On September 12 of 1717 an avalanche cascaded down the Troilet, Italy glacier, gaining speed on a cushion of air, reaching a falling velocity of 320 km/hr over a 3600 m fall. Two towns were destroyed, with 7 people killed and 120 cows lost. The slosh of the avalanche ran up the far side of the valley at a speed of 125 km/hr.
Snow avalanches, rock avalanches, debris flows, mud flows, and rock falls are failures of the surface under the action of gravity. The basic physics controlling the stability or instability of landforms is relatively simple and well understood, but the hazards are not always recognized, even when geological deposits document past slope failures in the region. In many cases surface instabilities of this type are compound events, associated with earthquake or volcanic processes, which enhance their catastrophic potential. From the surface geology perspective, landslides and debris flows are important landscape modifying agents, and play as large of a role in eroding topography and depositing debris as is played by other mechanisms such as rainfall and runoff.
There are two basic classes of surface failures:
These often are Blockfalls of individual rocks falling from an eroding surface, which is usually very steep, or 'oversteepened' beyond the stable angle of repose for the surface materials. The blocks pile up in an apron of debris, called a talus slope or talus cone.
Slow Flows - [creep, solifluction, earthflows]
Fast Flows - [snow avalanches, landslides, mudflows, debris flows]
Fast flows leave jumbled deposits, often piles upon piles, or hummocky surfaces that can be recognized in the geological record by their shape and the poorly sorted (many rock sizes intermixed) nature of the deposit. The fast flows tend to be fluidized, either by mixture of rock and air or rock and water. The relative components of the mixture determine the nomenclature: a mudflow is water with lots of clay and silt material, which is a runny mud, while a debris flow is a jumble of rock fragments with some water. The fluidized nature of the flow accounts for the fast flow velocities attained, and the associated hazard posed.
All falls are driven by gravity, pulling the surface downward, but the tendency for failure depends on many factors. Basically, the driving forces must exceed the resisting forces for a slope to fail. The driving force acting on a rock or soil mass is the product of the mass of the object and the gravitational acceleration component acting parallel to the surface. If the surface is vertically oriented, this acceleration is maximum, if it is horizontal, the acceleration is zero. Effectively, the driving force is the component of the objects weight acting parallel to the surface.
The resisting force is the friction, which involves cohesion of the object (things like roots, stickiness of the soil, protruding rocks, etc.) as well as the intrinsic resistance to sliding of the material on the contact surface. Effectively, it is the weight component acting perpendicular to the be times the friction coefficient.
Some of the factors influencing the balance of driving and resisting forces are:
Landslides often have some common features that allow them to be recognized. The actual failure surface is arcuate, tending to dip steeply at the highest level, or the headscarp, and to scallop under the surface on a single or multiple slip surface that emerges under the toe of the slide, which is hummocky, with compression ridges in it. Typically the landslides are lobate, or finger shaped, spreading out laterally in the toe as they expand into flatter surface areas.
The speed of flows is important for understanding their energy and destructive potential. In some places there are regular debris flows and avalanches, so films are made. In other cases, we can assess the flow energy by looking where the debris ends up, on the far wall of a valley, or over humps in the terrain. The flow velocity required to reach that final position can be estimated. For flows in well-defined channels, the angle of slosh, or superelevation of the flow as it goes around bends can be used to estimate the velocity of the flow.
The association of landslides and snow avalanches with precipitation is rather clear, as both a source of ground water pressure, erosion, and snow accumulation. Thus, flooding from heavy rains is often accompanied by landsliding.
Earthquakes and landslides are also commonly associated. This is mainly because the ground acceleration can act to temporarily reduce the weight of the unstable surface mass, either by directly accelerating the ground upward to counteract the force of gravity, or by increasing the water pressure by pumping the groundwater system. Accelerations of the ground can also break up cohesive bonding forces such as roots, or soil cementation. In situations where there is sandy soil, the ground vibrations can cause liquefaction of the subsurface leading to a loss of strength that causes a surface failure.
An example of an earthquake induced slide is the 1964 Sherman Slide, in which a huge rock avalanche was triggered by the March 27, 1964 Alaskan earthquake. The slide spilled out onto the Sherman Glacier, with the rupture surface following dipping bedding planes in the adjacent highlands. That great earthquake also induced much smaller scale slides within the city of Anchorage, which were responsible for much of the destruction and loss of life during the huge earthquake.
The 1903 Frank rockslide, in Alberta Canada also ruptured on dipping bedding planes in the hillside, and geologists can map the bedding geometries to assess the potential for such failures in different regions. The surface geometry is also readily assessed by mapping, and pathways of possible slides and debris flows can be charted out. Still, even in areas of repeated slope failures, the lessons are not always taken to heart. The 1970 Nevados Huascaran debris avalanche in the Andes of Peru followed a course well marked by predecessor events yet it still took a deadly toll. The mudflow on Nevado del Ruiz had a similar recognized history, but no effort had been made to avoid the inevitable recurrence of clearly documented past events.
Volcanoes like Nevado del Ruiz are commonly associated with landslides, mudflows and debris flows because they are steep-sided, active structures, with earthquakes, expanding cones, glaciers and heating events. The Mount St. Helens event involved a massive landslide on the north flank of the mountain due to slope instability, and the debris injected into the Toutle River resulted in a damaging mudflow. The same has happened in the past on Mt. Rainier, and threatens Tacoma for the future because there are past mudflows that penetrate right into town. Mt. Shasta has a massive flow deposit on the north side, which involved 26 cubic kilometers of debris avalanche (26 times larger than for Mt. St. Helens!). On Mt. Fuji, debris flows are seasonal events, and tend to follow regular courses down the hillside. Engineers have designed channels to guide the flows around surrounding towns.
Landslides and floods are often coupled, because the slides may block up rivers, providing temporary dams which fail once the water in the transient lake accumulates enough pressure to wash out the dam. Large flood can then ensue. Intermittent great floods occur on the Indus river as a result of avalanche dams in the Himalayas impounding lakes that fail again and again.
Debris flows are common in the Sierra Nevadas due to the high topography, the snowpack and the steep slopes. Occasionally these flows take a human toll, as in the August 1989 Olancha flow which blocked up the LA aqueduct.
One would expect that there are also large submarine slides, and this is indeed the case, although only a few such events are directly witnessed. One of the most famous cases was the 1929 Grand Banks (offshore Newfoundland) event, which involved a magnitude 7.2 earthquake/slide. The ocean bottom motions triggered a tsunami which took 27 lives, but there was also a massive turbidity current (an underwater debris flow) which cascaded down the continental slope, extending over 1700 km long. It was detected by the fact that the region of failure was criss-crossed with trans-Atlantic phone cables, which suffered 28 ruptures. The slump of material involved 500 billion cubic meters. Later investigations of sediment deposits on the seafloor suggest that such events recur about every 30,000-100,000 years.
Another example involved the 1975 Kalapana, Hawaii slump. This was an earthquake/landslide with a magnitude of 7.1 on the flank of Kilauea volcano. A mass of about 10exp(15) or 10exp(16) kg moved, which is about ten thousand times more mass than in the Mt. St. Helens slide. A local tsunami of 14.6 m height swept up the coast. Similar events appear to have taken place in 1823 and 1868, but it has now been found that vastly greater submarine slides have occurred offshore of Oahu and Maui. These events, perhaps the largest known landslides, are an important part of the erosional process wearing down the huge volcanic edifices built up as the Pacific plate has overridden the Hawaiian hotspot.
Closer to home, landslides in the Santa Cruz Mountains have been responsible for much of the damage in the 1989 Loma Prieta earthquake, as well as the cause of many moderately damaged homes being 'red-tagged' because it was determined that they are vulnerable to future destruction as they are built on active landslides. This has been a difficult social issue, as the potential loss of people's home under the unpredictable future threat of landslide failure is a difficult issue to resolve and to make policies about.
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