SIMULATION OF FUTURE ANDAMAN TSUNAMI INTO STRAITS OF MALACCA BY TUNA

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1 Journal of Earthquake and Tsunami, Vol. 3, No. 2 (29) 89 1 c World Scientific Publishing Company SIMULATION OF FUTURE ANDAMAN TSUNAMI INTO STRAITS OF MALACCA BY TUNA KOH HOCK LYE,, TEH SU YEAN,KEWLEEMING and NOR AZAZI ZAKARIA River Engineering and Urban Drainage Research Centre, REDAC School of Mathematical Sciences Universiti Sains Malaysia, 118 Penang, Malaysia hlkoh@cs.usm.my Accepted 14 January 29 The Andaman tsunami that occurred on 26 December 24 has initiated and sustained keen research interest on modeling the characteristics and impacts of tsunami, with particular reference to tsunami wave heights, velocities and travel times. We have developed an in-house tsunami simulation model known as TUNA based upon the shallow water equations SWE for the purpose of simulating these tsunami characteristics. In this paper we present simulated tsunami scenarios along Malaysian coasts in the Straits of Malacca due to a potential earthquake originating in the Andaman Sea. Linear shallow water equations (LSWE) are used in the deep ocean, without the friction and advection terms to reduce computational time. On the other hand, in regions with shallow depth and over the beaches, non-linear shallow water equations (NSWE) and moving boundary are used in TUNA. Simulation results with TUNA indicate satisfactory performance when compared to simulation results from COMCOT and on-site survey results for the 24 Andaman tsunami. Finally we discuss future enhancement of TUNA to improve its performance and to extend its applications to include ecological and water quality simulations. Keywords: TUNA; COMCOT; tsunami; Straits of Malacca. 1. Introduction Prompted by a desire to develop tsunami simulation capability in an effort to build tsunami resilience, we have developed a tsunami simulation package known as TUNA [Koh et al., 27; Teh et al., 26]. This simulation package consists of three components: TUNA-GE, TUNA-M2 and TUNA-RP for the simulations of the three phases of tsunami generation, propagation and beach runup respectively. We have previously simulated the tsunami due to the 26 December 24 north Sumatra earthquake, commonly referred to as the Andaman Tsunami. Tsunami runup heights simulated by TUNA-RP manage to match runup heights surveyed soon after the 24 Andaman Tsunami, with a runup amplification factor of up to 3.3 [Koh et al., 28]. Amplification factor refers to the extent to which an incoming wave at offshore is amplified as they run up the shallow beaches and over the dry-wet land. In this paper we will demonstrate close agreement between 89

2 9 H. L. Koh et al. simulation results performed by TUNA and COMCOT [27], an abbreviation for Cornell Multi-grid Coupled Tsunami Model, to establish some measure of credibility of TUNA. For this purpose we have chosen two scenarios, in which one has a hypothetical square domain of 2 km by 2 km, while the second is an elongated rectangle of 1.5km by 5km, schematically representing the Selat Johor situated between Singapore and south Malaysia. Propagation of tsunami in deep water is simulated by TUNA-M2, which is based upon the linear shallow water equations (LSWE), in which it is assumed that the wave height η must be much smaller than the water depth H, which in turn must be much smaller than the wavelength L, i.e. the condition η H L is fulfilled. However, propagation over shallow water depths and runup along wet-dry beaches are simulated by the runup component TUNA-RP, based upon the non-linear shallow water equations (NSWE) and moving boundary. 2. TUNA: Framework and Merits For the simulation of tsunami propagation in the deep ocean, the following shallow water equation (SWE) is typically used. M t + x N t + x ( M 2 D ( MN D ) + ( MN y D ( N 2 ) + y D ) + gd η η t + M x + N y =, (1) x + gn2 D M M 7/3 2 + N 2 =, (2) ) + gd η y + gn2 D N M 7/3 2 + N 2 =. (3) Here, discharge fluxes (M,N) inthex- andy-directions are related to velocities u and v by the expressions M = u(h + η) =ud, N = v(h + η) =vd, whereh is the sea depth and η is the water elevation above mean sea level. Further g is the gravitational acceleration and n is Manning friction coefficient. This set of equations is discretized by the staggered finite difference methods, the details of which may be referred to Teh [28]. Several tsunami simulation models are currently available, including COMCOT, TUNAMI and MOST, all of which are based upon the shallow water equations described above. The concept of an in-house tsunami simulation model TUNA was motivated by the desire to develop a set of efficient tsunami simulation models that is capable of simulating all three phases of tsunami evolution, with the flexibility of future enhancement and extension [Koh et al., 27]. Upon the completion of the basic tsunami evolution model TUNA, additional submodels will be incorporated so as to further extend its applications to include other related simulations, such as sediment transport associated with tsunami. Incidentally, some tsunami simulation models incorporate bottom friction and non-linear advection term in the propagation component. These bottom friction and non-linear advection terms typically consume

3 Simulation of Future Andaman Tsunami by TUNA 91 extra computational time. TUNA-M2 has an option to exclude these two terms to improve computational efficiency. The exclusion of these terms from TUNA-M2 typically reduces computational time by at least 5%, thus improving TUNA efficiency since computational time is often a critical factor in tsunami simulation. In reality these terms in the deep ocean contribute insignificantly to the simulation results, as the magnitude is much smaller than the magnitudes of other terms such as the gravity term (fourth term in Eqs. (2) and (3)). Sensitivity analysis has confirmed the contention that these terms in the deep ocean may indeed be ignored without sacrificing accuracy. Currently we are in the process of incorporating ecological and water quality submodels into the framework of TUNA to enable the simulation of ecological and water quality scenarios in estuaries and open seas, subject to tides, storm surges and tsunami. The flexible modular framework of TUNA offers the facility to incorporate the water quality and ecological submodels of WASP [Ambrose et al., 1993] into TUNA for this purpose. Further, large tsunamis have the tendency to drastically alter salinity regimes in affected coastal regions, which might lead to vegetation succession changes, most of which are detrimental. The ability to simulate potential vegetation successions is important in any effort to rehabilitate coastal vegetations that are destroyed by mega storm surges or tsunamis. Of particular interest is the role of salinity in shaping vegetation succession and the capability to predict such successions. We have therefore developed a simulation model known as MANHAM to predict such vegetation succession in the Florida Everglades [Sternberg et al., 28; Teh et al., 28]. The built-in flexibility of TUNA framework allows future modification and enhancement to incorporate vegetation succession simulation into TUNA. Finally simulations of tsunamis and associated ecological processes consume large computational resources, constraining the capability of a normal PC. To overcome this constraint, we have modified TUNA to run on the grid parallel platform by means of MPI, in which many PCs are linked together to improve computational speed and memory, the details of which are referred elsewhere. In summary TUNA was developed with the objectives of computational efficiency, coupled with the flexibility and facility to incorporate future enhancements. 3. Numerical Tests on a Closed Square In an earlier research we have demonstrated the capability of TUNA to simulate the 24 Andaman tsunami for the affected coasts of Malaysia, particularly Penang and Langkawi. To further establish credibility of TUNA we now perform numerical tests to compare simulation results performed by TUNA and another well established model COMCOT [Liu et al., 1998]. For this purpose of numerical experiment, we choose a square domain of 2 km by 2 km with a depth H of 5 m, with solid land boundary on all sides. This choice would allow the observation of reflected waves from the solid boundary. This choice of depth implies a celerity of m/s

4 92 H. L. Koh et al. calculated by g H, withg =9.87 m/s. The initial source is a Gaussian hump with a maximum height of 1 m located at the center, and a vertical wave distribution represented by a two-dimensional Gaussian hump with standard deviations σ x of 1 m and σ y of 5 m, following the choice of Yoon [22]. Hence the initial wave has the approximate form of a positive half sine curve in the x-direction with a half wavelength of about 4 σ x or 4 m. Similarly the initial wave has an approximate half wavelength of 2m in the y-direction. To provide adequate resolution, we therefore choose the grid size of 1 m in both x- andy-directions and the corresponding time step of 1. s to ensure numerical stability. A grid size of 1 m in this case would allow 4 grids in a half sine curve in the x-direction (2 in the y-direction), thus providing adequate resolution. As expected, grid size of 5 m or 25 m has not produced significantly different simulation results. Figure 1 shows the comparison between TUNA and COMCOT computational results, consisting of snapshots of the tsunami waves at s, 225 s and 9 s, indicating good agreement between the two models. Figure 2 shows the time series of tsunami heights computed by TUNA and COMCOT at two selected locations, showing good agreement between the two models and demonstrating symmetry between the two observation locations as expected TUNA COMCOT Fig. 1. Snapshots of the tsunami waves at s, 225 s and 9 s simulated by TUNA (top row) and COMCOT (bottom row).

5 Simulation of Future Andaman Tsunami by TUNA 93 (m) TUNA-M2 COMCOT Elevation at (5, 5) km (m) TUNA-M2 COMCOT Elevation at (15, 15) km Fig. 2. Elevation simulated by TUNA and COMCOT at (5, 5) and (15, 15 ). 4. Semi-Closed Rectangle Representing Selat Johor A secondary interest in this paper is to simulate potential tsunami propagation through the Straits of Malacca into the Selat Johor located in the southern tip of Peninsular Malaysia (Fig. 3). Because the channel is narrow and long, proper boundary conditions must be imposed on the solid (north, south and west side) and the open (radiation) boundary (east side). Figure 3 shows a schematic representation of Selat Johor with a dimension of 1.5 km by 5 km and a constant depth of 1 m, with the west end close and the east end open. For this numerical experiment, 15 I E L K H G D C B A J F Fig. 3. Location of Selat Johor between Malaysia and Singapore. Location of 12 observation points (A to L) along Selat Johor.

6 94 H. L. Koh et al. we choose an initial source of maximum height of 1 m located at B, with a vertical distribution represented by a Gaussian hump with standard deviations σ x of 35 m (x-direction) and σ y of 5 m (y-direction). As explained earlier, a grid size of 5 m would allow 28 grids in the half sine curve in the x-direction (4 in the y-direction), which is deemed appropriate to provide adequate resolutions. Time step of 1.25 s is used to ensure numerical stability. However, grid sizes of 1 m and 25 m do not produce significantly different results. Figure 4 shows the wave time series within Selat Johor at selected locations simulated by TUNA and COMCOT, indicating good agreement between the two models. Figures 5 and 6 depict the flux time series in the x- andy-directions at locations G and L respectively. We further test the.3.2 (A) TUNA: W Close - E Open.3.2 (A) COMCOT: W Close - E Open (G) TUNA: W Close - E Open (c) (G) COMCOT: W Close - E Open (d) (L) TUNA: W Close - E Open (L) COMCOT: W Close - E Open Fig. 4. (e) (f) Elevation at Points A, G and L simulated by TUNA (left) and COMCOT (right).

7 Simulation of Future Andaman Tsunami by TUNA (G) TUNA: W Close - E Open (G) COMCOT: W Close - E Open M (m 2 /s) M (m 2 /s) Fig. 5. Flux M (m 2 /s)atpointg simulated by TUNA (left) and COMCOT (right). N (m 2 /s) (L) TUNA: W Close - E Open N (m 2 /s) (L) COMCOT: W Close - E Open Fig. 6. Flux N (m 2 /s)atpointl simulated by TUNA (left) and COMCOT (right)..3.2 (A) TUNA: W Open - E Open.3.2 (A) COMCOT: W Open - E Open Fig. 7. Elevation atpointa simulated by TUNA and COMCOT for Selat Johor with both western (W) and eastern (E) boundaries opened. performance of TUNA and COMCOT in simulating scenarios with various boundary conditions. Finally Figs. 7 and 8 show the tsunami wave heights at location A subject to boundary conditions with both ends open and both ends close respectively. The waveforms at several locations indicate the interaction of several waves arising from waves reflection from the solid boundary on three sides of the rectangle. Standing waves are formed at certain locations when two opposing waves meet to cancel out each other. Our intention in this numerical experiment is to verify that

8 96 H. L. Koh et al..3.2 (A) TUNA: W Close - E Close.3.2 (A) COMCOT: W Close - E Close Fig. 8. Elevation atpointa simulated by TUNA and COMCOT for Selat Johor with both western (W) and eastern (E) boundaries closed. TUNA can indeed simulate this type of situations, where waves are reflected from solid boundary and pass out of open boundary. It is not intended to use TUNA or COMCOT to simulate dispersive waves, which is beyond the scope of TUNA and COMCOT. Finally, extensive sensitivity analysis performed on the square and on Selat Johor indicates good performance of both TUNA and COMCOT. 5. Results and Discussion Following the 26 December 24 Andaman tsunami, there are concerns regarding the next earthquake that might lead to the rupture of the trenches near the Andaman Islands. Further, the orientation of the rupture causing tsunami to propagate directly towards the Straits of Malacca is of major concern to Malaysia. Having demonstrated the credibility of TUNA, we now consider a potential earthquake occurring in the Andaman Sea that could generate tsunamis that pose great risks to Malaysia. For this purpose we will perform numerous tsunami simulations based upon various Okada fault configurations and parameters. As an example we consider an earthquake with the Okada fault parameters consisting of the following: fault length = 3 km, fault width = 1 km, dip angle = 8, slip angle = 11, strike angle = 45, focal depth = 3 m and displacement = 2 m. This is considered as one of the worst-case scenarios for Malaysian coasts as the source will cause the tsunami to propagate directly into the Straits of Malacca. This earthquake will generate an initial displacement of water as shown in Fig. 9. The wave propagates eastwards towards Malaysia and Thailand as a leading depression N wave, while an opposing wave propagates westwards as a leading elevation N wave. Figure 1 shows snapshot of the wave propagation into the Straits of Malacca. The simulated wave heights at three observed locations Penang, Langkawi and (c) Phuket are shown in Fig. 11, showing leading depression N waves. The waves offshore at about 5 m depth are 1. m, 1.2 m and 4. m for Penang, Langkawi and Phuket respectively. The high waves at Phuket are partly due to the refraction of waves towards Phuket induced by the bathymetry. Although Phuket is not in the direct propagation path for this hypothetical tsunami, the waves at Phuket

9 Simulation of Future Andaman Tsunami by TUNA 97 Fig. 9. Potential tsunami source in the Andaman Sea. remain high, giving rise to concern regarding the vulnerability of Phuket to future tsunamis originating in the Andaman seas. We perform various sensitivity tests by varying the Okada fault parameters and configurations over a wide range of possible scenarios. Simulation results indicate that the maximum tsunami wave heights for Penang and Langkawi at 5 m depth may reach 2. m to 2.5 m. Based upon these wave heights at 5 m depth offshore, it is possible for the runup wave heights along beaches in Penang and Langkawi to reach 7. m to 7.5 m in the worst-case scenario. This is because the runup wave heights may be amplified by a factor of up to 3.3, as was observed in the earlier section. These wave heights far exceed the maximum surveyed and simulated beach runup heights of 3 m to 4 m at both locations for the 24 Andaman tsunami. Waves of the magnitude of 7 m to 7.5 m do pose severe risks to coastal communities in the affected regions. Hence further research is deemed essential in view of the likelihood of future earthquake-induced tsunamis severely impacting Malaysia and Thailand to an extend far worse than what was recorded in the 24 Andaman tsunami. 6. Conclusion In this paper, we have demonstrated the capability of TUNA to simulate tsunami evolution to a degree of accuracy and consistency that might be expected from models of similar level of sophistication and resolution such as COMCOT. In particular, TUNA can simulate tsunami wave reflection on solid boundary properly

10 June 9, 29 9:5 WSPC/238-JET H. L. Koh et al. Fig. 1. Snapshot of tsunami propagation into the Straits of Malacca. and free passage of waves out of the open boundary. We are therefore confident in using TUNA to develop inundation and runup maps as well as evacuation routes in northwest Malaysia for the purpose of providing tsunami mitigation measures. These mitigation measures are considered essential because of the possibility of future tsunamis that might pose severe threats to Malaysia. Similarly research on simulation of tsunami originating in the South China Sea is equally important and will be performed later by means of TUNA. We plan to incorporate water quality and ecological submodels into the framework of TUNA to extend TUNA range of applications. In particular the well-developed and fully documented model WASP

11 Simulation of Future Andaman Tsunami by TUNA (m) (m) (m) (c) Fig. 11. Simulated tsunami wave heights near Penang, Langkawi and (c) Phuket. developed by the United States Environmental Protection Agency will be incorporated into TUNA for this purpose. Finally we hope to modify, enhance and link TUNA to MANHAM to provide the capability to simulate the impact on vegetation succession due to large salinity alterations caused by tsunamis and storm surges. Acknowledgment Financial support from Grant # 35/PMATHS/ and # 11/PMATHS/ is gratefully acknowledged. The comparison between TUNA and COMCOT was possible due to the goodwill of Prof. Philip Liu of Cornell University to whom we would like to extend our deep appreciation. References Ambrose, R. B. Jr., Wool, T. A. and Martin, J. L. [1993] The Water Quality Analysis Simulation Program, WASP5 Part A: Model Documentation, Part B: The WASP5 Input Dataset. Georgia, United States Environmental Protection Agency. COMCOT, COMCOT User Manual Version 1.6, School of Civil Engineering [27] Cornell University, Ithaca, New York, USA, p. 23.

12 1 H. L. Koh et al. Koh, H. L., Teh, S. Y. and Izani, A. M. I. [27] Tsunami Mitigation Management Technology, Asia Pacific Tech. Monitor, Nov Dec 27 24(6), Special Features, pp , The United Nations Asian and Pacific Center for Transfer of Technology (UN-APCTT), India. Koh, H. L., Teh, S. Y., Liu, P. L.-F., Izani, A. M. I. and Lee, H. L. [28] Simulation of Andaman 24 tsunami for assessing impact on Malaysia, Journal of Asian Earth Sciences (in press). Liu, P. L.-F., Woo, S. B. and Cho, Y. S. [1998] Computer Programs for Tsunami Propagation and Inundation, Sponsored by National Science Foundation, p. 14. Sternberg, L., Teh, S. Y., Ewe, S., Miralles-Wilhelm, F. and DeAngelis, D. [27] Competition between hardwood hammocks and mangroves, Ecosystems 1(4), Teh, S. Y. [28] Modeling Evolution of Tsunami and Its Impact on Coastal Vegetation, Ph.D. Thesis, School of Mathematical Sciences, Universiti Sains Malaysia. Teh, S. Y., Koh, H. L. and Izani, A. M. I. [26] A model investigation on tsunami propagation in Malaysian and Thailand coastal water, Association of Engineering Education in Southeast and East Asia and the Pacific (AEESEAP), Journal of Engineering Education 31, Teh, S. Y., DeAngelis, D., Sternberg, L., Miralles-Wilhelm, F. R., Smith, T. J. and Koh, H. L. [28] A simulation model for projecting changes in salinity concentrations and species dominance in the coastal margin habitats of the everglades, Ecological Modeling 213(2), Yoon, S. B. [22] Propagation of distant tsunamis over slowly varying topography, Journal of Geophysical Research 17 (C1), American Geophysical Union,