Every summer, monsoon rains come to southwestern India. This summer, however, the deluge brought the worst flooding in a century.
The resulting destruction killed nearly 500 people, inundated cities, and collapsed bridges. The rains also caused thousands of landslides in mountainous regions after torrents of water loosened soils from hillslopes. These slurries of water, soil, rock, and vegetation overwhelmed villages, downed power lines, and cut some communities off from receiving immediate aid.
Following the disaster, a team of geoscientists traveled to the devastated state of Kerala to survey landslide damage. Their ground survey, which was finished last month, uncovered new insights into what triggered the dangerous slides in the region.
“We observed that most of the landslides occurred in areas where recent construction had happened,” Thomas Oommen, associate professor at Michigan Technological University, told Eos. The team’s findings also discovered evidence of large subsurface channels flowing under the soil, a new factor they hope to incorporate in future hazard models.
But landslide hazards in the region aren’t over now that the summer monsoon has ceased. The region’s northeast monsoon is slated to begin later this month. More rain means that more slides could occur soon, the researchers warn.
The Monsoon of a Century
Summer monsoon rains began to fall in Kerala last May. In the months following, storm after storm brought 2.4 meters of rain to the state in just 87 days. The amount was well above the region’s usual monsoon levels and nearly triple the amount Elizabethtown, N.C., received from Hurricane Florence last month.
Oommen, who is from Kerala, watched the floods unfold from afar and knew that scientists needed to survey the effects of the rain quickly, while landslide scars were still fresh. So in late August after rains had ceased, he reached out to a fund sponsored by the National Science Foundation (NSF) that awards grants to those with the contacts to undertake rapid reconnaissance after disasters.
NSF agreed to send him and a team of landslide geoscientists—Sajinkumar K.S., Richard Coffman, and Vishnu C.L.—to the mountains of Kerala. One week later, Oommen packed his bags and boarded a plane to India.
A First Glimpse at the Floods
On 3 September, the team of researchers met in the Kerala city of Thiruvananthapuram. Their first stop was 3 hours north, in the state’s Alappuzha District, a region that sits along the coast between the inland Vembanad Lake and the waters of the Arabian Sea.
When they got there, the scientists were taken aback by what they found. “To our surprise, there were over 3,500 houses still flooded in the Kainakary area,” Oommen said. They did not expect to see houses still flooded nearly 2 weeks after the rain abated, he said.
The region sits below average sea level, and an embankment between agricultural fields and the city had been washed away in the flood. As the city worked to rebuild the wall and pump out water, the damage became apparent: At houses where the water had receded, the scientists saw foundations starting to sink into the ground.
Alappuzha “is known as the ‘Rice Bowl of Kerala’ for its paddy fields,” Oommen explained. The fields were almost ready for harvest before the monsoon came, he noted. But instead they were destroyed by the rains.
The floods, they knew, were just one symptom of the downpours. Another hazard lay upslope in the mountains from which the floodwaters came: landslides. As landslide geologists, the crux of their fieldwork lay there.
The researchers drove east, toward the lush Idukki District of Kerala. Idukki lies within the Western Ghats, a mountain range draped with tropical forests that stretches along the west coast of India.
They encountered what they had suspected: The heavy rain from the monsoon had saturated hillsides of the Ghats during the summer months, triggering more than 1,000 landslides according to initial reports that Oommen received from the government. Those estimates indicated that in Idukki alone, the slides buried 161 structures and damaged roughly another 2,000.
Traveling around Idukki, often on roads reduced to one lane because of debris, the scientists encountered landslides abutting roadsides and sluicing through villages. They saw gigantic slides that had left whole hillsides bare. The landslides also “severely damaged or totally destroyed” the roads in Idukki, said Oommen, leaving towns stranded for weeks at a time.
Mapping Land on the Move
The team of scientists set out to map some of the recent slides before the forest swallowed the evidence. “Being a tropical climate, all the manifestations of landslides will be erased very soon due to intense growth of bushy vegetation,” said Sajinkumar, a member of the reconnaissance team and an assistant professor of geology at the University of Kerala.
At many of the slides they encountered, the researchers stopped to take photographs, using a thermal camera to measure underlying moisture and drainage channels below the soil. They probed the soil on and around recent slides using a cone penetrometer to check the strength of the soil. And in some cases they traced the perimeter of the slide by foot, plotting the edges with GPS.
After the researchers studied roughly 40 slides, of which they took detailed measurements of nearly a dozen, the scientists began to notice two distinct causes for the landslides: one natural and one human caused.
The natural cause of the landslides was the simple reality of steep terrain being bombarded with too much water. When the torrential rain bore down on the slopes of the Western Ghats, the water percolated under the surface of the soil and tried to flow downhill. In certain areas, subsurface “pipes” formed, carving water channels under the soil. These pipes either quickly drained subsurface water, destabilizing hillslopes, or became clogged, creating a backlog of water that grew larger and larger until the entire slope gave way. In some places, the underground pipes “reached a maximum of 20 centimeters” in diameter, Sajinkumar said.
But these types of natural slides were in the minority, Oommen noted. The researchers found that human activity triggered most of the slides.
When people build new structures, Oommen explained, “the terrain is typically altered by the cutting of slopes and removal of soil.” Slicing into the slope transforms the runoff pattern and opens up new ways for water to seep underground. “The increased infiltration,” Oommen said, “destabilizes the slope and leads to landslides.”
Building on Shifting Ground
A study released 2 months ago in Natural Hazards and Earth System Sciences documented a steady rise in fatal landslides that had been triggered by human activities, particularly in Asia. Oommen and Sajinkumar saw this playing out in near-real time, with construction practices aggravating the existing landslide hazards.
For example, a massive landslide near Government College in Munnar in Idukki sent soil cascading down the hillside earlier this month, wiping out several newly built buildings, Sajinkumar noted. Fortunately, no one was injured in the landslide.
This slide came on the heels of other tragic losses in the community: More than a dozen monsoon-related deaths occurred during the month of August, including a family swept away by a landslide in the night.
Sajinkumar said that officials at the college should hardly be surprised by the most recent slide on campus. In that same location, a landslide had previously occurred in 2005, destroying a building in its path. After the 2005 slide, Sajinkumar published a study diagnosing the type of slide (a “head-ward retreating landslide”), and he warned that the slope would likely fail again in strong rainfall.
“However, the college authorities extended the campus within the landslide scarp, forgoing the warning from previous studies,” he said.
More Risks to Come?
Unlike the college, some communities may not have had studies that warn of possible disasters.
The scientists visited the village of Panniyarkutty, where five people were killed when a landslide swept over several houses. When the team of scientists visited, residents were still searching for the bodies of three victims.
“While speaking with the neighbors of the deceased, we saw fear on their faces,” Oommen said. “They are traumatized and live in constant fear of rain.”
The researchers said that the cause of the deadly slide in Panniyarkutty was not immediately clear. They didn’t find evidence of recent construction or natural piping. Oommen suspects that the area may serve as a valley that several slopes drain into, but he said that further hydrologic analysis is needed.
The researchers worry that the devastation might not be over for Panniyarkutty and other communities in high-risk zones. “The district is expecting another spell of rain in October” from the northeast monsoon, explained Sajinkumar. Slopes on the verge of collapse, called aborted landslides, could become destabilized and grow into full landslides when another rainstorm strikes.
“These aborted landslides can get reactivated in this spell of rain,” said Sajinkumar. Given the danger, he explained, “it is advisable not to have human habitations in these highly vulnerable areas.”
However, families in Panniyarkutty face substantial difficulties trying to rebuild or relocate. “Many low-income families lost their homes, in which they had invested all their savings,” said Oommen. “Due to the high population density,” Sajinkumar added, “people are forced to occupy the hilly areas without accounting for the landslide susceptibility.”
Turning Disaster into Knowledge
Oommen and Sajinkumar released an atlas of landslide hazards in Kerala last year, and they plan to update their book with the results of their ground survey last month. Specifically, they intend to add the piping phenomenon into their hydrologic estimates.
“I think incorporating these subsurface channels could improve our estimates,” Oommen said.
But despite their warnings, the researchers worry that local governments won’t act decisively to mitigate future hazards. Previous reports had indicated many vulnerable areas should be excluded from future development, Oommen noted. “But there has been a lot of pressure from the political parties not to implement that,” he said.
Sajinkumar agreed. The “absence of strong legislation” coupled with “the lack of will to enforce existing regulations” made the risks posed by these hazards worse, he said.
The government really needs to step up to resolve these issues,” Oommen added. Otherwise, he noted, “people just carry on the same way. That’s what we have seen in the past.”
But, “hopefully, this time it will be different,” he said.
—Jenessa Duncombe (@jenessaduncombe), News Writing and Production Intern
Duncombe, J. (2018), Making sense of landslide danger after Kerala’s floods, Eos, 99, https://doi.org/10.1029/2018EO108061. Published on 18 October 2018.
Text © 2018. The authors. CC BY-NC-ND 3.0
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