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Transcript of Landslide Risks S - 50(8) 20-27.pdf · PDF file TORY Landslide Risks Landslides are a...

  • SCIENCE REPORTER, AUGUST 2013 20

    SHANTANU SARKAR

    THERE has recently been a spurt in natural disasters and associated losses around the globe, in spite of the signifi cant advancement in hazard prediction and scientifi c solutions. Though the impact of fl oods and earthquakes is more severe than landslides, frequent occurrence of landslides in hilly regions causes immense loss to lives and property. In fact, in the hilly regions of India, the cumulative loss of life and property caused by landslides is higher than other natural disasters.

    Some landslide disasters in the Uttarakhand Himalayas in the recent past have been particularly devastating. Landslides in river valleys have also been accompanied by fl ash fl oods from breaching of temporary landslide dams. One of the most severe instances of this phenomenon is the recent fl ash fl ood along with debris fl ow at Kedarnath on 16 June 2013, which has claimed more than a thousand casualties.

    The exact cause of the disaster is not yet ascertained. Researchers hypothesize that this disaster has been caused by excess rain in the catchment associated with landslide events in a

    higher valley, which became channelised as debris fl ow. Another hypothesized cause is the breaking of a part of glacial lake situated upstream of Kedarnath.

    Can a landslide hazard be prevented or minimised? All landslide hazards may not be completely prevented but the consequences can be minimised if such events can be predicted and effi cient disaster mitigation, management and preparedness are adopted. This is particularly important for developing countries, which experience higher natural disaster casualties due to higher population densities and lack of disaster preparedness.

    C O

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    RY LandslideLandslide Risks Risks

    Landslides are a common and continuous process in the hilly regions of the world. The Indian Himalayan belt is highly prone to all kinds of natural hazards, including landslides, earthquakes, avalanches, fl ash fl oods, and glacial lake outbursts. Can effi cient disaster management reduce the losses due to such disasters?

  • SCIENCE REPORTER, AUGUST 201321

    What Causes Landslides? Landslide occurrence round the globe through the years and the observed climate change pattern suggest that increasing rainfall intensities and frequencies, coupled with population growth in hills, has signifi cantly increased landslide associated fatalities.

    Landslides comprise a wide variety of complex processes that result in downward and outward movements, under gravity, of materials on unstable slopes. Some forms of mass movements like fl ows, falls, spread, subsidence and creep are also considered as part of landslide events.

    Landslide can be triggered by both natural and human induced changes in hilly regions. Slope instability of a region are governed by geology, geomorphology, thrusts and faults, slope characteristics, topographic relief, drainage characteristics and land use. These are called preparatory or passive factors whose adverse nature make the slopes susceptible for landslides or in other words bring the slopes in a marginally stable condition. However, the main triggering factors that actually initiate landslides are rainfall, earthquakes, and anthropogenic activities.

    Landslides also form part of a multiple hazard effect with other natural disasters, where many disasters occur simultaneously or trigger each other. Landslides can be caused by earthquakes, fl oods, and volcanic activity. Landslides can generate a tsunami if a huge amount of suffi cient landslide material slides into a body of water to displace a large volume of water. Thus, multiple hazard susceptibilities should be considered in assessing risk or creating early warning systems for landslides.

    The Himalayas, one of the youngest mountains, present a dynamic geo- environment with varied rock types, seismically active tectonic zones, rugged topography, steep slopes and intense monsoon rainfall. The weak and fragile rocks along with thick overburden deposits on steep slopes are further subjected to severe weathering and toe erosion by a number of streams. In recent years, slope instability problems have been aggravated due to increased urbanization and haphazard road and hydropower construction activities.

    COVERCOVER STORY

    Uttarkashi landslide

    disaster at Varunavat

    Parvat (a) Pre disaster scenario

    and (b) Post disaster scenario

    (Source: Sarkar, S., Kanungo, D.P and Chauhan, P.K.S., 2011. Varunavat landslide

    disaster in Uttarkashi, Garhwal Himalaya, India (Quarterly

    Journal of Engineering Geology and Hydrology. Vol.

    44; pp. 17-22)

    (a)

    (b)The local community should be aware and prepared for the potential risks present in the area where they live. Local groups should also be identifi ed and trained to discern early warning signs and form another channel of the early warning system by informing the appropriate District Magistrate offi ce.

  • SCIENCE REPORTER, AUGUST 2013 22 (Source: Adhikari, Y. and Yoshitani, J., 2009. Global trend in water related disasters. The United Nations World Water Development Report 3, International Centre for Water Hazard and Risk Assessment, UNESCO, p.24.)

    (a) (b)

    Debris fl ow at (a) Phata and (b) Jakhla in Pithoragarh district [Source: Sarkar, S, and Kanungo, D.P., 2010. Landslide disaster on Berinag-Munsiyari Road, Pithoragarh District, Uttarakhand - an evaluation, Current Science, 98 (7), pp. 900-902]

    Although in India there are many research and academic institutions engaged in different aspects of landslide research, the research outcomes have not been adequately implemented in the fi eld. There is a need to transform research outcomes into engineering practices. Further, it is pertinent to strengthen the landslide database and hazard zonation maps for pragmatic landslide risk estimation.

    Landslide Hazard and Risk Assessment Landslide risk is a measure of the magnitude of probable hazards and expected damage, i.e., the vulnerability of exposed risk elements in the form of population, infrastructures, economic

    and social activities. Therefore, risk is negligible where there are no risk elements or they are not vulnerable even if potential hazard is high. On the contrary, for a large number of exposed risk elements with high vulnerability, the risk is high even if potential hazard is low.

    Assessment of landslide hazard and risk is an imperative task in the area of disaster management. A simple way to estimate the hazard is by creating a database of existing landslides. The most scientifi c way to assess the hazard is by landslide susceptibility mapping. Such an effort can produce landslide hazard zonation maps, which will classify the area into various hazard classes of landslide potential zones. Such

    maps of different regions of India have been prepared by various research and academic institutions.

    There are different techniques available, primarily depending on the nature of data and mapping scale. Some techniques most widely being used are qualitative map combination, information model, bi–variate and multivariate statistical methods, and soft computing

    COVER STORY

    N u m

    b e r

    o f d is

    a s te

    rs

    F a ta

    lit y

    in e a c h

    c o n ti n e n t

    G lo

    b a l to

    ta l fa

    ta lit

    y (T

    h o u s a n d s )

    Asia

    Asia

    0 0

    10 10

    20

    100

    30

    1,000

    40

    10,000

    50

    19 80

    -1 98

    2

    19 80

    -1 98

    2

    19 83

    -1 98

    5

    19 83

    -1 98

    5

    19 86

    -1 98

    8

    19 86

    -1 98

    8

    19 89

    -1 99

    1

    19 89

    -1 99

    1

    19 92

    -1 99

    4

    19 92

    -1 99

    4

    19 95

    -1 99

    7

    19 95

    -1 99

    7

    19 98

    -2 00

    0

    19 98

    -2 00

    0

    20 01

    -2 00

    3

    20 01

    -2 00

    3

    20 04

    -2 00

    6

    20 04

    -2 00

    6

    Africa

    Africa

    Americas

    Americas

    Europe EuropeOcearia Ocearia Total

    Trends in landslide disasters and fatalities by region from 1980 to 2006

    Often, the lack of sturdy foundations or the inherent instability of the landslide-prone slope is not given due attention by builders.

  • SCIENCE REPORTER, AUGUST 201323

    A panoramic view of Kedarnath (a) before and (b) after the disaster (Source: Internet)

    techniques. These maps are useful for safe land-use planning of hilly regions.

    However, the maps available in our country are of small scale (1:50,000- 1:25,000) and therefore detailed information about the hazards are mostly missing. Remote sensing integrated with Geographic Information System (GIS) is now extensively used for such studies. High resolution remote sensing images also help in providing detailed information about the terrain to produce large-scale hazard maps.

    Landslide hazard assessment in a highly susceptible region can also be achieved by delineating the active/

    COVER STORY

    A framework for landslide risk assessment and management

    (Source: Dai, F.C.., Lee, C.F. and Ngai, Y.Y. 2