Debris Flow Numerical Modelling using High Resolution Digital Terrain Models of Ilocos Sur, Philippines

 

Victoriano REALINO II *1,2, Francesca LLANES1,2, Peter Khallil FERRER1,2, Marvee DELA RESMA1,2, Julius OBRIQUE1, Romer Carlo GACUSAN1,2, Iris Jill ORTIZ1,2, Celestino QUINA1, Dakila AQUINO1, Rodrigo Narod ECO1,2, and Alfredo Mahar Francisco LAGMAY1,2

1Nationwide Operational Assessment of Hazards, Department of Science and Technology, Philippines
2National Institute of Geological Sciences, University of the Philippines, Quezon City, Philippines
*Corresponding Author, E-mail: vcrealino@gmail.com Phone: (+632) 434 4012 Telefax: (+632) 929 6047

 

Abstract

Debris flows are one of the most dangerous and destructive of all mass wasting phenomena. Travelling at speeds that range from 2 to 40 kilometers per hour, debris flows occur and get deposited at the base of a mountain drainage network, the same depositional area of alluvial fans. Debris flow and alluvial fan deposits in any given place are derived from the same mountain source but differ in terms of their conditions of formation and emplacement dynamics. In order to form, debris flows need to be triggered by extreme rainfall conditions or high seismic activity in unstable mountain slopes. In the Philippines, extreme rainfall and unstable mountain slopes are common, favorable conditions for the occurrence of debris flows. As such, it is important to identify locations prone to the impacts of this type of hazard.  Due to the complexity of the debris flow process, a number of numerical models have been developed to understand and simulate the debris flow behavior. Simulations were generated using FLO-2D, a flood and debris flow routing software over a 5-m Interferometric Synthetic Aperture Radar (IfSAR) digital terrain model (DTM). Using Ilocos Sur as a prototype for the 81 provinces of the Philippines, a debris flow hazard map was created for the province to identify communities exposed to this type of hazard. This method may serve as an example for best practice in disaster prevention and mitigation as this has never been done in the Philippines and is rarely applied elsewhere as a nationwide effort. Since the Philippines is visited on average by 20 typhoons every year, mapping debris flows is crucial for use in the disaster prevention and mitigation efforts of the country. Results show that 18 municipalities with a total area of 206.17 km2 may be affected by debris flows in communities built on alluvial fans.

Keywords: debris flow, geohazard, Flo-2D, landslide, hazard maps, Ilocos Sur

 

1. Introduction

A debris flow is a rapid mass movement of sediments carried by a finer matrix with speeds ranging from 2 to 40 kilometers per hour. Debris flows usually occur along fairly steep slopes at the mouth of a mountain drainage network and are then directly deposited on alluvial fans, which are found at the base of the mountains where water drains. As these flow downstream, the mixture often behaves like viscous ‘slurries’ and are analogous to the flow of ‘wet concrete’ (Varnes, 1978; Hutchinson, 1988). Along their flow path, they have the ability to remove and transport large sediments such as tree trunks, vehicles, and gravel and boulders, thereby increasing its sediment load and significantly enhancing its erosive capabilities (Johnson and Rodine, 1984). The occurrence of debris flows is largely influenced by meteorological conditions, topographical, geological, geotechnical, and hydrogeological factors (Nettleton, 2005) and are triggered mainly by extreme rainfall condition and high seismic activity in mountainous areas (Huang and Li, 2009).

The complexity of the debris flow process gave rise to the development of several numerical models to simulate movement behavior of debris flows. These models can either be single-phase models (Coussot, 1994; Hungr, 1995; Hungr and Evans, 1997; Naef et al., 1999; Rickenmann and Koch, 1997; Whipple, 1997) or two-phase models (Brufau et al., 2000; Lai, 1991; Morris and Williams, 1996; Nakagawa et al., 1997, 2000; Quan et al., 2011, 2012; Shieh, 1996; Takahashi, 1991; Takahashi et al., 1992; Zanre and Needham, 1996). The two-phase models assume the mixture as non-homogeneous while the monophase flow models are often used in situations with minimal morphological changes and have the advantage to attain the parameters from current debris flows (Wu, 2013). In this study, numerical modelling of debris flows was done using FLO-2D, a single-phase, two-dimensional, hydrologic and hydraulic flood routing model that simulates channel flow, unconfined overland flow, and street flow over complex topography. The rheological model of FLO-2D uses a quadratic shear stress model which can characterize the continuum of flow regimes from viscous to turbulent flow (O’Brien et al., 2006) and has the ability to avoid the modelling problem of not knowing the flow regime in advance (Cetina et al., 2006). FLO-2D has been applied to several countries such as Austria, Ecuador, Italy, Mexico, South Korea, Switzerland, Taiwan, Thailand, United States and Venezuela (O’Brien et al., 2006) and has been successfully used for practical cases of debris flow simulations on previous studies (Julien and O’Brien, 1997; Garcia and Lopez, 2005; Lin et al., 2005; Cetina et al., 2006), thus the use of the numerical model. Being of high risk with debris flows, the Philippines, which is part of the Pacific Ring of Fire located in the humid tropics that has intense rainfall, seismicity and volcanic activity, is one of the countries where numerical modelling of hazards is of utmost importance.

On 4 December 2012, typhoon Bopha made landfall in the southern island of the Philippines. Classified as a Category 5 typhoon, Bopha brought intense amount of rainfall and strong winds over the provinces of Davao Oriental, Compostela Valley, Agusan del Sur, Bukidnon, and Misamis Oriental. Despite early warnings and preparations, Supertyphoon Bopha brought heavy damage especially in the province of Compostela Valley which recorded the most number of deaths totalling to 612 (NDRRMC, 2012). Intense rainfall triggered the debris flow event and buried Brgy. Andap, municipality of New Bataan, Compostela Valley (Lagmay, 2012).

In light of this unfortunate event, several measures were conducted to raise public awareness on hazards specifically on landslides and debris flows. This work describes numerical models of debris flows simulated over high resolution topography and has never been done in the Philippines. There is a nationwide effort to map out alluvial fans and debris flows and Ilocos Sur was selected as the model for the 81 provinces of the country where the same methods for identifying debris flow hazards will be applied. Since the Philippines is visited on average by 20 typhoons every year, mapping debris flows is crucial for use in the disaster prevention and mitigation efforts of the country. Such a nationwide effort using the methods described in this work is rarely applied elsewhere and may serve as an example for best practice in disaster prevention and mitigation.

 

2. Geography, Geology and Geomorphology of the Study Area

The province of Ilocos Sur is located in the northwestern coast of Luzon island, approximately 300 kilometers north of Manila, the capital of the Philippines (Figure 1). It is one of the provinces of Ilocos region bounded by Ilocos Norte in the north, Abra in the northeast, Mountain Province in the east, Benguet in the southeast, La Union in the south and the Luzon Sea and Lingayen Gulf in the west. Ilocos Sur is classified to have a Type 1 climate in which there are two distinct seasons; dry from November to April and wet during the rest of the year (PAGASA, 2011).

As part of the stratigraphic sequence of the Ilocos Region, Ilocos Sur’s oldest formation is the Suyo Schist which dates back from the Cretaceous to the late Early Pliocene to Pleistocene. Overlying the Suyo Schist is the Ilocos Peridotite which is confined in deformation zones and consists mostly of serpentinized peridotites (Pinet and Stephan, 1990). Unconformable over this layer is the Bangui Formation which formed during the Late Eocene to Late Oligocene epoch. It consists mainly of volcanic sandstones interbedded with conglomerate and mudstone (Pinet, 1990). Formations younger than the Bangui Formation are primarily sedimentary units except for the Pasaleng Quartz Diorite (Aurelio and Pena, 2010). Uplifted coral reefs cover the top and youngest rocks of the sequence (Smith, 1907).

Map of the Philippines. Ilocos Sur is the red highlighted area inside the box. The yellow dot is situated in Metro Manila, the National Capital Region of the Philippines.

Figure 1: Map of the Philippines. Ilocos Sur is the red highlighted area inside the box. The yellow dot is situated in Metro Manila, the National Capital Region of the Philippines.

Researchers around the globe have studied the geology of the Philippines extensively. Being transected by a strike-slip fault system which is separated by an opposed subduction zones, the Philippines offers a complex geologic feature. The Philippines is widely considered to have formed part of an arc system at the edge of the Philippine Sea Plate before the Pliocene (Rangin et al., 1985). Unlike any other islands of the Philippines which is formed from an arc at the southern edge of the Philippine Sea Plate before Early Miocene, Luzon Island is believed to have formed in an arc on the north side of the Celebes Sea-West Philippine Basin (Hall 1996).  The formation of the Philippines started during the end of Early Eocene or 50 million years ago. It is until 45 million years ago when rapid clockwise rotation of the Philippine Sea Plate may have been linked to the subduction of the Northern New Guinea-Pacific ridge (Hall 1996). This clockwise rotation produced the island of Luzon on which the province of Ilocos Sur is part of.

The geomorphological setting of the province of Ilocos Sur is classified to be part of the sedimentary basins of the Philippines. It belongs to the Ilocos-Central Luzon Sedimentary Basin. Filled with a sedimentary sequence thick for around 8,000 meters (Saldivar-Sali, 1978), the Ilocos-Central Luzon Basin flanks western Luzon Island along a generally North-South axis. The basin is filled with Upper Oligocene to Middle Miocene marine deposits such as wacke, shale, and conglomerate derived from the Luzon Central Cordillera Range which is located east of the basin. The Ilocos-Central Luzon Basin is structurally controlled by the main branches of the northern segment of the Philippine Fault, notably the Vigan-Aggao Fault (Malaterre, 1989; Pinet, 1990). This segment of the Philippine Fault is of a transpressional regime where movement is both strike-slip and thrust faulting. Rivers from the Luzon Central Cordillera Range drain westward to the West Philippine Sea passing through major channels which is situated in an alluvial fan. The geomorphology of the province coupled with structural control gives an ideal setting for the possibility of a debris flow event to happen. The study area for the debris flow simulation is situated at the base of a slope of mountain drainage network. Debris flows are easily generated when the loose sediments from the sedimentary basin are saturated with heavy rainfall causing them to flow down river channels.

 

3. Methodology

In this study, the single-phase FLO-2D model was used for the simulation of debris flow movement behavior. This simple volume conservation model analyzes debris flows with two governing equations described as the continuity equation and the equation of motion (dynamic wave momentum equation):

continuity_motion_equations

in which h is the flow depth, V is the depth-averaged velocity in one of the eight flow directions x, and i is the excess rainfall intensity which may be nonzero on the flow surface. The friction slope component Sf, which is based on Manning’s equation, is a function of the bed slope (So), pressure gradient, and the convective and local acceleration terms. Being a multi-direction flow model, FLO-2D computes the average flow velocity across a grid element boundary one direction at a time using the one-dimensional depth averaged channel flow equation. There are eight possible flow directions (north, south, east, west, northeast, southeast, southwest, northwest) with each velocity computation independently solved (FLO-2D PRO Reference Manual, 2013).

After initial simulations, field assessment was conducted to validate the accuracy of the initial results. Debris flow hazard maps were then calibrated based on field work data.

This work describes numerical models of debris flows simulated over high resolution topography and has never been done in the Philippines. The method is rarely applied elsewhere and may serve as an example for best practice in disaster prevention and mitigation.

 

3.1 Debris Flow Simulations

The debris flow simulation was conducted over a 5-meter digital terrain model derived from an Interferometric Synthetic Aperture Radar (IfSAR) survey. Alluvial fans were identified in the area and their corresponding catchments used in the simulations processed by FLO-2D. The Grid Developer System (GDS), a pre-processor program under FLO-2D, was utilized for preparing the basic FLO-2D data files by generating a bounded grid system over the set of IfSAR elevation points, which were then interpolated for a grid cell size of 15 m (FLO-2D GDS Manual, 2007). The 5-meter digital terrain model was resampled to 15 meters to save computing time without altering much on the DTM. Manning’s coefficients of 0.20 and 0.05 were identified for forested areas and alluvial streams, respectively. Glenwood 4 coefficients and exponents of viscosity and yield stress were the input parameters used for simulating hyperconcentrated sediment flows. These are empirical coefficients defined by laboratory experiments (O’Brien and Julien, 1988). A 100-year return period rainfall data of 602 mm within 24 hours was obtained from the archive of the Philippine weather bureau in Laoag, Ilocos Norte and was used in the numerical model runs.

The hazard map produced by the FLO-2D program have three different colour highlights – red, orange, and yellow. Red refers to areas with a high hazard level where debris flows can be more than 1 meter depth. Orange indicates a moderate hazard level where debris flow range from 0.2 to 1 meter depth while yellow refers to areas that may be inundated with debris flows less than 0.2 meters thick.

 

3.2 Field Validation

Field validation was done after the initial simulation. The source of the alluvial fan was studied by mapping the fans from its toe to its apex. A hand-held GPS device was used to obtain the exact location and elevation of the source and outcrops along the main river channel and a rangefinder was used to determine the slope angle at which the alluvial fan is located. While distinguishing debris flows with flood or stream flows, it is important to recognize the probable historical event that occurred in an area by observing the outcrop’s characteristics such as its clasts’ sphericity, sorting, and grain size. Some of the field evidences present in a stream flow includes but not limited to imbricated clasts, subrounded to rounded grains, and moderate to good sorting, while debris flow has subangular to angular clasts, extremely poorly sorting, and matrix-supported. Determination of flow type is often made on the basis of field evidence preserved at the site since high-discharged flows are seldom witnessed (Pierson, 2005). Data acquired from the field were used to determine areas with possible historical debris flows and were then compared against the highlighted red and orange areas from the simulation of the same area.

 

4. Results

Six field sites were selected to validate the debris flow simulations. These sites are located in the municipalities of San Juan (Camanggaan-Asilang boundary), Magsingal (Brgy. Maratudo), and Bantay (Brgy. Lingsat, Brgy. Taleb, and Brgy. Banaoang), and the boundary of Magsingal and Santo Domingo (Brgy. Laoingen).

The first observed alluvial fan is located at the municipality of San Juan (Camanggaan-Asilang boundary) with an outcrop on its apex described as poorly-sorted and clast-supported. It is composed of oblate to prolate, pebble to boulder clasts embedded in a very fine to coarse sand matrix. The source of the alluvial fan in Brgy. Maratudo, Magsingal (Figure 2) was also observed and it was found that the outcrop has deposits that can be divided into four strata with layer 1 located at the bottom portion. Layer 1 is well sorted and clast-supported with well rounded, pebble to cobble-sized clasts, while layer 2 shows deposits to be matrix-supported with very poorly-sorted pebble to boulder clasts. Similarities of layers 1 and 3 are observed as the latter also exhibits well rounded cobble to boulder-sized clasts in a clast-supported region. As observed in the outcrop, the topmost layer is composed of sand mixed with a few pebble clasts.

The third alluvial fan inspected is located at Brgy. Laoingen, near the boundary of the municipalities of Santo Domingo and Magsingal (Figure 3). Five different strata are observed in the outcrop with layer 1 as the bottom layer. Layers 1 to 4 exhibit a clast-supported medium with rock sizes in the pebble to boulder range. Layer 2 was also deposited in a sedimentary environment with imbricated layers nearly parallel to the horizontal. The topmost layer is a matrix-supported stratum that is poorly sorted and has subrounded to angular pebble to boulder sized clasts embedded in coarse sand to very fine sand matrix. Boulders floating in the matrix are also evident.

debris_flow_hazard_map_1

Figure 2: Debris Flow Hazard Map of the municipalities of Magsingal and San Juan and parts of Santo Domingo using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dot indicates a probable historical debris flow site located at Brgy. Maratudo, Magsingal (Refer to Figure 5-D).

 

Figure 3: Debris Flow Hazard Map of the municipality of Santo Domingo using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dot indicates a probable historical debris flow site located at Brgy. Laoingen, near the boundary of the municipalities of Santo Domingo and Magsingal (Refer to Figure 5-B).

Figure 3: Debris Flow Hazard Map of the municipality of Santo Domingo using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dot indicates a probable historical debris flow site located at Brgy. Laoingen, near the boundary of the municipalities of Santo Domingo and Magsingal (Refer to Figure 5-B).

 

Figure 4: Debris Flow Hazard Map of parts of the municipalities of Bantay, Santa, Vigan City, Santa Catalina, and Caoayan using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dot indicates a probable historical debris flow site located at Brgy. Taleb, Bantay (Refer to Figure 5-C).

Figure 4: Debris Flow Hazard Map of parts of the municipalities of Bantay, Santa, Vigan City, Santa Catalina, and Caoayan using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dot indicates a probable historical debris flow site located at Brgy. Taleb, Bantay (Refer to Figure 5-C).

Moreover, the alluvial fan located in Brgy. Lingsat, Bantay has an outcrop which can be divided into two layers: the mudflow and coralline limestone. The mudflow layer, which unconformably overlies the limestone layer, is highly matrix-supported with angular coarse sand to pebble clasts embedded in silty, clay matrix (Figure 5). Located at Brgy. Taleb, Bantay (Figure 4), the outcrop in this location has angular cobble to boulder sized clasts embedded in a sandy matrix and is moderately matrix-supported. Unfortunately, the last fan to be validated, located at Brgy. Banaoang, Bantay is not accessible for field observations.

field_validation

Figure 5: Field validation. (A) A mudflow layer which unconformably overlies the coralline limestone layer located at Brgy. Lingsat, Bantay. (B) Outcrop located in Brgy. Laoingen, near the boundary of the municipalities of Santo Domingo and Magsingal (Figure3) which shows pebble to boulder-sized clasts embedded in a coarse to very fine sand matrix. Floating boulders are also evident. (C) Topography of the outcrop located at Brgy. Taleb, Bantay (Figure 4) with silt to boulder-sized debris flow deposits. (D) Outcrop observed in Brgy. Maratudo, Magsingal (Figure 2) with deposits that can be divided into 4 layers. Layers 1 and 3 are well-sorted and clast-supported with rounded, pebble to boulder-sized clasts, whereas layer 2 shows matrix-supported deposits with very poorly-sorted pebble to boulder clasts.

 

5. Discussions

Eleven alluvial fans were identified in Ilocos Sur and some were simulated for debris flow hazards. In the maps that were produced, municipalities located in the red areas have high debris flow hazard while the orange areas indicate moderate hazard by debris flows.

Figure 2 shows two alluvial fans located at the municipalities of San Juan and Magsingal. The San Juan fan has its source of debris at Brgy. Barbar which extends to the towns of Camanggaan, Asilang, Nagsupotan, and Nagsabaran. Based on the simulation, these five barangays have high debris flow hazard while some areas on the towns of Nagsupotan and Nagsabaran have moderate debris flow hazard.

On the other hand, the 5.01-square kilometer fan located at the municipality of Magsingal has its debris source at Brgy. Maratudo. The debris path passes the towns of Macatcatud, Maas-Asin, Caraisan, Barbarit, and Bungro and is expected to have a depth of at least a meter.

Alluvial fans located at Santo Domingo and at the area which covered some parts of the municipalities of Bantay, Santa, Vigan City, Santa Catalina, and Caoayan were also observed and were simulated for debris flows. The results are shown in Figure 3 and Figure 4, respectively. The flow of the debris in Santo Domingo has its tail generally at 0.2 to 1 meter depth which mainly affects the towns of Borobor and Laoingen with the latter as its source. Brgy. Lussoc, Santo Tomas, and Lagatit may also experience debris flows according to the simulations. Figure 4 shows an alluvial fan with debris sources located at the municipalities of Bantay (Brgy. Taleb) and Santa (Brgy. Banaoang). The debris flow simulated on this fan, which is mostly red and indicates debris with greater than 1 meter depth, covers some areas in the municipalities of Bantay, Santa, Vigan City, Santa Catalina, and Caoayan.

Out of the six sites in Ilocos Sur inferred to be high risk to debris flows, three showed outcrops with historical debris flows. These are observed in Brgy. Maratudo, Magsingal, Brgy. Laoingen (near Santo Domingo-Magsingal boundary) and Brgy. Taleb, Bantay which showed primary characteristics of debris flow deposits in the field. Due to the chaotic flow in the mass wasting event, outcrops in these sites exhibit extremely poor sorting, and matrix-supported subangular to angular clasts. Poor sorting is common in debris flow deposits since the turbulent flow causes intense mixing of sediments which disallows it to settle and sort by grain size. The same flow carries large amount of fine sediments for outcrops to be observed in a matrix-supported environment which also has the ability to carry large gravel and boulders. Large boulders on top of the 3-meter and reverse-graded sedimentary layer observed in Brgy. Laoingen are also evident (Figure 5). Boulders that appear to float are common in debris flows and are supported and suspended in a finer matrix because of mechanisms related to cohesive strength, buoyancy, and structural support (Costa and Fleisher, 1984). Unlike debris flows, stream flows are characterized in outcrops as well-sorted layers and composed mainly of rounded to subrounded clasts. These clast-supported outcrops may also show sedimentary structures such as imbrications. As shown in Figure 6, the outcrops, where signs of probable historical debris flow events were observed, are all located within the red or orange areas of the simulations.

A total of 18 municipalities may be affected by debris flow based on the presence of alluvial fans. These include the municipalities of Sinait, San Juan, Santo Domingo, Santa Catalina, Cabugao, Magsingal, Bantay, Santa, Nagbukel, Santa Maria, Burgos, Vigan City, Caoayan, Narvacan, Salcedo, Santa Lucia, Santa Cruz, and Tagudin.

Figure 6: Debris Flow Hazard Map of some parts of the northern section of Ilocos Sur using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dots indicate sites where probable historical debris flows occurred.

Figure 6: Debris Flow Hazard Map of some parts of the northern section of Ilocos Sur using 5-m resolution IfSAR DTM for the FLO-2D simulation. Yellow dots indicate sites where probable historical debris flows occurred.

 

Conclusion

 Results show that a total of 18 municipalities with alluvial fans making up a total area of 206.17 square kilometers may be at risk from flood and debris flows in the province of Ilocos Sur. Historical debris flows were identified from deposits of outcrops observed during field validation and are all located within the orange or red zones of the simulations. In order to mitigate disasters caused by debris flows, generating debris flow hazard maps using high-resolution imagery is important. A more accurate and detailed simulation result due to high-resolution imageries can save a number of lives in a country highly prone to natural hazards and calamities. In the Philippines, the significance of debris flows was largely unnoticed maybe due to its unfamiliarity and the long recurrence intervals between each event. By recognizing the debris flow-prone areas in Ilocos Sur, as well as in other provinces around the Philippines, safe zones and evacuation sites can be easily identified. Maps generated by these simulations are very important in disaster prevention and mitigation. A nationwide effort to map out alluvial fans and debris flows is ongoing. Debris flow simulations are conducted over high resolution topography and are validated with field observations. The example presented is for one of the 81 provinces of the Philippines.

 

Acknowledgement

The authors would like to thank the National Mapping and Resource Information Authority (NAMRIA) for the IfSAR data, and the University of the Philippines-National Institute of Geological Sciences. This study was funded by the Department of Science and Technology’s Project NOAH (Nationwide Operational Assessment of Hazards).

 

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