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LandSlides

Landslides are sudden, fast movements of a cohesive mass of soil, rock or regolith. Landslides occur in two types- the first of which is a translational slide. These involve movement along a flat surface parallel to the angle of the slope in a stepped-liked pattern, with no rotation. The second type of landslide is called a rotational slide and is the movement of surface material along a concave surface. Both types of landslides can be moist, but they are not normally saturated with water. It is a significant part of the process of erosion because it moves material from high elevations to lower elevations. It can be triggered by natural events like earthquakes, volcanic eruptions and flooding, but gravity is its driving force. Although gravity is the driving force of landslides, it is impacted mainly by the slope material’s strength and cohesiveness as well as the amount of friction acting on the material. If friction, cohesion and strength (collectively known as the resisting forces) are high in a given area, landslides are less likely to occur because the gravitational force does not exceed the resisting force. Once the force of gravity on a mass of rock or soil reaches the shear-failure point, it can fall, slide, flow or creep down a slope. These are the four types of landslides and are determined by the speed of the material’s movement downslope as well as the amount of moisture found.
Although most landslides occur via natural phenomena like earthquakes, human activities like surface mining or the building of a highway or shopping malls can also contribute to landslides. Whether human induced or natural though, landslides have caused damage in cities as well.

The causative factors of landslide are classified into two important groups – (i) Natural and (ii) Anthropogenic. The natural factors are also sub-divided into two broad classes: (a) Inherent (b) External. All these factors are described below:
(i) Natural
(ii) Lithology
(iii) Structure
(iv) Slope
(v) Drainage Density
(vi) Hydrogeology
(vii) Rainfall

Anthropogenic factors are as follows:
(i) Improper Land Use
(ii) Deforestation
(iii) Infrastructure Development

Risks & Mitigation

MITIGATION STRATEGIES
Hazard mapping will locate areas prone to slope failures. This will permit to identify avoidance of areas for building settlements. These maps will serve as a tool for mitigation planning. Land use practices such as:

Areas covered by degraded natural vegetation in upper slopes are to be afforested with suitable species. Existing patches of natural vegetation (forest and natural grass land) in good condition, should be preserved.

  • Any developmental activity initiated in the area should be taken up only after a detailed study of the region and slope protection should be carried out if necessary.
  • In construction of roads, irrigation canals etc. proper care is to be taken to avoid blockage of natural drainage
  • Total avoidance of settlement in the risk zone should be made mandatory.
  • Relocate settlements and infrastructure that fall in the possible path of the landslide
  • No construction of buildings in areas beyond a certain degree of slope.
Retaining Walls can be built to stop land from slipping (these walls are commonly seen along roads in hill stations). It's constructed to prevent smaller sized and secondary landslides that often occur along the toe portion of the larger landslides. Surface Drainage Control Works implemented to control the movement of landslides accompanied by infiltration of rain water and spring flows. Engineered structures with strong foundations can withstand or take the ground movement forces. Underground installations (pipes, cables, etc.) should be made flexible to move in order to withstand forces caused by the landslide Increasing vegetation cover is the cheapest and most effective way of arresting landslides. This helps to bind the top layer of the soil with layers below, while preventing excessive run-off and soil erosion.

COMMUNITY BASED MITIGATION
The most damaging landslides are often related to human intervention such as construction of roads, housing and other infrastructure in vulnerable slopes and regions. Other community based activities that can mitigate landslides are education and awareness generation among the communities, establishing community based monitoring, timely warning and evacuation system. Communities can play a vital role in identifying the areas where there is land instability. Compacting ground locally, slope stabilization (procedures such as terracing and tree planting may reduce damages to some extent), and avoiding construction of houses in hazardous locations are something that the community has to agree and adhere to avoid damage from the possible landslides.


DEBRIS FLOWS OR MUDSLIES
Fast-moving flows of mud and rock, called debris flows or mudslides, are among the most numerous and dangerous types of landslides in the world. They are particularly dangerous to life and property because of their high speeds and the sheer destructive force of their flow. Hazardous Areas Debris flows start on steep slopes-slopes steep enough to make walking difficult. Once started, however,

debris flows can travel even over gently sloping ground. The most hazardous areas are canyon bottoms, stream channels, areas near the outlets of canyons, and slopes excavated for buildings and roads.

  • Canyon bottoms, stream channels, and areas near the outlets of canyons or channels are particularly hazardous. Multiple debris flows that start high in canyons commonly funnel into channels. There, they merge, gain volume, and travel long distances from their sources.
  • Debris flows commonly begin in swales (depressions) on steep slopes, making areas downslope from swales particularly hazardous.
  • Roadcuts and other altered or excavated areas of slopes are particularly susceptible to debris flows. Debris flows and other landslides onto roadways are common during rainstorms, and often occur during milder rainfall conditions than those needed for debris flows on natural slopes.
  • Areas where surface runoff is channeled, such as along roadways and below culverts, are common sites of debris flows and other landslides.

Major types of Slope Failures in Darjeeling
The following important types of mass movements have been recognized by the investigator: a) Slump, b) Debris slide and c) Earth-rockflows.
a) Slump : A very good example is represented by a large slumping slope near Pagljhora (23 mile post from Siliguri). The gradual subsidence of the Hill Cart Road for a length of about 500 m has been noticed since 1966.
b) Debris Slide: The debris slides are generally of greater magnitude and are quite common. Slope failures are found in and around Tindharia and Chunbhati.
c) Earth-rock Flows :This type of slope failure has often been found in and around Tindharia and Mahanadi where saturated debris along with big blocks of rocks downwards to form a channel of its own.

Landslides in and around Tindharia
This region (in between 24 to 30 km north of Siliguri) is mainly composed of slightly altered Gondwana formation of which sandstone and shale are noteworthy. These have more or less been extended in ENE to WSW direction with varying depths inter-bedded with coal seams of few cm thick only. Altered quartzite, phyllite, slate and mica schist of Daling formation are found above sandstone. The landslides have been identified on the lower sandstone at 25.5 km and 29.33 km respectively from Siliguri on the left side of the Hill Cart road, whereas the landslide has been found on the upper sandstone of Gondwana formation.

Here the slightly re-crystallized and coarse grained sandstone is characterized by cataclastic deformation which destroyed the clastic texture with intense granulation along ramifying narrow zones of fracture.

Moreover, alternative thermal expansion and contraction gave rise to vertical cracks within the rock and horizontal beds of rock disintegrate into blocks. Water seeping into coarse textured sandstone helps in decomposing feldspar to form clay mineral, which acts as a lubricant for further sliding of rocks, which are steeply inclined towards roadside. The coal seams, hitherto unexposed has suddenly been unearthed after the removal of the surface layer of the soils by intense showers (100 mm in 52 hrs). The local people, suffering immensely due to lack of fuel, naturally get delighted at the sight of a new source of energy and had a tendency to collect fragmentary pieces of coal, creating large hollows at the crest of slopes. Such human interference readily ruptured the slope stability in case of landslides..


Landslides in and around Mahanadi

This region (in between 36-37 km north of Siliguri) is composed of altered metamorphic rocks of Daling formation of which quartzite, phyllite, slate and mica schist are important. Due to metamorphism and intense weathering coupled with high rainfall (140 mm in 24 hrs), the earthflows were formed.

Water of small springs following over these slides often go underground through the fragmentary pieces of rocks and perform considerable subsurface erosion. As a result, subsidence is a common feature in this region. The Hill Cart Road has been very much affected since 1985 and has gone down by 5 to 7 m disrupting the regular traffic. Landslides above Pankhabari

The southern portion of this region (3-4 km south of Kurseong) is composed of sedimentary rocks (sandstone and slate) of Gondwana formation and the northern portion is formed of metamorphic rocks (quartzite, phyllite, slate and mica-schist) of Daling formation.

The apexes of the slides of this region had long been deforested and turned into cultivated terraces by local inhabitants. Such terraces were seldom laid along the contours with proper protection walls. Moreover, these terraces had often been cultivated with potato, ginger and onion, which were harvested during September-October, when the monsoon was very much prevalent in this region. This particular practice not only disturbed the cohesiveness of the soil (totally saturated with water) but made it vulnerable to erosion.

Thus, the slope being devoid of proper vegetal mat remained fully exposed to heavy rainfall (83 mm in 24 hrs) and was susceptible to both sheet and gully-erosion. As a result fragmentary pieces of rock roll down to cause landslides.


Management Approach

There are several techniques to control or minimise landslide hazards. These are described below:

1. Careful and systematic site selection, process identification and hazard mapping are needed to define the level of risk and to decide upon the mitigative measures.
2. To use modern technique of Remote Sensing, Geographic Information System and Global Positioning System (RS, GIS and GPS) which can help a great deal in formulating action plans in disaster mitigation/ preparedness.
3. Bio-engineering Techniques are useful for erosion control and / or hill slope stabilisation.
4. Grass planting techniques for erosion control and/ or hill slope stabilisation (Downslope grass plantation, Turfing) is found to be useful.
5. Afforestation/ revegetation for erosion control and / or hill slope stabilisation is a must.
6. Other low cost techniques for erosion control or hill slope stabilisation for examples jute netting, or wire meshing, Rip-rap drain to provide good ground hold.
7. Law should be implemented to prevent making high-rise buildings along the road sides and in the unstable zones.
8. To control the road traffic movements during the rainy season.
9. To construct retaining walls, anchor retaining walls along the road sides particularly on steep slopes with proper weep-holes to drain out the excess water.


As far as their effects upon environmental management are concerned landslides are of three types : (i) slide which was not predicted and which has caused immense damage and grief. (ii) consists of those slides which are known about the threat and the need to control them being appreciated. (iii) which occurs either along proposed transport route or which potentially infuences the site of a projected development. The geomorphological contribution to the management of each of these three groups lies in

(a) predicting which areas are susceptible to landslide, and
(b)identifying the controlling environmental characteristics of known landslides. Identifying Landslide-prone Areas

In order to identify landslide-prone areas it is relevant to distinguish between situation and site. Those situations where landslides may be expected to occur include mountain fronts, steep-sided mountain valleys, and steep-hill slopes and scarps. By contrast situations not susceptible to large amounts of mass movement include relatively flat areas without deep river incision, Site, on other hand refers to location of a particular landslide, for example along a specified steep slope. General relief categories (e.g. intensely dissected mountains) are sufficient if it is necessary only to identify those areas within which sliding is likely to occur. These closely approximate to the land systems and the land-systems approach as a whole may be validly employed in situation mapping. One of the advantages of this approach is that land-systems boundaries can be defined from aerial photographs, thus saving much time and expense in field mapping.

The advantage of having a checklist, either in a field investigation or during air-photo interpretation, is that each of the main categories of influencing and controlling parameters is systematically examined. By carefully assessing the state of the relief, drainage, bedrock, and regolith, the incidence of earthquakes, legacies from past processes, and man-made feature the likelihood of landslide can be assessed.


Identifying and Controlling the Cause of a Landslide

It is important to distinguish between those physical characteristics of a site which make landsliding possible and the actual cause which initiates movement. For most slides given that the site possesses the appropriate physical characteristics, the trigger machanism for movement is an increase in pore-water pressure.

Excessive pore-water pressure can be relieved by adequate drainage of the slide, with the appropriate removal of the cause of these high pressures if it is at all possible. In addition to draining the slide itself diversion channels can be placed above the head of the slide to prevent surface runoff into the slide itself. Where overloading at the head of the slide is a contributing cause of movement it is necessary to relieve that load by removing it. If greater support is required at the toe then adequate retaining structures have to be built. If river undercutting is a contributory cause then its channel must be diverted. There is yet one other primary cause of landsliding over which man can exercise no control and that is the shock waves generated by earthquake. In such cases it is necessary to recognize that in the earthquake zones of the world it is better for human structures such as houses and dams to avoid sites which are themselves potentially unstable.

Preventing or controlling landslides is not always an economically viable proposition, but where such control is necessary it is of fundamental importance that the nature of slope instability should be fully understood.


Historical Records of LandSlides

Year Area Blocks in Darjeeling District Damages
1980   10 Blocks Population affected: 84825; Death: 47; Houses damaged: 1,085;.
1983     Population affected: 40011; Death: 05; Houses damaged: 345.
1987   02 Blocks Population affected: 52000; Houses damaged: 8,104.
1997   03 Blocks & 1 Municipalities Population affected: 795; Death: 17; Houses damaged: 765.
1998 Lodhoma Darjeeling-Pulbazar Population affected: 40000; Death: 21 Houses damaged: 3,498.
  labdah kolbong simbong division Rangli-Rangliot  
  All GP except gayabari & Sukna Kurseong  
1999 Samalbong Darjeeling-Pulbazar Population affected: 321; Death: 13 Houses damaged: 1,697.
  Sittong 1 & 3, Tindhoria Kurseong  
2000 Halhalay Darjeeling-Pulbazar No Causualities, but 50 houses damaged.
  Goethels & Sepoydhura Kurseong  
2002 Tista Vally, Poomung, Lamahatta Rangli-Rangliot Population affected: 72; Death: 07, Houses damaged: 05.
  Samdong 2 Kalimpong-II  
  Daragaon, Gayabari-I Kurseong  
2003 Dabaipani, Jinglam Rangli-Rangliot Population affected: 72; Death: 07 Houses damaged: 05.
  Sakhibhir Kalimpong-II  
2004 Maneydara, Labdaah Rangli-Rangliot Population affected: 103417; Death: 25; House damaged: 5499.
2006 Bloonfield Darjeeling-Pulbazar Population affected: 3029; Death: 13; House damaged: 2375
  Gairigoan, Sindepong Kalimpong-I  
2007 Mangalpuri, Nalbon Darjeeling-Pulbazar  
  Margaret's hop, Beach Gaon Jorebunglow-Sukiapokhari  
  Singringtam, Rangli Rangli-Rangliot  
  Upper khani, Mangal Basti, Lamini Kalimpong-I  
  Bindu, Patengodak, Panjabi Golai, Shermali Gorubathan  
  Bagogra, Kharay, Bhotey busty, Mirik-Karshing municipal areas Kurshing  
2008 5 Blocks and 1 municiplaities are affected Sukhiapokria, Kalimpong-I, Matigara, Khoribari, Phansidewa Blokcs and Kalimpong Municipality Population affected: 16,674; Death: 03; Houses damaged: 3173.
2009 AILA coupled with Landlside occurred in 555 villages in 8 blocks and 4 municipalities Bijanbari, Sukhiapokhuria, Takdah, Kurseong, Mirik, Kalimpong – I & II, Gorubathan Blocks and Darjeeling, Kurseong, Mirik, Kalimpong Municiplaities Population affected: 1,45,758; Death: 41; Houses damaged: 26595.

Dos and Don’ts
Prior to a Potential Landslide due to Intense Storms
  • Stay alert and stay awake. Many debris-flow fatalities occur when people are sleeping. Listen to a radio for warnings of intense rainfall. Be aware that intense short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
  • Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger flows. If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate debris flow activity upstream, so be prepared to move quickly.
  • Contact your local fire, police, or Public Works Department. Local officials are the best persons able to assess potential danger.
  • Inform affected neighbours.Your neighbours may not be aware of potential hazards. Advising them of a potential threat may help save lives. Help neighbours who may need assistance to evacuate.
  • Evacuate.Getting out of the path of a landslide or debris flow is your best protection.
During a Landslide
  • Quickly move out of the path of the landslide or debris flow. Moving away from the path of the flow to a stable area will reduce your risk.
  • If escape is not possible, curl into a tight ball and protect your head. A tight ball will provide the best protection for your body.

After a Landslide
  • Stay away from the slide area. There may be danger of additional slides.
  • Go to the established meeting point and follow instructions by the assigned responsible person.
  • Check for injured and trapped personsnear the slide, without entering the direct slide area.
  • Help a neighbour who may require special assistance — infants, elderly people, and people with disabilities. Elderly people and people with disabilities may require additional assistance. People who care for them or who have large families may need additional assistance in emergency situations.
  • Listen to local radio or television stations for the latest emergency information.
  • Watch for flooding, which may occur after a landslide or debris flow. Floods sometimes follow landslides and debris flows because they may both be started by the same event.
  • Look for and report broken utility lines to appropriate authorities. Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further hazard and injury.
  • Check the building foundation, chimney and, surrounding land for damage. Damage to foundations, chimneys or surrounding land may help you assess the safety of the area.
  • Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding.
  • Seek the advice of a geotechnical expert for evaluating landslide hazards or designing corrective techniques to reduce landslide risk. A professional will be able to advise you of the best ways to prevent or reduce landslide risk, without creating further hazard.

Construction Don’ts
  • Do not build on or at the base of unstable slopes
  • In or at the base of minor drainage hollow
  • At the base or top of an old fill slope
  • At the base or top of a steep cut slope
  • Developed hillsides where leach field septic
  • Minimize the number of trees and vegetation removed from the slope.

  • Do research: Learn about the geology and the history of your property. Talk to local officials, your neighbours, or visit the local library. Review geologic or slope stability maps of your area.
  • Do Get Advice: Get advice from a qualified geologist or geological engineer before buying a potentially unstable site or building your home. Although waterfront lots can be attractive sites, they often have severe natural limitations. They may also be subject to strict environmental and safety regulations.
  • Do leave a safe setback:Build a prudent distance from the top or bottom of steep slopes. Avoid sites that are too small to allow a safe setback from the slope. Allow adequate room for drainfields and driveways. Local setback requirements should be viewed as absolute minimums. Resist the urge to trade safety for a view.
  • Do keep plants: Maintain existing vegetation, both above and on steep slopes. Trees, shrubs, and groundcovers help anchor soils and absorb excess water. Get expert advice identifying and removing weeds.
  • Do Maintain Drainage: Collect runoff from roofs and improved areas and convey water away from the steep slope or to the beach in a carefully designed pipe system. Regularly inspect and maintain drainage systems.
  • Don't irrigate or put drain fields on a bluff: Avoid placing septic system drainfields or irrigation systems between a home and the edge of a bluff, where excess water or leakage could exacerbate slope instability.
  • Don't Dump on a slope : Do not place clearing debris, yard waste, or fill material on a steep slope. Even small accumulations of debris can become saturated and precipitate a larger slide.
  • Don't Change Natural Drainage: Avoid modifications of the ground that disrupt or alter natural drainage, unless based on the recommendations of a qualified geologist or engineer.
  • Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding.
  • Don't cut into the slope toe. Don't cut into a steep slope or excavate the toe of slope.
  • Don't overlook slide hazards: Do not be lulled into complacency by the lack of recent slides. Landslides typically only occur every few decades on a given site, and in some cases are even less frequent, but may remain a serious risk when heavy rains occur.
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