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FALLS (rock fall and rock avalanche)
SLIDES (rock slide and slump)
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.
To see video of the La Conchita disaster recorded by Los Angeles television station CBS channel 2, click on this link. This footage clearly shows the debris-flow nature of this event, and how rapidly the mass moved down slope. In addition there is footage of some of the damage and rescue activities undertaken almost immediately after the event.
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.
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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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