Mass Movement HazardIrwan Saputra T 1273021

Mass movement hazard is the most common hazard in many mountainous environments. It involves the displacement of surface materials down-slope under the force of gravity and occur in almost any environment in which slopes are present. These movements vary greatly in size ranging from a few cubic metres to over 100 cubic kilometers) and in speed (ranging from millimeters per year to hundreds of meters per second.Mass movement hazard is separated into two types based on material that form most of their mass namely landslides which consist mostly of rock and/or soil and snow avalanches are form predominantly from snow and/or ice. Most of mass movements are triggered by natural process, such as an earthquake, intense and/or prolonged rainfall or rapid snowmelt. However some of damaging landslides occur in material formed by human, such as mining waste, fill or garbage and people often play a key role in the causation and triggering of mass movements. Petley (2012) stated a total of 89,177 fatalities caused by landslides in period between 2002 and 2007. The annual global toll was almost 9.000 people per year. Kashmir earthquake in 2005 has contributed a dramatically increase number at 40,000 toll death as well as Wenchuan earthquake in 2008 (70,000 toll deaths). Meanwhile a progressive rise of number of winter avalanches fatalities in USA from 1950 to 2012 is due to a growing participation in winter sports activities which over half the victims were mountain climbers or back-country skiers.LandslideThe term landslide describes down-slope movements of soil and/or rock under the influence of gravity. It occurs through the process of rock or soil sliding on a distinct surface. There is a wide variety of types of movement, including falling, sliding and flowing. The type of movement depends upon the angle of the slope, the nature of the materials and the various stresses that act upon them.Landslides mostly occur in five major types of terrain (after Jones 1995):1. Upland areas subject to seismic shaking.2. Mountainous environments with high relative-relief. High levels of rock falls and landslides due to the steep terrain, deformed rock masses and the occurrence of orographic rainfall.3. Areas of moderate relief suffering severe land degradation caused by actions of humans.4. Areas with high rainfall. Intense or prolonged rainfall is trigger of slope instability. This process is especially active in humid tropical areas where rock weathering can penetrate tens of metres below the ground surface.5. Areas covered with thick deposits of fine grained materials.Forces in landslidesFigure 2 Forces in Landslide

In a slope, the forces that try to cause movement of the landslide are called shear forces. Shear forces derived from the force of gravity trying to pull the mass down the slope. There are two main forces that resist the movement of the landslide: (1) Cohesion, comes from the stickiness of particles or from interlocking. (2) Friction, arises from the resistance of particles to slide across each other. Friction and cohesion together provide the resistive forces that maintain stability in a slope. Movement of the landslide occurs when the shear forces exceed the resistive forces as shown in Figure 2.The location of Fatal Landslides in the WorldIn Europe, North America and Japan, number of fatalities tends to small (less than 5). Meanwhile in South Asia, Central America were characterized with scatter number. China also has high frequency of landslide but appears to have comparatively few small ones. Africa surprisingly has few recorded fatal landslide, but those that occur tend to be quite large. The number fatalities occur based on Figure 3 imply the different levels of mitigation of landslide around the world. It were revealed that in more developed world, large landslides tend to be very well mitigate (i.e engineering have been used to stabilize them) and a few fatal occur in small and remote location. In South and South-East Asia, mitigation is much less, and density of people in affected are result in fatality counts are much higher. It caused also be an effect of the both tectonic setting (high mountainous) and climate (i.e torrential monsoon might tend to trigger very energetic landslide).Figure 3 Map of location, the size of the impact of landslides, as measured by the number of fatalities in each event, for 2007

Classification of LandslideLandslides are generally classified according to the materials involved and the mechanism of movement (Table 1).Table 1 . Classification of LandslidesThe Major landslide types:1. Rock falls, involve the movement of the material through the air. They generally occur on steep rock-faces. The blocks that fall usually detach from the cliff-face along an existing weakness, such as a joint, bedding or exfoliation surface. The scale of rock falls varies from individual blocks through to rock avalanches.The triggering of rock falls is quite complex namely (1) Earthquakes can be an important factor because the seismic waves literally shake blocks off a cliff. (2) By the presence of water in joints and fissures which can apply a pressure to loose material. (3) In some cases, no landslide trigger is apparent.2. Flows, are movements of fluidized soil and rock fragments acting as a viscous mass. They occur when loose materials become saturated and start to behave as a fluid rather than a solid in very heavy rain. However in most cases, the flow actually starts as a different type of landslide. For example, in tropical environments, the heavy precipitation associated with the passage of tropical cyclones, when rainfall totals often exceed 600 mm day1 and intensities can reach 100 mm h-1(Thomas, 1994), can trigger large numbers of small, shallow translational landslides in the soil that mantles the hill slopes.The initial movement of the landslide allows the saturated soil mass to break up, changing the movement into a debris flow. Debris flows often accelerate rapidly down the slope, disrupting and entraining soil and regolith as they go. In this way, landslides of just a few cubic metres can turn into debris flows with a volume of tens of thousands of m3 and cause high levels of loss. Many tropical cities, such as Rio de Janeiro and Hong Kong, are at risk from both landslides and debris flows.

Causes of LandslidesThe causes of landslides, which are the factors that have served to render a slope susceptible to landsliding. The triggers of a landslide, which is the final event that initiated failure. Causes and triggers both serve to either decrease the strength of the slope materials or to increase the shear forces. Common causes of landslides are as follow: (1) Weathering; (2) An increase in slope angle and removal of lateral support at the slope toe. (3) Head loading (human-induced slope failures); (3) Changes to the water table destablise a slope; (4) Removal of vegetation causes the additional strength provided by the roots and the role of tree for controlling water on the slope disappear.Trigger of LandslidesA triggering event initiates the final failure. These key triggers are: (1) Intense and/or persistent rainfall increases pore water pressure in the slope materials to the point at which shear stress exceeds shear resistance. (2) Earthquake shaking increases the shear forces and reduce the resisting forces. In The 2005 Kashmir event in Pakistan and India, the vertical acceleration can exceed 1g. Landslide becomes weightless, reducing friction on the base to zero. Thus this triggers slope failures. (3) Human activity such as in quarries where excavation and blasting can destabilise a slope to the point of failure. Snow AvalanchesSnow avalanches occur when the shear stress exceeds the shear strength of the material, a mass of snow located on a slope (Schaerer, 1981). The strength of the snowpack is related to its density and temperature. Compared to other solids, snow layers have the ability to undergo large changes in density. The densification increases the strength of the snow, however the shear strength decreases as the temperature warms towards 0. Liquid melt-water is present in the pack, the risk of movement of the snow blanket increases. Triggers of pack failure tend to be heavy snowfall, rain, thaw or some artificial increase in dynamic loading, such as skiers traversing the surfaceAvalanche frequency is thus related to slope angle. Most events occurring on intermediate slope gradients of between 3045. Angles below 20 are generally too low for sliding to occur. Most slopes above 60 rarely accumulate sufficient snow to pose a major hazard.Because avalanches tend to recur at the same sites, the threat from future events can often be detected from the recognition of previous avalanche paths in the landscape. Clues in the terrain include breaks of slope, eroded channels on the hillsides and damaged vegetation.Disaster Preparedness Figure 3 summarizes disaster preparedness toward mass movement hazard. It contains 3 kind actions namely: (1) mitigation, (2) protection, and (3) adaptation. A few examples are described next. Figure 3 Mass Movement Disaster Preparedness Schema

There are four key types of avalanche defence structure (Figure 4): (1) Retention structures are designed to trap and retain snow on a slope and thus to prevent the initiation of an avalanche. (2) Redistribution structures are designed to prevent snow accumulation by drifting. (3) Deflectors and retarding devices are placed in the avalanche track and run-out zone. (4) Direct-protection structures such as avalanche sheds and galleries provide the most complete avalanche defence.Figure 4 Methods for Avalanche Hazard Reduction

Any avalanche reaching the highway is detected by sensors suspended from an overhead cableway. The relevant stretch of road can then be closed, and the highway authority alerted by telemetry (Figure 4).

ReferencesNature Publishing Group. (2013). Lesson 8: Landslides Hazards. Retrieved July 2, 2013, from Scitable by Nature Education: http://www.nature.com/scitable/topicpage/lesson-8-landslides-hazards-8704578Petley, D. (2012). Ten Years of Collecting Landslide Fatality Data. Retrieved June 15, 2012, from AGU Blogosphere: http://blogs.agu.org/landslideblog/2012/09/13/ten-years-of-landslide-fatality-data/Smith, K., & Petley, D. N. (2008). Environmental Hazards Assessing Risk and Reducing Disaster (Fifth ed.). London and New York: Taylor & Francis Group.