaving much like falling rock. As the avalanching rocks begin to slow and lose energy, the internal behavior of the mass becomes more like a fluid, with individual rock fragments moving randomly and rapidly within the mass. As the rock fragments bang into each other and Earth's surface beneath the flow, the mass will slow down and eventually cease movement. The sequence of diagrams below illustrates how a rock avalanche might evolve from a large-scale rock fall event.
This photograph of Nevado Huascaran, a tall volcano in Peru, shows the huge rock-avalanche deposit left behind by the major rock-fall event triggered by a powerful earthquake in 1970. This event is discussed in more detail under the heading of "Falls" within this web site.
Examples of rock avalanche deposits and paths are shown in pictures 1, 2 and 3 below. Walking an top of such deposits is not recommended however, because individual rocks tend to shift with your added weight. Also, since rock falls are unpredictable and the resulting rock avalanches are very fast-moving, you run the risk of being incorporated into a rock avalanche at any time - not a good thing!
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2) debris flow As the name implies, this type of flow contains a variety of particles or fragments, mainly small to large rock fragments but also trees, animal carcasses, cars and buildings. Debris flows usually contain a high water content which enables them to travel at fairly high velocity for some distance from where they originated. Debris flows tend to follow the paths of pre-existing stream channels and valleys, but debris flows are much denser than water, so they can destroy anything in their paths such as houses, bridges, or highways. Debris flows tend to originate from denuded slopes receiving heavy rainfall, but they also evolve from leading edges of large rock avalanches and fast-moving slumps. In volcanically active regions such as the Cascade Mountains of North America, the Andes Mountains of South America, or the islands of Indonesia, ash on the slopes of volcanoes can readily mix with water from rainfall or snowmelt. When this occurs, a low-viscosity debris flow, called by the Indonesian term lahar, can form and move very rapidly down slope.
Below are some pictures and descriptions of debris flows that have occurred in the western United States.
Cable Canyon, near San Bernardino, California, was the site of a deadly debris flow in December of 2003. In October of 2003, a wildfire had swept across the slopes of the San Bernardino Mountains above Cable Canyon, removing most of the vegetation that protected the slope from the impacts of rain drops, and held the slope sediment in place with its roots. As rain fell on these slopes in December, the water rushing down the mountain slopes picked up speed and sediment. As the muddy runoff became focused in small stream channels, its energy for transporting larger particles increased dramatically. As the raging flow entered Cable Canyon it became a true debris flow, carrying trees, boulders and anything in its path down canyon and eventually through a KOA campground. Campers in RV's and tents were swept up in the debris flow. Once the event was over, several people were dead and wrecked vehicles and trailers were littered up to a half mile down canyon from the campground. The pictures below are a record of this tragic mass-wasting event.
View of the burned foothills of the San Bernardino Mountains, and Cable Canyon in the foreground.
Below are views of the KOA campground office, pictures 1 and 2, and a nearby house, pictures 3 and 4. Both show the effects of having the debris flow moving into the structures. Note the tree that was rammed through the bedroom window of the house. The residents were not injured, but were very shaken by their experience. (The campground is closed until further notice.)
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The remaining photographs show some of the variety of destruction wrought by the debris flow, including a dead bear (first picture), views of the campground with huge boulders and smaller sediment deposited helter skelter, and some of the trailers and vehicles carried away by the flow. Especially interesting is the first picture in the third row, which, when enlarged, shows the depth of the debris flow by the muddy coating of sediment on the tree trunk - roughly ten feet deep!
Waterman Canyon is about 20 miles east of Cable Canyon. It also experienced a deadly debris flow at almost the same time as the Cable Canyon event. Unfortunately, there was a much greater loss of life from the Waterman Canyon debris flow, with 13 people being swept away and buried in the debris. After this event, access was limited to emergency personnel only. So, the pictures below show only some of the effects down canyon from the church camp where most of the devastation occurred.
These two pictures are viewing down into Waterman Canyon and the church camp where many people were swept away and killed by the debris flow. Note the charred slopes above the camp.
The pictures below show some of the effect of the debris flow moving through a pre-existing stream channel. Such events cause dramatic changes to a landscape. Note that vegetation was scoured from the edges of the stream, and large boulders were dumped randomly within the stream bed. Over time, plants will re-vegetate the stream banks, and normal stream flow will sort out the sediment into a more orderly stream bed and channel.
These last two pictures, showing Waterman Canyon to the right, have a sobering effect on people who understand the factors that can produce mass wasting. Clearly, the slopes have been denuded by fire, and it will take nature many years to again cover the slopes with vegetation. With this in mind, it would be wise for people to stay out of the canyon during southern California's rainy season, from December through April.
La Canada, California This debris flow occurred in the 1980's in the foothill community of La Canada. A recent fire had denuded the slopes in the area, making them susceptible to mixing of sediment with rain. The house in the first picture received only minor damage, but the house down slope and across the street experienced the debris flow from the inside out! Photographs by Dr. Burt Conrey.
Forest Falls, California In June, 1999 the mountainous slopes above the small community of Forest Falls received over four inches of rain in less than an hour. The rocks comprising the slopes in this area are highly fractured due to the close proximity of the San Andreas Fault. This factor, combined with the sparse undergrowth of plants beneath pine trees enabled the downpour of rain water to loosen the mountain sides, generating a series of debris flows that thundered through the Forest Falls area. Several people were killed, and many houses and cabins were destroyed or damaged by the debris flows. Picture 1 below shows Diane Kawahata pointing at the trunk of a tree. Beneath where she is pointing, tree bark has been worn away by the grinding effect of the debris as it flowed past the tree. Pictures 2, 3, and 4 show some of the damage that occurred to property in Forest Falls, and picture 5 shows Bruce Perry standing beneath a boulder transported within the debris flow.
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La Conchita, California In January of 2005, a series of storms soaked the slopes above La Conchita, adding mass to the unstable slopes and lubricating the sediment grains. The result was a catastrophic failure of a small portion of the slope above La Conchita. This event, labeled in news reports as a mudslide, began as a sudden rupture occurred within the slope, releasing a slump block which instantly crumbled into a fast-moving, fluid-rich debris flow which roared into the eastern part of La Conchita. Geologists estimate that a total of about 400,000 tons of earth moved during this event, which destroyed 13 homes and killed 10 people. The future of the entire community of La Conchita is now very much in doubt because of the ongoing threat of further mass-wasting events, and the prohibitive cost, estimated at $100 million dollars, to stablize the slope above La Conchita.
Below are series of photos taken soon after the 2005 La Conchita disaster. Pictures 1 through 4 are aerial photos of La Conchita, taken by Mr. Allen Krivanek, showing the path of the debris flow and the resulting destruction. Pictures 3 and especially 4 enable you to see what's left of the retaining wall constructed after the 1995 slump-earth flow event. It may have helped to shield some of La Conchita from the 2005 debris flow, most of which was deflected toward the right (east). A careful look at picture 2 suggests that the road that cuts a diagonal across the slope from left to right is the likely culprit for recent slumps above La Conchita: cutting a road into a slope over-steepens the slope and removes support from the slope mass above the road. See the enlarged "Features of a Slump" picture contained within the discussion of "Slides" on this web site for clarification of this simple but important concept.
Mt. St. Helens lahar During the major 1980 eruption of Mt. St. Helens volcano in Washington, flooding and lahars (debris flows associated with past or concurrent volcanic eruptions) caused a lot of property damage in the Toutle River drainage. Picture 1 shows a home swamped by this event. Picture 2 shows the path of a later lahar that swept across the snow-covered remnants of Mt. St. Helens in 1982.
1 Photograph by D.R. Crandell, USGS.
2 Photograph by Tom CasaDevall, USGS.
Ancient debris-flow deposits, Santa Monica Mountains, California As human population increases, people are moving further from cities and into previously unpopulated regions. As this occurs, it is important for geologists to evaluate home and subdivision sites for dangers such as threat of mass wasting. Even if a threat is not obvious, a geologist can determine what has happened in the past by carefully inspecting the sediment and rocks lying beneath a proposed home/subdivision location. Debris flows often leave a poorly layered, disorganized deposit of fine sediment and large, angular rocks that can be many feet in thickness. The photographs below show such multiple, stacked debris-flow deposits, now frozen in time as rock outcrops in the Santa Monica Mountains.
Controlling Debris Flows Reducing the hazard posed by debris flows to populated areas is of critical importance. Since it is nearly impossible to keep debris flows from occurring, the next best thing to do is to stop the flows before they move into populated areas. In the Los Angeles Basin region of southern California this has been accomplished with a combination of debris dams and collection basins. The expensive debris dams have been constructed where large streams flow out from the base of the Santa Monica, San Gabriel, and San Bernardino mountains into foothill communities. The debris dams are designed to allow water to flow through, but to trap solid matter such as sediment, rocks and logs carried by debris flows. Collection basins, large excavated depressions designed to hold or "catch" a debris flow, are nearly as effective as debris dams, and less expensive to construct. Both approaches can be compromised by large-scale debris flows, which can fill the debris-entrapment areas, and then continue down slope. Fortunately, this has yet to happen in southern California. Note that the debris-flow disasters of Cable Canyon, Waterman Canyon discussed above occurred where no protection was in place due to sparse permanent human populations in each location. For communities like Forest Falls and La Conchita that are at the immediate bases of steep unstable slopes, there is little that can be done to lessen the hazard. In such situations residents should become educated about the risks, and then carefully make a decision to live in the face of danger, or not.
Debris dam straddling a stream valley, San Gabriel Mountains, California.
Collection basin, San Bernardino, California (by Doug Morton, USGS). This basin initially did its job during a 1980 storm, but so much debris moved through the drainage that it overflowed into the neighborhood.
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3) earth flow Earth flows typically develop at the low end of a large slump, where the slump block breaks apart and material continues moving down slope. This down-slope movement can be rapid and short-lived, as a debris flow (example: the La Conchita event of 2005), or the movement can be slow and variable, and prolonged over a long period of time (example: the Portuguese Bend earthflow). The speed of an earth flow can be controlled by several factors, the most important being the amount of water introduced into the earth flow - the more water, the faster it will move. Other factors that can speed the movement of an earthflow include the shaking from an earthquake or the removal of the toe of an earth flow due to erosion or human activity. Earth flows can move up to 100 feet per day, or not at all depending on local conditions. Large earth flows can be complex structures with individual blocks moving at different speeds, and with slumps, fissures, and ponded drainages common.
The Portuguese Bend earth flow, widely referred to as the Portuguese Bend landslide, is pictured below. It originates high on the slopes of Palos Verdes Peninsula, southern California, and moves with variable speed into the Pacific Ocean. Though the Portuguese Bend area had been mapped as a landslide complex before the 1950's, 100's of homes were built on and above the unstable rock and soil in the early 1950's. Each home had its own sewage treatment facility (cesspool or septic system) and home owners established lawns and gardens on their properties. These human activities introduced a lot of ground water beneath the homes, lubricating a layer of bentonite clay formed by the subsurface weathering of volcanic rock called tuff. Slippage in the Portuguese Bend area began in 1956, coincident with the construction of a road (Crenshaw Boulevard) along the top of the ancient landslide complex. During this construction, excavated sediment was dumped onto the upper slopes of the complex, initiating new down-slope movement which continues to the present. A successful suit was filed by area homeowners in 1961, winning $10 million dollars in compensation against Los Angeles County, the responsible party for the road construction. Strangely, the presiding judge ignored the actions of the homeowners, which almost certainly contributed to the severity of down-slope movement and resulting damage to property.
Though no one has been directly injured by this earth flow, many people had to abandon their homes due to structural damages caused by the incessant movements of the earth flow. Due to the natural beauty and wonderful climate of the Portuguese Bend area, many homeowners decided to stay as long as possible before abandoning their homes (picture 1, below). Note the irregular surface of the earth flow in picture 1. Now, houses that remain on this active earthflow are equipped with heavy-duty, wall-supporting jacks that can be adjusted to compensate for the sinking or rising of the ground beneath their homes, as is shown in pictures 2 and 3 below.
1 2 3
Efforts, including dewatering wells and improved surface drainage, were begun in the 1980's to remove water from the earth flow as quickly as possible during and after rainy weather have slowed the Portuguese Bend earth flow to a slow crawl - just a few feet per year. Stopping the movement is the long-term goal, but this is unlikely to occur due to the continued erosion of the toe of the earth flow by wave activity. Picture 1 shows the drain pipe where it delivers rain runoff to the ocean at the toe of the earth flow. Here, rocks of all sizes are constantly breaking off from the fractured slopes, falling and avalanching down to the beach - not a good place to lay out your beach towel! Picture 2 provides a glimpse of Palos Verdes Drive North, the main road crossing the earth flow. Due to constant horizontal and vertical movement of the earth here, the road needs frequent patching and grading to make it safe for drivers and bicyclers.
Below is a composite view of Portuguese Bend earth flow showing a drainage pipe that carries water runoff from near the top of the earth flow complex directly out to the ocean. Note the numerous slump blocks that comprise part of the earth flow in the right-hand photo, and Palos Verdes Drive North which has to be straightened every few years as the ground beneath it shifts and breaks. See picture 2 above, for a closer view of the road.
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4) creep This is the slowest type of mass wasting, requiring years of gradual movement to have a pronounced effect on a slope. Slopes creep due to the expansion and contraction of surface sediment, and the pull of gravity. The pull of gravity is a constant, but the forces causing expansion and contraction of sediment are not. The presence of water is generally required, but in a desert lacking vegetative ground cover even dry sediment will creep due to daily heating and cooling of surface sediment grains. Below are the two primary factors causing active creep of a slope:
(1) freeze-thaw cycle Freezing of water in between sediment grains causes the water to expand by 9% in volume, forcing grains upward. As the ice melts, gravity pulls the grains downward.
(2) wet-dry cycle The addition of water to dry sediment can force the grains to separate and move upward. As the slope sediment dries, gravity pulls the grains downward.
Even when one of these processes occurs on a daily basis, the down-slope movement of grains is very slow - a few inches to several feet per year. Although creep is not a life-threatening form of mass wasting, it can damage the foundation of a building, eventually leading to expensive repairs or even abandonment of the structure.
identifying a creeping slope
(1) Slopes that experience creep can usually be identified by trees that have an unusual bend near the bases of their trunks. This results from the active creep of surface sediment that occurs as the roots of a young tree begin to penetrate deeply enough underground into bedrock, anchoring the tree to that location. If creep continues, it will cause the top of the tree to tilt down slope. As the top of the tree grows upward, and creep keeps tilting the tree down slope, a pronounced bend may develop in the tree's trunk. This effect is shown in the pictures below.
(2) The tops of telephone poles and even fence posts will tilt down slope if their bases are sunk deeply enough into non-moving sediment or rock, with creep of the surface sediment pushing the pole or post over. So, wherever you see tilted telephone poles or fence posts, think "creep". The pictures below show this effect on fences constructed on slopes.
(3) Almost any human structure (building foundation or retaining wall) can suffer from the effects of a creeping slope. It's not the speed of the down-slope movement so much as the weight of the creeping sediment that does the damage, exerting tremendous force on construction materials (metal, wood, or concrete), eventually causing them to fail. Pictured below is a trail retaining wall showing the effects of creep. Note that the retaining wall is anchored by metal posts that were driven into more stable ground several feet down.
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