Section 1 :
Landslides: Introduction

‘Landslide’ is a general term for a variety of downslope movements of earth materials that result in the perceptible downward and outward movement of soil, rock and vegetation under the influence of gravity. The materials may move by falling, toppling, sliding, spreading, or flowing. Some landslides are rapid, occurring in seconds, whereas others may take hours, weeks, or even longer to develop.

It includes various types of slope failure including earth and debris flows, slumps, slides, and soil and rock fall. Landslides are one of the normal landscape building processes in undulating terrain and are common in Himalaya and Western Ghats regions in India. It includes any detached mass of soil, rock, or debris that moves down a slope or a stream channel. They are classified according to the type and rate of movement and the type of materials that are transported. Two types of forces are at work: (1) driving forces combine to cause a slope to move, and (2) friction forces and strength of materials act to stabilize the slope. When driving forces exceed resisting forces, landslides occur. It is one of the common natural hazards with devastating effects. They become a problem when they interfere with human activity resulting in damage to property and loss of life, evident from recent episodes in Karwar. In order to minimise the losses due to landslide it is necessary to identify and analyse the most important determining factors leading to slope failure. They occur as “on-site” hazards and “off-site” hazards, and should be distinguished to effectively plan for future hazard situations.

• On-site hazards occur on or near the development site and are typically the slower moving landslides that cause most of the property damage in urban areas. They include features called slumps, earth flows and block slides.

• Off-site hazards typically begin on steep slopes at distance from homes or developments, and are often rapidly moving.

Different Types of Landslides

Landslides are classified by causal factors and conditions, and include falls, slides and flows, which are described below. There are many attributes used as criteria for identification and classification including rate of movement, type of material and nature of movement. A combination of characteristics can also contribute to an increased risk of landslide hazards.

1. Falls: Falls move through the air and land at the base of a slope. In falls, material is detached from a steep slope or cliff and descends through the air by free fall or by bouncing or rolling down slope. Rock fall, the most common type, is a fall of detached rock from an area of intact bedrock.
2. Slide: Slides move in contact with the underlying surface. Slides include rockslides – the down slope movement of a rock mass along a plane surface; and slumps – the sliding of material along a curved (rotational slide) or flat (translational slide) surface. Slow-moving landslides can occur on relatively gentle slopes, and can cause significant property damage, but are far less likely to result in serious injuries.
3. Flows:  Flows are plastic or liquid movements in which mass (e.g., soil and rock) breaks up and flows during movement. Debris flows normally occur when a landslide moves down slope as a semi fluid mass scouring, or partially scouring soils from the slope along its path. Flows are typically rapidly moving and also tend to increase in volume as they scour out the channel.

Causes of Landslides: Causes of landslides can be broadly distinguished in the following two types and the probability of landslide occurrence depends on both the intrinsic and extrinsic variables.

• Internal factors or Intrinsic variables : Geology, slope gradient; slope aspect, elevation, soil geotechnical properties, vegetation cover, and long-term drainage patterns are intrinsic variables that contribute to landslide susceptibility. The steeping of the slope, water content of the stratum and mineralogical composition and structural features, which tend to reduce the shearing strength of the rocks, are also vital factors for causing landslides.
• External factors or Extrinsic variables : A slight vibration or jerk to the mass would greatly add up against the frictional resistance and the mass would become unstable. The heavy traffic in a hilly terrain could be a contributing factor towards causing the imbalance of the masses. The extrinsic variables may change over a very short time span, and are thus very difficult to estimate. If extrinsic variables are not taken into account, the term susceptibility could be employed to define the likelihood of occurrence of a landslide event.

Causal Factors : The causal factors that contribute to the landslides in the area are (i) preparatory and (ii) triggering. These mainly consist of:

• Fissured Materials- Loose soil, rock and fragmented materials.
• Mass discontinuities- Adversely oriented slip control, bedding lineaments, faults, etc.
• Water and soil erosion of the slope toe- Due to streams or human anthropogenic activities.
• Intense short period rainfall- Heavy rainfall for a shorter duration on a weak plane.
• Prolonged high precipitation- High rainfall, dam construction and alteration in river course.
• Loading of the slope at its crest- Construction on the top of slope, heavy earthmoving equipments, etc.
• Shrink and swell of expansive clay- Soil of clayey or clay loamy with high permeability and porosity.
• Water leakage- Seepage of water from any construction or natural resources.

Understanding of these factors in a region helps in adopting at stabilization approaches.

In order to minimise the damage due to landslides, it is required to identify the regions, which are susceptible for landslides. The proneness of the terrain to produce slope failures and susceptibility is usually expressed in a cartographic way is defined as landslide susceptibility. A landslide susceptibility map depicts areas likely to have landslides in the future by correlating some of the principal factors that contribute to landslides with the past distribution of slope failures. This requires spatial and temporal data related to the region. Geospatial technologies such as Geographic Information System (GIS) and Remote Sensing (RS) help in the analysis of spatial and temporal data. Remote sensing provides the spatial data at regular intervals, while GIS helps in the analysis.

The Landslide Hazard Zonation map can be derived through interpretation of satellite imagery and by using the factors leading to the occurrence of the potentially damaging phenomenon. With the increasing availability of high-resolution spatial data sets along with GIS it is possible to partially automate the landslide hazard mapping process that eliminates the need of longer time required in fieldwork and extensive expert input. The essential steps followed in landslide susceptibility zoning are:

• A landslide distribution mapping, differentiated according to type, activity, dimensions, etc., and based on information covering, when possible, a time span as large as possible.
• Spatial mapping of the relevant terrain parameters related to the occurrence of landslides.
• The analysis of the terrain conditions which are responsible for the occurrence of the different types of landslides.
• The assignment of weights to the causal factors, depending on their role in landslides.
• Formulation of decision rules and designation of susceptibility classes.

Landslides: Review

Landslides, around the world, take a heavy toll on life and property every year. Indeed, they are one of the most significant contributors to loss of life and aggregate national losses caused by natural disasters associated with earthquakes, severe storms and heavy rainfall in mountainous terrain (Lundgren, 1986; Swiss Re, 2000). Sediment disasters by debris flows, mud flows, and landslides occur almost every year during the rainy and typhoon period of July to October in Japan. In July 1982, a heavy rainfall of 488 mm in a day caused 4300 debris flows in Kyushu Island killing 299 people. Many debris flows caused by intensive rainfall during July 1983, in Honshu Island killed 199 people. Both created also enormous loss of property (Abe and Ziemer, 1999).

Landslides are particularly frequent in active oregenic belts like the Himalayas, which are the youngest, tallest and the most fragile mountains in the world. The North-Eastern Hill regions and the Western Ghats are also landslide prone areas. Scores of human lives are lost in these areas every year, livestock perish, houses and property and agriculture destroyed. Transportation and communication networks are also affected costing enormous loss to the states. Geologists say that seismic movements are constantly taking place in the Himalayas. Thousands of landslides of medium to large dimensions have been occurring in the Himalayas every year. On an average, nearly 200 earthquakes of smaller magnitude occur every year in the Uttarakhand region of mid-Himalayas alone. Most of them are undetected by the local communities. According to V.C. Thakur, then Director of the Wadia Institute of Himalayan Geology, the uncontrolled downhill flow of water after heavy rains, particularly along barren slopes, was an important causative factor in these landslides. Apart from geological reasons, changes in the land use pattern in the mountains have also led to the increase in the frequency and magnitude of landslides. The most obvious of these changes, according to Thakur, is the rapid destruction of forests which has left large tracts in the entire Himalayan region with denuded slopes (Kazmi, 1998).

Many factors contribute to landslides, which include geology, gravity, weather, groundwater, wave action and human actions. Landslide may accompany highway and building excavations, collapse of mine waste piles, and slope failures associated with quarries and open-pit mines. Typically a landslide occurs when several factors converge. Gravity works more effectively on steeper slopes, but more gradual slopes may also be vulnerable. Geological setting like the presence of permeable sands and gravels above impermeable layers of silt and clay, or bedrock may precipitate landslides. Water seeps downward through the upper materials and accumulates on the top of the underlying units, forming a zone of weakness. Heavy and prolonged rainfall is the most common triggering factor for landslides. Slides often occur following intense rainfall, when storm water runoff saturates soils on steep slopes or when infiltration causes a rapid rise in groundwater levels. Groundwater may rise as a result of heavy rains or a prolonged wet
spell. As water tables rise, some slopes become unstable. When earthquakes occur on areas with steep slopes, many times the soil slips causing landslides. Debris flows caused by earthquakes can also trigger mass movement of soil. Of the various human factors that cause landslides are

• Destruction of natural vegetation (including deforestation),
• Inappropriate drainage systems,
• Drastic land cover changes - faulty land use patterns and
• Cutting and deep excavations on hill slopes.

A landslide is a complex dynamic system. An individual landslide characteristically involves many complex processes operating together. Heavy rains, earthquakes and volcanic eruptions alone or in combination are the major reasons for slope failures. The high annual rainfall, steep slopes, high weathering rates and slope material with a low shear resistance or a high clay content are often considered the main preconditions for mass movements in East Africa (Knapen et al., 2006). Rainfall and earthquakes are the most common triggering factors for landslides (Cannon and Ellen, 1988; Corominas and Moya, 2008).

Humans are reported to have precipitated landslides in many areas because of soil removal, quarrying and mining, deforestation, road networks, tunneling etc. The vulnerability to landslides or earthquakes is greater in zones what are geologically termed as active faults and fractures, shear zones, lineaments, joints or folds. Debris flows could be greater along narrow streams on steep areas. If basement of the hills/mountains are cut the chances of landslides increase. Occurrence of highly fractured bedrocks increases landslide susceptibility. Tectonically and seismologically sensitive areas, if not carefully managed by humans, stand greater chance of suffering from landslides, especially from heavy and prolonged rains, vibrations from movement of heavy vehicles, excavation work etc. Wherever drainage density is high the water pressure pushes the slope material and generates pore water pressure along joints. As failure starts the rough joints widen and water pressure pushes slope materials forward causing slides. Younger geological formations with unconsolidated soils stand greater chance for slides. In any case action of water during heavy rains often acts as the main triggering cause (Deoja et al., 1991; Naithani et al., 2000; Meunier et al., 2008).

The main causal factors for slope failures can be divided into preparatory and triggering causal factors (Glade and Crozier, 2004). Preparatory causal factors, i.e. factors making slopes susceptible to movement over time without actually initiating it, often reported for the East African region include the increasing population pressure with slope disturbance and deforestation as a consequence and the reduction in material strength by weathering. Triggering causal factors on the other hand can be seen as external stimuli responsible for the actual initiation of mass movements. The triggering causal factors in the region can be earthquakes, excessive rainfall events and human disturbance such as slope excavation and terracing, inconsiderate irrigation and water leakage (Knapen et al., 2006).

Rainfall and slope instability: Slope instability due to rainfall is a common geotechnical problem in tropical and subtropical areas. Many slope stability studies have indicated that the infiltration of rainwater into a slope decreases the stability of the slope. The mechanism that leads to slope failures is that the negative pore–water pressures start to increase when water starts to infiltrate the unsaturated soil.  (Tsaparas et al., 2001; Gasmo et al., 2000). Slope failures in the tropical regions like Malaysia are commonly triggered by frequent rainfall (Lee et al., 2009). Analysis by Dahal and Hasegawa (2008) of 193 landslides in the Himalayan locations, where rainfall data was available, showed that when the daily precipitation exceeded 144 mm, the risk of landslides was high. However, rainfall alone cannot be taken in isolation as various other factors are involved in causing landslides during rainfall. They include hydraulic, physical and mechanical properties of the terrain and other geomorphological factors such as slope, vegetation cover, micro-climatic characteristics of the area, and perhaps other factors.

Vegetation clearing by fires and logging favours slope failures. Changes in the vegetative cover on steep slopes have increased debris-flow frequency (DeGraff, 1991; Guthrie, 2002). Deforestation is reported to have reduced slope stability on the densely populated Mount Elgon region of Uganda (Knapen et al., 2006).

Vulnerable places in India: Hill-side instability is a common problem in the geo-dynamically sensitive belts of Himalayas. Several major landslides have occurred in the recent past resulting in large-scale damages to life and property. Cloudburst occurs in areas where the mountains are high. As the floating clouds cannot find a passage, after some time a thick layer of clouds accumulate and one day it bursts creating landslides (Deoja et al., 1991; Naithani et al., 2002).

Largely made up of Precambrian geological formations with variable cover of Jurassic to Quaternary sedimentary rocks and Cretaceous-Eocene volcanics, the peninsular Indian shield was long held to be immune from seismicity. Nevertheless, many earthquakes have been recorded from the coastal margin of the Indian peninsula during the last 200 years. Geologists ascribe point out various reasons for such disturbances:

1. The continental fragment is being pushed northeastward by the Carlsberg and Central Indian ridges;
2. The Indo-Myanmar subduction zone is exerting vigorous slab pull towards the east;
3. Repeated cycles of sea level change during the Quaternary have also induced continuing hydro-isostatic adjustment due to variable melt water loading in the Bay of Bengal and the Arabian sea. All these forces produce space-time fluctuations of strain around many small to large faults, which occur in the upper crust of the shield (Banerjee et al., 2001).

Karwar landslides
The landslides which took place in the Karwar taluk on 2nd October 2009 may be categorized as a mudflow. Also known as debris flows mudflows are fluid mass of rock, earth and other debris saturated with water. A mudflow develops when water rapidly accumulates in the ground, such as during heavy rainfall changing the earth into a flowing river of mud or slurry.

Of the several causes that can be attributed for the landslides are:

• Very heavy rainfall : Most important triggering cause for landslides. Antecedent rainfall over a longer time period is considered, as well as the geological and geomorphic setting of the area, landsliding susceptibility can be easily regarded as a natural tangible landscape process.
• Earthquakes : In mountainous regions all over the world earthquakes toll a heavy toll of human lives through house collapse and landslides.
• Human actions impeding drainage of storm water : The high wall of INS Kadamba is reported to have blocked the surge of storm water towards the sea, because of insufficient outlets provided in the wall. The wall itself collapsed in many places and in one place the local people broke the wall to pave way for fast drainage.
• Slope cutting and deep excavations : Natural slope of hills have been cut to make the roads like NH-17, and the Baithkol fisheries port road. Toe support of the most vulnerable Zariwada hill has been cut by the local residents for housing. There has been also digging of the hill for soil.
• Deforestation is widespread in the coastal areas. Tree cutting amounts to removal of a protective mantle. Moreover, after interval of some years when the root network disintegrates the compaction of soils and rocks by root mass weakens causing slope failure. The hollows created by stump and root disintegration become a major cause for water seepage into the soil and trigger off landslides during spells of heavy rainfall.
• The entire city of Karwar is situated on beach ridges formed due to the influence of Kali River and Arabian Sea confluence during the past. It seems during the formation of these beach ridges at Majali and Karwar the accumulation of sediment as ridges, the above processes played major role compared to the Holocene sea level fluctuation (regression).

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