15 Disaster Management for Landslides and Avalanches

Prof. Masood Ahsan Siddiqui

Objectives

To understand the phenomenon of landslides and avalanches
To explore the causes and impacts of landslides and avalanches
To describe the measures for their prevention and management

Rationale:

The mountainous stretches and hills of North and North Eastern India as well as the Ghat region in Western and Southern India are highly prone to Landslides. The threat of Avalanches is eminent in the snow-capped Himalayan Region. This module overviews the phenomenon of landslides and avalanches and provides a basic knowledge on how to manage them.

Introduction:

In many mountainous environments, the most common hazard is that of mass movement. This is the displacement of surface materials down-slope under the force of gravity occurring in almost any environment in which slopes are present. Both landslides and avalanches are types of mass movements. Landslides consist mostly of rock and/or soil and snow avalanches are formed predominantly from snow and/or ice. These movements vary greatly in size (ranging from a few cubic metres to over 100 cubic kilometres) and in speed (ranging from millimetres per year to hundreds of metres per second). They are responsible for large amounts of damage, with rapid mass movements generally causing the greatest loss of life but slower movements cause most of the long term costs.

Most mass movements are triggered by the natural processes, such as an earthquake; intense and/or prolonged rainfall; or rapid snowmelt. However, some of the most damaging landslides occur in materials formed by humans such as mining waste, fill or garbage .People often play key role in the creation and triggering of mass movements.

Landslides:

The term landslide describes down-slope and outward movement of soils and/or rocks under the influence of gravity. Whilst many landslides do occur through the process of rock or soil sliding on a distinct surface, this is not necessarily the case, and thus the term can be something of a misnomer. In fact, there is a wide variety of the types of movement that includes 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 (Smith, 2013).

Figure 1.Landslide progression.

The above figure shows a landslide progression. The loosened mass located in the area of initial failure moves down the slope and deposits in the fan at the foot hill region .

Landslides mostly occur in five major types of terrain (Jones 1995):

Upland areas subject to seismic activities .

Mountainous environments with high relative-relief.

Areas of moderate relief suffering severe land degradation. Areas with high rainfall.

Areas covered with thick deposits of fine grained materials.

Causes:

There are many ways in which slopes may fail, depending on the angle of slope, the water content, the type of earth material involved, local environmental factors such as ground temperature etc, yet , landslides are commonly regarded as the most predictable geological hazards. Few parameters are particularly important for monitoring all kinds of mass movements-

Ground cracks are the surface manifestation of a variety of mass movements. In plan, they are commonly concentric or parallel, and have widths of a few centimetres and lengths of several meters, which distinguish them from the much shorter desiccation cracks. The formation of cracks and any increase in their rate of widening is a common measure of impending slope failure.

The appearance and increases in ground subsidence or upheaval is also a good indicator of impending failure.

Special conditions and processes exist in the permafrost terrains. Landslides and mudflows of permafrost regions are mobilized and shaped by the freezing and thawing of pore water in the active layer, the base of which acts as a shear discontinuity. Failure here can occur on slopes as low as 1° C. Gelifluction (a form of solifluction, the slow down slope movement of waterlogged soil and surficial debris) is the regular downslope flow or creep of seasonally frozen and thawed soils. Gentle to medium slopes with blankets of loose rock fragments overlying frozen ground may be subjected to mass movements such as rock glaciers and rock streams. In this case catastrophic slope failure can expose new frozen ground, setting off renewed mass wasting.

Climate change may accelerate or slow down the natural rate of slope failure, through changes in precipitation or in the vegetation cover that binds loose slope materials. Wildfires can also promote mass movements by destroying tree covers. However, it is difficult to generalize where information is lacking on the present distribution and significance of landslides, and because many parameters, in addition to climate change, contribute to slope stability.

The area of slope failure is a measure of the extent of landsliding in any region. Changes over time may both reflect significant environmental stresses (e.g. deforestation, weather extremes) and provide important clues about landscape and ecosystem degradation.

Mass movements (landslides, mass wasting) may take place suddenly and catastrophically, resulting in debris and snow avalanches, lahars, rock falls, slides and flows (debris, quick clay, loess, and dry or wet sand and silt). For example, the initial velocity of mudflows can reach 30m/second in a few seconds, slowing to several m/days.

Slower movements result in slides (debris, rock blocks), topples, slumps (rock, earth), complex landslides and creep.

Major landslides in recent years:

1998 (Bangladesh, China): Multiple high impact landslides due to heavy summer rains. Around 1,200 people were killed in Bangladesh and around 3,600 in China.

October 30, 1998 (Nicaragua): In a major landslip in Casitas northwest of the Nicaraguan capital Managua, 2,200 people died in the wake of Cyclone Mitch.

March 1998 (Pakistan): 1,500 people were killed due tolandslides induced by flooding in southwestern Pakistan.

February 17, 2006 (Philippines): In the massive landslide in the Philippine village of Guinsaugon, more than thousand dies, of which133 bodies were recovered and 973 people left missing, presumed dead

November 10, 2001 (Algeria): In the major landslide in Bab El-Oued quarter of Algiers, more than 700 people dies and around 100 were missing.

July 20, 2000 (China): Mudslides and landslides due to heavy seasonal rain in the north of China killed 625 people.

June 5, 1996 (China): In two landslides in a gold mine in the Chinese province of Yunnan, more than 200 people were dead or missing

December, 2004 (Philippines): Following a series of tropical storms, over 200 people were killed after floods induced mud slides in the eastern Philippines

Avalanches:

As with slope failures in rock and soil, a snow avalanche occurs when the shear stress exceeds the shear strength of the material (in this case 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.

Thus, a layer deposited with an original density of 100 kg m3 may densify to 400 kg m3during the course of a winter, largely due to the weight of over-lying snow, pressure melting and the recrystallizationof the ice. This densification increases the strength of the snow. On the other hand, the shear strength decreases as the temperature warms towards 0° C. As the temperature rises further, such that liquid melt-water is present in the pack, the risk of movement of the snow blanket increases.

There are two basic types of avalanches – loose snow avalanches and slab avalanches. These are further sub-divided according to whether the snow involved is dry, damp or wet, whether the snow slide originates in a surface layer or involves the whole snow cover (slides to the ground), and whether the motion is on the ground, in the air, or mixed.

Three distinct sections of an avalanche track can usually be identified. These are the starting zone where the snow initially breaks away, the track or path followed and the run out zone where the snow decelerates and stops.

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. In heavily forested mountains, avalanche paths can be identified by the age and species of trees and by sharp ‘trimlines’ separating the mature, undisturbed forest from the cleared slope. Once the hazard is recognised, a wide range of potential adjustments is available, some of which are shared with landslide hazard mitigation.

Causes:

The gradual loading of snows take place on slopes ,which gives the snow pack an opportunity to adjust by deformation because of its plastic nature, without any damaging failure. The most important triggers of pack failure tend to be heavy snowfall, rain, thaw or some artificial increase in dynamic loading, such as skiers traversing the surface. However, the commonly held perception that avalanches can be triggered by sound waves, such as the noise generated by an overflying aircraft, is a myth. For failure to occur in a hazardous snowpack, the slope must also be sufficiently steep to allow the snow to slide. Avalanche frequency is thus related to slope angle, with most events occurring on intermediate slope gradients between 30–45°. Angles below 20° are generally too low for sliding to occur and most slopes above 60° rarely accumulate sufficient snow to pose a major hazard. Most avalanches start at fracture points in the snow blanket where there is high tensile stress, such as a break of ground slope, at an overhanging cornice or where the snow fails to bond to another surface, such as a rock outcrop.

Avalanches are released (spontaneously or artificially) by an increase in stress (e.g., by fresh snow) and/or a decrease in strength (e.g., by warming or rain). Though internal metamorphism or stress development may sometimes initiate a snow rupture, avalanches are often dislodged by external triggers. Ice fall, falling cornices, earthquakes, rock falls, thermal changes, blizzards, and even other avalanches are common natural triggers. Avalanches can also be triggered by loud sounds such as shouts, machine noise, and sonic booms. In the absence of external triggers, unstable snow may revert to stability with the passage of time as long as no avalanche occurs. The rheology of snow cover is similar to that of ice as both are visco-elastic materials that exhibit creep behaviour over time. Snow deforms continually without fracturing as the load on top of it increases. However, the loading rate is critical. Heavy snow fall over a short duration leads to a greater probability of avalanche occurrence. A snow fall of 1m in one day is far more hazardous than 1m over three days.

Landslides and Avalanches in the Indian Context

India has the highest mountain chain on earth, the Himalayas, which are formed due to collision of Indian and Eurasian plate, the northward movement of the Indian plate towards China causes continuous stress on the rocks rendering them friable, weak and prone to landslides and earthquakes. The Indian crust is moving slowly at about 5 cm/year which accumulates stress and many natural disasters are attributed to this. Some landslides make unique and unparalleled catastrophes. Besides the Himalayas, the North-eastern hill ranges, the Western Ghats, the Nilgiris, the Eastern Ghats and the Vindhyas, in that order, covering about 15 % of the landmass of India, is affected by landslides. (NDMA)

The Himalayas alone count for the most number of landslides per year which vary in size, speed, extent and every possible characteristics. Landslides in the north eastern region of West Bengal and the hill states of Sikkim, Mizoram, Tripura, Meghalaya, Assam, Nagaland and Arunachal Pradesh pose chronic problems, causing recurring economic losses worth billions of rupees. A different variety of landslides, characterized by a lateritic cap, pose constant threat to the Western Ghats in the South, along the steep slopes overlooking the Konkan coast besides Nilgiris, which is highly landslide prone.

Major tragedies occurred in Varnavat landslide (Uttarkashi District) Malpa landslide (Pithoragarh district) Okhimath landslide (Chamoli district) UK and Paglajhora (Darjeeling district) as well as Sikkim, Aizawl sports complex, Mizoram.These are some of the more recent examples of landslides

(source- NDMA)

Prevention and Management:

To provide protection against landslides, the design and construction of measures to prevent slope failure is a routine task within geotechnical engineering. For example, within the 1,100 sq.km area of Hong Kong, over 57,000 slopes have been engineered to prevent failure. Similarly, the railway agency in the UK, Network Rail, has to maintain over 16,000 km of earthworks designed to prevent slope failures. Methods of slope protection are well developed and include the following:

Drainage: As slope failures are generally linked to the presence of high water pressures in a slope, drainage is a key technique for improving stability. The aim is to either prevent water from entering a critical area of slope by installing gravel-filled trench drains around that area or to remove water from within a slope by installing horizontal drains. In most cases, drainage is effective but problems often arise due to lack of maintenance. Drains can easily become blocked with fine particles or even by animals using them as burrows. In addition, small amounts of movement in a slope can cause drains to become cracked or broken and so leak water into a slope at critical locations.

Regrading: In many cases, the landslide threat can be minimised by reducing the overall slope angle. This can be achieved by excavating the upper parts of the slope or by placing material at the toe, an approach often used during road construction in upland areas. In some cases, good results can be achieved by removing the natural slope soil or rock and replacing it with a lighter material. Whilst effective, such approaches are often technically challenging and expensive.

Supporting structures: Piles, buttresses and retaining walls are widely used for slopes lying adjacent to buildings and transportation routes. Although effective, this is an expensive and visually intrusive way to stabilise a slope. Increasingly there is a move towards the use of measures that sit within the soil or rock rather than on the surface. Examples include soil nails and rock bolts, both of which seek to increase stability by increasing the resistance to movement. In addition, structures can be designed to deflect landslides around vulnerable facilities. For example, diversion walls are often constructed around electricity pylons in mountain areas in order to deflect small debris flows.

Vegetation of slopes perform several functions. Plant roots help to bind soil particles together and provide resistance to movement, the vegetation canopy protects the soil surface from rain splash impact whilst transpiration processes reduce the water content of the slope. In recent years, a new breed of ‘bioengineering’ has emerged. It is critically important to ensure that the used plant species can maximise the beneficial effects and thrive in the environment in which they are planted. Thus, the preference is to use local species of trees and plants. Bioengineering is also considered to be more environmental conscious than traditional engineering approaches and to provide better visual aesthetics.

Protection against avalanches:

There are two main protective measures against avalanches viz. artificial release and defence structures.

Artificial Release:

In most cases, artificial release is accomplished through the use of small explosive charges to trigger controlled avalanches. This technique is used surprisingly often; in the USA about 10,000 avalanches are triggered through artificial release each year. The main advantages of artificial release are:

Snow release occurs at pre-determined times, when the downslope areas affected are closed.

Measures to allow snow clearance can be put in place before the avalanche occurs, minimising inconvenience.

Snow pack can be released safely in several small avalanches rather than allowing the build-up of a major threat.

Defence structures:

The use of defence structures has become the most common adjustment to avalanches throughout the world. In Switzerland alone, the total amount spent on avalanche defence structures in the period 1950–2000 was approximately 1 billion (Fuchs and McAlpin, 2005). There are four key types of avalanche defence structure:

Retention structures are designed to trap and retain snow on a slope and thus to prevent the initiation of an avalanche or to stop a small avalanche before it can develop fully.

Redistribution structures are designed to prevent snow accumulation by drifting. In particular, they are used to prevent the build-up of cornices that often break off steep slopes and initiate an avalanche.

Deflectors and retarding devices are placed in the avalanche track and run-out zone. They are usually built of earth, rock or concrete and are designed to divert flowing snow from its path.

Direct-protection structures such as avalanche sheds and galleries provide the most complete avalanche defence. They are designed to allow the flow to pass over key built facilities and avalanche sheds typically act as protective roofs over roads or railways.

NDMA guidelines for landslides and avalanches in India:

The main objectives of the NDMA guidelines are to institutionalise the landslide hazard mitigation efforts, to make society aware of the various aspects of landslide hazard in the country and to prepare the society to take suitable action to reduce both risks and costs associated with this hazard. The recommendations include:

Continuously updating the inventory of landslide incidences affecting the country.

Landslide hazard zonation mapping in macro and meso scales after consultation with the Border Roads Organization, state governments and local communities.

Pilot projects to be taken up in different regions of the country to carry out detailed studies and monitoring of select landslides to assess their stability status and estimate risk.

Setting pace setter examples for stabilization of slides and also setting up early warning systems depending on the risk evaluation and cost-benefit ratio.

Completion of site specific studies of major landslides and plan treatment measures, and encourage state governments to continue these measures.

Institutional mechanisms have to be set up for generating awareness and preparedness about landslide hazard among various stakeholders.

Training and capacity building measures to be taken up for professionals and organizations working in the field of landslide management.

New codes and guidelines to be developed on landslide studies and existing ones have to be revised.

An autonomous national centre for landslide research, studies and management has to be established.

Implementation of the above action points would increase efficacy in the management of landslides and avalanches in the country. The above measures should be duly backed by requisite operational, legal, institutional, and financial support.

Measures for rehabilitation:

Landslides and avalanches can cause huge loss of life and property. Livelihoods of many people are disrupted and many are displaced or become homeless. Thus, proper rehabilitation measures are needed to be implemented.

Measures for the rehabilitation of a community affected by landslides or snow avalanches will depend very much on the extent of the damage done by the disastrous event.

If the damage is not severe, rehabilitation in the form of short-term relief is provided to restart normal activities. Long-term measures are taken so that any future landslides or snow avalanches do not harm the community or at least not in a great extent.

Reduction in the risk of the site through technical (engineering) measures like “strengthening or modifying the slopes, removing fragile and unstable portions, securing snow accumulations by snow fences, snow nets or by cribbing, and improvement of drainage.”

Prohibition of indiscriminate quarrying and mining in mountain areas.

Afforestation in zones prone to landslides and “snow avalanches so that trees and vegetation provide a binding force to prevent slippage of debris, rock, and snow.”

Creation of a voluntary, community-based preparedness-system of watch, monitoring and alert. This will not only be useful in times of a disaster but will provide enough self-confidence (and thereby self-reliance) which is an essential objective of an effective rehabilitation programme.

Provision of assistance for economic rehabilitation by arranging work, employment, loans, and grants.

In extreme cases where severe damage has occurred to a community by a landslide or snow avalanche, the site can be marked as totally ‘unsafe and unusable’. In that case, rehabilitation in the form of relocation and reconstruction occurs. In such an event, the new site is chosen to minimize vulnerability and risks.

Summary

Landslides and avalanches are two most significant hazards in India.
The speed of onset and size of the affected area can vary significantly.
Landslides can be predicted utilising signs like formation of ground cracks, appearance of and increases in ground subsidence or upheaval etc.
Climate change and environmental degradation might enhance the process of landslides.
A snow avalanche occurs when the shear stress exceeds the shear strength of the mass of snow located on a slope.
Avalanches occur mostly in slopes between 30 – 45°.
Measures to protect against landslides include improving drainage, regrading, supporting structure development and improving vegetation cover.
Avalanche protection measures include artificial release and development of defence structures like Retention Structures, Redistribution Structures, Deflectors and Retarding devices and direct protection structures etc.
NDMA has developed guidelines for both prevention and mitigation of landslides and avalanches.
Rehabilitation and relocation of the people affected (or people who might get affected) by the landslide depend on the size and impact (expected impact) of the event.

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References

Brabb, E.E. 1984. Innovative approaches to landslide hazard and risk mapping. Proceedings of IV International Symposium on Landslides 1: 307-324.\Berger, A.R. & W.J. Iams (eds) 1996. Geoindicators: Assessing rapid environmental changes in earth systems. Rotterdam: A.A. Balkema (see papers by Nesje, Rasch et al., Romanovskii et al.).
Coppola, D. P. (2007). Introduction to International Disaster Management . Oxford : Elsevier.
Etkin, D. (2016). Disaster Theory An Interdisciplinary Approach to Concepts and Causes. Oxford: Elsevier.
How Do Landslides Occur. (2009). Retrieved July 16, 2017, from Natural Disasters: http://www.naturaldisasters.myewebsite.com/articles/mudslides_-avalanches-and-landslides.html
Iasija, M. (n.d.). Complete information on the Landslides and Snow avalanches in India. Retrieved from Preserve Your Article: http://www.preservearticles.com/201101143276/landslides-and-snow-avalanches-in-india.html
NDMA. (2009). National Disaster Management Guidelines—Management of Landslides and Snow Avalanches. New Delhi: NDMA.
NDMA. (n.d.). Landslides. Retrieved from National Disaster Management Authority:
http://www.ndma.gov.in/en/media-public-awareness/disaster/natural-disaster/landslides.html
Novosad, S. & P. Wagner (eds) 1993. Landslides. Proceedings of 7th International Conference on Landslides. Rotterdam:Balkema. (especially papers by Glawe et al. and Moriwaki).
Smith, K., & Petley, D. (n.d.). Assessing risk and reducing disaster, Environmental Hazards. 2008: Routledge