Hazard Analysis of Landslides Triggered by Intense Rainfall in Albay Province, Philippines

Arlene Mae P. Tengonciang^1,3, Armelle Reca C. Remedio²,
Joel D. de Mesa² and May Celine T. M. Vicente²

¹National Institute of Geological Sciences, University of the Philippines,
Diliman, Quezon City 1101
²Manila Observatory, Ateneo de Manila University Campus,
Loyola Heights, Quezon City 1108
³Department of Physical Sciences, University of the Philippines,
Baguio City 2600

Abstract

Between 14 February and 3 March 2008, the combined effects of a low pressure area and the tail-end of a cold front moving from northern Mindanao to the eastern Visayas region brought severely heavy rainfall over Albay, Philippines. Numerous landslides were triggered on 21-23 February along steep mountain slopes in various portions of the province, while basins and low-lying areas were inundated with floodwaters. Within the municipality of Manito, smaller landslides occurred along steep slopes in the mid-afternoon of 22 February, while the largest slope failure took place at about 4:00 AM of 23 February, two days after the maximum amount of accumulated rainfall was recorded for that month. According to locals in Barangay Burabod, Libon, an old landslide was reactivated during the height of the intense rainfall, with cracks developing on the main body destroying a reservoir upslope of the barangay hall and cutting off the water supply to the barangay. In Barangay Abella, Ligao City, a large landslide occurred on 27 March, about a month after the intense rainfall. Minor rainfall events after the landslide transpired caused the slope to fail further, increasing the height of the main scarp. Rapid hazards assessment was conducted in two field campaigns, on 15-16 March and 19-24 April, in response to reports of property damages and minor casualties caused by the heavy rainfall. A preliminary inventory of landslide locations and characteristics was made along the major roadways adjacent to steep mountain slopes. Interviews with residents established the timing and approximate magnitude of the landslides and related phenomena, such as damage to roads, houses and other infrastructure. An analysis of weather and climate patterns, occurrence of earthquakes, regional geology and active tectonic structures, pre-landslide slope gradient, and soil characteristics helped to constrain the actual trigger and possible contributory causes of the slope failures.

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