Super Typhoon Bopha was the world's worst storm in 2012. In December of that year, its torrential rains on the southern Philippine island of Mindanao triggered an enormous debris flow in the Mayo River watershed that devastated Barangay (village) Andap in New Bataan, a municipality of Compostela Valley province. Debris flows, although among the world's most destructive natural phenomena, are remarkably misunderstood. Technically, debris flows are a type of landslide (Pierson and Costa, 1987; Cruden and Varnes, 1996; Hungr et al., 2001), but using the generic term “landslide” as a synonym for “debris flow” makes most people mistakenly think of rock masses detaching from a cliff and accumulating near its base.

Debris flows are also often mistakenly called floods, mudslides or mudflows, not only by the media, but by decision makers as well. In fact, the official descriptions of the disaster incorrectly termed and treated it as a “flash flood”, and relocation sites were initially evaluated in that context (MGB, 2012). In 2012 it was still not widely recognized that the conic-shaped alluvial fans with apices at the mouths of mountain gorges are built over long periods by rarely occurring debris flows, and are thus unsafe sites to occupy. Such a lack of understanding may have tragic consequences for communities like Andap in mountainous terrain. To address this deficiency, we review debris flows and their deposits in general, and exemplify them with a detailed description of the Mayo River event.

Beyond the huge volume and rapidity of the flow itself, human factors contributed to this catastrophe. Such events are rare in Mindanao, and New Bataan was settled much too recently for its founders and inhabitants to be familiar with super typhoons and debris flows. It is worrisome that the rapidly growing Philippine population continues to expand into increasingly disaster-prone areas, and it does so with insufficient hazard evaluation. Unregulated logging deforested the steep slopes, facilitating runoff, erosion and the landslides that fed the debris flow.

As part of Project NOAH (Nationwide Operational Assessment of Hazards), the Philippine government's disaster assessment program, we studied the Mayo River debris flow until most of our resources and attention were urgently diverted to a new major Philippine disaster event. That was the world's worst storm of 2013, Super Typhoon Haiyan in December, which generated the catastrophic storm surge that destroyed Tacloban and damaged many other municipalities on the Visayan Islands, killing thousands of people.

Many questions about the Andap disaster still await our attention; in the interim this report describes for the larger community of disaster-mitigation specialists Super Typhoon Bopha, the Andap catastrophe and its detailed geologic bases. We review the historical role that population growth and insufficiently guided settlement continue to play in generating “natural” disasters in the Philippines.

We then address the possibility that climate change will bring similar large storms and debris flows more frequently to Mindanao and to other subequatorial areas that are similarly unused to them. We present the sparse record of tropical cyclones that made landfall on Mindanao since 1945 and associated records of the Pacific El Niño–Southern Oscillation (ENSO). Our review of the literature pertinent to the question is an invitation for commentary and advice from climatologists and meteorologists to guide our thinking as we proceed.

We describe our new program, an outgrowth of the Andap disaster, that has identified over 1200 alluvial fan areas in the Philippines that are susceptible to debris flows, together with communities at risk from them. The program has already had significant success. Finally, we discuss what else might be done to protect Mindanao and other vulnerable subequatorial populations from climate-related hazards.

On 23 November 2012, a large area of convection began forming at 0.6∘ N latitude, 158∘ E longitude (NASA, 2012). While still unusually close to the Equator at 03.6∘ N, 157∘ E, 2 days later it was categorized as a tropical depression (Fig. 1a). On 26 November, while at 04.4∘ N, 155.8∘ E, it was upgraded to Tropical Storm Bopha. It was too close to the Equator for the weak Coriolis effect there to develop its rotation quickly, but on 30 November, while still at 3.8∘ N, 145.2∘ E, it was upgraded to a typhoon. Bopha intensified into a Category 4 Super Typhoon on 1 December while at 5.8∘ N, 138.8∘ E. On 2 December, it attained Category 5 wind speeds of 259 km h-1 while at 7.4∘ N, 128.9∘ E, the record proximity to the Equator for that category. As it passed south of Palau Island on 3 December, Bopha weakened into a Category 3 typhoon, and then re-intensified to Category 5. It entered the Philippine area of responsibility at 08:00 local time (LT) on 2 December and was given the local name of Pablo.

On 4 December at 04:45 LT, Bopha arrived at the eastern Mindanao coast at about 7.7∘ N (Fig. 1b), the landfall closest to the Equator for all Category 5 tropical cyclones of record. Its average wind speed and gust wind speed were 185 and 210 km h-1, respectively. Once onshore, Bopha weakened rapidly as it expended much of its energy generating great havoc. Many fisherfolk were lost at sea and many coastal dwellers were drowned. The National Disaster Risk Reduction and Management Council of the Philippine government (NDRRMC, 2012) also attributed numerous deaths and severe injuries to flying trees and debris, but by far the greatest cause of death and destruction was wreaked by the debris flow that Bopha's intense rains generated, which is described in this report (Fig. 1c). After passing through Mindanao, Bopha crossed the Sulu Sea and Palawan Island, entered the West Philippine Sea (South China Sea), and then reversed course towards northern Luzon, but dissipated before making landfall there.

On 12 February 2013, the United Nations Office for the Coordination of Humanitarian Affairs reported that while in the Philippines, Bopha killed 1146 people with 834 missing, and displaced 925 412 others. It totally or partially damaged 233 163 houses and caused USD 1.04 billion of damage; the most costly typhoon in the nation's history up to that time, only to be superseded by Super Typhoon Haiyan the following year.

Among the world's most destructive natural phenomena, debris flows are fast-moving slurries of water, rock fragments, soil and mud (Takahashi, 1981; Hutter et al., 1994; Iverson, 1997; Iverson et al., 1997). They can be triggered by sudden downpours such as those commonly delivered by tropical cyclones, reservoir collapses (Lagmay et al., 2007) or landslides dislodged by earthquakes into streams. Many debris flows (Table 1) are associated with volcanoes (Vallance, 2000; Rodolfo, 2000; Lagmay et al., 2007). Casualties can be light or even non-existent in a poorly populated area, such as Mount St. Helens, or where people are familiar with the hazard, such as with lahars at Mount Pinatubo.

When rainfall on slopes exceeds critical thresholds of intensity, duration and accumulation it dislodges soil, sediment and rock masses into landslides that may coalesce to form debris flows, which are slurries of sediment and water with the consistency of freshly mixed concrete. Water content rarely exceeds 25 % by weight and may be only 10 %, which is just enough to provide mobility. Gravel and boulders constitute more than half of the solids, and sand typically makes up about 40 %. Silt and clay normally constitute less than 10 % and remain suspended in the water (Pierson and Scott, 1985; Smith and Lowe, 1991).

While flowing in a channel, a striking debris-flow characteristic is how easily it transports large boulders, owing only in part to the buoyancy provided by the density of the slurry. Boulders repeatedly bounce up from the channel floor or away from its sides into the central near-surface “plug” of the flow where friction with the channel is minimal and flow velocity is greatest. Thus, in a mountain gorge they tend to migrate to the front of the flow, where they create a high, moving dam consisting largely of boulders, logs and tree debris.

Behind it, the moving frontal dam ponds the main flow body, which is richer in sand, silt and clay, and progressively becomes more dilute toward the rear, undergoing transitions into hyperconcentrated flows, so called because they carry much more sediment than normal streams do. Sand, silt and clay commonly comprise up to 75 % by weight of hyperconcentrated flows, which look similar to normal, turbid flood waters, but flow twice as fast or more, typically 2 to 3 m s-1 (Pierson and Scott, 1985). Having no strength, they can transport gravel only as bed load. Hyperconcentrated flows in turn are succeeded by normal, turbid stream flow. Confusingly, “debris flow” sometimes refers to only a true debris-flow phase and sometimes to an entire hydrologic event including its hyperconcentrated and normal stream-flow phases, which we do here in reference to the Mayo River debris flow.

Emerging from a mountain gorge, a debris flow spreads out, and increased basal friction slows it down. It drops some of its sediment load, adding to a conical alluvial fan, expressed on topographic maps by contour lines that are convex toward the downstream direction (Fig. 2). Even after it spreads out, it continues to transport large boulders by combined flotation, pushing, dragging and rolling. The flow may extend beyond the fan for many kilometers, especially its hyperconcentrated and normal-flood phases. Debris flows vary in volume by many orders of magnitude, from 1000 to 100 thousand m3 for the most frequent ones to more than 100 million m3 (Table 1; Jakob, 2005). Importantly, debris-flow sizes correlate positively with velocities, which range from 2 to 100 km h-1 (Pierson, 1998; Rickenmann, 1999).

An important distinguishing characteristic of debris-flow deposits is “reverse grading”: boulders tend to be smaller at the base and increase in size upwards. Large boulders commonly jut out at the top of a deposit (Fig. 3a). In addition to the buoyancy they experience from the dense slurry, the best mechanism advanced to explain reverse grading is kinetic sieving (Gallino and Pierson, 1985; Savage and Lun, 1988; Hutter et al., 1994; Vallance, 2000). While the flowing slurry is undergoing shear at its base, void spaces of different sizes continuously open and close, and particles of equivalent sizes migrate into them while they last. Smaller voids form more frequently and are filled by smaller solid particles, so larger boulders migrate up to the flow surface. Debris-flow deposits are also characteristically “matrix-supported” (Fig. 3b); the larger rock fragments are separated by a mixture of the finer sediment that constituted the bulk of the flowing slurry that carried them. Pierson (2005) has published a useful guide for distinguishing the effects of debris flows from those of floods.

Prior to our field work, we mapped out the extent of the debris flow deposits using high-resolution optical satellite imagery acquired through Sentinel Asia, the collaborative initiative between space agencies and disaster management agencies, that applies remote sensing and Web-GIS technologies to support Asian Pacific disaster management. In the images, large boulders and other coarse debris easily discerned in the main debris flow body facilitated its delineation from the hyperconcentrated-flow deposits (Fig. 4c). The only available maps were 1 : 50 000 scale maps dating from the 1950s. Therefore, we commissioned a lidar survey to generate detailed topographic maps of the affected areas for our fieldwork.

In the field, we analyzed the new deposits, ascertained that they were indeed left by a debris flow and found evidence that enabled us to determine its velocity when it hit Andap. We also found and described old deposits that confirm that debris flows had happened long before New Bataan was established. The Bopha event was described for us in detail by residents and eyewitnesses we interviewed. We asked those who have lived in New Bataan since the 1960s whether similar events had happened before; they had not. Data gathered from these surveys and interviews were used to analyze and reconstruct the event.

Upstream of New Bataan and Andap, the Mayo River drains a mountainous watershed of 36.5 km2 with a total relief of about 2320 m and with slopes commonly steeper than 35∘ (Fig. 2). The Mayo River passes northward through a narrow gorge to join the Kalyawan River, which flows in the Compostela Valley that it shares with several other tributaries of the Agusan River.

A site 8 km downstream of the Mayo junction, near the eastern edge of the Compostela Valley, was informally known as Cabinuangan after its many enormous, valuable Binuang (Octomeles sumatrana) trees. This old-growth forest drew the attention of the logging industry in the early 1950s (Ea et al., 2013). As the loggers rapidly expanded their road networks, immigrant farmers from Luzon and the Visayan Islands followed closely behind, planting the cleared land mainly with coconuts, rice, corn, bananas, coffee, cacao, abaca and bamboo.

In 1966 the government subdivided the public lands of Compostela Valley into municipal areas, including one of 55 315 ha that was further subdivided into farm lots and a 154 ha central town site in Cabinuangan. When this new municipality comprising 16 barangays was formally established by an act of Congress on 18 June 1968, it was named New Bataan in honor of Luz Banzon-Magsaysay, the widow of President Magsaysay and a native of the Luzon province of Bataan, who had lent her influence to proponents of the town. The central town retained “Cabinuangan” for its barangay name. In 1970, two years after its founding, the population of New Bataan was 19 978 (National Census and Statistics Office, 1970); by 1 May 2010 it had increased 238 % to 47 470, including 10 390 in Cabinuangan and 7550 in Andap (National Statistics Office, 2010).

Cabinuangan was laid out thoughtfully, with streets radiating out from a circular central core for government and social functions (Fig. 4a), but the founders of New Bataan were not aware of the natural hazards it faced. No one, including the government, realized that the Kalyawan River portion of Compostela Valley had served as an avenue for ancient debris flows. Indeed, debris flows were not widely understood at that time, and even the government-issued hazard map of New Bataan available in 2012 (MGB, 2009) was only concerned with landslides and floods. This lack of geomorphologic knowledge would prove fatal during Super Typhoon Bopha (Fig. 4b).

Barangay Andap was established at the head of the valley 3 km upstream of Cabinuangan, on high ground that was not recognized as an alluvial fan but is clearly expressed as such in Fig. 2 by contour lines that are convex downstream where they cross the valley. Characteristically reverse-graded, matrix-supported debris-flow deposits of unknown but ancient age built up the fan (Fig. 3).

Rain-gauge data from Maragusan municipality 17 km south of Andap are proxies for the rainfall that triggered the debris flow (Fig. 5). From midnight on 4 December until the flow occurred at 06:30 LT that morning, the Mayo River watershed above the alluvial fan received 120 mm of rain, falling as intensely as 43 mm h-1, and accumulated 4.4 million m3. These values greatly exceeded the global initiation thresholds for debris flows, including those at the Philippine volcanoes Mayon and Mount Pinatubo (Rodolfo and Arguden, 1991; Van Westen and Daag, 2005), and Taiwan (Guzzetti et al., 2008; Huang, 2013).

After the debris flow began, it was sustained until 07:00 LT by another 24 mm of torrential rainfall that peaked at 52 mm h-1 at 06:45 LT. This delivered an additional 900 000 m3 of runoff. Substantial discharge from the 17.7 km2 Mamada River watershed joined the debris flows about 450 m downstream of the Mayo Bridge; this, along with discharge from other Kalyawan River tributaries, diluted the western portions of the debris flow into hyperconcentrated flows that reached 2 km beyond Cabinuangan.

Other factors facilitated the debris flows. The rocks are extensively fractured because the watershed lies in the broad, left-lateral Philippine Fault zone of which the Mati Fault in Fig. 2 is a major splay. Its steep slopes have been largely deforested by mining and logging, which facilitated numerous landslides, both shallow and involving bedrock, that were triggered by Bopha's heavy rains. Powerful typhoon winds uprooted trees on the upper watershed, enhancing infiltration-triggered soil slips and erosion by runoff, providing additional bulk that included clay, which increases debris-flow cohesion, mobility and runout distance (Costa, 1984). Abundant, ancient and easily remobilized debris-flow deposits underlay the path that the flows took (Fig. 3b).

At about 06:30 LT, Andap resident Eva Penserga watched in horror as the 16 m high front of a full-fledged debris flow emerged from the Mayo River gorge and obliterated a 100 m long concrete bridge 1.5 km upstream of Andap, carrying away a truck bearing 30 construction workers. Shortly thereafter, people in Andap witnessed the arrival of the debris flow, which lasted only about 5 to 10 min. Unfortunately, alerts radioed the night before had directed about 200 people from outside Andap proper to seek shelter from floods at the community center where they joined many local inhabitants; 566 people were swept away, equivalent to 7.5 % of the village population counted by the 2010 census.

Amateur video footage (available at https://youtu.be/figGMlzDt0s) and the 5 km length of the debris-flow deposit indicate a flow velocity of 60 km h-1. No structures survived the main flow, but battered trees standing in the debris field 30 m from its eastern edge and 70 m upstream from the obliterated community center document slower flows there. The heights to which the flows rose up against the trees yield their velocity (Arguden and Rodolfo, 1990): assuming that all of the kinetic energy of the flow was converted to potential energy as it rose up against these obstacles, the 1.7 m run-up height h indicates a velocity v of 5.8 m s-1, or 21 km h-1, from v = (2gh)1/2. This value is only minimal because the formula considers neither channel roughness nor internal friction.

From satellite imagery and our post-Bopha lidar mapping and field measurements, the volume of the Andap debris-flow deposit is 25 to 30 million m3, ranking it among the largest ever experienced worldwide (Table 1). The deposit is 0.2–1 km wide and 0.25–9 m thick. Debris with boulders up to 16 m in diameter (Fig. 3c) covered 500 ha and buried Andap as deep as 9 m. Downstream, clast sizes decrease and the deposits thin, grading into sandy, laminated hyperconcentrated-flow deposits less than 0.5 m thick. These finer-grained deposits cover 2000 hectares and extend 8 km beyond Cabinuangan. Where these finer-grained sediments dominate, associated tree debris clogs streams and creeks. In Cabinuangan, dozens of corpses were recovered from a tangle of fallen trees and logs (Fig. 3d).

The global increase in death and damage from natural calamities may be due in part to the effects of anthropogenic climate change, but another, more likely reason is the growth of populations in high-risk areas (Huppert and Sparks, 2006). Developed nations in Europe and North America are not immune from increasing incidences of landslide disasters (Cascini et al., 2005; Di Martire et al., 2012; Lari et al., 2012). The trend, however, is especially pronounced in places that experience tropical-cyclone landfalls (Weinkle et al., 2012). Nowhere is this better exemplified than in Mindanao by the Andap disaster.

The founding of the newer Mindanao settlements including New Bataan was largely driven by the pressure of rapid population growth, well described by Dolan (1993). In 1950 the Philippine land–population ratio was about one cultivated hectare per agricultural worker; by the early 1980s the ratio had been cut in half. The 1980 census documented that 6 of the 12 Philippine provinces experiencing the fastest growth were in western, northern and southern Mindanao.

When New Bataan was settled in 1968, the annual Philippine population growth rate was 2.98 % and Filipinos numbered 36 424 000. By 2014 the population had almost tripled, to 107 668 000 (United States Census Bureau International Programs, 2014). A Reproductive Health Care congressional bill was filed in 2003, its main purpose being to provide contraception to the poor. After strenuous opposition from the clergy in this predominantly Roman Catholic country, the bill was finally passed in December 2012, coincidentally the month that Bopha arrived. The annual growth rate has dropped to 1.83 %, but that means that the country will still need to provide for another two million people in 2016, and similar numbers every year for some time to come. Among these needs, housing will be extremely difficult to find because hardly any hazard-free areas remain.

In February 2013 the office of the Philippine president organized Task Force Pablo, a multiagency group of geologists and engineers from the Mines and Geosciences Bureau (MGB) and Project NOAH, to conduct field analyses of the Andap disaster and search for safe relocation sites for the people of New Bataan and other municipalities of the province of Compostela Valley. Task Force Pablo identified 31 resettlement sites using lidar-derived digital terrain models and rainfall intensity–duration frequency data from the national weather service. The phenomenal event at Barangay Andap required special attention from us to identify relocation sites safe from future debris flows in New Bataan. The task is a daunting one; the Kalyawan floodplain is susceptible to floods and debris flows, and the valley margins and adjacent high grounds are susceptible to landslides.

A positive outgrowth of the Andap disaster is the compilation by NOAH of all alluvial fan areas in the Philippines (Aquino et al., 2014). Alluvial fans were delineated from high-resolution digital terrain models by analyzing geomorphic features, slopes, gradients and stream networks. So far, more than 1200 alluvial fans have been identified throughout the country, and communities under the threat of debris flows are being educated about them. The results can be accessed online for free in the NOAH portal at http://noah.dost.gov.ph.

In October 2015, Typhoon Koppu (Lando) generated devastating debris flows on alluvial fans in the Nueva Ecija province (Eco et al., 2015). Fortunately, communities living on those alluvial fans had been warned and evacuated. No one was killed. In December 2015, Typhoon Melor (Nona) struck Mindoro in the central Philippines, also triggering massive debris flows. Houses and buildings were buried or washed out in several communities on alluvial fans, but no one died because of timely warnings and evacuations (Llanes et al., 2016).

Future fluctuations between extreme El Niños and La Niñas pose other threats. Philippine rainfall is modulated by ENSO; El Niños bring droughts and La Niñas cause excessive rainfall (Lyon et al., 2006). During a protracted El Niño drought, rock debris accumulates on slopes that heavy rains of the succeeding La Niña wash down, causing landslides and debris flows. Additionally, excessive La Niña rainfall encourages strong forest growth that a succeeding protracted drought dries out and renders susceptible to fire.

Mindanao has 8 active (PHIVOLCS, 2008a) and 12 potentially active volcanoes (PHIVOLCS, 2008b) that are popular tourist destinations, productive geothermal areas and mining districts. Many are situated in watersheds with important agriculture and large populations. However, like Mount Pinatubo on Luzon island before its disastrous 1990 eruption, these volcanoes have not yet been fully studied or instrumentally monitored, and their populations are not used to eruptions. As Table 1 shows, some of the world's largest debris flows are lahars generated on volcanoes by intense rainfall during an eruption or even decades afterwards. Whether or not typhoons will visit Mindanao more frequently in future, any large eruption there will inevitably be succeeded by a major storm. Even without eruptions, Mindanao's larger, taller volcanoes pose serious threats, being structurally and mechanically weak (Herrero, 2014) and are thus susceptible to landslides and debris flows during exceptionally strong rainstorms.

Bopha formed abnormally close to the Equator. It developed into a Category 5 Super Typhoon and made landfall at record proximities to the Equator for all tropical cyclones of that category anywhere in the world. In only 7 h, it delivered more than 120 mm of rain to the Mayo River watershed, generating a debris flow that deposited a dry volume of 30 million m3, the world's seventh largest of record. The village of Andap was devastated and 566 of its inhabitants were killed.

Debris flows are among the most lethal of natural hazards. They are remarkably poorly recognized in the Philippines, especially in Mindanao, which lies in the southern fringe of the western North Pacific typhoon track and thus has been infrequently visited by typhoons and debris flows. This unfamiliarity exacerbated the loss of life caused by the Mayo River debris flow.

“Every health centre or school that collapses in an earthquake and every road or bridge that is washed away in a flood began as development activities” (UNDP BCPR, 2004). The people and government authorities who established New Bataan and Andap in 1968 did not know that they were building on ancient debris-flow deposits, and they were unaware of the hazardous process that produced the deposits. The lack of awareness about debris flows persisted until Bopha approached, when many people were advised to seek refuge from flooding on high ground in Andap. Even after the disaster, the government personnel initially designated to explain the tragedy and select relocation sites treated it as a “flash flood”, not as a debris flow (MGB, 2012).

The rapid growth of the Philippine population provided the impetus for the establishment of New Bataan and Andap in the late 1960s. A Reproductive Health Care congressional bill filed in 2003 was finally passed in 2012, though how successful it will be in curbing population growth remains to be seen. Meanwhile, the population continues to expand into more areas susceptible to natural hazards. Drawing upon Andap and numerous other recent disasters, the government must more rigorously assess the hazards posed to new settlement sites and infrastructure.

Western North Pacific tropical cyclone data have been archived accurately since 1945. The frequency of Mindanao landfalls has doubled since 1990, a possible indication that anthropogenic global warming is making such events more frequent. Learning whether this is true or not is obscured by irregular climatic rhythms on the ENSO timescale of a few years in the western North Pacific. Additionally, most tropical cyclones that affect Mindanao do not arrive in the main typhoon season of July through October and most are only tropical depressions, which most climatologists and meteorologists do not include as data for their models. The typhoon regimens of Mindanao and other, more densely populated low-latitude areas need more attention.

Typhoons make Philippine landfalls most frequently during La Niña episodes during the July–October main season. In Mindanao, however, they arrive during the off season from November to June. Current models suggest that extreme El Niños and La Niñas will succeed each other more frequently, a prime example of how Earth systems, kept in balance by a myriad of interacting phenomena, fluctuate strongly when disturbed. Thus, Mindanao and the Philippines as a whole should prepare their populations for more frequent hazards associated with these events, including landslides, debris flows and forest fires.

Developing countries have difficulty funding mitigation measures, and the best and least costly recourse is to enable each family to develop its own emergency plans, with accurate, accessible, understandable and timely government input. Among NOAH's mandated tasks are evaluating the numerous natural hazards that confront every region of the Philippines, educating every community about the hazards they face, and advising them on how to prepare and protect themselves when the threats materialize. Thus, as a consequence of our work on the Mayo debris flow, Project NOAH has examined detailed topographic maps for the entire Philippine archipelago and identified more than 1200 alluvial fans and associated communities that may be threatened by debris flows (Aquino et al., 2014). We have also simulated potential flow paths of debris flows on all the alluvial fans and identified communities threatened by them. This work has already helped to mitigate the effects of two major Philippine debris flows in 2015.