journal of earthquake and tsunami, vol. 3, no. 2 (2009) 89–100...
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June 9, 2009 9:5 WSPC/238-JET 00047
Journal of Earthquake and Tsunami, Vol. 3, No. 2 (2009) 89–100c© World Scientific Publishing Company
SIMULATION OF FUTURE ANDAMAN TSUNAMIINTO STRAITS OF MALACCA BY TUNA
KOH HOCK LYE∗,‡, TEH SU YEAN†, KEW LEE MING†and NOR AZAZI ZAKARIA∗
∗River Engineering and Urban Drainage Research Centre, REDAC†School of Mathematical Sciences
Universiti Sains Malaysia, 11800 Penang, Malaysia‡[email protected]
Accepted 14 January 2009
The Andaman tsunami that occurred on 26 December 2004 has initiated and sustainedkeen research interest on modeling the characteristics and impacts of tsunami, with par-ticular reference to tsunami wave heights, velocities and travel times. We have developedan in-house tsunami simulation model known as TUNA based upon the shallow waterequations SWE for the purpose of simulating these tsunami characteristics. In this paperwe present simulated tsunami scenarios along Malaysian coasts in the Straits of Malaccadue to a potential earthquake originating in the Andaman Sea. Linear shallow waterequations (LSWE) are used in the deep ocean, without the friction and advection termsto reduce computational time. On the other hand, in regions with shallow depth andover the beaches, non-linear shallow water equations (NSWE) and moving boundaryare used in TUNA. Simulation results with TUNA indicate satisfactory performancewhen compared to simulation results from COMCOT and on-site survey results for the2004 Andaman tsunami. Finally we discuss future enhancement of TUNA to improveits performance and to extend its applications to include ecological and water qualitysimulations.
Keywords: TUNA; COMCOT; tsunami; Straits of Malacca.
1. Introduction
Prompted by a desire to develop tsunami simulation capability in an effort tobuild tsunami resilience, we have developed a tsunami simulation package knownas TUNA [Koh et al., 2007; Teh et al., 2006]. This simulation package consists ofthree components: TUNA-GE, TUNA-M2 and TUNA-RP for the simulations ofthe three phases of tsunami generation, propagation and beach runup respectively.We have previously simulated the tsunami due to the 26 December 2004 northSumatra earthquake, commonly referred to as the Andaman Tsunami. Tsunamirunup heights simulated by TUNA-RP manage to match runup heights surveyedsoon after the 2004 Andaman Tsunami, with a runup amplification factor of upto 3.3 [Koh et al., 2008]. Amplification factor refers to the extent to which anincoming wave at offshore is amplified as they run up the shallow beaches andover the dry-wet land. In this paper we will demonstrate close agreement between
89
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90 H. L. Koh et al.
simulation results performed by TUNA and COMCOT [2007], an abbreviation forCornell Multi-grid Coupled Tsunami Model, to establish some measure of credibil-ity of TUNA. For this purpose we have chosen two scenarios, in which one has ahypothetical square domain of 20 km by 20 km, while the second is an elongatedrectangle of 1.5 km by 50 km, schematically representing the Selat Johor situatedbetween Singapore and south Malaysia. Propagation of tsunami in deep water issimulated 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 thanthe 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 waterdepths and runup along wet-dry beaches are simulated by the runup componentTUNA-RP, based upon the non-linear shallow water equations (NSWE) and movingboundary.
2. TUNA: Framework and Merits
For the simulation of tsunami propagation in the deep ocean, the following shallowwater equation (SWE) is typically used.
∂η
∂t+
∂M
∂x+
∂N
∂y= 0, (1)
∂M
∂t+
∂
∂x
(M2
D
)+
∂
∂y
(MND
)+ gD
∂η
∂x+
gn2
D7/3M
√M2 + N2 = 0, (2)
∂N
∂t+
∂
∂x
(MND
)+
∂
∂y
(N2
D
)+ gD
∂η
∂y+
gn2
D7/3N
√M2 + N2 = 0. (3)
Here, discharge fluxes (M, N) in the x- and y-directions are related to velocities u
and v by the expressions M = u(h + η) = uD , N = v(h + η) = vD , where h isthe sea depth and η is the water elevation above mean sea level. Further g is thegravitational acceleration and n is Manning friction coefficient. This set of equationsis discretized by the staggered finite difference methods, the details of which may bereferred to Teh [2008]. Several tsunami simulation models are currently available,including COMCOT, TUNAMI and MOST, all of which are based upon the shallowwater equations described above.
The concept of an in-house tsunami simulation model TUNA was motivated bythe desire to develop a set of efficient tsunami simulation models that is capableof simulating all three phases of tsunami evolution, with the flexibility of futureenhancement and extension [Koh et al., 2007]. Upon the completion of the basictsunami evolution model TUNA, additional submodels will be incorporated so as tofurther extend its applications to include other related simulations, such as sedimenttransport associated with tsunami. Incidentally, some tsunami simulation modelsincorporate bottom friction and non-linear advection term in the propagation com-ponent. These bottom friction and non-linear advection terms typically consume
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Simulation of Future Andaman Tsunami by TUNA 91
extra computational time. TUNA-M2 has an option to exclude these two termsto improve computational efficiency. The exclusion of these terms from TUNA-M2typically reduces computational time by at least 50%, thus improving TUNA effi-ciency since computational time is often a critical factor in tsunami simulation. Inreality these terms in the deep ocean contribute insignificantly to the simulationresults, as the magnitude is much smaller than the magnitudes of other terms suchas the gravity term (fourth term in Eqs. (2) and (3)). Sensitivity analysis has con-firmed the contention that these terms in the deep ocean may indeed be ignoredwithout sacrificing accuracy.
Currently we are in the process of incorporating ecological and water qualitysubmodels into the framework of TUNA to enable the simulation of ecological andwater quality scenarios in estuaries and open seas, subject to tides, storm surgesand tsunami. The flexible modular framework of TUNA offers the facility to incor-porate the water quality and ecological submodels of WASP [Ambrose et al., 1993]into TUNA for this purpose. Further, large tsunamis have the tendency to dras-tically alter salinity regimes in affected coastal regions, which might lead to vege-tation succession changes, most of which are detrimental. The ability to simulatepotential vegetation successions is important in any effort to rehabilitate coastalvegetations that are destroyed by mega storm surges or tsunamis. Of particularinterest is the role of salinity in shaping vegetation succession and the capabil-ity to predict such successions. We have therefore developed a simulation modelknown as MANHAM to predict such vegetation succession in the Florida Ever-glades [Sternberg et al., 2008; Teh et al., 2008]. The built-in flexibility of TUNAframework allows future modification and enhancement to incorporate vegetationsuccession simulation into TUNA. Finally simulations of tsunamis and associatedecological processes consume large computational resources, constraining the capa-bility of a normal PC. To overcome this constraint, we have modified TUNA torun on the grid parallel platform by means of MPI, in which many PCs are linkedtogether to improve computational speed and memory, the details of which arereferred elsewhere. In summary TUNA was developed with the objectives of com-putational efficiency, coupled with the flexibility and facility to incorporate futureenhancements.
3. Numerical Tests on a Closed Square
In an earlier research we have demonstrated the capability of TUNA to simulatethe 2004 Andaman tsunami for the affected coasts of Malaysia, particularly Penangand Langkawi. To further establish credibility of TUNA we now perform numericaltests to compare simulation results performed by TUNA and another well estab-lished model COMCOT [Liu et al., 1998]. For this purpose of numerical experiment,we choose a square domain of 20km by 20 km with a depth H of 50m, with solidland boundary on all sides. This choice would allow the observation of reflectedwaves from the solid boundary. This choice of depth implies a celerity of 22.14m/s
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92 H. L. Koh et al.
calculated by√
g · H , with g = 9.807m/s. The initial source is a Gaussian humpwith a maximum height of 1m located at the center, and a vertical wave distri-bution represented by a two-dimensional Gaussian hump with standard deviationsσx of 1000m and σy of 5000m, following the choice of Yoon [2002]. Hence the ini-tial wave has the approximate form of a positive half sine curve in the x-directionwith a half wavelength of about 4× σx or 4000m. Similarly the initial wave has anapproximate half wavelength of 20 000m in the y-direction. To provide adequateresolution, we therefore choose the grid size of 100m in both x- and y-directionsand the corresponding time step of 1.0 s to ensure numerical stability. A grid size of100m in this case would allow 40 grids in a half sine curve in the x-direction (200 inthe y-direction), thus providing adequate resolution. As expected, grid size of 50mor 25m has not produced significantly different simulation results. Figure 1 showsthe comparison between TUNA and COMCOT computational results, consistingof snapshots of the tsunami waves at 0 s, 225 s and 900 s, indicating good agreementbetween the two models. Figure 2 shows the time series of tsunami heights com-puted by TUNA and COMCOT at two selected locations, showing good agreementbetween the two models and demonstrating symmetry between the two observationlocations as expected.
0
2
4
6
8
10
12
14
16
18
20TUNA
0 2 4 6 8 10 12 14 16 180
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 20
COMCOT
-0.050.10.250.40.550.70.85 1 -0.12-0.075-0.030.059 0.23 0.41 -0.66 -0.29 -0.021 0.12 0.38
Fig. 1. Snapshots of the tsunami waves at 0 s, 225 s and 900 s simulated by TUNA (top row) andCOMCOT (bottom row).
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Simulation of Future Andaman Tsunami by TUNA 93
Elevation at (5, 5) km
-0.05
0
0.05
0.1
0.15
0.2
0.25
0 0.02 0.04 0.06 0.08 0.1 0.12
Time (h)
(m
)
TUNA-M2
COMCOT
Elevation at (15, 15) km
-0.05
0
0.05
0.1
0.15
0.2
0.25
0 0.02 0.04 0.06 0.08 0.1 0.12Time (h)
(m
)
TUNA-M2
COMCOT
Fig. 2. Elevation η (m) simulated by TUNA and COMCOT at (5000, 5000) and (15 000, 15 000).
4. Semi-Closed Rectangle Representing Selat Johor
A secondary interest in this paper is to simulate potential tsunami propagationthrough the Straits of Malacca into the Selat Johor located in the southern tip ofPeninsular Malaysia (Fig. 3(a)). Because the channel is narrow and long, properboundary conditions must be imposed on the solid (north, south and west side) andthe open (radiation) boundary (east side). Figure 3(b) shows a schematic represen-tation of Selat Johor with a dimension of 1.5 km by 50 km and a constant depth of10m, with the west end close and the east end open. For this numerical experiment,
(a)
0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000
0
1500ABDE
FGHIJ
KL C
(b)
Fig. 3. (a) Location of Selat Johor between Malaysia and Singapore. (b) Location of 12 obser-vation points (A to L) along Selat Johor.
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94 H. L. Koh et al.
we choose an initial source of maximum height of 1m located at B, with a verticaldistribution represented by a Gaussian hump with standard deviations σx of 350m(x-direction) and σy of 500m (y-direction). As explained earlier, a grid size of 50mwould allow 28 grids in the half sine curve in the x-direction (40 in the y-direction),which is deemed appropriate to provide adequate resolutions. Time step of 1.25 sis used to ensure numerical stability. However, grid sizes of 100m and 25m do notproduce significantly different results. Figure 4 shows the wave time series withinSelat Johor at selected locations simulated by TUNA and COMCOT, indicatinggood agreement between the two models. Figures 5 and 6 depict the flux time seriesin the x- and y-directions at locations G and L respectively. We further test the
(A) TUNA: W Close - E Open
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(A) COMCOT: W Close - E Open
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(a) (b)
(G) TUNA: W Close - E Open
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(G) COMCOT: W Close - E Open
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(c) (d)
(L) TUNA: W Close - E Open
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(L) COMCOT: W Close - E Open
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1 2 3 4 5 6 7Time (h)
η (
m)
(e) (f)
Fig. 4. Elevation η (m) at Points A, G and L simulated by TUNA (left) and COMCOT (right).
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Simulation of Future Andaman Tsunami by TUNA 95
(G) TUNA: W Close - E Open
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2 3 4 5 6 7
Time (h)
M (
m2/s
)
(G) COMCOT: W Close - E Open
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2 3 4 5 6 7
Time (h)
M (
m2/s
)
(a) (b)
Fig. 5. Flux M (m2/s) at Point G simulated by TUNA (left) and COMCOT (right).
(L) TUNA: W Close - E Open
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0 1 2 3 4 5 6 7
Time (h)
N (
m2/s
)
(L) COMCOT: W Close - E Open
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0 1 2 3 4 5 6 7
Time (h)
N (
m2/s
)
(a) (b)
Fig. 6. Flux N (m2/s) at Point L simulated by TUNA (left) and COMCOT (right).
(A) TUNA: W Open - E Open
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(A) COMCOT: W Open - E Open
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6 7
Time (h)
η (
m)
(a) (b)
Fig. 7. Elevation η (m) at Point A simulated by (a) TUNA and (b) COMCOT for Selat Johorwith both western (W) and eastern (E) boundaries opened.
performance of TUNA and COMCOT in simulating scenarios with various bound-ary 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 respec-tively. The waveforms at several locations indicate the interaction of several wavesarising 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 tocancel out each other. Our intention in this numerical experiment is to verify that
June 9, 2009 9:5 WSPC/238-JET 00047
96 H. L. Koh et al.
(A) TUNA: W Close - E Close
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6 7
Time (h)
η (
m)
7
(A) COMCOT: W Close - E Close
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 1 2 3 4 5 6
Time (h)
η (
m)
(a) (b)
Fig. 8. Elevation η (m) at Point A simulated by (a) TUNA and (b) COMCOT for Selat Johorwith both western (W) and eastern (E) boundaries closed.
TUNA can indeed simulate this type of situations, where waves are reflected fromsolid boundary and pass out of open boundary. It is not intended to use TUNA orCOMCOT to simulate dispersive waves, which is beyond the scope of TUNA andCOMCOT. Finally, extensive sensitivity analysis performed on the square and onSelat Johor indicates good performance of both TUNA and COMCOT.
5. Results and Discussion
Following the 26 December 2004 Andaman tsunami, there are concerns regard-ing the next earthquake that might lead to the rupture of the trenches near theAndaman Islands. Further, the orientation of the rupture causing tsunami to prop-agate directly towards the Straits of Malacca is of major concern to Malaysia.Having demonstrated the credibility of TUNA, we now consider a potential earth-quake occurring in the Andaman Sea that could generate tsunamis that pose greatrisks to Malaysia. For this purpose we will perform numerous tsunami simulationsbased upon various Okada fault configurations and parameters. As an example weconsider an earthquake with the Okada fault parameters consisting of the following:fault length = 300km, fault width = 100 km, dip angle = 8◦, slip angle = 110◦,strike angle = 45◦, focal depth = 30 000m and displacement = 20m. This is con-sidered as one of the worst-case scenarios for Malaysian coasts as the source willcause the tsunami to propagate directly into the Straits of Malacca. This earth-quake will generate an initial displacement of water as shown in Fig. 9. The wavepropagates eastwards towards Malaysia and Thailand as a leading depression N
wave, while an opposing wave propagates westwards as a leading elevation N wave.Figure 10 shows snapshot of the wave propagation into the Straits of Malacca. Thesimulated wave heights at three observed locations (a) Penang, (b) Langkawi and(c) Phuket are shown in Fig. 11, showing leading depression N waves. The wavesoffshore at about 50m depth are 1.0m, 1.2m and 4.0m for Penang, Langkawi andPhuket respectively. The high waves at Phuket are partly due to the refractionof waves towards Phuket induced by the bathymetry. Although Phuket is not inthe direct propagation path for this hypothetical tsunami, the waves at Phuket
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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 futuretsunamis originating in the Andaman seas. We perform various sensitivity tests byvarying the Okada fault parameters and configurations over a wide range of possi-ble scenarios. Simulation results indicate that the maximum tsunami wave heightsfor Penang and Langkawi at 50m depth may reach 2.0m to 2.5m. Based uponthese wave heights at 50m depth offshore, it is possible for the runup wave heightsalong beaches in Penang and Langkawi to reach 7.0m to 7.5m in the worst-casescenario. This is because the runup wave heights may be amplified by a factor ofup to 3.3, as was observed in the earlier section. These wave heights far exceedthe maximum surveyed and simulated beach runup heights of 3m to 4m at bothlocations for the 2004 Andaman tsunami. Waves of the magnitude of 7 m to 7.5mdo pose severe risks to coastal communities in the affected regions. Hence furtherresearch is deemed essential in view of the likelihood of future earthquake-inducedtsunamis severely impacting Malaysia and Thailand to an extend far worse thanwhat was recorded in the 2004 Andaman tsunami.
6. Conclusion
In this paper, we have demonstrated the capability of TUNA to simulate tsunamievolution to a degree of accuracy and consistency that might be expected frommodels of similar level of sophistication and resolution such as COMCOT. In par-ticular, TUNA can simulate tsunami wave reflection on solid boundary properly
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98 H. L. Koh et al.
Fig. 10. Snapshot of tsunami propagation into the Straits of Malacca.
and free passage of waves out of the open boundary. We are therefore confident inusing TUNA to develop inundation and runup maps as well as evacuation routesin northwest Malaysia for the purpose of providing tsunami mitigation measures.These mitigation measures are considered essential because of the possibility offuture tsunamis that might pose severe threats to Malaysia. Similarly research onsimulation of tsunami originating in the South China Sea is equally important andwill be performed later by means of TUNA. We plan to incorporate water qualityand ecological submodels into the framework of TUNA to extend TUNA range ofapplications. In particular the well-developed and fully documented model WASP
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Simulation of Future Andaman Tsunami by TUNA 99
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
0 0.5 1 1.5 2 2.5 3 3.5 4Time (h)
(m
)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0 0.5 1 1.5 2 2.5 3 3.5Time (h)
(m
)
(a) (b)
-3.0-2.0-1.0
0.01.0
2.03.0
4.05.0
0 0.5 1 1.5 2Time (h)
(m
)
(c)
Fig. 11. Simulated tsunami wave heights near (a) Penang, (b) Langkawi and (c) Phuket.
developed by the United States Environmental Protection Agency will be incorpo-rated into TUNA for this purpose. Finally we hope to modify, enhance and linkTUNA to MANHAM to provide the capability to simulate the impact on vegetationsuccession due to large salinity alterations caused by tsunamis and storm surges.
Acknowledgment
Financial support from Grant # 305/PMATHS/613131 and # 1001/PMATHS/817025 is gratefully acknowledged. The comparison between TUNA and COMCOTwas possible due to the goodwill of Prof. Philip Liu of Cornell University to whomwe would like to extend our deep appreciation.
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