**2. Prolog to disaster: geomorphologic setting and history of New Bataan**

Southeastern Mindanao is a rugged coastal range (**Figure 1**). About 35 km west of the coast, and 3 km upstream of Andap, the Mayo River drains a rugged, 36.5 km<sup>2</sup> watershed on the western slopes of the coastal range. In the Mayo watershed, many slopes are steeper than 35° and total relief is about 2320 m. Flowing northward, the Mayo River debouches through a narrow gorge to join the Kalyawan River, which flows northward along the Compostela Valley, as do other Agusan River southern tributaries.

A site 8 km below the Mayo-Kalyawan junction in the eastern Compostela Valley called "Cabinuangan" because of its many huge Binuang (*Octomeles sumatrana*) trees began to be logged in the early 1950s [2]. As the loggers rapidly expanded their road networks, immigrant farmers from Luzon and the Visayan Islands followed closely behind, planting the cleared land mainly to coconuts, but also to rice, corn, bananas, coffee, cacao, abaca, and bamboo.

numerous associated fractures in a broad zone along its length.

**Figure 1.** Physical setting of the Andap disaster. Gray area enclosed by dashes is the Mayo River watershed. All steep slopes are contoured at 50-m intervals. Below 700-m elevations, the contour interval is 20 m to better define the gentler valley surfaces. New deposits of "true" debris flows are shown in solid black; associated hyperconcentrated-flow deposits are shaded in gray. Note that the topographic contour lines from the Mayo Bridge to Andap are convex northward, defining the surface of an alluvial fan just upstream from Andap. The trace of the Mati Fault is only generalized; it has

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The Philippine government divided the public lands of Compostela Valley into formal municipal areas beginning in 1966. One covering 55,315 ha in Cabinuangan was named New Super Typhoon Bopha and the Mayo River Debris-Flow Disaster, Mindanao, Philippines… http://dx.doi.org/10.5772/intechopen.81669 87

**1. Introduction**

86 Climate Change and Global Warming

from all these gathered data.

**Bataan**

University of the Philippines in Diliman, Quezon City.

that threaten Mindanao and other subequatorial areas.

as do other Agusan River southern tributaries.

On 4 November, 2012, Super Typhoon Bopha generated a massive debris flow that devastated *barangay* (village) Andap in the Mindanao municipality of New Bataan and killed hundreds of people. In early 2013, we were designated as a field disaster-analysis team by Project NOAH (Nationwide Operational Assessment of Hazards), the disaster-assessment program of the

Prior to our field work, we gathered high-resolution optical satellite imagery for mapping out the extent of the debris flow deposits and commissioned a Light Detection and Ranging (LiDAR) survey to generate detailed topographic maps of the area. In the field, we analyzed and plotted the new deposits on our new maps. They were clearly left by a debris flow, and we determined its velocity when it hit Andap from scarring on impacted trees. Old deposits were left in the area by debris flows that occurred long before New Bataan was established. Eyewitnesses recounted the Bopha event for us in detail, and long-time residents informed us that similar events had never happened before. We analyzed and reconstructed the event

An initial report we published in 2016 [1] described the Super Typhoon, the Mayo River debris flow, and the detailed geologic reasons for it. We also discussed how and reviewed how population growth and inadequate geological analysis of settlement sites contribute to Philippine "natural" disasters. Our report discussed how climate change may be bringing more frequent major typhoons and debris flows they trigger to Mindanao and to other vulnerable subequatorial areas. We did so by examining the sparse record of tropical cyclones that made landfall on Mindanao since 1945, associated records of the Pacific El Niño-Southern

Oscillation (ENSO), and all western North Pacific tropical cyclones from 1945 to 2015.

**2. Prolog to disaster: geomorphologic setting and history of New**

and 3 km upstream of Andap, the Mayo River drains a rugged, 36.5 km<sup>2</sup>

Here, we update that evaluation with additional data from 2016 through February 2018. A positive outgrowth of this research is Project NOAH's new program that has identified more than a thousand Philippine alluvial fans and associated communities that might experience debris flows. This program already helped to mitigate debris flows on Luzon and Mindoro islands. We conclude by exploring possible protective measures for climate-related hazards

Southeastern Mindanao is a rugged coastal range (**Figure 1**). About 35 km west of the coast,

western slopes of the coastal range. In the Mayo watershed, many slopes are steeper than 35° and total relief is about 2320 m. Flowing northward, the Mayo River debouches through a narrow gorge to join the Kalyawan River, which flows northward along the Compostela Valley,

watershed on the

**Figure 1.** Physical setting of the Andap disaster. Gray area enclosed by dashes is the Mayo River watershed. All steep slopes are contoured at 50-m intervals. Below 700-m elevations, the contour interval is 20 m to better define the gentler valley surfaces. New deposits of "true" debris flows are shown in solid black; associated hyperconcentrated-flow deposits are shaded in gray. Note that the topographic contour lines from the Mayo Bridge to Andap are convex northward, defining the surface of an alluvial fan just upstream from Andap. The trace of the Mati Fault is only generalized; it has numerous associated fractures in a broad zone along its length.

A site 8 km below the Mayo-Kalyawan junction in the eastern Compostela Valley called "Cabinuangan" because of its many huge Binuang (*Octomeles sumatrana*) trees began to be logged in the early 1950s [2]. As the loggers rapidly expanded their road networks, immigrant farmers from Luzon and the Visayan Islands followed closely behind, planting the cleared land mainly to coconuts, but also to rice, corn, bananas, coffee, cacao, abaca, and bamboo.

The Philippine government divided the public lands of Compostela Valley into formal municipal areas beginning in 1966. One covering 55,315 ha in Cabinuangan was named New Bataan in 1968 because Luz Banzon-Magsaysay, a native of the Luzon province of Bataan and President Magsaysay's widow, had espoused its establishment. New Bataan was subdivided into 16 *barangays* (villages) comprising farm lots. A 154-ha area at the center of New Bataan was designated the town site and given the *barangay* name of "Cabinuangan." In 1970, 2 years after its founding, the population of New Bataan was 19,978 [3]; by 1 May, 2010, it had increased 238% to 47,470, including 10,390 in Cabinuangan and 7550 in Andap [4].

The town planners made a nice design for Cabinuangan, its streets fanning out geometrically from its central core of government and social buildings (**Figure 2A**). Unfortunately, the planners knew little about natural hazards. Even government authorities did not know that the Kalyawan River had been a conduit for ancient debris flows; as late as 2012, the official hazard map of New Bataan [5] evaluated only landslide and flood risks. This lack of geomorphologic knowledge was fatal during Bopha (**Figure 2B**).

*Barangay* Andap was established at the head of Compostela Valley on high ground 3 km upstream of Cabinuangan. That site was not recognized as an alluvial fan, a landform built up by successive debris flows. Our field work documented that the fan was built up by characteristically reverse-graded, matrix-supported debris-flow deposits of unknown but ancient age (**Figure 3**).

### **2.1. Debris flows**

Among the world's most destructive natural phenomena, debris flows are fast-moving slurries of water and rock fragments, soil, and mud [6–9]. Many debris flows (**Table 1**) [10] are associated with volcanoes [11, 12]; many others are not, including the Mayo River event. All that is required to generate a debris flow is an abundance of loose rock debris and soil and a sudden large influx of water. They can be triggered by sudden downpours such as commonly delivered by tropical cyclones, by reservoir collapses [13], or by landslides dislodged by earthquakes into streams.

The lethality and capacity for damage of a debris flow is not determined by its size alone. If its path is sparsely populated, such as at Mount St. Helens, or if the people in harm's way are familiar with the hazard, such as at Pinatubo Volcano, even large debris flows may not inflict

**Figure 3.** Debris-flow deposits in the New Bataan area. (A) Boulder in ancient reverse-graded debris-flow deposit. Wellestablished trees indicate an age of some decades prior to the settlement of the town. (B) Old debris-flow deposits underlying New Bataan—Andap high-way. Boulders and cobbles are separated from each other by a matrix of finergrained sediment, as they were while still flowing. For scale, the concrete is 15-cm thick. The coarse sediments atop the highway are new debris-flow deposits from Typhoon Bopha. (C) Boulder-rich deposits of debris flows that destroyed

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Rain on mountain slopes that falls strongly and lasts long enough will dislodge soil and loose rock into landslides. These may coalesce into debris flows, which are slurries of sediment and water that look and behave like concrete pouring out of a delivery truck. By weight, the water rarely exceeds 25%; only 10% may be 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 [21, 22]. Students of debris flows frequently say "In stream floods, the water carries the sediment; in debris flows,

While a debris flow is contained in a mountain channel, it carries large boulders with remarkable ease. In part, this is because of the high buoyancy of the dense slurry. Additionally, boulders in the flow repeatedly bounce away from the channel floor and sides up into the "central plug" of the flow near the surface, where friction with the channel is minimal and the

casualties.

the sediment carries the water."

much of the barangay, at the site of the destroyed Mayo River bridge.

**Figure 2.** New Bataan. A= Andap, Google image of Cabinuangan (the central district of New Bataan) before the debris flow. B= Southward facing three-dimensional terrain diagram of Andap and Cabinuanga after the Mayo River disaster. Red areas are boulder-rich "true debris flow; orange areas are deposits of more dilute " hyperconcentrated" flows.

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Bataan in 1968 because Luz Banzon-Magsaysay, a native of the Luzon province of Bataan and President Magsaysay's widow, had espoused its establishment. New Bataan was subdivided into 16 *barangays* (villages) comprising farm lots. A 154-ha area at the center of New Bataan was designated the town site and given the *barangay* name of "Cabinuangan." In 1970, 2 years after its founding, the population of New Bataan was 19,978 [3]; by 1 May, 2010, it had

The town planners made a nice design for Cabinuangan, its streets fanning out geometrically from its central core of government and social buildings (**Figure 2A**). Unfortunately, the planners knew little about natural hazards. Even government authorities did not know that the Kalyawan River had been a conduit for ancient debris flows; as late as 2012, the official hazard map of New Bataan [5] evaluated only landslide and flood risks. This lack of geomorphologic

*Barangay* Andap was established at the head of Compostela Valley on high ground 3 km upstream of Cabinuangan. That site was not recognized as an alluvial fan, a landform built up by successive debris flows. Our field work documented that the fan was built up by characteristically reverse-graded, matrix-supported debris-flow deposits of unknown but ancient

Among the world's most destructive natural phenomena, debris flows are fast-moving slurries of water and rock fragments, soil, and mud [6–9]. Many debris flows (**Table 1**) [10] are associated with volcanoes [11, 12]; many others are not, including the Mayo River event. All that is required to generate a debris flow is an abundance of loose rock debris and soil and a sudden large influx of water. They can be triggered by sudden downpours such as commonly delivered by tropical cyclones, by reservoir collapses [13], or by landslides dislodged

**Figure 2.** New Bataan. A= Andap, Google image of Cabinuangan (the central district of New Bataan) before the debris flow. B= Southward facing three-dimensional terrain diagram of Andap and Cabinuanga after the Mayo River disaster. Red areas are boulder-rich "true debris flow; orange areas are deposits of more dilute " hyperconcentrated" flows.

increased 238% to 47,470, including 10,390 in Cabinuangan and 7550 in Andap [4].

knowledge was fatal during Bopha (**Figure 2B**).

age (**Figure 3**).

**2.1. Debris flows**

88 Climate Change and Global Warming

by earthquakes into streams.

**Figure 3.** Debris-flow deposits in the New Bataan area. (A) Boulder in ancient reverse-graded debris-flow deposit. Wellestablished trees indicate an age of some decades prior to the settlement of the town. (B) Old debris-flow deposits underlying New Bataan—Andap high-way. Boulders and cobbles are separated from each other by a matrix of finergrained sediment, as they were while still flowing. For scale, the concrete is 15-cm thick. The coarse sediments atop the highway are new debris-flow deposits from Typhoon Bopha. (C) Boulder-rich deposits of debris flows that destroyed much of the barangay, at the site of the destroyed Mayo River bridge.

The lethality and capacity for damage of a debris flow is not determined by its size alone. If its path is sparsely populated, such as at Mount St. Helens, or if the people in harm's way are familiar with the hazard, such as at Pinatubo Volcano, even large debris flows may not inflict casualties.

Rain on mountain slopes that falls strongly and lasts long enough will dislodge soil and loose rock into landslides. These may coalesce into debris flows, which are slurries of sediment and water that look and behave like concrete pouring out of a delivery truck. By weight, the water rarely exceeds 25%; only 10% may be 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 [21, 22]. Students of debris flows frequently say "In stream floods, the water carries the sediment; in debris flows, the sediment carries the water."

While a debris flow is contained in a mountain channel, it carries large boulders with remarkable ease. In part, this is because of the high buoyancy of the dense slurry. Additionally, boulders in the flow repeatedly bounce away from the channel floor and sides up into the "central plug" of the flow near the surface, where friction with the channel is minimal and the


alluvial fan. Debris flows vary in volume by many orders of magnitude (**Table 1**), the most

An important distinguishing characteristic of true 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, as observed at New Bataan (**Figure 3A**). In addition to the buoyancy they experience from the dense slurry, the best mechanism advanced to explain reverse grading is *kinetic sieving* [7, 11, 24–26]. While flowing, shear at the base of a debris flow continuously causes temporary void spaces of different sizes to open, and particles of equivalent sizes migrate into them. Smaller voids form and are filled by smaller solid particles more frequently, and so larger boulders migrate up toward the top of the flow. Another characteristic of debris-flow deposits that distinguish them from the deposits left by normal streams, in which particles grade upward from coarse to fine, is "matrix support" (**Figure 3B**). A mixture of the finer sediment that constituted the bulk of the flow separates the larger rock fragments from each other. A useful guide for distinguishing the effects of debris flows from those of floods was published by Pierson [27].

On 23 November, 2012, a large area of convection began forming at 0.6°N latitude, 158°E longitude [28] (**Figure 4A**). Two days later, while still unusually close to the equator at 03*.*6°N, 157°E, it was categorized as a tropical depression. It was upgraded to Tropical Storm Bopha three days later on 26 November, while at 04*.*4°N, 155*.*8°E, a latitude where the Coriolis effect was too weak to quickly cause it to rotate. Only four days later, on 30 November, while Bopha

Bopha then rapidly gained in intensity. On 1 December, while at 5*.*8°N, 138*.*8°E, it had intensified into a C4 Super Typhoon. On 2 December, wind speeds were 259 km/h, those of a C5 Super Typhoon. Notably, this happened while Bopha was at 7*.*4°N, 128*.*9°E, closer to the equator than any Category 5 tropical cyclone ever had before. On 3 December, as Bopha interacted with Palau Island, it weakened temporarily into a C3 typhoon before reintensifying back to C5. On 2 December, Bopha entered the Philippine area of responsibility at 8 a.m. local

Bopha crossed the eastern Mindanao coast at about 7*.*7°N on 4 December at 0445H, the global record proximity to the equator for all C5 tropical cyclones (**Figure 4B**). Average wind speeds and gusts were 185 and 210 km/h, respectively. Many fisher folk at sea were lost, and many

Once onshore, Bopha weakened rapidly as it expended much of its energy in wreaking great havoc. Numerous deaths and severe injuries were attributed to flying trees and debris [29]; however, by far, the greatest cause of death and destruction was the Mayo River debris flow

Bopha passed through Mindanao, entered the Sulu Sea, and crossed Palawan Island to enter the West Philippine Sea. There, it reversed course and approached northern Luzon but dis-

was still at 3*.*8°N, 145*.*2°E, it did grow into a Category 1 typhoon.

time and was assigned the local name of Pablo.

that the typhoon rains generated (**Figure 4C**).

coastal dwellers were drowned.

sipated before reaching it.

and the largest more than a 100,000,000 m<sup>3</sup>

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[10].

91

frequent ones being only a 1000–100,000 m3

**3. Super Typhoon Bopha**

**Table 1.** The global record of the 10 largest debris flows, ranked by decreasing volume. Modified and updated from [10].

flow is fastest, enabling them to migrate quickly to the front of the flow. There, they become part of a moving dam of boulders, logs, and tree debris being pushed along by the flowing mass contained 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 what are called *hyperconcentrated* flows, somewhat confusingly because they carry much more sediment than do normal streams. In hyperconcentrated flows, sand, silt, and clay typically comprise up to 75% by weight. Such flows look like normal, turbid flood waters, but their velocities are much greater, typically 2–3 m/s [23]. They are too dilute to transport boulders and can transport gravel only by pushing and rolling it on the channel floor. To the rear, hyperconcentrated flows are succeeded by even more dilute, turbid flood water. In the literature, somewhat confusingly, "debris flow" sometimes refers to only a true debris-flow phase. Sometimes, however, the term means an entire hydrologic event consisting of debris-flow, hyperconcentrated, and normal stream-flow phases, as we do here in reference to the Mayo River debris flow.

When a debris flow emerges from the mountains, it spreads out, and the increased basal friction slows it down. Some of its sediment load drops out and adds volume to an *alluvial fan*, a cone-shaped feature that topographic maps show as contour lines that are convex in the downstream direction, as seen in **Figure 1**. Even after the debris flow spreads out, large boulders (**Figure 3**) continue to be transported by combined flotation, push, drag, and rolling. The hyperconcentrated and normal-flood phases may extend many kilometers beyond the alluvial fan. Debris flows vary in volume by many orders of magnitude (**Table 1**), the most frequent ones being only a 1000–100,000 m3 and the largest more than a 100,000,000 m<sup>3</sup> [10].

An important distinguishing characteristic of true 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, as observed at New Bataan (**Figure 3A**). In addition to the buoyancy they experience from the dense slurry, the best mechanism advanced to explain reverse grading is *kinetic sieving* [7, 11, 24–26]. While flowing, shear at the base of a debris flow continuously causes temporary void spaces of different sizes to open, and particles of equivalent sizes migrate into them. Smaller voids form and are filled by smaller solid particles more frequently, and so larger boulders migrate up toward the top of the flow. Another characteristic of debris-flow deposits that distinguish them from the deposits left by normal streams, in which particles grade upward from coarse to fine, is "matrix support" (**Figure 3B**). A mixture of the finer sediment that constituted the bulk of the flow separates the larger rock fragments from each other. A useful guide for distinguishing the effects of debris flows from those of floods was published by Pierson [27].
