Preliminary findings of researchers from the National Centre for Earth Science Studies (NCESS), who inspected the major landslide sites of Kavalappara, Puthumala, Vilangad, and Pathar in north Kerala, say that there are no traces of soil piping in any of the incidents. All of the landslides were either translational or debris flow in character, with the absence of any kind of soil pipes.
Soil piping, a process through which water erodes beneath the ground surface and leading to underground tunnels which can enlarge over a period of time, has been cited in recent weeks as a potential threat to hillocks in Kerala. There have also been reports that the landslides, particularly the ones at Kavalappara and Puthumala which caused the most casualties, are a result of alarming soil piping.
“We have not seen traces of soil piping in these areas. In Kavalappara, the landslide is translational in nature as the entire face of a hillock has come down. Here, the water (from the rainfall) somehow got accumulated at the top and seeped into the soil. Since the soil couldn’t hold it anymore, it came sliding down. On the way down with its momentum, it began carrying the entire soil profile beneath and also a lot of trees were uprooted,” said Dr KK Ramachandran, who heads the department of atmospheric processes at NCESS in Thiruvananthapuram.
At the same time, the other three landslide sites of Puthumala, Vilangad and Pathar were examples of ‘debris flow’ incidents where huge amounts of debris came down the sloping area as the soil cover underneath the boulders and rocks was ‘very thin’, he said. Due to the burst of extreme rainfall within a short period of time, the boulders cannot hold the erosion of soil and it flows down.
Both the districts of Malappuram and Wayanad, which reported a majority of the landslides, had received excess rainfall in the days following August 8.
“If you take all other factors such as soil, rock, land-use as static, then the triggering mechanism (for the landslide) could only be a dynamic factor like rainfall. Normal rainfall in those parts never crosses 5-6 cms a day. But this time, on one day it went up to 15 cms, another day 12 cms,” said Dr Ramachandran.
But at the same time, he stressed that changes in land-use on such hillocks was the ‘culprit’.
“In Kerala, we don’t have any regulations for housing in hilly regions from a disaster point of view. In coastal areas, we have the coastal regulation zone. In paddy lands, we have the paddy-wetland act. For big infrastructure projects, we carry out an environment impact assessment. When people are aware themselves and try to be modest with nature, there’s no issue. If they are not aware, there has to be a regulation,” he added.
A three-member research team from NCESS had visited the landslide sites over the past few weeks and will compile a preliminary report on the same. The researchers will attempt to conduct further studies on the type of landslides and the slopes vulnerable to it as well as the susceptibility of different kinds of soil profile.