Types of Landslides

There is some landslide terminology you should know about. Figure 56 shows a few of the features you might see after one has happened. They are labeled from "head" to "toe", so to speak.

Crown cracks are seen above the landfall where the earth has been tugged by the weight of the fall before it fell. The crown is the top of the earth just above the landslide. A scarp is a cliff face; there can be a main scarp and one or more minor scarps. At the top of a minor scarp is a ledge called a head.

The upper part of the fall is the zone of depletion, where rock has been removed from a spot. The bottom part is the zone of accumulation, where rock or debris has been added. You will be able to see the surface of the rupture and its main body, where the debris is missing. More debris is added to existing debris at the foot of the landfall. The very ends of the landfall you see after it has traveled are the tips of the toes, but the toes themselves are the bottom edge of the rupture itself.

The surface of separation is below the rupture. Expect to see some transverse cracks from side to side as well as radial cracks that run up and down along the zone of accumulation. In between cracks, you can have buildup that forms ridges of debris near the base of the landfall.

TYPES OF LANDSLIDES

Most landslides happen in more remote areas where people are not significantly affected. Landslides are not all dramatic and catastrophic. You can have various types of rapid or slow mass movement of earth. The material that falls and the mode of falling both go into the classification of a landslide. These include the following different types of phenomena:

• Slides or sliding events – these are true landslides in restrictive sense. There is usually a single weak point or weak zone that causes a separation of earth from its original elevation. Rotational slides involve a concave slope left over after the slide has happened. There is a greater slope at the top of the slide than the bottom. A translational slide is simpler, with just a fall down an inclined plane. Of

these, the rotational slide is more spectacular, with large chunks of earth being displaced.

• Falls – these are sudden movements, usually of rocks and/or boulders that separate from cliffs or mountain slopes. Bedding planes, joints, or faults can be sources of these freely falling and bouncing rocks. These are deadly because of the numbers of boulders, their speed, and their weight. Water in the earth and mechanical weathering contribute to this kind of fall.

• Topples – this is a pivoting or rotating of a bunch of rock due to instability and the effects of gravity. Fluid in cracks can cause chunks of a cliff to topple off their original location.

• Flows – these are more complex and involve different types of materials and speed. These are the different flows you might encounter:

o Debris flow – these are rapidly moving mixtures of rock, organic material, water, and dirt flowing downhill. They are often due to heavy rains or snowmelt and can arise after a landslide has upset the ground. Gullies are often linked to these kinds of dangerous flows. The slope generally must be more than 20 degrees to have any type of flow like this. They will continue to creep if the slope stays at 10 degrees or more, depending on how wet the debris slurry is. Most of these begin as a translational slide, leaving some rubble or debris in their wake as they are completed. Most of these are not very large. o Debris avalanche – this is just an extreme debris flow that moves very fast. It is highly dangerous. o Earthflow – we will talk more about these in a minute. These are hourglassshaped flows with fine-grained debris or clay that forms a long flow on a moderately-steep slope. o Mudflow – this is a type of earthflow where the material is very wet. There needs to be at least 50 percent fine particles like silt or clay. o Creep – these are flows that can be imperceptible at times and involves either soil, rock, or both. There is enough shear stress to have gradual movement