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Llanes F.,University of the Philippines at Diliman | Ferrer P.K.,University of the Philippines at Diliman | Gacusan R.,University of the Philippines at Diliman | Realino V.,University of the Philippines at Diliman | And 3 more authors.
ACRS 2015 - 36th Asian Conference on Remote Sensing: Fostering Resilient Growth in Asia, Proceedings | Year: 2015

On 29-30 November 2006, Supertyphoon Durian caused floods and lahars that swept the southern and eastern flanks of Mount Mayon in southeastern Luzon. The weather station in Legazpi City, located at the base of the volcano, recorded a total rainfall of 495.8 mm over a period of 36 hours. Heavy and intense rains during the first 18 hours breached six dikes that formed new pathways for the water and debris to flow through. Several communities downstream were buried in thick volcanic deposits, causing most of the 1,266 fatalities reported. Pre- and post-disaster SPOT 5 images were used to map the extent of lahars. Affected areas were barangays (villages) Maipon and Tandarora in Guinobatan municipality, Sua in Camalig municipality, Budiao and Busay in Daraga municipality, Pawa and Padang in Legaspi City and San Antonio in Sto. Domingo municipality. Changes in the topography after the event will invariably change the flow dynamics of future floods and lahars. We used FLO-2D, a two-dimensional numerical modeling software for floods and debris flows, to simulate the lahars using different rainfall scenarios. Rainfall data from Legazpi City based from 5-year, 10-year, 25-year, 50-year, and 100-year return periods was simulated over 5-meter spatial resolution airborne IfSAR-derived digital terrain model acquired in 2013, more than six years after the event. The simulations using the 100-year return rainfall had an accumulated amount of 490.3 mm in a span of 24 hours and was set as the worst-case scenario for the lahar extents. The hazard maps show possible lahars in some of the old channels as well as new pathways where lahars could flow in succeeding events. Dikes that were constructed since the 2006 event are part of the topography of the 2013 IfSAR image and may also have affected the results of the simulation.

Lapidez J.P.,Nationwide Operational Assessment of Hazards | Tablazon J.,Nationwide Operational Assessment of Hazards | Dasallas L.,Nationwide Operational Assessment of Hazards | Gonzalo L.A.,Nationwide Operational Assessment of Hazards | And 7 more authors.
Natural Hazards and Earth System Sciences | Year: 2015

Super Typhoon Haiyan entered the Philippine Area of Responsibility (PAR) on 7 November 2013, causing tremendous damage to infrastructure and loss of lives mainly due to the storm surge and strong winds. Storm surges up to a height of 7 m were reported in the hardest hit areas. The threat imposed by this kind of natural calamity compelled researchers of the Nationwide Operational Assessment of Hazards (Project NOAH) which is the flagship disaster mitigation program of the Department of Science and Technology (DOST) of the Philippine government to undertake a study to determine the vulnerability of all Philippine coastal communities to storm surges of the same magnitude as those generated by Haiyan. This study calculates the maximum probable storm surge height for every coastal locality by running simulations of Haiyan-type conditions but with tracks of tropical cyclones that entered PAR from 1948-2013. One product of this study is a list of the 30 most vulnerable coastal areas that can be used as a basis for choosing priority sites for further studies to implement appropriate site-specific solutions for flood risk management. Another product is the storm tide inundation maps that the local government units can use to develop a risk-sensitive land use plan for identifying appropriate areas to build residential buildings, evacuation sites, and other critical facilities and lifelines. The maps can also be used to develop a disaster response plan and evacuation scheme. © Author(s) 2015.

Escape C.M.,Nationwide Operational Assessment of Hazards | Escape C.M.,University of the Philippines at Diliman | Rabonza M.,Nationwide Operational Assessment of Hazards | Luzon P.K.,Nationwide Operational Assessment of Hazards | And 2 more authors.
ACRS 2015 - 36th Asian Conference on Remote Sensing: Fostering Resilient Growth in Asia, Proceedings | Year: 2015

The effectiveness of terrain stability mapping through SINMAP was tested by calculating density ratios between most likely landslide initiation points (MLIP) generated from slope and SINMAP stability index thresholds, and landslide areas mapped within a few barangays in Compostela Valley, Philippines. A precise model for hazard mapping meant greater accuracy of the resulting shallow landslide hazard maps. The landslides in Compostela were wholly delineated with no differentiation between initiation, run out and depositional areas. Due to this lack of distinction, evaluations of terrain stability mapping based on these inventoried landslides could be challenging because the stability index employed in stability models assesses landslide initiation potentiality. Through the method employed by Tarolli and Tarboton (2006) to produce MLIPs, which are grid cells with the most critical stability index along a downslope path, this problem could be overcome. The density of MLIPs within and outside the mapped landslide areas quantifies the model's ability to determine zones where landslides are most likely to initiate. From the Interferometric Synthetic Aperture Radar (IfSAR) 10 m resolution DTM and SINMAP stability index, there were several trends that had been observed from the threshold results. 1) As the slope threshold increased, the percentage of terrain with slope greater than the chosen threshold that occurred within the mapped landslide area also increased. This means that there was a huge fraction of highly sloped terrain located within the mapped landslides and that the slope thresholds correctly distinguished these high sloped landslide scar regions. 2) As the SINMAP stability index threshold is reduced, the percentage of terrain with stability index less than the threshold that occurred within the landslide areas also increased. This demonstrates that the stability index threshold had been able to identify a huge fraction of areas with low stability index at the same mapped scar areas. It had also been observed that at less than or equal to 0.5 SINMAP generated stability index threshold, the ratio of density between points inside and outside the mapped landslide scars was around 2.5, which in itself is a demonstration of the effectively of the stability index in delineating landslide initiation zones. However, compared to the ratio of density inside and outside mapped landslide scars using MLIP, which was around 5.89, MLIP is obviously more effective in quantifying SINMAP's ability to discriminate locations of landslide initiation than using SINMAP stability threshold alone. These results reflect the general competence and accuracy of SINMAP as a terrain stability model and a useful means of shallow landslide hazard mapping. The precision of the results could be further improved by using newer and more localized parameters instead of nationwide generalized values.

Tapales B.J.M.,University of the Philippines at Diliman | Lagmay A.M.F.A.,Nationwide Operational Assessment of Hazards | Lagmay A.M.F.A.,University of the Philippines at Diliman | Mendoza J.,Nationwide Operational Assessment of Hazards | And 6 more authors.
ACRS 2015 - 36th Asian Conference on Remote Sensing: Fostering Resilient Growth in Asia, Proceedings | Year: 2015

Flooding has been a perennial problem in the city of Marikina. In response to this, the city has been investing in their flood disaster mitigation program in the past years. As a result, flooding in Marikina was reduced by 31% from 1992 to 2004. [1] However, these measures need to be improved so as to mitigate the effects of floods with more than 100 year return period, such as the flooding brought by tropical storm Ketsana in 2009 which generated 455mm of rains over a 24-hour period. Heavy rainfall caused the streets to be completely submerged in water, leaving at least 70 people dead in the area. In 2012, the Southwest monsoon, enhanced by a typhoon, brought massive rains with an accumulated rainfall of 472mm for 22-hours, a number greater than that which was experienced during Ketsana. At this time, the local government units were much more prepared in mitigating the risk with the use of early warning and evacuation measures, resulting to zero casualty in the area. Their urban disaster management program, however, can be further improved through the integration of high-resolution 2D flood hazard maps in the city's flood disaster management. The use of these maps in flood disaster management is essential in reducing flood-related risks. This paper discusses the importance and advantages of integrating flood maps in structural and non-structural mitigation measures in Marikina City. Flood hazard maps are essential tools in predicting the frequency and magnitude of floods in an area. An information that may be determined with the use of these maps is the locations of evacuation areas, which may be accurately positioned using high-resolution 2D flood hazard maps. This paper also discusses proposals for a more efficient exchange of information, allowing for flood simulations to be utilized in local flood disaster management programs.

Tablazon J.,Nationwide Operational Assessment of Hazards | Tablazon J.,University of the Philippines at Diliman | Caro C.V.,Nationwide Operational Assessment of Hazards | Lagmay A.M.F.,Nationwide Operational Assessment of Hazards | And 11 more authors.
Natural Hazards and Earth System Sciences | Year: 2015

A storm surge is the sudden rise of sea water over the astronomical tides, generated by an approaching storm. This event poses a major threat to the Philippine coastal areas, as manifested by Typhoon Haiyan on 8 November 2013. This hydro-meteorological hazard is one of the main reasons for the high number of casualties due to the typhoon, with 6300 deaths. It became evident that the need to develop a storm surge inundation map is of utmost importance. To develop these maps, the Nationwide Operational Assessment of Hazards under the Department of Science and Technology (DOST-Project NOAH) simulated historical tropical cyclones that entered the Philippine Area of Responsibility. The Japan Meteorological Agency storm surge model was used to simulate storm surge heights. The frequency distribution of the maximum storm surge heights was calculated using simulation results of tropical cyclones under a specific public storm warning signal (PSWS) that passed through a particular coastal area. This determines the storm surge height corresponding to a given probability of occurrence. The storm surge heights from the model were added to the maximum astronomical tide data from WXTide software. The team then created maps of inundation for a specific PSWS using the probability of exceedance derived from the frequency distribution. Buildings and other structures were assigned a probability of exceedance depending on their occupancy category, i.e., 1% probability of exceedance for critical facilities, 10% probability of exceedance for special occupancy structures, and 25% for standard occupancy and miscellaneous structures. The maps produced show the storm-surge-vulnerable areas in Metro Manila, illustrated by the flood depth of up to 4 m and extent of up to 6.5 km from the coastline. This information can help local government units in developing early warning systems, disaster preparedness and mitigation plans, vulnerability assessments, risk-sensitive land use plans, shoreline defense efforts, and coastal protection measures. These maps can also determine the best areas to build critical structures, or at least determine the level of protection of these structures should they be built in hazard areas. Moreover, these will support the local government units' mandate to raise public awareness, disseminate information about storm surge hazards, and implement appropriate countermeasures for a given PSWS. © 2015 Author(s).

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