What caused the Baybay landslides during the Tropical Depression Agaton on April 11, 2022?

The Baybay landslides on April 11, 2022, brought about by the Tropical Depression Agaton, have already claimed 116 lives, and many victims are still missing. Social media is buzzing with explanations about what caused the landslides. As usual, deforestation is claimed as the top culprit. And as always, the deforestation issue is politicized. But is it the cause of the landslides? Several factors cause landslides. In the case of the Baybay landslides, the most important are geology (rock type), topography (slope), soil characteristics, land use (and vegetation cover), and rainfall. Let me explain the role of each factor.

Geology

All the landslides in Baybay are located on the western slopes of the central highlands of Leyte, also called the Leyte Cordillera. This mountain range is volcanic, and the rocks consist mainly of pyroclastic rocks, specifically basalt and andesite. Pyroclastic rocks are fragmented or unconsolidated rocks produced by volcanic eruptions. Consequently, the slopes underlain by these materials are generally weak and prone to slope failure. This situation is aggravated by the presence of the Philippine fault line along the central highlands, which has caused the shearing of the rocks.

The common type of pyroclastic rocks in the central highlands of Leyte

Topography

The western slope of the central highlands is generally rugged and mountainous. The steepness of a mountain slope is a major determining factor in whether the slope will fail or not. The steeper the slope, the less stable it is. In the presence of a triggering factor such as a heavy rainfall event, steep slopes (>25%) may fail, thereby causing landslides. From the topographic maps available on the internet, one can easily see that the source areas of the Baybay landslides have steep slopes. 

Topographic maps show the steep slopes of the source areas
of Bunga & Kantagnos landslides

Soil

Soils vary in the stage of development from young (poorly weathered) in the plains to old (highly weathered) soils in the mountains. Young, very porous, and unstable volcanic soils (65% porosity) prone to land sliding are widespread in the upper mountain slopes, generally above 300 meters elevation, such as in Mailhi. Except for the Mailhi landslide, most of the Baybay landslides occurred on the old and highly weathered soils. These soils are highly friable, clayey, and prone to shallow landslides. When saturated with water, the clay serves as a lubricant for the sliding mixture of soil and rock debris. And also, regardless of soil type, the soil can turn into a liquid state when supersaturated with water resulting in mudslides.

The source area of the Bunga landslide with its highly weathered soil, deep-seated
characteristic, and mixed vegetation cover. (Photo Source: jbatravelvlog)

The source area of the deep-seated Kantagnos landslide with its highly weathered soil and relatively thick mixed vegetation. (Photo Source: Dan Michael Castanares)

The deep Mailhi landslide with its unstable young volcanic soil and mixed
vegetation cover. (Photo Source: jbatravelvlog)

Land use 

Vegetation cover, particularly trees, can prevent soil erosion and shallow landslides, which generally occurs within the root zone. No doubt, forest trees can minimize shallow landslides better than shallow-rooted plants like coconut and grasses like cogon. Studies have revealed that roots increase water permeability and the mechanical stability of shallow pyroclastic soil cover (Alfonso-Dias, 2019). Zhang et al. (2019) found that the 23-year-old reforest in the mountain in Tacloban, Leyte, positively affected the hillslope hydrological functioning. But deep landslides that occur below the root zone are beyond the control of the roots of the vegetation cover (Zhuang et al., 2022). In such a case, the failure of the land surface is controlled more by the steep slope and by the weak geological and soil foundation. This explains why landslides can occur under any type of vegetation cover or land use. For example, on a clear day, one can see several old landslide scars in the forest on Mt. Pangasugan. In the Bunga and Kantagnos landslides, the largest Baybay landslides, the source area in the upper part of the mountain is still covered with mixed vegetation consisting of trees and coconuts. And according to Forbes and Broadhead (2013), the forest cover will not affect the occurrence of landslides during extreme events such as heavy rainfall.

Rainfall

Excessive soil water content from heavy rainfall is generally considered the primary cause of slope failure (Forbes and Broadhead, 2013). The tremendous volume of rain dumped by Agaton in three days from April 9 to 11 was 907mm, which is one-third of the annual rainfall in Baybay (Source: VSU-PAGASA Agromet Station). This is close to a meter deep water poured into the land surface in Baybay in only 72 hours. This volume of water is equivalent to 9,000 cubic meters of water per hectare or 200 gallons per square meter. No vegetation type can absorb and evaporate this tremendous amount of water in so short a time. Likewise, no soil can either hold or percolate this volume of water in just 72 hours. Even the very porous young volcanic soil in Mailhi, which has an average porosity of 65% and moisture content at a field capacity of 40%, can only potentially absorb 58 gallons of water per square meter calculated to a depth of 1 meter. The old soil under the forest can potentially hold a maximum of only 61 gallons per square meter to a depth of 1 meter, while the old soil under coconut can hold only 55 gallons per square meter. These values are based on the assumption of a flat surface and fast infiltration rates which are not the case in the landslide areas. This means that the actual amount of water that the soils on the mountain slopes can hold is much lower than the values mentioned. As soon as the soil is saturated with water, the incoming rainwater cannot be accommodated in the soil pores and flows as surface runoff, causing the landslides and the flooding, for the first time in about four decades, the city center of Baybay. 

Severe flooding of the city center of Baybay due to Agaton on April 10, 2022.
This is the first severe flooding of Baybay that I have witnessed in
nearly four decades. (Photo Source: Discover Baybay City).

Summary

The Baybay landslides were triggered by the extremely heavy rainfall and enhanced by the unstable and highly weathered soils (or the young unstable volcanic soil in the case of Mailhi), weak geologic material composed of unconsolidated pyroclastic rocks, and the steep slopes. The role of vegetation cover is not straightforward since it can partly prevent shallow landslides but not deep-seated landslides such as those in Bunga, Kantagnos, and Mailhi. This means that the coconut's shallow root system (about a meter deep) may not have effectively prevented the landslides, but the same can be said of the trees with their 2-3 m deep rooting zone since the landslides are several meters deep in their source areas. Landslides are natural geologic processes on the land surface. Catastrophic landslides like those we have witnessed in Baybay may occur again anytime at any place with the above environmental conditions regardless of the vegetation cover. I suspect that several incipient landslides (landslides in the initial stage) were produced in various areas during Agaton but are hidden by the vegetation cover. Another typhoon may trigger these incipient landslides to become full or even catastrophic landslides. Thus, it is crucial that residents living in landslide-prone areas be given proper and timely advice. 

References:

Alfonso-Dias 2019. Dissertation, University of Montpellier, France.

Forbes, K. and J. Broadhead. 2013. RAP Publication 2013/02. FAO Regional Office, Bangkok.

Zhang et al. 2019. Geoderma 333: 163-177.

Zhuang et al. 2022. Engineering Geology 298