Peter Khallil Ferrer1,2, Francesca Llanes1,2, Marvee dela Resma1,2, Victoriano II Realino1,2, Julius Obrique1, Romer Carlo Gacusan1,2, Iris Jill Ortiz1,2, Celestino Quina1, Dakila Aquino1, Rodrigo Narod Eco1,2, and Alfredo Mahar Francisco Lagmay, Ph.D.1,2
1Nationwide Operation Assessment of Hazards, U.P. NIGS, C.P. Garcia Ave., U.P. Diliman, Quezon City, 1101 Philippines
2National Institute of Geological Sciences, University of the Philippines, C.P. Garcia corner Velasquez street, U.P. Diliman, Quezon City. 1101 Philippines
*Corresponding author: Email address: [email protected]
On December 4, 2012, Super Typhoon Bopha left 1,067 people dead and USD 800 million worth of damage. Classified as a Category 5 typhoon by the Joint Typhoon Warning Center (JTWC), Bopha brought intense rainfall that triggered debris flows, a type of landslide with a consistency of wet-concrete that rapidly moves in a steep channel and into alluvial fans at the base of the slope of a mountain drainage network. In Barangay (village) Andap, New Bataan municipality, Compostela Valley, debris flows destroyed school buildings and an evacuation center with ~550 refugees. In response to the Bopha disaster, measures were immediately devised to improve available geohazard maps to raise public awareness on debris flows. Many rural communities in the Philippines, such as Barangay Andap, are situated at the apex of alluvial fans and in the path of potential debris flows. To address this type of hazard, alluvial fans in the Provinces of Pangasinan and Aurora were delineated and Flo-2D simulations of debris flow scenarios were made using a 5-m resolution Interferometric Synthetic Aperture Radar (IfSAR)-derived Digital Terrain Model (DTM). The study shows that a total of 135 barangays with a population of 252,405 in Pangasinan and 25 barangays with 34,495 people in Aurora are susceptible to debris flows. This information can be used for proper zoning of alluvial fans in these provinces in order to inform communities on the risk from this type of hazard.
On December 4, 2012, Super Typhoon Bopha devastated the southern region of Mindanao, Philippines. The Category 5 typhoon brought intense rainfall and strong winds as it crossed the southern part of the Philippines. The eye of the typhoon traversed the provinces of Agusan del Sur, Bukidnon, Davao Oriental, Misamis Oriental and Compostela Valley (Figure 1). Compostela Valley was the worst hit. Intense rainfall saturated the ground and triggered voluminous landslides, which developed into debris flows in New Bataan, Compostela Valley. In barangay (village) Andap, municipality of New Bataan, debris flows overwhelmed school buildings, and an evacuation center killing more than 500 evacuees and causing damage worth USD 800 million. There were 1,067 casualties from the typhoon with 612 recorded in Compostela Valley (NDRRMC, 2012).Mass wasting events in a tropical country like the Philippines are quite frequent especially during times of heavy rainfall. Considered as one of the most dangerous of all mass wasting events, debris flows are saturated, non-plastic debris that rapidly move in a steep channel (Figure 2) and into alluvial fans. (Kowalski, 2008).
Seismic activity and extreme rainfall conditions are the main triggers for debris flows in steep and mountainous areas (Huang and Li, 2009). When they occur, debris flows reach velocities up to hundreds of meters per hour and can inundate an entire town in a matter of minutes (Adegbe et al., 2013). Many are at risk from debris flow hazards and can inflict a high death toll as well as widespread damage to property.
After the Bopha disaster, measures were immediately implemented to improve available geohazard maps. Many rural communities in the Philippines are at risk from debris flows. The need to identify debris flow hazard zones in the Philippines is of urgent concern because current hazard maps of the country does not include physical models of debris flows. Using high-resolution, digital terrain models (DTMs), debris flows were simulated to produce maps that depict this type of hazard phenomenon. This work focuses on the debris flow hazard map simulations of the provinces of Pangasinan and Aurora as primary examples of a nationwide effort to map debris flows.
2. Geology and Geography of the Study Area
The provinces of Pangasinan and Aurora are found in the central portion of Luzon Island in the Philippines (Figure 3). Pangasinan is situated on the western side whereas Aurora is in the east of Luzon. The province of Pangasinan is mostly underlain by Late Pliocene sedimentary rocks such as tuffaceous sandstone, interbedded siltstone, shale, minor lenses of limestone, and conglomerate. On the other hand, Aurora is composed of igneous rocks such as andesite, diorite, and basalt.
These two provinces have many potential failure surfaces because of the presence of bedded sedimentary units and faults (Adegbe et al., 2013) (Figure 3). The main structure crossing the study areas is the Philippine fault (Figure 4), which consists of four left-stepping fault segments with a total distance of approximately 150 km. These are the San Manuel, San Jose, Digdig, and Gabaldon faults(Tsutsumi and Perez, 2013). The San Manuel and San Jose Fault traverse Pangasinan Province whereas the Dingalan Fault and Casiguran Fault traverse the province of Aurora.
Three steps were used in generating debris flow hazard maps (Figure 5). After the watersheds were identified, debris flows were simulated using FLO-2D, a flood-routing software that can model floods and debris flows. Results are then converted into hazard maps and validated in the field. The elevation data set used is a 5-meter resolution Interferometric Synthetic Aperture Radar (IfSAR) Digital Terrain Model (DTM) with 0.5 meter vertical accuracy, which was used to identify the watersheds in the study area. It was also used as terrain for flow modeling using historical rainfall with a 100-year return period (cite PAGASA). Outlines of alluvial fans were delineated by following fan-shaped contour lines.
Debris flows are simulated using the dynamic-wave momentum equation and finite-difference routing scheme (Adegbe et al., 2013), which was run in a model prepared with a Grid Developer System (GDS). A 15 x15-meter grid (FLO2D, 2007) with Manning coefficients and green-ampt infiltration attributes was used in the numerical model. Resulting debris flow simulations were converted into hazard maps with colors assigned depending on the hazard level. Red refers to a high level of hazard with a flow depth greater than 1 meter, while an orange color indicates an intermediate hazard with flow depth ranging from 0.2 to 1 meter. The debris flow hazard maps were validated in the field.
To validate the simulation results, alluvial fans in Pangasinan and Aurora were investigated for the presence of debris flow and hyperconcentrated flow deposits. In the case where debris flow deposits are observed, its exact location and distribution are measured. Field data are then used to calibrate the model simulations.
The province of Pangasinan has 7 alluvial fans. These are called the Sison, Pozzorubio, Binalonan, San Manuel, San Nicolas, Aguilar, and Mangatarem alluvial fans (Figure 6).
The extent of the Sison alluvial fan encompasses the municipalities of Sison, Pangasinan and Rosario, La Union with its apex located in Barangay Dungon. Twenty one barangays are susceptible to debris flows in the Sison alluvial fan. These are barangays Agat, Sagunto, Artacho, Cauringan, Poblacion Norte, Pobalcion Central, Poblacion Sur, Camangaan, Tara-tara, Amagbagan, Pindangan, Cabaritan, Esperanza, Binmeckeg, Tabtabungao, Puzon, Udiao, Nangcamotian, Camp One, Concepcion, and Bacani.
The alluvial fans of Pozzorubio, Binalonan, and San Manuel are located side-by-side in the central portion of Pangasinan. Nine barangays within the Pozzorubio alluvial fan are susceptible to debris flows. These are barangays Labayug, Calunetan, Villegas, Buneg, Kilo, Alipangpang, Imbalbalatong, Poblacion I and Poblacion III are susceptible to debris flow with Barangay Labayug as the location of its apex.
In the Binalonan alluvial fan, debris flow hazards are observed in Barangays Alibeng, Rosario, Camangaan, Moreno, and Santa Catalina. Out of these five villages, barangay Alibeng is located at the mouth of the mountain drainage network.
The San Manuel alluvial fan has seven barangays at risk from debris flows. These are barangays Lapalo, Mangcasuy, San Felipe Central, Santo Domingo, and Nagsaag. Lapalo located at the apex of the fan deposit.
The San Nicolas alluvial fan shares encroaches the municipalities of San Manuel and San Nicolas with its apex located in Barangay Calanutian. Other barangays that may be affected by debris flows within the fan are barangays Camindoroan, Narra, San Bonifacio, and Santo Tomas.
In the southwest portion of the province of Pangasinan at the eastern footslopes of the Zambales range is a relatively small alluvial fan in the municipality of Aguilar. Barangay Bantay, which is located at the apex of the fan is the most vulnerable to debris flows with the adjacent barangays of Bantocalining and Mueleng also at risk.
The Mangatarem alluvial fan’s apex is located in Barangay Lawak Langka. This is the most vulnerable barangay to debris flows but the downstream communities of Cabarabuan and Catarataraan can also be affected by this hazard..
Six (6) small alluvial fans were identified in the northern part of Aurora (Figure 7). One alluvial fan is located in the municipalities of Casiguran and Dinalungan, and four in the municipality of Dipaculao. The Casiguran alluvial fan has its apex located in Barangay Bianuan. Bianuan will be the first to be hit by debris flows if ever they form but can also impact the downstream Barangays of Calabgan and Esteves. On the other hand, the Dinalungan alluvial fan has its apex located in Barangay Zone II with Barangay Dibaraybay and Zone I as the locations of the path of a possible debris flow event. In the municipality of Dipaculao, Barangay Dibutunan, Ditale, Diabarasin, Toytoyan, and Puangi are susceptible to debris flows.
The total number of alluvial fans observed in the southern portion of Aurora is five. Two are located in the municipality of San Luis, one in Maria Aurora, and two in Dingalan (Figure 8). Barangay Ponglo is the location of the apex of the Maria Aurora alluvial fan. Santo Tomas and Diaman are the barangays, which can be highly affected by debris flows. In the San Luis alluvial fan, Barangays Ditumabo, Nonong Senior, and L. Pimental are susceptible to this landslide hazard. Barangays that may be affected in case of a debris flow event in the municipality of Dingalan are Dikapanikian, Caragsacan, Poblacion and Aplaya.
Fieldwork validation (Figure 9) was conducted after the initial debris flow hazard map was generated. Out of the seven alluvial fans presented in the section above, only four were successfully validated. The fans that were assessed are Sison, Pozzorubio, San Manuel, and San Nicolas. A river deposit found along the southern portion of Pangasinan was also observed.
The Sison alluvial fan’s apex (Figure 6) is located near Camp 1 bridge in Benguet, Philippines. Bued River is the tributary system that contributes to the deposition of sediments. Outcrops along the Bued River are boulder-sized and shows slight reverse grading. The scouring seen in the wallrock about 1.7 m. Southeast of the Sison alluvial fan is the Pozzorubio alluvial fan (Figure 6). Although the investigated area is relatively far from the Pozzorubio’s fan apex, coarse sand to boulder-sized rocks are observed especially in pointbars. Further southeast of Pozzorubio, after the Binalonan alluvial fan is the San Manuel alluvial fan. Brown silt-sized deposits to boulder-sized rocks are observed within the fan area. The last fan to be assessed is the San Nicolas (Figure 6) alluvial fan. It is located northeast of San Manuel. The area is directly downstream of the San Roque Dam, which is major dam in Luzon, Philippines. The apex of the fan is within the confines of the San Roque Power Corporation. All of the fans mentioned are part of the northern Pangasinan Bajada, a system of coalescing alluvial fans which form a continuous landform.
In southwest Pangasinan, a couple of minor alluvial fans were identified. These are the Aguilar and Mangatarem (Figure 6) alluvial fans. A river located downstream of one of the mini fans show deposits that are volcanic in origin. It shows poor sorting, boulder-sized imbrications, and composed of cobbles and boulders. Also, a pointbar with similar deposits is observed.
The discussion on the field assessment conducted in Aurora can be divided into three areas: Northern Aurora (Casiguran and Dinalungan), Mid-Aurora (Dipaculao, Maria Aurora, and San Luis), and Southern Aurora (Dingalan). Aurora has a total of 11 alluvial fans. Both Northern and Southern Aurora have 2 alluvial fans each while 7 alluvial fans are spread out around the municipalities of Dipaculao, Maria Aurora, and San Luis.
There are two observation points in the northernmost alluvial fan of Aurora. Located primarily at Barangay Calabgan, in the municipality of Casiguran (Figure 7), the deposits fan out in a southeast direction leading to the coast. The two observation points are relatively far apart, with one located near the foot of the fan while the other is located near the apex. The deposits found are mostly silt-sized but a few boulders were able to make its way to this area. There is no definite grading in the deposits buried in the soily matrix but the boulders can be found at the top. The matrix is unconsolidated and sandy. Meanwhile clasts are polymictic with the lithology ranging from andesite to diorite. The second alluvial fan is located in Barangay Dibaraybay, in the municipality of Dinalungan (Figure 7). There are plenty of cobble to boulder-sized deposits that are igneous in origin. An outcrop investigated in the vicinity of this observation point is comprised of three layers and is 1.5 meters in height. The topmost layer is clast-supported and poorly sorted with pebble to boulder-sized clasts. The layer in between is composed of sandy dark brown soil in which plant remains can be identified. The bottommost layer is composed primarily of boulders that are clast-supported.
In Mid-Aurora, four of the seven fans are located in Dipaculao (Figure 7). There are more cobble-sized deposits. Facing downstream, more boulders can be found. The deposits found at this point is similar in lithology as the floats found in the two fans in Northern Aurora. The next alluvial fan is located in Barangay Dibutunan. The river at this point is very narrow and is surrounded by boulder-sized deposits. Brecciated boulders have also been observed in the area. An outcrop found near the area is composed of poorly sorted pebble to boulder-sized clasts that are subangular to rounded. The outcrop is matrix-supported when it comes to the smaller deposits. The fan in Barangay Ditale in Dipaculao has deposits of igneous origin such as diorite, andesite, and breccias– similar to the igneous deposits of the northern alluvial fans. The nearest outcrop has deposits that are very poorly sorted and ranges in size from pebbles to boulders. The outcrop is matrix-supported and shows normal grading. The sub-angular to rounded clasts can be seen to have imbrication pointing downstream. The alluvial fan assessed in Maria Aurora (Figure 8) is located in Barangay Ponglo. Cobble-sized to boulder-sized igneous rocks are observed in the river deposits. The stream is braided suggesting it is a high-energy fluvial environment. Outcrop along the river reaches up to 1.5 meters in height.
In southern Aurora (Figure 8), the alluvial fan assessed is located in Barangay Aplaya, Dingalan. The observation point is relatively far from the apex around 2.1 kilometers northwest. However, the outcrop observed in this area shows poor sorting, angular to sub-rounded rocks, clast-supported igneous to sedimentary type of rocks. It is slightly imbricated and the height of the outcrop is almost one (1) meter. Figure 10 shows some of the photos taken during the field validation in Aurora.6. Conclusions
Debris flow simulations are very important in geohazard mapping. Using high-resolution imageries, rapid assessment of an area which are prone to debris flows can be done. The extent and the height of the debris are mapped in a regional scale. In the past, the old method of geohazard mapping in the Philippines did not include the simulations of debris flow prone areas. Knowing and understanding the debris flow prone areas in one place can potentially save many lives. Simulations make use of the advances in technology which provide a better way to understand debris flow hazards. It is also a rigid way of identifying such type of hazards and possible scenarios. In this case study, the province of Pangasinan and Aurora are given as examples. Results show that debris flows zones colored in red indicate a high level of hazard. Flow depth in this level is greater than one (1) meter. Orange areas indicate intermediate hazard with flow depth ranging from 0.2 to 1 meter. Most of the red-colored hazard zones in the simulations are found on the base of the slope of a mountain drainage network. The data on the number of population that may be affected by a potential debris flow event is based on the Philippine Statistics Authority. Results show that a total of 135 barangays with 252,405 people in Pangasinan and 25 barangays with 34,495 people in Aurora exposed to debris flow hazards. As of the present, debris flow simulation for the rest of the country is on-going.
The authors would like to thank the DOST Project NOAH (Nationwide Operational Assessment of Hazards) Landslide Hazard Mapping Team of the University of the Philippines (UP) National Institute of Geological Sciences (NIGS).
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