Cause And Effects Of Landslides Environmental Sciences Essay

Nature has given us beautiful mountains which stood safe to centuries. In the primitive ages, when the population was low, people in Sri Lanka mostly lived in valleys and basins. No one ever disturbed the lush green mountains apart from meeting their basic requirements. As a result of the rapid increase of population, more land was required for housing and cultivation. Due to lack of land people started building their houses on the slopes and they even started construction on old landslides. They started felling trees to clear land for housing, roads etc. To meet their daily demand for fuel wood they started felling trees. Their sole motive was to fulfill their needs. Eventually these slopes were rendered barren and the process of erosion was set in motion. It is identified that these slopes are now frequented by landslides. In the past, when the landslides occurred on the higher slopes they were hardly noticed because there was no inhabitance on the higher slopes. But now a days these higher slopes are been inhabited by the people due to take over of land and the landslides have become a threat to the people.

These haphazard development activities and inadequate attention to construction aspects have lead to landsides and consequently property damage and loss of life. Most landsides can be prevented if proper investigations are performed in time. The cost of preventing landslides is less than the cost of correcting them. Most of the landslides can be handling by normal maintenance procedures. Studies carried out in several areas in Sri Lanka and suggested that small landsides could be easily avoided by paying a little more attention during construction.

In some hilly Districts, in Sri Lanka, the National Building Research Organization (NBRO) has already delineated areas according to the potential risk for development activities. When construction is planned in any of these hilly areas, it is necessary to adopt prudent measures that would enhance the stability of the landmass and any structure built on it. In same instance, prior to implementation any such measures may need detailed studies and significant input from professionals such as geotechnical engineers and engineering geologists.

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Now the National Building Research Organization (NBRO) is to issue new guidelines for development, construction and human settlement activities in all landslide prone areas. What this means is in future no settlements will be allowed in locations designated as landslide prone. A landslide hazard zonal map is being prepared in 20 districts that have been identified as landslide prone to guide local decision makers in permitting constructions in such areas.

This is a sound move considering the recent spate of landslides even in areas hitherto considered safe that caught people off guard. The shift in the ecological balance over the years due to environmental degradation has now made even once landslide free areas vulnerable today. Therefore steps are needed to put a halt to indiscriminate development ignoring environmental factors. The maps will now help sort out the danger zones so that they could be cleared of all human habitation. The public will also be sufficiently warned to avoid such areas and use alternate routes in their travel.

Based on research the NBRO has also revealed that 90 percent of the recent landslides in the country was due to human activity. This is hardly surprising given the mindless destruction of the country’s natural environment over the past few decades in the name of development and progress.

WHAT IS LAND SLIDE

The term “landslide” describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing. Figure 1 shows a graphic illustration of a landslide, with the commonly accepted terminology describing its features.

Figure 1. An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide. Source: USGS

TYPES OF LANDSLIDES

Although many types of mass movements are included in the general term “landslide,” the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material. The two major types of slides are rotational slides and translational slides.

Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide (Figure 3A).

Translational slide:  In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting (Figure 3B).

Block slide: This is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass (Figure 3C).

Figure 3. The most common types of landslides. Source: USGS

Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs (Figure 3D). Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water.

Toppling failures are distinguished by the forward rotation of a unit or units about some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks (Figure 3E).

Earthflows have a characteristic “hourglass” shape (Figure 3H). The slope material liquefies and runs out, forming a bowl or depression at the head. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. However, dry flows of granular material are also possible.

. Mudflow: A mudflow is an earth flow consisting of material that is wet enough to flow rapidly and that contains at least 50 percent sand-, silt-, and clay-sized particles. In some instances, for example in many newspaper reports, mudflows and debris flows are commonly referred to as “mudslides.”

Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. There are generally three types of creep: (1) seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; (2) continuous, where shear stress continuously exceeds the strength of the material; and (3) progressive, where slopes are reaching the point of failure as other types of mass movements. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.

Rows of bricks offset by fault creep on a pateo area on the north side of the campus of Contra Costa Community College. Source: USGS

Lateral spreads are distinctive because they usually occur on very gentle slopes or flat terrain (Figure 3J). The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. The failure is caused by liquefaction, the process whereby saturated, loose, cohesion less sediments (usually sands and silts) are transformed from a solid into a liquefied state. Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may undergo fracturing and extension and may then subside, translate, rotate, disintegrate, or liquefy and flow. Lateral spreading in fine-grained materials on shallow slopes is usually progressive. The failure starts suddenly in a small area and spreads rapidly. Often the initial failure is a slump, but in some materials movement occurs for no apparent reason. Combination of two or more of the above types is known as a complex landslide.

The Landslide Triggering Mechanisms

The soils on most hillsides are in a stable state. If they were unstable a landslide would have taken them to the bottom of that hillside a long time ago. The landslides that we see today occur when natural processes or human activities disturb a stable slope. The processes and activities that cause landslides are frequently referred to as “triggering” conditions. Although people trigger a few landslides most are triggered by natural processes. A summary of landslide triggering conditions is given below. The columns of this table separate the two landslide triggering categories (natural and human), the rows of the table are the different landslide triggering processes, and the cells within the table give examples of situations that can caused landslides.

Natural Triggers

Human Triggers

Removal of 

Support

erosion at the base of a slope by streams, waves, glaciers

excavation at the base of a slope or excavation on a hillside

Removal of Vegetation

forest fires

timbering

Addition of Moisture

rainfall or snowmelt

sewage or runoff disposal, broken water pipes, improper grading

Addition of Weight

heavy snowfall, volcanic ash, landslides

placement of fill

Oversteepening

(this term is sometimes used in the same context as “removal of support”)

placing fill at a gradient that exceeds the angle of repose

Vibrations

earthquakes, thunder

blasting, operation of heavy equipment

LANDSLIDES IN SRI LANKA

In Sri Lanka, most of the landslides, rock and cutting failures occur in the central highland of the country. The central region of Sri Lanka is hilly and mountainous with highly fractured and folded basement rock overlain by residual soil and colluvium. The elevation of the hilly region of the country ranges from 185m to 2717m above mean sea level. It is about 20% of the total land area and is occupied by 30% of the total population of the country. Landslides, slope failures and rock falls are frequent phenomena in these areas causing severe damages to life and property. Therefore, occurrence of frequent Landslides and slope failures could be considered as the most significant natural disaster in Sri Lanka. They are likely to have a greater economic impact in the urban and semi urban environment when there is a possibility of damages and losses to the investments on various development projects, infrastructure facilities and more importantly to lives. The total extent of loss of forest cover, wild life and damage to the eco-system by landslides cannot be estimated and will probably remain unknown. Blogspot.com/_hO/GaOhh0IA/TSogkq3_zvI/AAAAAAAAG

A FEW PREVIOUS INCIDENTS

The 1986 January landslides in ……… claimed 51 lives, rendering nearly 100 families homeless and affecting more or less all the seven landslide-prone districts of the hill country. A little over three years later, that was in may and June 1989, the landslide events surpassed all the earlier events in severity and magnitude, claiming more than 300 lives, rendering a large number of poor families homeless over an extensive part of the hill country. During October 1993, the landslide at Helauda in Ratnapura district resulted in the loss of 29 lives and destruction of many houses. A staggering statistics for the total loss due to landslide in 1989 alone was estimated to be in the region of Rs. 142 million. http://wHYPERLINK “http://www.adrc.asia/”ww.adrc.asia

More than 100 people lost their lives during May 2003 floods in the Ratnapura district when intense precipitation of more than 300mm fell within a period of few hours on unstable hill slopes.

“At least 15 houses have been destroyed at 2nd RajasinghaMawatha in Kandy by an earth slip while two persons were killed and some 15 have been injured. A landslide occurred in Ampitiya, Mariyawatta resulting in the death of a 9 year old girl. The girl’s father, who is reported to be critically injured, has been admitted to the Kandy General Hospital. Two houses in Bogambara, Kandy have also been gravely damaged leaving one person seriously wounded. There are also threats of landslides and earth slips in several areas in the Central Province including Dodangala and Leiwella. Approximately 40 houses have been completely or partially destroyed in Kandy and surrounding area owing to inclement weather conditions and landslides.  Derana January 9, 2011 

315 people have been evacuated to two shelters in Ella and Mahiyangana in Badulla district due to land slide threats. Among them are 30 Buddhist monks from the Mawalangala Aramaya. Landslide warnings have also been issued in Yatawatte, Rattota, and Pallepola of Matale district.  -Colombo Page 3 Feb 2011

THE AREAS PRONE TO LAND SLIDES

In the case of occurrences of Landslides in Sri Lanka, ten major districts have been identified as Landslide prone areas. Those are Badulla, Nuwara-Eliya, Rathnapura, Kegalle, Kandy, Matale, Matara, Galle, Hambanthota and Kalutara. More than 12,500 Km2 of highly prone area to Landslides are spread over these ten districts. During the last rainy season, hilly areas belonging to Matara, Galle and Hambanthota districts were severely affected by large-scale Landslides and slope failures. Major Landslides that occurred during the past two decades have taken thousands of lives and about 175,000 people became homeless.

Most of Sri Lankan landslides are caused by the percolation of rain water into the fractures of weathered rocks (e.g gneisses). The landslides are thus associated with accumulating and/or flowing water on or below the surface in mountainous terrains. Landslides are thus associated with surface or subsurface streams or water flows originating from the mountains. Newly formed landslides and reactivated ancient landslides have been observed in the Sri Lankan highlands.

(http://www.recoverlanka.net/background/hazards.html)

& (http://www.srilankanparliamentonnaturaldisasters.org/Meeting_Prof%20Kapila%20Dahanayake.htm)

NATURAL CAUSES SRI LANKA

The intensity of rainfall is the triggering factor in bringing about the occurrence of landslides and other mass movement. In Sri Lanka the highest rainfalls of more than 100-150 mm can occur in a day in the south western sector of the island. This is the wet zone. How ever the intermediate zone is also of high rainfall. For example, heavy rainfall occurred from 17 to 25 December 1957 in the north- eastern part of Central Highland and many stations had 500mm of rainfall. As a result 15 major landslides occurred within an area of 59 sq km in the Hunnasgiriya – Teldeniya area alone. Other main landsides occurred due to high rainfall are, Keenakelle (elevation 1433m) in the Buddlla District 06 January 1986 (annual rainfall 2644 mm) , Ginigathena area in Nuwara Eliya district (elevation 762m) in 03 June 1989 (annual rainfall 5086mm) and Patulapana in the Ratnapura district (elevation 34m) in 08 June 1982 (annual rainfall 3749mm)

Other key factors that lead to the occurrence of landslides are the geology of Sri Lanka, and more especially at the rock types that are commonly associated with landsides. Sri Lanka is made up predominantly of crystalline rocks, which underlie about nine tenths of it. The crystalline rocks are of Precambrian age (i.e. over 570 million years old) and they are made up of interlocking minerals. Hence these rocks are resistance to the weathering.

The Precambrian rocks are subdivided in to three major units, namely the highland complex (HC), the Wanni complex (WC) and the Vijayan Complex (VC). The HC underlies the entire hill country. Most of it falls within the wet and intermediate climatic zones, where the heaviest rainfalls are experienced and it is in HC rocks that nearly all landslides in Sri Lanka occur. The WC is on the west of the HC and the VC on the east. Both are made up of granite rock which is highly resistant to weathering. Further more these two units lay completely in the dry zone of island and rainfall intensity are never high enough to cause landsides.

Jointing is a common property of nearly all rocks of the HC and the presence of joints impart a degree of permeability to these crystalline rocks. When present joints cause solid rocks to be broken up into block of various sizes and shapes, they also enable water to penetrate to various depths and so cause deep weathering of the rocks. In a study of several landslides, it has been noted the presence of two or more joints in each case. Examples for a purview of this fact are Watawala earthslide in 3 June 1992 and the rockslide occurred on the Pattipola Ohiga railay line on 13 June 1992.

With the analyzing of these land slides, in Sri Lanka found most of landslides are due to erosion and action of ground water supplied from infiltration of precipitation rain water. Erosion creates steep slopes that make the downward and out word movement of rock and soil masses. Grand water can cause landslides by disrupting slope stability by means of weight lubrication hydrostatic pressure and piping. The flowing ground water in pores and fractures of loose rock and soil materials could produce an increase of weight, which cause a slump or earth flow. A sudden influx of around water in to a normally dry material can destroy its coherence and produce an earth flow or mudflow. Increase in hydrostatic presence in joints and faults due to an influx of water could generate a large outward and upward force which triggers off slides

MAN MADE CAUSES

Man made causes loom larger in the recent landslides in the Hill Country. Among the man made causes poor land management, depletion of forest cover, unsystematic drainage, silting water bodies on unstable hill slopes, road cuts (not well planed) and blasting operations. It is seen that those affected by landslides are the vulnerable groups living at subsistence or poverty levels. One can observe that these groups are affected by socio economic deprivations as well as landslides.

The rural sector in the Nuwara Eliya District is worst affected by land shortage or landlessness. Since the commencement of the plantation under the British during the 1820’s the indigenous populations of the up-country area had been restricted to pockets between mountains. With the expansion of the population this peasantry sub-divided their limited lands to meet the need of the growing families until landlessness and poverty became their way of life. As a result the actual man land ratio in these rural communities affected by landslides will be very much higher. As per the NBRO results, since 1970, 36 out of the 57 landslides in Badulla District and 43 out of 71 landslides in Nuwara Eliya District have occurred near human settlements and agricultural lands.

The depletion of forest cover, in the plantation sector, tree crops can play a vital role in the stabilizing slopes through the binding effect of their roof systems where the roots can penetrate the zone of rock fractures. It has been mentioned that under conditions of soil saturation up to 80% of the total shear strength of a soil is attributed to tree roots. In other situation trees can enhance slope instability due to their weight and effect of wind action. In any case retention of forest cover cannot guarantee landslides on many steep slopes. This is why one can notice landslides even in dense forest areas such as those on the knuckles Range. In some situations a grass cover with well-developed rock systems can be more efficient than a forest.

It could be argued that efficient management of excessive water from sloping lands may be more important in preventing landslides than land use treatments. The intricate system of irrigation channels that existed in the hill country during the last century undoubtedly served the dual purposes of irrigation as well as slope stabilization, thereby minimizing the occurrence of landslides. The rehabilitation of these channels may go a long way in minimizing landslide hazards.

The construction of highland reservoir systems are also possible impacts to induced landslides. The slope instabilities observed in reservoirs constructed in landslide prone areas such as Kotmale, indicated the effects of the fluctuations of the reservoir level in the surrounding areas which was not adequately foreseen by the design engineers. This led to the evacuation and resettlement of large numbers of families resident in the reservoir periphery areas. This has also raised the issue of management of land in the reservoir margins.

Similarly, in developing the road and railway network in the Hill County, early British engineers paid serious attention to the design and maintenance of culverts to evacuate excess water and minimize slope instability. These practice particularly the maintenance of culverts, and drains have been neglected in recent decades and it may be contended that this is an overriding reason for the increasing trend of landslides to day. The landslides occurred in 3 Jan. 1992 at Watawla Railway station is a typical example for the above causes. That was basically due to the lack of maintenance of the drainage. Gem mining is another man-induced cause, carelessly abandoned quarries, often results in unexpected earth slips.

FORECASTED DAMAGE INCLUDING AREAS DESIGNATED AS UNSAFE

METHODS OF MINIMIZING THE EFFECTS OF LANDSLIDES

As a first mode of minimizing the effects of landslides we have to identify and map out the vulnerable areas in the hilly regions. The evacuation of dwellers from the landslide prone areas and resettling them in safe location would minimize the number of death and destruction caused by landslides. You should introduce successful land use methods and enforce new laws and order to implement them. Controlling of clearing of forests for firewood and timber would minimize the landslides up to a considerable extent. Prohibiting the cultivation of the steeps of mountainous could be another mode of reducing the effects of landslides. Afforestation could be another step that we can take to protect soil layers. An active drainage system has to maintain in unstable slopes.

People tend to make use of the unstable slopes for gem mining. This also paved the way of for landslides. So propitiation of gem mining in unstable slopes could also minimize this problem. Due to the industrialization and easy transportation now a days excavations are made for roads on unstable slopes. This should be prevented and also the prohibition of rock blasting works on unstable slopes could minimize this problem. Soil layers could be stabilized by the cultivating of special crops such as grass, Bamboo etc. on unstable slopes, it is important to adhere to the rules given on soil conservation act especially in landslide prone areas.

The counter forming project should be encouraged. The construction of ditches along contour lines also caused minimizes the effect of landslides. The cracks on slopes should be filled prevent the infiltration of rain water or runoff water. By using rubble or boulders according to the technical guidelines and approval beams can be constructed along the contour lines.

People chop trees for timber and furniture requirements. Some times the trees are totally uprooted. As a result of that holes are created in the slopes. So the re-plantations in the created holes avoid the filling of rainwater. Mostly in Sri Lanka small scale irrigation channels and drains are neglected. By maintaining these channels and drains we can reduce the landslides. Retaining walls and rock lying should be constructed methodically and when constructing them you should take the necessary methods and instructions. Vigilance committees should be established to observe rainfall data during rainy seasons.

The dwellers who live in the landslide prone areas are mainly uneducated people. It is good, if you can make them aware the causes for landslide by providing a good knowledge about landslide. The agriculture department has to form quick growing and deep-rooted trees. So it is advisable to plant quick growing and deep-rooted trees to prevent soil erosion. Unstable rocks and crops could be anchored and this could also minimize the effects of landslides. The stabilizing of unstable slopes with the use of cement and lime could be minimized the landslides effects up to a considerable extent.

CONCLUSION

For landslides and others types of mass movements to take place, certain condition of rainfall, slope rock type and rock structure must be present. These conditions are present in the Hill Country of Sri Lanka, where there is a coincidence of land over 100m in highland rainfall of over 2000mm per annum and rocks which weather easily in to highly clayey earth material. In the foregoing we have attempted to show that geology plays a key role in these landslides and mass movements, providing the specific geological Conditions for specific types of movements. In order therefore to be able to delimit landslide prone areas and to improve our ways of predicting landslides and of giving early warming of their occurrence a proper understanding of the geology and geological structure of such areas is an essential pre requisite.

More specifically, it should be determined whether a landslide has occurred in solid rock, weathered overburden or colluviums, a clear distinction to make between the latter two conditions. Furthermore in the case of overburden the depth to solid rock, the strike and dip of the rocks and of the joints should be analyzed. Only then it will it be possible to determine the exact geological conditions of landslides in future.

The most critical issue that emerge from the discussion are related to the future utilization of land use and landslide hazard maps, the need for concentration of effects on soil water management, the importance of rehabilitating stream and reservoir reservation and the questions of scientific land use in the areas identified as landslide prone.

The acceleration of present landslide hazards and related others environmental degradation is due to inadequate investigation of geological structure during the constructional activities. During the present decade the forest felling, inappropriate agricultural practices, rapid increase in housing and other constructional activities will continue with direct and indirect influence by both state and private sectors. For the period of 1950 – 1982 loss of life damage to property caused by landslides is not so severe as compared to the period between 1983 and 1993. This trend reveals acceleration on landslide hazards in Sri Lanka in the coming years.