Abstract
4 Debris avalanches pose some of the most destructive geologic hazards that threaten both 5 urban and rural populations around the world. On 20 September 2018, villages in Naga City, 6 Cebu, Philippines, were devastated by a landslide that claimed 78 lives with 6 missing, joining 7 other catastrophic landslides in the country like the 1628 Iriga and the 2006 Guinsaugon debris 8 avalanches. Understanding the mechanism of these gargantuan landslides and their correct 9 nomenclature are useful for hazard prevention and mitigation. In this study, we compare the 10 deposit characteristics of the Naga City landslide with analog models and well-known historical 11 debris avalanche events/deposits in the Philippines to understand factors that led to the landslide 12 disaster in Naga City. Physical characteristics obtained from aerial and satellite imagery, ground 13 surveys, recorded footage, borehole data, and lithologic maps provided a detailed dataset for 14 analyzing the conditions that led to the mass movement and the observed characteristics of 15 the Naga landslide deposits. Comparison with analog models of hummock formation and the 16 description of historical debris avalanche deposits show striking similarities, which were used 17 to demonstrate that the Naga landslide was a Rockslide-Debris Avalanche. The equations of 18 Corominas (1996) and Dade and Huppert (1998) for long-runout rockfalls support this analysis. 19 The Naga landslide event is an example of a well-documented debris avalanche, complete with 20 all the characteristics of this type of rapid mass movement. It is consistent with the descriptions 21 found in the literature with respect to its deposit features and mechanical behavior as de.ned by 22 laboratory models and empirically-derived equations. This study helps us understand historical 23 and future long-runout debris avalanches in order for scientists and authorities to .nd ways to 24 save lives. Unfortunately, there was lack of appropriate hazards assessment on the site, which 25 had warnings in the form of the development of fractures at the headscarp of the landslide, a 26 month prior to the disaster.
Highlights
On 20 September 2018, a massive landslide devastated Naga City, Cebu
We explore the relationship of the volume with the runout length of the Naga landslide and compare the deposits with the worldwide dataset of debris avalanches (Corominas, 1996; Dade and Huppert, 1998) and existing analog models (Paguican et al, 2014)
A DJI Mavic 2 Pro drone equipped with a Hasselblad L1D-20c camera with a field of view (FOV) of about 77 deg, aperture of f/2.8-f/11 and shooting range of a minimum of 1 m was used to fill in gaps of the initial Digital Elevation Models (DEMs)
Summary
On 20 September 2018, a massive landslide devastated Naga City, Cebu. The Naga City Landslide claimed the lives of 78 villagers and injured 18 while six people remained missing and are presumed dead. The majority of the fatalities were recovered at the landslide toe about 1.2 km from the 200 meter-high headscarp. This was due to the unexpectedly large landslide volume and unusually long-runout, which surprised villagers at the landslide toe (Figure 1). Despite early warnings from developing fractures near the headscarp a month prior to the disaster, no action was taken by villagers at the landslide toe in Sitio Sindulan, Barangay Tinaan and those in an adjacent area at the base of a cliff in Barangay Naalad (Figure 1). Residents in Sitio Tagaytay, situated near the headscarp of the landslide, were evacuated by authorities the night before the mountain collapsed
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