Flood Susceptibility Assessment of Mt. Makiling, Philippines Using Two-Dimensional Meteorological and Hydrological Modelling
Richard L. Ybañez, Bernard Alan B. Racoma, Audrei Anne B. Ybañez, and Maria Ines Rosana D. Balangue-Tarriela
National Institute of Geological Sciences, University of the Philippines Diliman, Quezon City 1101 Philippines
In a data-poor, hazard-prone country like the Philippines, interpolating distant data points and computer modelling have become the go-to methods for determining the hazards that may affect an area. The absence of monitoring stations and gauges necessitates the application of modelling techniques to build on the little data available and generate reliable hazard maps. In this study – the devastating Sep 2009 Tropical Cyclone Ketsana (local name: Ondoy) event, its atmospheric characteristics, and its effects near Mt. Makiling, Laguna – is analyzed utilizing two modelling software: the Weather Research and Forecasting (WRF) model to assess the amount of rainfall, and FLO-2D to map the flood hazard areas around the volcano using the output of the WRF. A lone meteorological observation station on Mt. Makiling provided rainfall data for comparison with the results of the meteorological and hydrological models. The WRF model yielded a mean rainfall amount in the study area of 129.92 mm over 24 h for the storm against the observed rainfall amount for the same duration at 182.3 mm from the meteorological station. The flood model using the WRF data yielded minimal inundated areas, while the flood model of the observed rainfall data showed several low-lying urban areas inundated by up to 1.5 m of floodwaters. Comparison with flood data collected by responding agencies and groups after the event shows good correlation of affected areas and flood heights, with discrepancies being attributed to the swelling of Laguna de Bay because of excess runoff from other surrounding provinces – a factor that the models could not consider. Despite this, the WRF model generated from global atmospheric data and the flood model using the WRF product appears as a feasible substitute in the absence of on-site observation points and monitoring stations.
Located in the tropics, the Philippines is subject yearly to tropical cyclones (TCs) and monsoon rains. In the Philippine Area of Responsibility (PAR) – an area stretching from Taiwan to Palau in which the Philippine Atmospheric, Geophysical, and Astronomic Services Administration (PAGASA) monitors TC activity – as much as 20 TCs enter annually with seven to eight of these making landfall (Yumul et al. 2011). While the total number of TCs that pass through the Philippines has slightly decreased in recent years according to David and colleagues (2014), the current 10-year moving average number of TCs is listed at 28.4 annually. Based on this number, TC formation in the Western Pacific, and east to west movement of most of the TCs, the Philippines is at significant risk from TCs and extreme rainfall related . . . . . read more
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