Identification of Storm Surge Vulnerable Areas in the Philippines through Simulations of Typhoon Haiyan-Induced Storm Surge Using Tracks of Historical Typhoons

Lapidez, J.P., Suarez, J. K., Tablazon, J., Dasallas, L., Gonzalo, L.A., Santiago, J. Cabacaba, K.M., Ramos, M.M.A., Lagmay, A.M.F., Malano, V.

Department of Science and Technology, Nationwide Operational Assessment of Hazards

Abstract

Super Typhoon Haiyan entered the Philippine Area of Responsibility (PAR) 07 November 2013, causing tremendous damage to infrastructure and loss of lives mainly due to the typhoons storm surge and strong winds. Storm surges up to a height of 7 meters 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, the flagship disaster mitigation program of the Department of Science and Technology, Government of the Philippines, to undertake a study to determine the vulnerability of all Philippine coastal communities to storm surges of the same magnitude as those generatedby 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. DOST-Project NOAH used the Japan Meteorological Agency (JMA) Storm Surge Model, a numerical code that simulates and predicts storm surges spawned by tropical cyclones. Input parameters for the storm surge model include bathymetric data, storm track, central atmospheric pressure, and maximum wind speed. The simulations were made using Haiyans pressure and wind speed as the forcing parameters. The simulated storm surge height values were added to the maximum tide level obtained from WXTide, software that contains a catalogue of worldwide astronomical tides, to come up with storm tide levels. The resulting water level was used as input to FLO-2D to generate the storm tide inundation maps. One product of this study is a list of the most vulnerable coastal areas that can be used as basis for choosing priority sites for further studies to implement appropriate site-specific solutions. 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.

I Introduction

Storm surge is the abnormal rise of water over and above the predicted astronomical tide generated by the force of the wind cyclonically moving around the storm (NWS- NOAA, 2013). The specific factors affecting the height of the generated surge are the following: The storms central pressure, wind intensity, translational forward speed, storm radius, storm approach angle, coastline shape, and the local bathymetry. The resulting surging flood induced by storm surge is a major cause of casualties and damages to coastal regions. The Philippines, being an archipelagic country consisting of roughly 7,107 islands with 36, 289 kilometers of coastline, is highly susceptible to storm surges. Its low lying islands, long stretches of coastal areas, concave and gently sloping coastlines contribute to the enhancement of storm surge. The country’s geographical location also increases its exposure to storm surge hazard – It lies in the south western part of the Northwest Pacific basin which is considered to be the most active ocean basin, generating an average of 26 tropical cyclones per year (NOAA, 2006). Typhoon Haiyan is the 25th typhoon that entered the Philippine area of responsibility (PAR) in 2013. It started as a low pressure region in the West Pacific Ocean early 02 November 2013. Favorable environmental conditions prompted the atmospheric disturbance to undergo rapid intensification, upgrading the typhoon to category 5 on 07 November 2013 (NOAA, 2013). The intense wind, torrential rainfall and several meter-
high storm surge generated by the typhoon, resulted in widespread devastation in the central
Philippines. This extreme event emphasized the necessity to forecast storm surge height and inundation in the Philippine coastal regions. The study’s objective is to identify the areas in the Philippines that are most susceptible to extreme storm surges. The maximum probable storm surge height for every coastal locality is calculated by running simulations of storms using the intensity of Haiyan and tracks of tropical cyclones that entered PAR from 1948-2013. This would give an idea of the probable extent of damage if a Haiyan intensity storm hits a certain area. Once the vulnerable coastal areas are identified, appropriate site-specific solutions to storm surge hazards can be studied and planned. Outputs can also be used as a basis to develop a risk-sensitive land use plan to identify appropriate areas in building residential buildings, evacuation sites and other critical facilities. Inundation maps and hazard maps based on the worst case scenario for every area can also be used to develop a disaster response plan and evacuation scheme.

II Methodology

1. The Japan Meteorological Agency (JMA) keeps an archive of typhoon best track data. These data are publicly available and can be downloaded from their website: http://www.jma.go.jp/jma/jmaeng/jma-center/rsmc-hp-pub-eg/besttrack.html. A best track data text _le contains information about all typhoons formed in the North western pacific basin for a specific year. The pertinentinformation in the best track data that are essential to the storm surge simulation are the following: the location of the typhoon center throughout its lifetime, the central pressure and maximum sustained wind speed values, and the radii to 50 knot and 30 knot winds. For this research, the best track data from the year 1951 to 2013 were downloaded. For each typhoon, the information about the location of its center from the time of formation until the time of dissipation were extracted and were used as the basis of the tracks ofthe hypothetical typhoons used in the storm surge simulations.

2. Data about typhoon Haiyan were taken from the 2013 best track data of the Japan Meteorological Agency http://www.jma.go.jp/jma/jma-eng/jma-center/rsmc-hp-pub-eg/Besttracks/bst2013.txt. The central pressure, maximum sustained wind speed values, and the radii to 50 knot and 30 knot winds of Haiyan will be the values used for hypothetical typhoons.

3. The best track data of JMA from 1951 to 2013 was cross-referenced to the list of typhoons
that entered the Philippine area of responsibility (PAR) as recorded by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA). Only the
typhoon tracks that crossed the PAR were used in the study.

4. Hypothetical typhoons were created using the tracks of the selected typhoons and the intensity parameter of Haiyan. A total of 861 hypothetical typhoons were generated for this study.

5. Storm surge simulations for the 861 hypothetical typhoons were done using the JMA Storm Surge Model. The model was developed by the JMA to simulate and predict the heights of storm surges generated by inland and offshore tropical cyclones. The model’s numerical scheme is based in two-dimensional shallow water equations. JMA Storm Surge Model calculates wind and pressure fields using empirical distribution formula and gradient wind relation. It computes storm surges that are produced by the wind set up due to the above strong sea surface onshore winds and the inverse barometer effect associated with the sudden decrease of pressure in the atmosphere (Hasegawa, et al., 2012). Model assumes that sea levels and the static level of local surface pressures are balanced, where a difference in sea level is supposed to make inflow and outflow currents that moves as gravitational waves (Kohno, 2013). The inputs used to run the storm surge simulations are the typhoon best track data, domain bathymetry, and station files. The bathymetric data used in the simulations was the 2-min Global Gridded Elevation Data (ETOPO2) of the National Oceanic and Atmospheric Administration (NOAA). The station _le was used to specify the locations at which storm surge time series was calculated. The station file contains a list of points inside the computational domain where the storm surge will be computed. A total of 4996 points corresponding to barangays along the entire coastline of the Philippines were listed in the station file used in this study. The JMA storm surge model simulation produces storm surge maps and time series plots. Storm surge maps show the storm surge height distribution inside the computational domain for each time step of the simulation. Time series plots are also generated for each point in the station file.

6. For each of the 4996 station points, the maximum storm surge height developed by simulating all the 861 typhoons was ranked and tabulated. This result, together with the population density of areas within a 10m low elevation coastal zones (insert source), was used to identify the priority sites for the development of inundation maps and hazard maps.

7. WXTide32 was used to incorporate tide data for the study. It is a free Windows tide and current prediction program that contains a catalogue of world-wide astronomical tides. It is based on the UNIX program XTide version 1.6.2 designed by Dave Flater. It has tide data from 1970 and can predict tides through 2037 with its more than 9,500 stations world-wide.It can produce daily, monthly, and incremental tides with tide steps from 1 to 1,440 minutes. There are only 149 WXTide stations inside the Philippine area of responsibility. Tide values were computed for each of the 4996 surge points. Three tide stations were chosen to be used for interpolation for each surge point. The grouping was based on geographical proximity while maintaining that there should be no land mass obstruction between the points. Distance weighted averaging was used to compute for the interpolated tide.

8. The FLO-2D two-dimensional flood routing model was used to simulate the storm tide inundation in the selected priority sites. FLO-2D is a simple volume conservation model that uses the continuity equation and the dynamic wave momentum equation as its governing equations (FLO-2D PRO Reference Manual, 2013). The input parameters for inundation are the time series results from the JMA Storm Surge Model and the astronomical tide levels from WXTide. Airborne IfSAR-derived Digital Elevation Models (DEM) with a spatial resolution of 5 m was used to represent the topography of the study area. Since inundation starts at the shoreline, the detailed shorelines of the cities were also traced using Google Earth aerial photos. These were identified in the grid system of the model and assigned the time-stage storm tide data.

III Results and Discussion

The maximum storm surge heights for each of the 4996 points are represented in the bubble chart shown in figure 1.

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The maximum storm surge heights for the top seven provinces in table 1 are represented in the bubble charts shown in figure 2.

The distribution of the maximum storm surge heights in the capital city Metro Manila are represented in the bubble chart shown in figure 3.

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The provinces of Leyte, Iloilo, and the city Metro Manila were chosen for storm surge inundation modelling and storm surge hazard mapping. The three areas were chosen because of their potential to generate high storm surge heights and their high LECZ population density as seen in table 1. The simulated storm surge heights for the areas were added to the astronomical tide values generated using WxTide. The resulting storm tide timeseries were used as input for the inundation modelling using Flo-2D.

Iloilo
The highest storm surge height in Iloilo is produced using the track of 2008 tropical depression Rolly shown in figure 4. The resulting inundation map and hazard map is shown in figure 5 and figure 6 respectively.

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Leyte
The highest storm surge height in Leyte is produced using the track of 2013 typhoon Haiyan shown in figure 10. The resulting inundation map and hazard map is shown in figure 11 and figure 12 respectively.

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The description of the color levels used in the hazard maps are summarized in table 2.

In figure 1, it is seen that the points that produce the highest surges concentrate in the central part of the country including the entire Visayas, some parts of southern Luzon, and some parts of northern Mindanao. Further investigation in the provinces showed in figures 2 and 3, reveals that the shape and characteristics of the coast contribute to the potential to generate high surges. Shallow bays, such as in the case of Samar, Leyte, Palawan, Biliran, Camarines sur, Quezon, and Manila, are highly vulnerable to occurrence of high surges. Barrier islands, on the other hand, can provide protection as seen in the northern part of Iloilo with the southern part being covered by the neighboring island Negros.

In the inundation modelling, the flow of water is mainly controlled by the topography of the land over which the water flows. Thus, it is worthy to investigate the topographic factors that contribute to the depth and extent of the flooding. Figures 13, 14, and 15 show the flood maps with topographic elevation profiles along several transects.

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In transect A{A’ of Iloilo (figure 13), it is seen that the land elevation in the seaward direction is above 2.5 m, higher than the inland elevation of about 1.0 m. This explains why the flooding in this area is much lower compared in the areas around B{B’ and C{C’ of Iloilo. The low elevation in the seaward direction of B{B’ is reason for high flooding in the area. The land is also almost at which contributes to lengthening the extent of inundation. C{C has the worst condition. It has the lowest land elevation in the seaward direction, a at landscape, and is situated near two rivers.

Transect A{A’ of Manila (figure 14) has the lowest elevation among the three transects which is why the highest flooding occurs in this area. There is also a river directly crossing A{A’ which further adds water volume in the flooding when it overflows. There are large rivers in the north and south of B{B’ adding water volume in the area. The elevation in the landward direction of C{C’, about 2.5 m, is higher than the elevation in the landward direction of B{B’ of 2.0 m. This forces the water to flow from the area near C{C’ to B{B’. Transects A{A’,B{B’, and C{C’ show that the landscape in entire region has gentle slopes because of urbanization allowing the flood water to reach farther inland.

Land masses that extend outward in the sea such as those in transects A{A’ and C{C’ of Leyte (figure 15) are vulnerable to flooding because water from the sea can enter from several directions at once. B{B’ has a steep slope near the coast which effectively reduced the inundation extent in the area. D{D’ has relatively higher elevation but also has a at landscape, this would result to lower flood depths but with farther extent of inundation.

IV Validation

Representatives from the Japanese Society of Civil Engineers (JSCE) and Philippine Institute of Civil Engineers (PICE) conducted a joint survey on Tacloban, Leyte to gather data about the inundation depth and extent during the Haiyan flooding. The results of their survey were used to validate the simulations of this study. Their results of their survey are summarized in figure 16.

v15Figure 16: Results of the JSCE-PICE joint field survey

Comparing the survey results to the simulation results shows that there are areas that the simulation underestimated the flooding depth. This may be due to wave run-ups that the model cannot capture. There is also a discrepancy in the inundation extent which may be due
to the value of the roughness coefficient used in the inundation modelling. Land cover survey
should be conducted to correct the roughness coefficient used for modelling. These discrepancies are shown in figures 17 and 18.

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V Conclusion

Coastal areas in the central Visayas (Samar, Leyte, Iloilo, Palawan, Cebu, Negros, Bohol), southern Luzon (Bicol, Quezon, Metro Manila, Bulacan), and northeastern Mindanao (Surigao) are the most vulnerable to high storm surges. This is because these regions have the characteristic of gently sloping coasts, shallow bays and are also frequently passed by typhoons. These areas should be subjected to detailed storm surge studies to implement appropriate site-specific solutions.

The resulting storm tide inundation maps and hazard maps can be used by the local government units 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.

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