Vol. 4 (2015), pp. 1-7, ISSN 2362 7409 Debris Flow Numerical Modelling using High Resolution Digital Terrain Models of Ilocos Sur, Philippines V Realino a,b,* , F Llanes a,b , PK Ferrer a,b , M Dela Resma a,b , J Obrique a , RC Gacusan a,b , IJ Ortiz a,1 , C Quina a , DT Aquino a , RN Eco a,b , AMF Lagmay a,b a Nationwide Operational Assessment of Hazards, Department of Science and Technology, Philippines b National Institute of Geological Sciences, University of the Philippines- Diliman 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 car- ried by a finer matrix with speeds ranging from 2 to 40 kilo- meters 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 abil- ity to remove and transport large sediments such as tree trunks, vehicles, and gravel and boulders, thereby increasing its sed- iment load and significantly enhancing its erosive capabilities (Johnson and Rodine, 1984). The occurrence of debris flows is largely influenced by meteorological conditions, topographi- cal, geological, geotechnical, and hydrogeological factors (Net- tleton et al., 2005) and are triggered mainly by extreme rain- ✩ Published online on 21 July 2015 at http://blog.noah.dost.gov.ph/2015/07/21/debris-flow-numerical-modelling- using-high-resolution-digital-terrain-models-of-ilocos-sur-philippines/ * Corresponding author Email address: vcrealino@gmail.com (V Realino) fall 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 move- ment 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 and Takahashi, 1997; Nakagawa et al., 2000; Luna et al., 2011, 2012; Shieh et al., 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 of- ten used in situations with minimal morphological changes and have the advantage to attain the parameters from current de- bris flows (Wu et al., 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 and has the ability to avoid the modelling problem of not know- ing the flow regime in advance (Cetina et al., 2006). FLO-2D 1