BAYBAY CITY—Virginia Queza remembers sharply the Day of the Landslide, for that was when she lost her husband, house and almost everything else she had.
“I kept shouting for help while we were deeply covered with mud, surrounded by dead bodies. My husband was still alive. But all my pleas were ignored,” Queza, 60, recalled, speaking in her native Cebuano.
On April 11, unusually heavy rainfall brought by Tropical Storm “Agaton” (international code-name: Megi) soaked and weakened the mountain walls of central Leyte, sending tons of earth and trees crashing down on a number of villages.
Queza’s house in Barangay Kantagnos was carried away by a torrent of thick porridge-like mud, boulders and trees that fell from a mountain and roared toward a river. She managed to clutch a tree branch and hold on to dear life.
Communities at the foot of Leyte’s central mountains may not have been aware of Nature’s grim threat or may have gambled with their lives and property in the face of soil experts’ constant warnings of killer landslides.
“It’s easy to pin the blame on deforestation or shallow-rooted plants like coconut; this I’ve seen on social media. As a soil scientist, I’d like to provide a more nuanced, research-based perspective on what happened,” said Victor B. Asio, a professor of soil science and geo-ecology and the dean of the College of Agriculture and Food Science at Visayas State University (VSU) in Baybay City.
Asio, 59, obtained a doctoral degree in soil science and site ecology from the University of Hohenheim in Germany and had postdoctoral studies in soil science at the University of Halle-Wittenberg, also in Germany, and National Taiwan University.
In an interview with CoverStory, Asio said he had been studying the central highlands of Leyte, the site of the recent landslides, for the last 30 years. “We have accumulated data on the physical, chemical and mineralogical properties of these soils,” he said, adding that these “are closely related to the occurrence of landslides.”
Landslides occurred in Barangays Kantagnos, Bunga and Mailhi on the western slopes of the central highlands of Leyte, also called the Leyte Cordillera. The casualty count: at least 175 people dead and 110 missing.
“This mountain range is volcanic, and consists mainly of pyroclastic rocks (specifically basalt and andesite), which are generally fragmented rocks produced by volcanic eruptions,” said Asio, who inspected ground zero in Bunga where 17 people were killed and eight houses were buried.
Several factors triggered the landslides, he said, citing the most important ones as geology, topography, soil characteristics, land use and vegetation cover, as well as rainfall.
According to the VSU-Pagasa Agromet Station, Agaton dumped 907 millimeters of rain from April 9 to 11, or about one-third of the annual rainfall in Baybay. The total rain volume is equivalent to 9,000 cubic meters of water per hectare, or about 200 gallons per square meter.
4 months of rain in 3 days
“That’s about four months of rain in three days,” Asio said. Floodwaters reached waist-high in the city for the first time in four decades, prompting officials to declare a state of calamity.
Old soil under the forest can potentially hold a maximum of only 61 gallons per square meter at a depth of one meter, and under coconut trees, only 55 gallons per square meter, Asio said.
“No vegetation type can absorb and evaporate this tremendous amount of water (200 gallons per square meter) in so short a time,” he said. “Likewise, no soil can hold or percolate this volume of water in just two days.”
On the rugged and steep mountain slopes in the landslide areas, the soil can hold even less amount of water, Asio said. Once the soil is fully drenched, rainwater cannot be accommodated in the pores and flows as surface runoff, causing landslides.
Most of the landslides occurred on old and highly weathered soils, which, according to Asio, are quick to crumble, are moist or clayey, and are prone to shallow landslides, as in the case of Barangay Mailhi. When saturated with water, the clay serves as a lubricant for the sliding mixture of soil and rock debris.
Any soil can turn liquid when “supersaturated” with water, resulting in mudslides, Asio said. Trees and plants can help prevent only shallow landslides, but not deep-seated ones such as those that occurred in Bunga, Kantagnos and Mailhi, he said.
About two hectares of hillsides in Bunga were eroded, said Bryan Gapasin, an instructor at VSU and the extension project leader of VSU’s Department of Business and Management. His team was among those who rushed to Bunga to bring relief and assistance to families, rescuers and members of a women’s association whose root crop processing facility was damaged.
Gapasin said the villagers, who depend mainly on peddling products for their livelihood, did not expect the tragedy, having lived in Bunga for more than 15 years.
Barangay officials have declared the area inhabitable and proposed the relocation of its residents.
Landslides are natural geologic processes on the land surface, Asio said, adding: “Catastrophic landslides like those we witnessed recently in Baybay may occur again any time at any place with the above environmental conditions regardless of the vegetation cover.”
The scientist said he believed that several “incipient” (formative stage) landslides were produced in various areas during Agaton’s onslaught but were hidden by the vegetation cover. Another storm or typhoon may trigger “full or even catastrophic landslides,” he said.
Asio said it was crucial that people in landslide-prone areas be given proper and timely attention. But he is uncertain if local government units have been warned about the hazard zones.
“I strongly believe that a detailed survey to identify and map landslide-prone areas in Baybay and other places in Leyte is urgently needed,” he said. “In particular, areas with incipient landslides need to be located before the onset of the typhoon season.”