Mass wasting or mass movement -II

For part I refer link  –

Types of Mass Movement

The down-slope movement of material, whether it be bedrock, regolith, or a mixture of these, is commonly referred to as a landslide. Mass movement processes can be divided into two broad categories.

1 . Slope Failures – a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.

2 . Sediment Flows – debris flows down hill mixed with water or air.

Slope Failures

1 . Slumps – types of slides wherein downward rotation of rock or regolith occurs along a concave-upward curved surface . The upper surface of each slump block remains relatively undisturbed, as do the individual blocks. Slumps leave arcuate scars or depressions on the hill slope.

Slumps can be isolated or may occur in large complexes covering thousands of square meters. They often form as a result of human activities, and thus are common along roads where slopes have been oversteepened during construction. They are also common along river banks and sea coasts, where erosion has under-cut the slopes. Heavy rains and earthquakes can also trigger slumps.

2 . Falls – Rock falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope. Debris falls are similar, except they involve a mixture of soil, regolith, vegetation, and rocks. A rock fall may be a single rock or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff. Because this process involves the free fall of material, falls commonly occur where there are steep cliffs. At the base of most cliffs is an accumulation of fallen material termed talus.

RockFall.GIF (8317 bytes)

3 . Slides – Rock slides and debris slides result when rocks or debris slide down a pre-existing surface. Piles of talus are common at the base of a rock slide or debris slide. Slides differ from slumps in that there is no rotation of the sliding rock mass along a curved surface.

Slides.GIF (11197 bytes)

Sediment Flows

Sediment flows occur when sufficient force is applied to rocks and regolith that they begin to flow down slope. A sediment flow is a mixture of rock, and/or regolith with some water or air. They can be broken into two types depending on the amount of water present.

1 . Slurry Flows- are sediment flows that contain between about 20 and 40% water. As the water content increases above about 40% slurry flows grade into streams. Slurry flows are considered water-saturated flows.

2 . Granular Flows – are sediment flows that contain between 0 and 20% water. Note that granular flows are possible with little or no water. Granular flows are not saturated with water.

Each of these classes of sediment flows can be further subdivided on the basis of the velocity at which flowage occurs.

Slurry Flows

1 . Solifluction – flowage at rates measured on the order of centimeters per year of regolith containing water. Solifluction produces distinctive lobes on hill slopes. These occur in areas where the soil remains saturated with water for long periods of time.

2 . Debris Flows – these occur at higher velocities than solifluction, with velocities between 1 meter/yr and 100 meters/hr and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow down hill forming lobes with an irregular surface consisting of ridges and furrows.

3 . Mudflows – these are a highly fluid, high velocity mixture of sediment and water that has a consistency ranging between soup-like and wet concrete. They move at velocities greater than 1 km/hr and tend to travel along valley floors. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Thus after a heavy rain streams can turn into mudflows as they pick up more and more loose sediment. Mudflows can travel for long distances over gently sloping stream beds. Because of their high velocity and long distance of travel they are potentially very dangerous. Mudflows can also result from volcanic eruptions that cause melting of snow or ice on the slopes of volcanoes, or draining of crater lakes on volcanoes. Volcanic mudflows are often referred to as lahars. Some lahars can be quite hot, if they are generated as a result of eruptions of hot tephra.

Granular Flows

1 . Creep – the very slow, usually continuous movement of regolith down slope. Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles.

2 . Earthflows – are usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day. They usually remain active for long periods of time. They generally tend to be narrow tongue-like features that begin at a scarp or small cliff.

3 . Grain Flows – usually form in relatively dry material, such as a sand dune, on a steep slope. A small disturbance sends the dry unconsolidated grains moving rapidly down slope.

4 . Debris Avalanches – These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. They move down slope and then can travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions.

Snow Avalanches are similar to debris avalanches, but involve only snow, and are much more common than debris avalanches. Snow avalanches usually cause hundreds of deaths worldwide each year.