Why landslide mitigation is important
The monsoons may bring life and prosperity to our agrarian societies, but it also has harmful consequences, especially for hilly regions. The monsoons have already begun to wreak havoc this year with persistent heavy rains leading to floods and landslides in many parts of Northeastern India that have claimed more than 80 lives according to government officials in the month of July 2017 (Indian Express, 2017), along with large numbers of people displaced by floods or isolated due to landslides.Prominent among these is a large landslide in Arunachal Pradeshâ€™s Papum Pare district on 11th JulyÂ that claimed at least 5 lives with many more missing (PTI, 2017). Large landslides have also occurred in the hills through which NH29 runs, connecting Nagaland to Manipur.
Landslides in highways create problems in sending aid and supplies to people who have been affected by other disasters in places that may be connected by these roads. Such was the case here, with timely aid and supplies failing to reach people affected by other landslides in Manipur (Indian Express, July 13th 2017). Such landslides triggered by heavy downpour had blocked the Manali-Leh highway on June 30th, 2017, reports the Times of India, with a number of landslides blocking access to the highway for tourists (ToI, 2017).
India has witnessed a number of landslides in the period between the end of June and the start of July, with the latest in Devidhar village in Shimla district, Himachal Pradesh on 18th July, 2017 claiming the life of a child (S. Sharma, ToI, 2017). Landslides across western Maharashtra, Jammu & Kashmir, Himachal Pradesh and northeast India have caused havoc so far in the 2017 Monsoon season. People living in mountainous regions fear landslides as it is sudden and devastating. However, a concerted effort to understand landslides can help predict it and enable mitigation measures to be put in place. Possible mitigation measures to check occurrence of landslides and the possible triggers for themcan then be enabled.
What are landslides?
Landslides are the downslope movement of soil, rock and organic materials due to gravitational force. Landslides occur across the world in all possible climactic conditions. Contrary to the assumption that landslides are triggered only by soil moisture, there is a wide range of factors working together that trigger different types of landslides.
Fig: Landslide Hazard map of India
Landslides are classified according to the particular method of slope failure, which is grouped into characteristic failure types. There are different types of movement associated with a particular landslide type. These movements can be fall, topple, slide, spread or flow.
Fall landslides are the decoupling of soil or rock from a steep slope or from a surface where there is minimal shear displacement, and as such, the material falls. They occur on steep or vertical slopes. Other than undercutting by water bodies, fall landslides can occur due to weathering, disturbances due to natural events like earthquakes, or by human activities such as excavations and building. Mitigation measures for fall landslides can include installing protective covers such as metal meshes over the sensitive zone, blasting with explosives of hazardous zones, or the removal of materials from such, anchoring of cliffs with rock bolts, and scaling. A good practice is setting up warning signs near hazardous zones (L.M. Highland, 2008).
Topple landslides usually occurin volcanic terrain or at banks of water bodies that are steep. In this form of landslide, a mass of soil or rock rotates outwards from a slope such that the mass does not fall directly downward, where the axis is situated at less than the center of gravity for that mass. Triggered by water flowing through cracks in the mass, topple landslides can also be caused by gravitational forces acting through objects above the mass, undercutting, vibration, differential weathering, erosion and human excavations. Mitigation efforts should focus on checking the stability for such masses, and look for contributory factors such as seepage and drainage networks. Mechanical methods of securing these masses such as rock bolts and metal meshes could prove useful.
Slide landslides are downward movement of soil and rock on a slope occurring due to rupture over sometimes thin sheets with characteristically intense shear strain. This form of landslide gathers material as it ruptures from the point of origin. Slide landslides are usually caused due to the action of water through intense rainfall or snowfall, excess groundwater, etc at the base of the slopes. Another trigger for this form of landslide can be earthquakes. Mitigating this form of landslide is extremely difficult, as it is extremely difficult to predict exactly where such a rupture could take place. While measures such as construction of retaining walls can be taken in areas with an expectation of risk, analyzing large swathes of area for detecting risks and predicting the exact point of rupture is a very difficult task.
Translational landslides are when the mass of soil or rocks movesoutwards or downwards a planar surface with the distinction from slide landslides in that there is no rotational motion in the mass. Provided that there are no barriers and the plane is sufficiently inclined, the mass in this form of landslides can travel a considerable distance. The trigger mechanisms are similar to slide landslides as due to action of water or vibrations by earthquakes. Mitigation efforts are similarly difficult as for slide landslides, although managing drainage in problem areas could help mitigation efforts to a great extent.
Spread landslides occur due to the subsidence of cohesive soil or rock masses into softer, less cohesive or vacant areas and masses. Its triggers include liquefaction, gravitational forces due to exerted weight above, changes in the underlying layer that cause them to subside or dissipate, and liquefaction of underlying layer. Its mitigation efforts are focused on mapping liquefaction in soil and in underground levels below the surface soil.
Flow landslides are the flowing motion of masses of soil and rock in manner similar to the flow of liquids frequently due to the presence of high amounts of liquids in the mass. These landslides are triggered by intense surface water flows and can also be by-products of previous landslides. These landslides, due to their sudden nature, are extremely unpredictable, except in places of likely or frequent occurrence, where canals and basins can be built to mitigate the event of a flow landslide. Volcanic lava flows can also be categorized as a form of flow landslide.
Fig: Anatomy of a Landslide
Source: US Geological Survey
Indian examples of devastating landslides
The flood mudslides that occurred in Uttarakhand in India in 2013 is listed as the 5th highest number of casualties from landslide events in the world, with 5,700 deaths. The chief cause for the extent of the damage, scientists and environmentalists point out, was the unbridled human intervention in the region. Hridayesh Joshi in his book â€˜Rage of the River: The Untold Story of the Kedarnath Disasterâ€™ says that human intervention in the region in the form of dams, commercial logging and open-cast mining created the pre-conditions that exacerbated the disaster (R. Guha, 2016). RamachandraGuha in The Hindu (2013) also talks of how a report by the prominent ecologist MadhavGadgil on an erosion strategy for Uttarakhand was requested and subsequently ignored by the then Union Ministry of Environment and Forests. Guha talks about a constant ignorance by the parties concerned of warnings by experts regarding development and environment in Uttarakhand.
The flash floods and landslides in Ladakh in 2010 is another landslide event in India of note. A cloudburst during the Monsoons on August 6, 2010 triggered flooding and landslides in the Himalayan region of Ladakh in Jammu & Kashmir that claimed at least 257 lives (NDMA, undated) and destroyed property in Leh town and several villages. According to estimates by Reuters more than 25,000 people were affected in what due to the inaccessibility of Ladakh during this period quickly turned into a humanitarian tragedy. Thomas Chandy of Save the Children, India (CEO) talking of rescue operations is reported to have told the press that â€œMore than 5,000 children have been affected by this humanitarian tragedy in Leh alone. We still do not know the fate of hundreds in surrounding villages as they remain inaccessible.â€ (Reuters, 2011). The true tragedy in simultaneous landslide events, especially after heavy showers, is often the difficulty faced by aid workers in carrying out relief work, in accessibility to affected areas and the delays in reaching troubled areas.
Technologies for landslide mitigation
There are however, a wide range of technologies available for landslide mitigation. These include equipments for emergency response, geological reconnaissance of landslide-prone areas, local monitoring services, site investigation with borings and test pits, slope stability analyses, seismic analysis of slopes, technical assistance in construction of buildings, roads, pipes, etc, design assistance for drainage systems, and erosion modelling (Landslide Technology, 2017).
Indiaâ€™s National Disaster Management Agency (NDMA) has a focus on site-specific landslide mitigation that involves making geological investigations on selected sites (NDMA, 2015). Its approach is based on co-operation with various government agencies such as the Geological Survey of India to designate certain areas as landslide-prone areas. India has not as yet witnessed a comprehensive hands-on technological approach to landslide mitigation. However, technology for this is available, and the problem is a matter of managing costs with the possibility of a high failure rate. On the other side however, a few timely evacuations might open a new vista for the NDMA in terms of disaster preparedness.