Sliding development

A massive landslide happened at 10:45 pm on August 6 at Pettimudi, a hamlet in Rajamala ward under Munnar village panchayat in Kerala's Idukki district crushing a settlement of 81 tea estates and the death toll in the tragedy rose to 52. According to the Idukki district administration officials, the searches are continuing to retrieve the remaining 19 residents who are still missing. The landslide was triggered in a shola forest region in the national park.

Landslides are closely related to the slope angle. If the slope angle is small, the shear stress on the slope is smaller, and the slope is more stable and less prone to landslide. Hence, mountains, plateaus, basins and hills that have a large slope angle are more prone to landslides. No assessment has been made to explore the susceptible area of landslide hazards and thereby no preventive measure is in place. In this context, it is pertinent to mention that the location at Pettimudi where the landslide was triggered has a 40-degree slope, and any slope above 20 degrees is vulnerable to landslides during heavy rain according to the Kerala Geology Department.

This is not the first time in Kerala landslides occurred. In 2019, 59 persons died in Kavalappara, Malappuram. In Puthumala, Wayanad, another landslide killed 17. Madhav Gadgil, the head of the Western Ghats Ecology Expert Panel (WGEEP), said if the Kerala Government had heeded the report's recommendations, the extent and the intensity of landslides in Kerala in 2019 and 2020 could have been much lower.

In 2011, WGEEP had designated this part of Kerala a region of highest ecological sensitivity, or ecologically sensitive zone 1 (ESZ1), which is susceptible to landslides. Despite this warning, continuous human interventions have resulted in a heavy load on the slopes that have made the area vulnerable. Another major cause of landslides is shockwaves from blasting rocks in quarries leading to a gradual weakening of other rocks. There are 5,294 quarries occupying 7,157.6 ha in Kerala. Of this, Central Kerala had 2,438 quarries covering 3,610.4 ha. Idukki district, in particular, had 328 quarries over 258.59 ha.

The landslides triggered by human activities are not only restricted to Kerala. These are on the sharp rise around the world including India. Heavy rainfall-induced landslides are one of the most dangerous gravity-induced surface processes resulting in severe damage to dwellings, roads, and other lifelines. Landslides contribute directly or indirectly to about 17 per cent of all disaster-related fatalities worldwide.

Land urbanisation increases the frequency and severity of landslides in the mountain area of the developing countries due to inadequate and unreasonable landslide prevention measures. The replacement of forests by agriculture and settlements is thought to cause severe landslides. The factors governing landslides can be grouped into potential factors (such as geology, topography, earthquake tendency, and groundwater characteristics) and triggering factors (including climate, rainfall, stream scouring, and human development). Climate change may cause the changes in the future to rainfall distribution and intensity, which will affect a variety of environmental processes that may be the concerns of decision-makers and politicians, especially for their associated consequences, including the geo-hydrological hazards.

Among these factors, ground surface conditions and vegetation cover are highly influential. For example, vegetation cover works to retain rainfall, not only reducing the ground surface erosion but also improving the cohesive force of root systems to stabilise the soil mass on slopes. Thus, the slope stability of areas with complete vegetation cover is often higher than that of areas with scarce vegetation. The velocity of water flow along a slope with a dense vegetation cover is approximately 1/4 of water flow along the same slope without vegetation in the same rainfall conditions; therefore, the erosive action that varies with the square of velocity may decrease to 1/16. The roots reinforce the soil increasing soil shear strength.

The development of forest roads with the removal of vegetation through the excavation of slope toes leads to changes in surface runoff and slope continuity, increasing the probability of landslides. Road construction changes the terrain at the toe of a slope leading to disruption of the continuity of water flow and reduces the ability of the slope to drain, in turn reducing its stability. Changes in the forest landscape or large scale logging also change the soil infiltration and ground evapotranspiration rates, thus indirectly affecting the water contents in soil and reducing slope stability.

In the hilly area, hydropower projects have a strong influence on landslides because excavation in hydropower projects for construction is usually done through traditional 'drill and blast' techniques. Use of heavy machinery and blasting during construction makes the soil in the project area unstable due to vibrations. As a result, landslides during construction are quite common. In addition to that, landslides may also occur during the operation of the project because soil strata in the project area become fragile as good construction practices are not followed. Sometimes water pressure or underground geological activity may cause cracks/fissures inside lining of tunnels leading to leakage of water. Water from this tunnel saturates the soil surrounding the tunnel that causes lowering of shear strength and bearing capacity of this soil.

Despite the occurrence of frequent landslides, the concerned governments do not any sincere attempt to delineate landslide susceptible areas for improving disaster preparedness and damage prevention. It is astonishing to note that the governmental agencies always appreciate proposals from the research groups/experts only after the calamities occur and constitute expert committees to frame the policy for prevention but nevertheless, no sound scientific mechanism is in place

Despite repeated warnings of the occurrence of devastating landslides, the Government remains silent. No one knows where the buck ought to stop.

The writer is a former Senior Scientist, Central Pollution Control Board. Views expressed are personal