[OPINION] Nepal landslides and recalling past Philippines events - Rappler

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On August 26, 2026, the day the Nepal landslides happened, the initial posts on social media claimed that “flash floods” had killed nine individuals and left hundreds missing. A quick review of these initial posts and other news feeds showed that the incident was not about flash floods per se, but debris flows and landslides instead. The flash floods were merely a consequential process.

The death toll and number of missing individuals would rise rapidly in the next days. Nearly a month since, more than 1,400 are now reported dead with more than 6,000 individuals missing, including hundreds of foreign nationals.

A landslide is a phenomenon that involves the downward movement of earth materials due to gravity. It is one among several types of what is generally referred to as “mass movement” in engineering geology. Certain areas are inherently prone to landslides, such as those with steep slopes and poor rock and soil integrity.

They can be triggered by several other phenomena. In the Philippines, the more common triggers are intense rainfall and earthquakes. In temperate and polar regions, avalanches are a common trigger.

Most post-event studies of the Nepal landslides now point to the collapse of a glacier in the Lantan National Park located high up in the Himalayan Mountains near the Nepal-Tibet border as the culprit. The glacier collapse triggered avalanches that mixed snow, rocks and soil scraped from steep mountain slopes turning into debris flows as the earth mass flowed down river to warmer elevations.

At this stage, the debris flows were estimated to be travelling at speeds peaking at about 180 km/hr. Further mixing with more river water allowed the debris flow to become more fluid and transformed into flash floods at the head of the onrushing rock-mud-water mixture. The first videos posted on social media showed footages of this fluid material, hence the initial captioning as flash floods. By then, the river gradient had become more gentle at lower elevations and the flow speed had decreased below 100 km/hr. The volume of eroded materials was so vast that the landslide travelled and filled the river channel of the Lende-Trishuli Khola river valley for more than 100 kilometers.

Sometime in late 1995, an investigative team climbed the active volcano Mt. Parker in South Cotabato with the objective of reaching the crater then called Lake Maughan (now Lake Holon). The climb was undertaken by a team consisting of government scientists and officials, escorted by a squad of Citizen Armed Force Geographic Unit and a couple of junior officers of the Philippine Army.

Today, Lake Holon is a tourist destination accessible through a 3-hour-hike trail, which otherwise required at least two nights and two days in 1995, not to mention that the region then was still a hotbed of insurgency. 

The climb was in response to instructions from authorities at the Department of Environment and Natural Resources to investigate the cause of a massive September 1995 flooding at the northwest flank of Mt. Parker, causing the death of more than 50 individuals, and rendering hundreds of families homeless, damaging infrastructure and devastating agricultural lands in the Allah Valley. 

Official reports indicated that the flooding resulted from the natural overflow of the crater lake after an artificial dam at the crater wall breached as it could no longer bear the weight of excess water impounded in the lake after days of heavy rainfall. Earlier reports indicated the possibility of blasting activities by treasure hunters wanting to drain the lake in search of the infamous Yamashita Treasure as the culprit. 

Regardless of the real cause of the tragedy, the deadly Allah Valley floods resulted from the release of excess water impounded upstream after days of heavy rainfall that could no longer be contained by natural barriers.

In the first week of August of 1999, Cherry Hills Subdivision, a low-cost residential village in eastern Antipolo, a suburban mountain city east of Metro Manila, was engulfed by a landslide emanating from the collapse of a steep hillslope on the eastern flank of the village. Reports indicated that the subdivision was constructed over a former aggregate quarry. This hillside slope was an excavation left open at the edge of the quarry property.

More than 50 lives perished in this disaster, with the entire village left practically inhabitable. Prior to the landslide, eastern Antipolo and vicinity were battered by torrential rains for three days. It was classified as a freak rainfall event because the estimated amount of rainfall of almost 1,000 millimeters exceeded the two-month rainfall average during the rainy season of the region. There was no weather disturbance at the time, except for the southwest monsoon that was then commencing.

The already inherently slide-prone steep slope left by quarry excavation became extremely saturated by the three-day rains, adding to the weight of the slope that promoted its failure. The tragedy was exacerbated by the loose quarry materials upon which the village houses were built.

In early 2006, the village of Guinsaugon in the town of Saint Bernard in Southern Leyte was buried in debris flow emanating from the steep cliffs of the Southern Leyte mountain range transected by the active Philippine Fault. This landslide event killed more than 1,000 individuals, most of whom were elementary pupils in a school located nearest to the source of the debris flow.

Debris flows are a mixture of water, rocks and soil that slide down from high elevations, usually using river systems as the channel way. When debris flows reach low and flat elevations, they pile up and bury the area with boulders and thick mud that together usually form hummocky hills when they dry out.

The Guinsaugon landslide was preceded by at least a week of continuous heavy rains over southern Leyte. It deposited debris flows about three meters thick over an area three kilometers long and two kilometers wide. This is roughly equivalent to the area of the central business district of Makati City in Metro Manila. 

Several other deadly debris flow and landslide events would occur later, including those that affected Baco, Oriental Mindoro and Gabaldon, Nueva Ecija both in 2015; Naga, Cebu in 2018; New Bataan, Compostela Valley (now Davao de Oro) in 2017; Baybay, Leyte in 2022, to name some. Like the Allah Valley, Cherry Hills and Guinsaugon disasters, all these events were preceded by extreme rainfall.

In the same way that, as we commemorate today the 54th anniversary of the declaration of Martial Law on Monday, September 21, we should not forget the lessons learned from this era marred by political suppression and extensive human rights abuses, we should as well remember the lessons learned from past landslide disasters.

The Nepal landslides were triggered by a glacier collapse high up in the Himalayas, generating deadly debris flows and flash floods as the rock-mud-water mixture traveled downstream for about 100 kilometers. While the Philippines is not expected to experience the same phenomenon due to the absence of glaciers in any of the Philippine mountain ranges, debris flows can emanate from landslides in steep mountain slopes at high elevations that may be induced by continuous heavy rains and/or earthquakes. 

The Mt. Parker- Allah Valley flash floods taught us that crater lakes at the summit of volcanoes must be protected and monitored, to avoid breaches and collapses of the crater wall preventing uncontrolled overflow of lake water that can induce flash floods downslope.

When building subdivisions and housing villages, especially in places earlier meant for a different land use, slopes must be kept at safe gradients and protected, and the subsurface must be rendered capable of bearing the foundation of houses. Village design must integrate extreme weather conditions such as anomalous rainfall and intense ground shaking due to earthquakes. – Rappler.com

Mario A. Aurelio, PhD is a professor at the National Institute of Geological Sciences-University of the Philippines. Then a geologist of the Department of Environment and Natural Resources, he was part of the team that climbed Mt. Parker to investigate the 1995 Allah Valley flash flood.