At least 25 people have died in multiple landslides and floods in the districts of Kottayam and Idukki in Kerala which had been witnessing heavy rainfalls under the influence of a low-pressure formation over the Arabian sea. In Koottickal, Kottayam, two landslides reported at Plapalli and Kavali, which are two kilometres apart, resulted in the death of 13 persons. Kokkayar, which is barely three kilometres away from Koottickal but falls in Idukki district, reported two landslides at Makkochi and Poovanchi resulting in the loss of seven lives.
For seven days from October 7 to October 13, three days before the landslides happened, the area received more than 50mm of rainfall in two hours. Meteorologists have coined the term 'mini-cloud bursts' to highlight their intensity. IMD defines cloudburst as an event when an area receives 100 mm of rain in an hour. Peermade in Idukki, had recorded 240.5mm rain on October 16.
Are landslides new in Kerala?
Landslide in the Western Ghats in Kerala have been on the rise over the past few years causing massive loss of life and destruction of property.
In 2018, the year in which heavy floods ravaged the state, Kerala recorded 155 deaths due to landslides alone. Between August 1 and 19, the state received 758.6mm of rain, an excess of 164%. Most numbers of deaths happened in Idukki district (47) followed by Malappuram (28) and Thrissur (27). In 2019, two major landslides at Puthumala and Kavalappara together resulted in 81 deaths. In 2020, the Pettimudi landslide caused the death of 66 tea estate workers after a torrent of debris swept away their line houses.
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A total of 1,848 square kilometres, 4.75% of the state's total area, has been identified as High Landslide Hazard Zone by the Kerala State Disaster Management Authority (KSDMA).
What triggers landslides in Kerala?
A recent study by scientists at the National Remote Sensing Centre (NRSC), Hyderabad, which analysed six major landslides in the state during the 2018-20 period, has found that the single-day rainfall, and 3-days and 7-days of antecedent rainfall ranging from 200 to 600 mm played a critical role in triggering landslides in Wayanad, Malappuram and Idukki districts.
The analysis, led by Nirmala Jain, shows that Puthumala landslide on August 8, 2019 was triggered by one-day rainfall (244 mm) and that Kavalappara had received a 3-day antecedent rainfall (236.2mm) on three days from August 5 to August 7, 2019. A single-day rainfall (229 mm) played a significant role in triggering the Pettimudi landslide.
The other three landslides they analysed -- Pancharakkolly (7-day, 37 mm to 305mm/day), Manniyankunnu (3-day, 370mm) and Kurchermala (3-day, 173.4 mm) too showed a similar trend.
Are rains the only reason that cause landslides?
No. Even when triggered by intense rain, landslides are born out of the cumulative impact water has on topography, geology, soil and vegetation. The Western Ghats in Kerala is characterised by thick soil cover but human interventions over a period of time have made them vulnerable, says G Sankar, a former scientist with National Centre for Earth Science Studies (NCESS).
"During monsoon, the overburden soil gets saturated due to percolating rainwater, making it unstable. When the water content reaches a threshold level, the soil mass becomes weak triggering debris flows," says Sankar. Human interventions that block natural drains down the slope have heightened the risk.
"There are small drainage channels present on the surface of the hilly areas. Erecting buildings and contour bunds block their natural flow. During the rains, the water that percolates get concentrated leading to increased pore pressure along the channel," he said. Pore pressure refers to the pressure of water held within soil or rock and in gaps between particles. "All mountain drainage channels should be free helping easy runoff. We should not construct buildings on or near these channels or at least give leverage of 50m on both sides," he said.
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The high ranges in Kerala began experiencing massive migration from the midlands in the 19th century. Human interventions like monocropping, deforestation and hill-toe modifications like slope cutting made them unstable.
What other factors are responsible for landslides in Kerala?
The steep slope between 19° and 35° is another causal factor for the occurrence of landslides, says the NRSC study. The presence of lateritic deposits, weathered rocks, and features such as joints and fractures further increased their vulnerability. The velocity of debris flow in these six landslides ranged from 12.9 m/s to 29.6 m/s. The runout length varied from 0.82 to 3.3 km.
The largest debris flow, in terms of velocity, flow height and pressure, was observed during Puthumala landslide, the study noted.
What about unbridled quarrying?
While environmental scientists and geologists have for long flagged the presence of quarries as a crucial factor that induces landslides, no major studies seem to have been undertaken that establish a correlation.
A critical mapping initiative in 2017 by T V Sajeev, a scientist with Kerala Forest Research Institute (KFRI), showed that there are 1,486 granite quarries in Eco-Sensitive Zone-1 (ESZ-1), I 169 in ESZ-2 and 1,667 in ESZ-3, identified by the Western Ghats Ecology Expert Panel (WGEP). "There have been some efforts that probed the linkages, which looks evident but a large study seems imperative now," he said. In Koottickal panchayat, an eco-sensitive zone, there are around 12 granite quarries.
Can landslides get reactivated?
During the field work, researchers from NRSC found that the crown of Chembanoda landslide, which was triggered in 2018, has worsened further and shifted 200 m due to heavy rainfall in 2019. At Puthumala, wide cracks were found near the crown area and dislodged soil mass. Landslide flanks and crown of Kurchermala has also become active following heavy rainfall in 2019, which indicates that they are prone to reactivation.
What can be done to prevent landslides?
Sankar says that there is no dearth of studies on causal factors of these landslides but what is missing is preventive action. "Koottickal, where a landslide occurred on October 16 was identified as part of Highly Susceptible Zone (HSZ) in the 90s, while I was with the Centre for Earth Science Studies. This was one of the first projects we undertook," he said.
The KSDMA and NCESS has mapped locations susceptible to landslides in the state. "It is possible to calculate a threshold for each and every location, and the intensity of water one particular area can sustain. This would help us issue timely warnings and evacuate people," he said.
If every local body had a rain gauge, a simple instrument used by meteorologists to measure the amount of water that falls, cost-effective and efficient system of warning can be developed, he said.