Journal of Waterway, Port, Coastal, and Ocean Engineering

A large earthquake (Mw 9.3) struck northern Sumatra on December 26, 2004. The generated tsunami caused significant casualties and damages in many countries around the Indian Ocean. This is a report on the subsequent rapid reconnaissance survey of the Southeast Indian Coast. A wide variety of field observations—from hydrodynamics to human behaviors—made within 2 weeks of the event are discussed. We found that individual tsunami effects of this event were not extraordinary but similar to previous smaller events; what makes the December 26 event distinct is the extent of the affected area.

The December 2004 Sumatra earthquake triggered massive tsunami waves in the Indian Ocean and the Bay of Bengal. The tsunami waves traveled primarily in the east-west direction and caused major damage along the coasts of southern India, Sri Lanka, Thailand, and Indonesia. It has proved to be the most disastrous tsunami in recorded history, and scientists and engineers around the world set out to record signatures of the tsunami from which the characteristics of the tsunami waves at the time of landfall could be deciphered. Such information is extremely useful in validating mathematical models for understanding the characteristics of tsunamis and their effects along the coast. The writers conducted surveys along the east and west coasts of peninsular India—focusing on the states of Tamil Nadu and Kerala and around Andaman and Nicobar Islands, where the devastation caused by the tsunami was unparalleled. The analysis of data gathered during the post-sunami survey focused on consolidating the primary parameters of the tsunami arrival time, runup height, and the extent of inundation.

The December 26, 2004 tsunami was perhaps the most devastating tsunami in recorded history, causing over 200,000 fatalities and widespread destruction in countries bordering the Indian Ocean. It was generated by the third largest earthquake on record $(M_w=9.1–9.3)$ and was a truly global event, with significant wave action felt around the world. Many measurements of this event were made with seismometers, tide gauges, global positioning system stations, and a few satellite overpasses. There were numerous eyewitness observations and video digital recordings of coastal tsunami impact, as well as subsequent coastal field surveys of runup and flooding. A few ship-based expeditions also took place in the months following the event, to measure and map seafloor disturbances in the epicenter area. Based on these various data sets, recent seismic analysis estimates of rupture propagation speed, and other seismological and geological constraints, we develop a calibrated tsunami source, in terms of coseismic seafloor displacement and rupture timing along $\mathrm{1,200}\mathrm{km}$ of the Andaman–Sunda trench. This source is used to build a numerical model of tsunami generation, propagation, and coastal flooding for the December 26, 2004 event. Frequency dispersion effects having been identified in the deep water tsunami wavetrain, we simulate tsunami propagation and coastal impact with a fully nonlinear and dispersive Boussinesq model (FUNWAVE). The tsunami source is specified in this model as a series of discrete, properly parameterized, dislocation source segments [Okada, 1985, Bull. Seismol. Soc. Am., 75(4), 1135–1154], triggered in a time sequence spanning about $\mathrm{1,200}\mathrm{s}$ . ETOPO2’s bottom bathymetry and land topography are specified in the modeled ocean basin, supplemented by more accurate and denser data in selected coastal areas (e.g., Thailand). A $1\min$ grid is used for tsunami simulations over the Indian Ocean basin, which is fine enough to model tsunami generation and propagation to nearshore areas. Surface elevations simulated in the model agree well, in both amplitude and timing, with measurements at tide gauges, one satellite transect, and ranges of runup values. These results validate our tsunami source and simulations of the December 26, 2004 event and indicate these can be used to conduct more detailed case studies, for specific coastal areas. In fact, part of the development of our proposed source already benefitted from such regional simulations performed on a finer grid $\left(15\mathrm{s}\right)$ , as part of a Thailand case study, in which higher frequency waves could be modeled (Ioualalen et al. 2007, J. Geophys. Res., 122, C07024). Finally, by running a non-dispersive version of FUNWAVE, we estimate dispersive effects on maximum deep water elevations to be more than 20% in some areas. We believe that work such as this, in which we achieve a better understanding through modeling of the catastrophic December 26, 2004 event, will help the scientific community better predict and mitigate any such future disaster. This will be achieved through a combination of forecasting models with adequate warning systems, and proper education of the local populations. Such work must be urgently done in light of the certitude that large, potentially tsunamogenic, earthquakes occur along all similar megathrust faults, with a periodicity of a few centuries.

This paper presents a sensitivity analysis of the effects of different fault plane parameters on earthquake generated tsunamis in the far field. The parameters studied include the location of epicenter, rake angle, dip angle, strike angle, fault plane dimensions, slip displacement, and focal depth. The study was carried out by applying the verified Cornell COMCOT tsunami model to simulate distant tsunamis generated by earthquakes in three different seismic regions, namely, Japan, Aleutian Islands, and Chile in the Pacific Basin. In this study, each fault plane parameter was varied and its effect on the tsunami wave height at a distant location (Hawaii offshore waters as a case study) was examined. Our results showed that under the same earthquake magnitude, in general, variations within a reasonable range of uncertainty in rake and dip angles, epicenter location, and focal depth do not affect the resulting tsunamis significantly while a variation in the fault dimensions, strike angle, and slip displacement can cause a large change in the wave heights in the far field. The study revealed that the sensitivity of the far field wave height to certain earthquake parameters including the fault plane dimensions, slip displacement, and the strike angle does not always decrease as the distance between the earthquake and the far field increases. These results indicate the importance of accurate earthquake information for accurate tsunami predictions and that even for wave heights in a far field thousands of miles away, the earthquake may not be treated as a simple point source described by its epicenter location and magnitude alone.

Large scale, three-dimensional, laboratory experiments are performed to study tsunami generation by rigid underwater landslides. The main purpose of these experiments is to both gain insight into landslide tsunami generation processes and provide data for subsequent validation of a three-dimensional numerical model. In each experiment a smooth and streamlined rigid body slides down a plane slope, starting from different initial submergence depths, and generates surface waves. Different conditions of wave nonlinearity and dispersion are generated by varying the model slide initial submergence depth. Surface elevations are measured with capacitance gauges. Runup is measured at the tank axis using a video camera. Landslide acceleration is measured with a microaccelerometer embedded within the model slide, and its time of passage is further recorded at three locations down the slope. The repeatability of experiments is very good. Landslide kinematics is inferred from these measurements and an analytic law of motion is derived, based on which the slide added mass and drag coefficients are computed. Characteristic distance and time of slide motion, as well as a characteristic tsunami wavelength, are parameters derived from these analyses. Measured wave elevations yield characteristic tsunami amplitudes, which are found to be well predicted by empirical equations derived in earlier work, based on two-dimensional numerical computations. The strongly dispersive nature and directionality of tsunamis generated by underwater landslides is confirmed by wave measurements at gauges. Measured coastal runup is analyzed and found to correlate well with initial slide submergence depth or characteristic tsunami amplitude.

A study is presented to examine the one-horizontal dimension effect of a shallow shelf obstacle on nonlinear long wave runup. Due to the large horizontal-vertical aspect ratio of this problem, it is not well suited for experimental analysis, and therefore this study is purely numerical. Simulations are performed for various incident wave conditions, obstacle height and widths, and final beach slopes. Many of the setups involve breaking, either through approaching the obstacle as a large breaking bore, incipient breaking on top of the obstacle, or breaking during the beach uprush. The general conclusion of this study is that, for highly nonlinear waves ( $ϵ$ =wave height/shelf water depth $⩾0.5$ ), the obstacle will always act to reduce the runup and the maximum overland velocity. However, for very small obstacle lengths, particularly for extremely large waves, this reduction may be practically inconsequential. Interestingly, for weakly nonlinear waves $(ϵ\approx 0.1)$ , due to front steepening over the obstacle, greater overland velocities can result from increasing obstacle length. Consistent with previous studies, it is found that the final beach slope is of primary importance for determining the runup.

The storm surge associated with Hurricane Katrina caused tremendous damage along the Gulf Coast in Louisiana, Mississippi, and Alabama. Similar damage was observed subsequent to the Indian Ocean tsunami of December 26, 2004. In order to gain a better understanding of the performance of engineered structures subjected to coastal inundation due to tsunami or hurricane storm surge, the writers surveyed damage to bridges, buildings, and other coastal infrastructure subsequent to Hurricane Katrina. Numerous lessons were learned from analysis of the observed damage, and these are reported herein. A number of structures experienced significant structural damage due to storm surge and wave action. Structural members submerged during the inundation were subjected to significant hydrostatic uplift forces due to buoyancy, enhanced by trapped air pockets, and to hydrodynamic uplift forces due to wave action. Any floating or mobile object in the nearshore/onshore areas can become floating debris, affecting structures in two ways: impact and water damming. Foundation soils and foundation systems are at risk from shear- and liquefaction-induced scour, unless designed appropriately.