Literature On The Impacts Of Landslides Environmental Sciences Essay

Objective of this chapter is to present all the literature available related to landslides and it’s impact to socio environment. It would include what are natural disasters, priority of occurrence of natural disasters, some thing about Sri Lanka and natural disasters in Sri Lanka; something about landslides, landslides is the one of main natural disasters for Hill country, reasons for landslides in Sri Lanka specially in Ratnapura district, socio impact of natural landslides in Sri Lanka, mitigation of landslides in Sri Lanka.

A natural hazard is a threat of an event that will have a negative effect on people or the environment. In recent years, growing population and expansion of settlements and life-lines over hazardous areas have largely increased the impact of natural disasters both in industrialized and developing countries. (Guzzetti.F, et al, Landslide hazard evaluation: a review of current techniques and their application in a multi-scale study, Central Italy). Specially Asia and the Pacific is the world’s most disaster-prone region, accounting for 91 per cent of deaths from natural disasters in the past century and 49 per cent of the resulting economic damage (ASIA – PACIFIC ENVIRONMENT OUTLOOK). It can be illustrated as follow. (Figure 2.2.1)

Figure 2.2.1: Impacts of natural disasters in the period 1900-2005

Source: EM – DAT, the OFDA/CRED International Disaster Database, Univercity Catholique de Louvain, Brussels, Belgium.

Each year for the past 15 years, an average of 41,000 people have died in the region from natural disasters, which annually inflicted $29 billion worth of damage. Of the world’s 10 most severe natural disasters in 2004, five occurred in the Asian and Pacific region, causing damage amounting to $55 billion, about 70 per cent of the total damage, estimated at $80 billion. Statistical Yearbook for Asia and the Pacific in 2007 presented major natural disaster events in the Asia and Pacific region from1988 to 2007.

(Figure 2.2.2)

Figure 2.2.2 – Major natural disaster events in the Asian and Pacific region, 1988- 2007

Source: ASIA – PACIFIC ENVIRONMENT OUTLOOK

At the meeting of Management of Landslide hazard, Japan Landslide Society (JLS) presented two reasons for that situation occurred in the Asia-Pacific Region. This include countries difficulty to face the high costs of controlling natural hazards through major engineering works, rational land-use planning and different part of landscape had been subjected to wide range of disaster from time to time through out the history. The droughts, landslides, cyclone, wind, storms, soil erosion, Tsunami and coastal erosion are the common natural disasters while terrorism, violence, civil conflicts, explosions, industrial accidents, epidemics, gem mining, sand mining, fires & deforestation are the man made disaster that affect the community of the Asia-Pacific.

Since 1990 the region has suffered 50 per cent of the world’s major natural disasters when International Decade for Natural Disaster Reduction began. The total number of deaths due to natural disasters in the region has exceeded 200,000 and the estimated damage to property over this period has been estimated at US$ 100 billion (ASIA – PACIFIC ENVIRONMENT OUTLOOK).

2.2.1 Flood

A flood is an overflow of an expanse of water that submerges land, a deluge. Floods are the most common climate-related disaster in the region and include seasonal floods, flash floods, urban floods due to inadequate drainage facilities and floods associated with tidal events induced by typhoons in coastal areas. In Bangladesh, one of the most flood-prone countries in the region, as many as 80 million people are vulnerable to flooding each year (ESCAP, 1995a). In India, where a total of 40 million hectares is at risk from flooding each year, the average annual direct damage has been estimated at US$ 240 million, although this figure can increase to over US$ 1.5 billion with severe flood events. (ASIA – PACIFIC ENVIRONMENT OUTLOOK).

Flooding is the most common natural hazard in Sri Lanka also. Heavy monsoon rain has affected the South and South West of Sri Lanka, leading to the most serious flooding and landslides in over 50 years. Six districts are affected by the floods. This includes Ratnapura, Galle, Kalutara, Matara, Nuwara Eliya, and Hambantota disricts. Recent records indicate that Sri Lanka experiences large scale flooding every 2 to 3 years on average about 200,000 people are affected every year. The Government of Sri Lanka Disaster Operations Center has provided the following data on the affected areas:

(Table 2.2.1.1)

Table 01: flooding areas

District

Affected Families

Deaths

Houses destroyed

Houses partially damaged

Hambantota

1,812

429

932

Kulatara

21,550

7,658

Galle

36,703

1,273

560

Matara

68,075

8,850

17,822

Ratnapura

47,756

137

5,726

6,902

Nuwara Eliya

132

286

Source: The Government of Sri Lanka Disaster Operations Center

There are 103 river basins in the island of which about 20 can be considered major basins of the five main rivers, the Kelani, Kalu, Gin, Nilwala and Mahaweli, which experience annual floods.

Floods in Sri Lanka occur mainly due to excessive rainfall during the monsoons, and they normally occur during the Southwest monsoon and during the inter-monsoonal rains, just before and after the Northeast monsoon. Therefore, rivers located on the Western slope of the country become prone to flooding. . (Malalgoda M, et al, ITDG – South Asia, 2002)

2.2.2 Earthquake

An earthquake is a sudden and sometimes catastrophic movement of a part of the earth’s surface. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the earth’s upper crust. (Cecil H, et al, Green Institute of Geophysics and Planetary Physics at Scripps Institution of Oceanography). Earthquakes are recorded with a seismometer, also known as a seismograph. At the Earth’s surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.(Spence, William; S. A. Sipkin, G. L. Choy (1989). “Measuring the Size of an Earthquake”, United States Geological Survey, 2006).

Earthquakes are short-lived, menacing and the most feared natural hazards because of their sudden impact and devastation in a matter of few seconds inflicting immense losses of life and property. Therefore since we are not in a position to predict an earthquake, we must at least try to find out the scientific causes that can lead to such catastrophic earthquakes. If not disaster due to earthquakes affects all segments of the community including individuals causing significant economic & social impacts, severe damage to life & human suffering, loss of property & material wealth, environmental devastations, devastation of physical facilities (Banerjee R, et al Geomatics in Earthquake Mitigation, ESHD/MWRG-RESA, Space Applications Centre (ISRO), Ahmedabad).

2.2.3 Cyclones

Cyclones are huge revolving storms caused by winds blowing around a central area of low atmospheric pressure. In the northern hemisphere, cyclones are called hurricanes or typhoons and their winds blow in an anti-clockwise circle. In the southern hemisphere, these tropical storms are known as cyclones, whose winds blow in a clockwise circle. Cyclones develop over warm seas near the Equator. Air heated by the sun rises very swiftly, which creates areas of very low pressure. As the warm air rises, it becomes loaded with moisture which condenses into massive thunderclouds. Cool air rushes in to fill the void that is left, but because of the constant turning of the Earth on its axis, the air is bent inwards and then spirals upwards with great force. The swirling winds rotate faster and faster, forming a huge circle which can be up to 2,000 km across. (Natural Disasters by Cindy, Lisa and Stacey).

The Eastern Coast of Sri Lanka is prone to disastrous cyclones. The cyclone season is from November to December. About 83% of occurrences in the past have been during this period. In the instances where cyclones have hit Sri Lanka the effects have been devastating. The cyclone of 1964 hit the districts of Polonnaruwa, Anuradhapura, Mannar, Batticaloa and Amparai. This cyclone resulted in 75,000 people being affected and 280 deaths. The cyclone of 1978 affected nearly 1 million people and an estimated 100,000 people were made homeless. . (Malalgoda M, et al, ITDG – South Asia, 2002)

2.2.4 Droughts

Droughts are caused by lack of rain over a long period of time. Most droughts tend to occur during summer, as the weather is hot and water is quickly evaporated. It has been observed that the impact of droughts differs widely between developed and developing countries because of the influence of such factors as water supply and water-use efficiency. The majority of the estimated 500 million rural poor in the Asia-Pacific region are subsistence farmers occupying mainly rain-fed land (ESCAP, 1995a). The drought-prone countries in this region are Afghanistan, Iran, Myanmar, Pakistan, Nepal, India, Sri Lanka and parts of Bangladesh. The Philippines, Thailand, Australia and the Pacific islands of Fiji, Vanuatu and Samoa also contain drought-prone areas. (ASIA – PACIFIC ENVIRONMENT OUTLOOK)

Dry spells and droughts are a recurrent feature of the natural climate of Sri Lanka.

In the extreme northwest Mannar and Kalpitiya areas and in the extreme southwest Yala area the mean annual rainfall drops below 1000mm. Hambantota, Moneragala and Puttlam are the administrative districts where drought is a regular phenomenon.

2.3 Natural disasters in Sri Lanka

2.3.1 About Sri Lanka

Sri Lanka is an island in the Indian Ocean, located in Southern Asia, southeast of India, in a strategic location near major Indian Ocean sea lanes. It has a total area of 65,610 km², with 64,740 km² of land and 870 km² of water. Its coastline is 1,340 km long. Sri Lanka’s climate includes tropical monsoons: the northeast monsoon (December to March), and the southwest monsoon (June to October). Its terrain is mostly low, flat to rolling plain, with mountains in the south-central interior. The highest point is Pidurutalagala at 2,524.13 m. Natural resources include limestone, graphite, mineral sands, gems, phosphates, clay, hydropower.

Extensive faulting and erosion over time have produced a wide range of topographic features, making Sri Lanka one of the most scenic places in the world. Three zones are distinguishable by elevation: the Central Highlands, the plains, and the coastal belt.

Sri Lanka’s climate can be described as tropical, and quite warm. Its position between 5 and 10 north latitude endows the country with a warm climate, moderated by ocean winds and considerable moisture. The mean temperature ranges from a low of 16°C in Nuwara Eliya in the Central Highlands (where frost may occur for several days in the winter) to a high of 32°C in Trincomalee on the northeast coast (where temperatures may reach 38 °C). As a whole the average yearly temperature for the country ranges from 28 to 30 °C. Day and night temperatures may vary by 4 to 7. January is the coolest month, causing people, especially those in the highlands, to wear coats and sweaters. May, the hottest period, precedes the summer monsoon rains.

The rainfall pattern is influenced by the monsoon winds of the Indian Ocean and Bay of Bengal and is marked by four seasons. The first is from mid-May to October, when winds originate in the southwest, bringing moisture from the Indian Ocean. When these winds encounter the slopes of the Central Highlands, they unload heavy rains on the mountain slopes and the southwestern sector of the island. Some of the windward slopes receive up to 2500 mm of rain per month, but the leeward slopes in the east and northeast receive little rain. The second season occurs in October and November, inter monsoonal months. During this season, periodic squalls occur and sometimes tropical cyclones bring overcast skies and rains to the southwest, northeast, and eastern parts of the island. During the third season, December to March, monsoon winds come from the northeast, bringing moisture from the Bay of Bengal. The northeastern slopes of the mountains may be inundated with up to 1250 mm of rain during these months. Another inter monsoonal period occurs from March until mid-May, with light, variable winds and evening thundershowers.(Library of Congress Country Studies, which are United States government publications in the public domain, Geography of Sri Lanka).

2.3.2 Natural disasters in Sri Lanka

As a results of geological factors, weather condition, human activities recent records indicate that Sri Lanka is a disaster prone country since historical time. Different part of landscape had been subjected to wide range of disaster from time to time through out the history. But not only natural disasters had been caused to introduce Sri Lanka as a disaster prone country, mane made disasters also subject to this situation. While the droughts, Landslides, Cyclone, Wind, Storms, Soil erosion, Tsunami and costal erosion are the common natural disasters; terrorism, violence, civil conflicts, explosions, industrial accidents, epidemics, gem mining, sand mining, fires & deforestation are the man made disasters that affect the community of the country.(Jayaweera S, National Building Research Organization).

Those natural disasters had been affected all segments of Sri Lankan community including individuals causing significant economic and social impacts, sever damage to life and human suffering, loss of property and material wealth, environmental devastation of physical facilities such as water supply, power generation and distribution, telecommunication, arterial roads linking the affected areas to other part of the country, township, health and education facilities. (Natural Disaster Risk Management Programme. Community Based Disaster Reduction and Recovery. Through Participation of Communities and Local Self Governments).

According to the relief distribution to the victims during past two decades, the most devastating natural hazard has been landslides and floods in terms of deaths and cost. Disaster due to landslides affects all segments of the community including individuals causing significant economic & social impacts. During the past few years landslide incidents seem to be increasing in number and severity casing vulnerability of communities. ( Jayaweera S, National Building Research Organizatio)

2.4 Landslides

2.4.1 Something about Landslide

A landslide is the down slope movement of sediment and rock. The word ‘landslide’ also refers to the landforms that result from this action. Landslides are reflecting the many diverse landscapes (Evans, S.G, Landslides; in A Synthesis of Geological Hazards in Canada, G.R. Brooks; Geological Survey of Canada, Bulletin, 2001)

Landslides and mass movements are common phenomena in the world, from the recent past. Especially landslides spread from the Central hills to neighboring districts predominantly. This can be occurred in different sizes and it can be range form a single boulder that fell off a cliff to a large area encompassing tens of square kilometers and millions of cubic meters of debris. And the speed of movement can range from extremely slow to extremely rapid. The slowest movement, ‘creep’, is on the order of a few centimeters to a few tens of centimeters per year. The fastest type of landslide, the rock avalanche, may reach velocities of up to 100 meters per second (360 kilometers per hour), which exceeds the speed of a race car. (Herath H.M.D.R, Department of Sociology University of Peradeniya)

Landslides move down slope under the influence of gravity; although, if the geological material is particularly weak or sensitive or is saturated with water, gravity is less important. How the slope will fail and how the material will move is dependent on the specific geology and topography of the region. Some landslides move only a short distance, coming to a stop near the base of the slope; others can travel several kilometers from the source. Some landslides will trigger sequential failures retrogress the head scarp back into the slope. (Evans, S.G. (2001),G.R. Brooks; Geological Survey of Canada, Bulletin).

Centers for Disease Control and Prevention (CDC) Atlanta, presented a specific areas which are more likely to experienced landslides. They are:

Areas where wildfires or human modification of the land have destroyed vegetation;

Areas where landslides have occurred before;

Steep slopes and areas at the bottom of slopes or canyons;

Slopes that have been altered for construction of buildings and roads;

Channels along a stream or river;

Areas where surface runoff is directed.

2.4.2 Types of landslides

Figure 2.4.2.1: Rock Slide

Diagram by J. Appleby, R. Kilbourne, and T. Spittler after Varnes, 1978

A landslide involving bedrock in which the rock that moves remains largely intact for at least a portion of the movement. Rock slides can range in size from small and thin to very large and thick, and are subject to a wide range of triggering mechanisms. Rock slides commonly occur on relatively steep slopes in competent rocks. Slope gradients are commonly from 35% to as steep as 70%.

Figure 2.4.2.2: EARTH FLOW

Diagram by J. Appleby, R. Kilbourne, and T. Spittler after Varnes, 1978

Earth flows commonly occur on moderately steep slopes. Slope gradients are commonly from 10% to as steep as 30%, although steeper slopes may be found in head scarp and toe areas. Earth flows typically are initiated by periods of prolonged rainfall and sometimes don’t initiate until well after a storm or the rainy season has passed. They are characteristically slow moving, in the millimeters or centimeters per day range, and may continue to move for a period of days to weeks after initiating.

Figure 2.4.2.3: DEBRIS SLIDE

Source:Diagram by J.Appleby, R. Kilbourne, and T.Spittler after Varnes, 1978

Debris slides typically move initially as shallow intact slabs of soil and vegetation, but break up after a short distance into falls and flows. Movement of the slide mass as a shallow slab leads to a smooth, steep, commonly curved scar. The debris is deposited at the base as a loose hummocky mass, although the deposit may be rapidly removed by erosion. Debris slides commonly occur on very steep slopes, as steep as 60% to 70%, usually in an area where the base of a slope is undercut by erosion.

Figure 2.4.2.4: DEBRIS FLOW

Diagram by J.Appleby, R. Kilbourne, and T.Spittler after Varnes, 1978

Debris flows are most often triggered by intense rainfall following a period of less intense precipitation, or by rapid snow melt. High pore water pressures cause the soil and weathered rock to rapidly lose strength and flow down slope. Debris flows can move very rapidly, at rates ranging from meters per hour to meters per second and travel relatively long distances, making them a significant threat to life and property. Debris flows commonly begin as a slide of a shallow mass of soil and weathered rock. Their most distinctive landform is the scar left by the original shallow slide. The path of the debris flow may be marked by a small drainage that has been stripped of vegetation. The debris flow may not leave any deposit if it flows directly into a larger creek and is immediately eroded away. Many debris flow deposits are ephemeral, but in some cases successive debris flows may deposit material in the same area thereby forming a debris fan, which resembles a small, steep alluvial fan.

Figure 2.4.2.5: ROCK FALL

Diagram by J.Appleby, R. Kilbourne, and T.Spittler after Varnes, 1978

Rock falls occur on steep slopes of hard, fractured rock. The scar left by a rock fall on the slope may be no more apparent than an area of rock that is less weathered than the surrounding rocks. Rock fall deposits are loose piles of rubble that may be easily removed by erosion. Because neither the scar nor the deposit are distinctive, and because the most frequently occurring rock falls are typically small, individual rock falls are usually not shown on regional-scale (1:24,000 and smaller) landslide maps. Though infrequent, moderate- to large-volume rock falls can be extremely dangerous and sometimes fatal. Large slabs of rock impacting a hard ledge after a long drop can rapidly break apart, leading to air entrainment and long run outs, induced air blasts, airborne projectiles (fly rock) and severe dust clouds.

(Varnes, D.J., 1978, Slope movement types and processes. In, Special Report, R.L. Schuster and R.J. Krizek, National Research Council, Washington)

2.5 Landslides is one of the main natural disasters in hill country

While flooding is most significant for lowland, Landslides are the serious geologic hazard common to hill country. Recent rainfall in several parts of the country caused havoc by way of landslides in the hill country and floods in low lands including Colombo City. Present landslide density in the central highlands and the wet zone of Sri Lanka is estimated to be in the order of 1-2 landslides per sq. km and approximately 20% of the total land area of the country, which supports nearly 30% of the total population, is subject to landslides.

Landslide prone areas are located in 7 districts in and adjacent to the central highlands, and 7-9 districts located in the wet zone are prone to landslides. Namely Nuwara Eliya, Badulla, Kegalle, Kalutara, Kandy, Matale and recently Matara, Galle and Hambantota are prone to landslides while Ratnapura district is taking important place.(Malalgoda M, et al, ITDG – South Asia, 2002)

Poorna S. Yahampath, technical officer of disaster preparedness illustrated that Most of areas in Ratnapura district are affected by landslide (Figure 08)

Map 2.5.1: Disaster prone district

Source:compiled by the author

Source: Poorna S. Yahampath; Technical Advisor – Disaster Preparedness (2008)

Because of above situation in Ratnapura district National Building Research Organization (NBRO) have choose Ratnapura for launch demonstration project (landslides, frequent flooding, erosion, pollution and contamination of water resources, ground subsidence etc.

2.6 Landslides in Ratnapura district

Landslides occurred in many areas such as Ratnapura, Pelmadulla, Kalawana, Nivithigala and Kahawatta in Ratnapura district. Twenty Grama Sewa Niladhari (GSN) Divisions were affected due to landslides. According to investigations carried out by NBRO there are 135 high-risk areas, 69 medium risk areas and 67 low risk areas in the whole of Ratnapura District. The hazard analysis carried out by the Ratnapura Disaster Management Steering Committee (RDMSC) in 1999 under the SLUMDMP identified the following hazards in the order of frequency of occurrence shown as percentage on the graph below. This data reveals that Ratnapura is vulnerable to multiple hazards. According to that investigation disasters due to landslide took a 2nd place.

Figure 2.6.1: multiple hazards in Ratnapura District

Source: Asian Disaster preparedness center

Table: 2.6.1 Landslides in Ratnapura district

Date

DS Division

Village

Deaths

1979

Kuruwita

Akurana

2/6/1981

Pelmadulla

Handurukanda

8/6/1981

Pelmadulla

Pathalpana kanda

2/11/1983

Ratnapura

Mihitiya

1985

Elapatha

Kohomba kanda

2/6/1985

Kuruwita

Wetumala

30/5/1989

Hettikanda

8/10/1993

Ratnapura

Helauda kanda

28/5/1994

Ehaliyagoda

Asgangula

21/6/1995

Balangoda

Kumaragama

13/10/1999

Kalawana

Ayagama

5/10/2002

Balangoda

Puwakgahawela

17/5/2003

Elapatha

Palawela

56 18/5/2003

Kalawana

Panapola

26 18/5/2003

Ratnapura

Muwagama

7 Source: Poorna S Yahampath, UK – South Asia scientist and Practitioners (2008)

The landslides in Ratnapura District, Sri Lanka in May 2003 again showed the high vulnerability of the community living in the area. On 17th May 2003, Ratnapura had extremely heavy and unusual rainfall of 347.2 mm within 24 hours. Many landslide occurrences have been observed within the Ratnapura district surrounding the municipality area. Sliding of Lawulu Kella, Panapola Kanda, Muwagama Kanda were take priority. This was recorded as the most severe event. These three landslides were the most vulnerable that have ever recorded since historically. (Asian disaster preparedness center: case study on mitigating disasters in Asia and the Pacific)

2.7 Reasons for landslides in Sri Lanka specially in Ratnapura district

Landslides commonly occur in connection with other major natural disasters such as earthquakes, volcanoes, wildfires, and floods. Landslides occur due to natural phenomena and man made activities also contributes to reactivate old landslide. Hilly area of Sri Lanka are affected or threatened by landslides when rains persist with increased intensity. Observations on rainfall data of land slide affected regions suggest that in the earlier times intensive precipitation of (200m – 300m) in 24 hours triggered landslide. However, with increased human intervention the tendency for the occurrence of a landslide was high with a precipitation of 100 mm/ hrs. (Jayaweera S, National Building Research Organizatio)

Natural phenomena can be divided in to three categories. They are geological factors, morphological factors and physical factors. Weak materials, sensitive materials, weathered materials, sheared materials, jointed or fissured materials, adversely orientated discontinuities, permeability contrasts, material contrasts are can be identified as a geological factor. slope angle, uplift, rebound, fluvial erosion, wave erosion, glacial erosion, erosion of lateral margins, subterranean erosion, slope loading, vegetation change are categorized as a Morphological causes. Physical factors are the factors that intense rainfall , rapid snow melt, prolonged precipitation, rapid drawdown, earthquake, volcanic eruption, thawing, freeze-thaw, shrink-swell, ground water changes.(Malalgoda M, et al, ITDG – South Asia, 2002)

Man made factors also directly subject to landslides. The reason for increasing man made factors is population growth in Sri Lanka. According to Annual Report of Central Bank resent years’ population and density rate in Sri Lanka is higher than 2001. So this is a clear indication of the heavy pressure, particularly on agricultural lands. Environment problems will be created with population growth distribution and density. The human population destroys wildlife habitats in many ways such as due to housing, built roads clearing forests or any kind of development tracts on land. There is a close relation between demographic pressure in hilly areas growing environmental degradation and landslides. The haphazard development and inappropriate resources and land utilization, Civil constructions on hilly terrain without proper geotechnical investigation non-provision of suitable drainage systems or blockage of drainage systems and /or natural watercourses, earthwork which alters the shape of a slope are contributory factors to landslide.

2.8 Socio impact of natural landslides in Sri Lanka

Natural disasters have a profound impact on the quality of life through their destruction of food crops and livestock, and forced dislocation of households and communities. Their toll on lives and the instant poverty they cause are among their most devastating impacts. The region accounts for a majority of the total number of deaths caused by natural disasters globally. The number of natural disasters has been increasing around the world, but the trends in lives lost and damage caused have changed in recent years due to effective use of technology, communication and transportation systems. Over the past five decades, the average number of deaths caused annually by natural disasters was (Statistical Yearbook for Asia and the Pacific 2007) about 100,000; in contrast, the average over the last 15 years has been about 41,000.

As low-income countries of South Asian countries consider as a more threaten counties by Natural Disasters. The sustained efforts for economic growth of those countries are often interrupted by recurrent natural disasters with considerable impact exacerbating poverty conditions in different parts of the region.

Since times of unrecorded history South Asian countries have been facing the onslaught of natural disasters such as floods, cyclones, earthquakes, landslides, etc. The low-income countries of South Asia i.e. Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan and Sri Lanka accommodate a large population of over a billion. The sustained efforts for economic growth of these countries are often interrupted by recurrent natural disasters with considerable impact exacerbating poverty conditions in different parts of the region. Among above that Asian Countries Sri Lanka consider as a one of main affected country by Natural disasters. Flood and landslides are most common natural disaster which has occurred in each and every year since 1981 to 2007 along with other natural disasters in Sri Lanka. Brammer, Bangladesh Country Study, 1991).

Landslides and mass movements are common phenomena in Sri Lanka, from the recent past. The consequences of landslides cause in terms of casualties and extensive damage to the roads, bridges, dams, buildings, forests, paddy fields farm plots and animals. Once these events occurred in uninhabited area is only of scientific interest and not considered as a disaster. When the such events kill and injure people, hurt communities, destroy homes, make stress, cause economic hardships, make vulnerability of people such events cause disaster. Landslides hazardous operate on varying scales. In order to decreasing severity, the following threats can be recognized.

Hazard to people 2. Hazards to goods

3. Hazards to environment 4. Hazard to development

Common impact associated with landslides is danger to life, property and infrastructure. Refugee loses their relatives and neighbors, property, houses and lands, employment opportunities, income, education and so on. The temporary accommodation areas have to share common resources and unfamiliar areas causing severe social problems such as limitations of basic needs, congestion, noise and damage to health, privacy conflict and ultimately the reduction of quality of life. Finally any type of natural disasters caused to Psychological stress of people, Social insecurity of women, Robberies and looting.

Landslide and floods are concomitant events of intensive rain. The blocking of drainage canals due to landslides reduces the capacity of the drainage, stream and rivers result in unexpected flooding.

Majority of the community of the hill slope is main income source is agricultural products. Large scale mass movements take place due to long term soil erosion resulting in entire cultivation enroot gets distracted. In addition damage to agriculture cause sever social problem.

Landslide cause sudden and large scale soil erosion, block road ways, streams and other natural drainage system

Finally this directly subject to economic destruction and poverty in Sri Lanka. It’s affect to development of Sri Lanka.

(Herath H.M.D.R, Department of Sociology University of Peradeniya).

2.9 Mitigation of landslides in Sri Lanka

Landslide mitigation works are conducted in order to stop or reduce the landslide movement so that the resulting damages can be minimized. The landslide mitigation works are broadly classified into two categories: 1) control works; and 2) restraint works.

2.9.1 Landslide Control Works

Surface Drainage Control Works: The surface drainage control works are implemented to control the movement of landslides accompanied by infiltration of rain water and spring flows. The surface drainage control works include two major works: drainage collection works and drainage channel works. The drainage collection works are designed to collect surface flow by installing corrugated half pipes or lined U-ditches along the slopes, and then connected to the drainage channel. The drainage channel works are

designed to remove the collected water out of the landslide zone as quickly as possible, and are constructed from the same materials as the drainage collection works.

Subsurface drainage Control Works: The purpose of the subsurface drainage control works is to remove the ground water within the landslide mass and to prevent the inflow of ground water into the landslide mass from outside sources. The subsurface drainage control works include shallow and deep subsurface drainage control works.

Soil Removal Works: This is one of the methods where the most reliable results can be expected, and generally applies to small to medium sized landslides. Except for special cases, the soil removal is focused on the head portion the slide.

Buttress Fill Works: The buttress fill is placed at the lower portions of the landslide in order to counterweight the landslide mass. It is most effective if the soils generated by the soil removal works are used.

River Structures: Degradation and channel bank erosion reduce earth stability and often tends to induce slide activity. In such cases, check dams, groundsill and bank protection can be constructed to prevent further erosion.

2.9.2 Landslide Restraint Works

Pile Works: The pile works consist of driving piles as keys to tie together the moving landslide and the stable ground to restrain the movement. Generally, a thick walled steel pipe is used as the pile, and is then filled with concrete

Large Diameter Cast-in-Place Pile Works: The large diameter cast-in-place works function similar to those of the pile works and are designed to tie the moving landslide and the stable ground together. However it involves much larger diameters. The construction is similar to the drainage well, and generally consists of pile of 1.5 to 6.5m in diameter and filed with reinforced concrete. Compared to the piles, the large diameter cast-in-place type are much more resistant to bending stresses

Anchor Works: The anchor works utilize the tensile force of anchor bodies embedded through the slide mass and into stable earth, and are connected to thrust blocks located on the ground surface. The thrust blocks are anchored with a tendon that counteracts the driving forces of the landslide to restrain the slide movement. The advantage is that large restraint forces can be obtained from a relatively small cross sectional tendon

Retaining Walls: Retaining walls are constructed to prevent smaller sized and secondary landslides that often occur along the toe portion of the larger landslides. Because of the large-scale earth-movement and numerous springs that are expected in landslide terrain, crib walls are common instead of conventional reinforced concrete retaining walls

(Cruden D.M., VARNES D. J. (1996) – Landslide types and processes. In: Turner A.K.; Shuster R.L. (eds) Landslides: Investigation and Mitigation).

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