Landslides in a changing climate

The environment is currently witnessing a discernible change in its intrinsic attributes as a result of changing global climate. The earlier alarming prediction of scientists on changing climate is coming true in the form of reducing and retreating glaciers and rising sea levels, harsh heat waves, ungovernable forest fires, and other eccentric cataclysms. These changes may be attributed to either natural internal processes or continual anthropogenic disruption in the composition of substratosphere or that of land. However, based on scientific estimations, human activities are at the helm of major disruptions in the present-day environment including unambiguous warming making natural calamities more intense and frequent. Increased temperatures and heavy downpour events have created an urgency of understanding the changes in climate variables to measure and predict geohydrological hazards.

Understanding landslides

The landslide ubiquity has been playing an important role in the transformation of landscapes inclusive of posing a significant threat to the human population and the geo-environment. Landslide refers to any mass wasting process or the movement of rock and soil, down the slope, under the influence of gravity initiated on natural or engineered slopes either by naturogenic or anthropogenic processes or by a combination of both. United Nations Development Programme (UNDP 1996) counted landslides as the second most vital natural hazard among all other contemplated hazards. The scale of economic losses associated with landslides is generally high than identified. Additionally, the increase in population pressure as well as insufficient land to accommodate the settlements has forced people to move further uphill in landslide-prone areas hence worsening disaster effects. The recorded landslide fatalities between 2004 and 2016 were 4862 with 75% of landslides in Asia alone along the Himalayan arc. According to the International Disaster Database (EM-DAT 2008) out of all the natural disasters, landslide accounts for 4.9% between 1990 and 2015. Despite alarming consequences, landslides are still recognized as more controllable and manageable disasters unlike storms and volcanic eruptions.

Landslides and changing climate

It is debatable, how climatic change has affected the frequency and occurrence of landslides. Imperatively knowing the facts for what it takes to change the stability of natural slope? How do climatically governed processes trigger landslides? Supposedly, climate change has the full potential to change slope stability. Irrespective of substantive causes for climate change, this issue needs more urgent attention. It has been proved that the active downslope movement is caused by the moisture saturation of slope-forming material resulting in soil strength failure. Conceding that increase in the climatically controlled processes such as incessant rainfall, accelerated snow melting, attenuation of glaciers, etc. will eventually escalate the frequency of landslides.  

How climate change affects slope movements

According to the report by Intergovernmental Panel on Climate Change 2013, changes in climate variability due to global warming are ascertained in the form of numerous geohazards including landslides. Heavy and frequent downpours in steep hilly regions have induced a higher probability of frequent landslides risking not only human life and related assets but also disturbing the geo-environment as well. It is proclaimed that changes in rainfall patterns are the results of the changing global climate. According to a study by Mishra et al., 2020, the global rainfall statistics for the last 50 to 100 years show an annual decrease in the average number of rainfall days, while a considerable increase in the number of days was observed when the heavy downpour occurred. These heavy precipitations for shorter periods have increased the risk of landslides induced by incessant rainfall on steep slopes. Furthermore, studies also reveal that temperatures in mountain regions has increased, hereby modifying the tree and snow line and altering the permafrost and glaciers. The hypothesis of climate change contributing to slope instability is also validated by IPCC 2014 report which states: “There is high confidence that changes in heatwaves, glacial retreat, and/or permafrost degradation will affect slope instabilities in high mountains and medium confidence that temperature-related changes will influence bedrock stability. There is also high confidence that changes in heavy precipitation will affect landslides in some regions.” 

The elemental theory of slope stability lies in the factor of safety which is expressed as sΤ where ‘s’ is shear strength and ‘Τ’ is shear stress. 

When the factor of safety is less than unity, the slope is contemplated as unstable, which can fail by any triggering factor. Coulomb in 1776 defined and expanded the shear strength as            

s = c + γ ⋅ z ⋅ cos 2 β−u ⋅ tan φ

whereas the shear stress is expressed as

τ = γ ⋅ z ⋅ sin β ⋅ cos β

The stability factors in the above equations that are attainable to be affected by the changing climate predominantly the prolonged and heavy rainfall which include cohesion (c) where excess water affects the soil suction, bulk density (γ) in which the weight of slope forming material gets affected by percolated water, slope angle (β), pore water pressure (u), internal friction angle (φ).

Over a longer duration of time, climate change augments the weathering processes such as shrinkage and expansion, drying and wetting of soils, etc which eventually depreciate the strength and structure of slope material, affecting the cohesion and internal friction. The slopes with prior high-water content require less water from any hydro-climatic event to lose stability. Corresponding to the water infiltration responsible for slope failure, rising global temperatures play a significant role in slope instability. Intermittent high temperatures contribute to the thermal breakdown of rocks leaving behind deep cracks which in the course of time reduce the strength of rock. The water or any other unconstrained material entering the cracks and fissures of rock further acts as a wedge and slacks the coherent rock blocks. The soaring temperatures also escalate evaporation on bare soil surfaces rendering the soils drier and creating more unstable conditions in deeper soils.

In addition to this, warming exacerbates the wet and dry period cycles which becomes a mode in widening the gaps in the soil thus creating a favorable condition for slopes to fail. Rapid and earlier glacier melting on steep higher altitudes expose the unvegetated cover of soil which become vulnerable to failures at the hands of triggering factors. The snow cover absorbs rain and protects the soil surface from getting eroded by raindrops, the receding glaciers devoid the soil of this protective covering while, on the other hand, paving the way to more water content inside the soil thereby increasing the probability of landslides. The wildfires that leave behind the ingrained plant roots for a period of more than three years after the fire also weaken the slopes.

Havoc of landslide ubiquity 

The ubiquitous nature of landslides and related damages is more pronounced when the people and their linked structures are in the proximity of landslide-prone areas. During the period from 2004 to 2016, 55997 people got killed in different 4862 global incidents of non-seismic landslides.  The Indian peninsula is no different in having a sensational record of such serious geohazards with 15% of total land affected by landslides alone. Among all the landslides that have occurred in India, the major disastrous landslides are directly linked to incessant rainfalls. Out of all the physiographic divisions of the Indian peninsula, the Himalayan arc is observed to have the highest rate and increased frequency of such disasters compared to the Southern mountain ranges of India which include the Western and Eastern Ghats and Nilgiri hills. Every year huge amount of productive land is lost to sliding during the monsoon period in India. While observing the rainfall versus landslides data from 1901 to 2010, it becomes clear how the landslides are associated with heavy downpours during the monsoon period in Himalayan states (Figure 1).

Figure 1: Rainfall distribution and Landslide/Rockfall events of Indian states from 1901-2010. (Source www.imd.gov.in)

Landslides in J&K

Jammu and Kashmir in particular has a history of some disastrous landslides that have claimed huge life and property losses from time to time. Most of these landslide events occurred after incessant rainfall. For example, as per IMD records the catastrophic rains of 7-12 cm within 24 hours in Jammu and Kashmir region Sadal Village of Udhampur district in September 2014 witnessed a huge landslide disaster in which 75 houses were damaged killing 40 people. National highway NH1A connecting the entire valley with the rest of the country gets frequently blocked during monsoon and winter periods due to recurring landslides. Landslides and rockfall events on the national highway not only have claimed human lives in the course but also disengage the only mucho used road network leading to inflation and shortage of imperative goods within the valley with thousands of people stranded on the highway for multiple days and nights. Although, authorities ensure road clearance by removing the debris and restoring the smooth movement of traffic along NH1A with more veracity than what it used to be one decade back. However, just making road connectivity available isn’t a permanent solution to the problem till the problem isn’t scrutinized from its core and excogitated. 

Conclusion

Landslide studies entail an intricate research and multidisciplinary approach to figure out the dynamics of an ongoing process that further demands data generation both qualitatively and quantitatively from discrete branches of knowledge. Landslides, at the beginning of the post-war periods, were seen as “engineering problems” which require “engineering solutions” by using structural techniques with a major focus on retaining walls but with the advancement of techniques, this structural approach was diversified to include more sophisticated and ingenious techniques like that of soil nailing. Also, a shift towards “soft engineering” has been conspicuous over the last few decades involving novel sustainable and non-structural solutions. While considering the appropriate remedial measures, sustainability and environmental measures should become the prime focus, aiming to reduce any impact on the local environment or visual hindrance of scenic beauties. One such example in this regard is the stabilization of slopes by bio-engineering methods.

In the real sense, the concept of utilizing the green cover to stabilize slopes is not new, from ages, vegetation cover has been shielding the slopes by rainfall interception and groundwater transpiration thus maintaining the drier conditions of the soil. Besides controlling these hydrological processes, the roots reinforce the soil and provide a mechanical effect to the soil by increasing the shear strength. The vetiver grass system is one such example of a bioengineering method that is being used on sensitive slopes of the Kashmir Himalayas to replace conventional structural measures for slope stability.

References

EM-DAT. (2008). List of Landslides in India.        

IPCC – Intergovernmental Panel on Climate Change: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 5 Geneva, Switzerland 151 pp, 2014.

IPCC – Intergovernmental Panel on Climate Change: Climate. (2013). Climate Change 2013: Synthesis Report.   

Mishra, V., Thirumalai, K., Singh, D., Aadhar, S., 2020. Future exacerbation of hot and dry summer monsoon extremes in India. npj Climate and Atmospheric Science 3, 10.

UNDP. (1996). Human development report. 1996.