Numerical simulation and field survey of inundation due to 2004 Indian Ocean tsunami in Trincomalee,Sri Lanka

This paper outlines the field measurements and numerical modelling carried out to develop a high-resolution tsunami inundation map, as a case study, for the city of Trincomalee on the east coast of Sri Lanka, which was devastated by the 2004 tsunami. We employ the deterministic approach together with numerical simulations based on the probable worst-case scenario to derive the inundation map. Linear and non-linear versions of shallow-water equations have been utilized to simulate tsunami propagation and onshore inundation, respectively. The field data considered in the present paper comprise the extent of inundation, the tsunami heights and the arrival times whilst the model results include the spatial distribution of the flow depth, the peak current speeds and the momentum flux. The computed extent of onshore inundation reproduces the observed overall pattern of inundation in most areas barring the south-eastern part of the city. Further, the model simulations suggest maximum flow depths up to about 2 m in most areas of the city whilst patches of flow depths exceeding 2 m can be seen in a narrow strip along the coastline. The computed current speeds also exceed 3 m/s at some locations adjacent to the shoreline.     

Distribution, origin and transport process of boulders deposited by the 2004 Indian Ocean tsunami at Pakarang Cape, Thailand

The tsunami of 2004 in the Indian Ocean transported thousands of meters-long boulders shoreward at Pakarang Cape, Thailand. We investigated size, position and long axis orientation of 467 boulders at the cape. Most of boulders found at the cape are well rounded, ellipsoid in shape, without sharp broken edges. They were fragments of reef rocks and their sizes were estimated to be < 14m³ (22.7t). The distribution pattern and orientation of long axis of boulders reflect the inundation pattern and behavior of the tsunami waves. It was found that there is no clear evidence indicating monotonous fine/coarse shoreward trends of these boulders along each transect line. On the other hand, the large boulders were deposited repeatedly along the three arcuate lines at the intertidal zone with a spacing of approximately 136m interval. This distribution pattern may suggest that long-lasting oscillatory flows might have repositioned the boulders and separated the big ones from small. No boulders were found on land, indicating that the hydraulic force of the tsunami wave rapidly dissipated on reaching the land due to the higher bottom friction and the presence of a steep slope. We further conducted numerical calculation of tsunami inundation at Pakarang Cape. According to the calculation, the sea receded and the major part of the tidal bench (area with boulders at present) was exposed above the sea surface before the arrival of the first tsunami wave. The first tsunami wave arrived at the cape from west to east at approximately 130min after the tsunami generation, and then inundated inlands. Our calculation shows that tsunami wave was focused around the offshore by a small cove at the reef edge and spread afterwards in a fan-like shape on the tidal bench. The critical wave velocities necessary to move the largest and average-size boulders by sliding can be estimated to be approximately 3.2 and 2.0m/s, respectively. The numerical result indicates that the maximum current velocity of the first tsunami wave was estimated to be from 8 to 15m/s between the reef edge and approximately 500m further offshore. This range is large enough for moving even the largest boulder shoreward. These suggest that the tsunami waves that were directed eastward, struck the reef rocks and coral colonies, originally located on the shallow sea bottom near the reef edge, and detached and transported the boulders shoreward.     

The Indian Ocean tsunami of December 26, 2004: observations in Sri Lanka and Thailand

On December 26, 2004 a great earthquake (M _W 9.3) occurred off the western coast of Sumatra triggering a series of tsunami waves that propagated across the Indian Ocean causing damage and life loss in 12 countries. This paper summarizes the observations of lifeline performance, building damage and its distribution, and the social and economic impact of the tsunami made by the Earthquake Engineering Field Investigation Team (EEFIT) in Thailand and Sri Lanka. EEFIT operates under the umbrella of the UK’s Institution of Structural Engineers. It is observed that good engineering practice can reduce economic losses, but additional measures are required to reduce risk to life.     

Flow conditions of the 2004 Indian Ocean tsunami in Thailand,inferred from capping bedforms and sedimentary structures

The 2004 Indian Ocean tsunami deposited a sheet of sand with surficial bedforms at the Andaman coast of Thailand. Here we show the recognition of bedforms and the key internal sedimentary structures as criteria of the tsunami supercritical flow condition. The presence of well‐preserved capping bedforms implied a dominant tsunami inflow. Sets of internal sedimentary structures including parallel lamination, seaward and landward inclined‐laminations, and downstream dipping laminae indicated antidune structures that were generated by a supercritical flow current in a depositional stage during the inflow. A set of seaward dipping cross‐laminations containing sand with mud drape on the surface of one depositional layer are a unique indication of an outflow structure. A majority of deposits show normal grading, but in some areas, localized reverse grading was also observed. The recognition of these capping bedforms and determination of the internal sedimentary structures provides new key criteria to help derive a better understanding of tsunami flow conditions.     

Identification and characterization of tsunami deposits off southeast coast of India from the 2004 Indian Ocean tsunami: Rock magnetic and geochemical approach

The December 2004 Indian Ocean Tsunami (IOT) had a major impact on the geomorphology and sedimentology of the east coast of India. Estimation of the magnitude of the tsunami from its deposits is a challenging topic to be developed in studies on tsunami hazard assessment. Two core sediments (C1 and C2) from Nagapattinam, southeast coast of India were subjected to textural, mineral, geochemical and rock-magnetic measurements. In both cores, three zones (zone I, II and III) have been distinguished based on mineralogical, geochemical and magnetic data. Zone II is featured by peculiar rock-magnetic, textural, mineralogical and geochemical signatures in both sediment cores that we interpret to correspond to the 2004 IOT deposit. Textural, mineralogical, geochemical and rock-magnetic investigations showed that the tsunami deposit is featured by relative enrichment in sand, quartz, feldspar, carbonate, SiO ₂, TiO ₂, K ₂O and CaO and by a depletion in clay and iron oxides. These results point to a dilution of reworked ferromagnetic particles into a huge volume of paramagnetic materials, similar to what has been described in other nearshore tsunami deposits (Font et al. 2010). Correlation analysis elucidated the relationships among the textural, mineral, geochemical and magnetic parameters, and suggests that most of the quartz-rich coarse sediments have been transported offshore by the tsunami wave. These results agreed well with the previously published numerical model of tsunami induced sediment transport off southeast coast of India and can be used for future comparative studies on tsunami deposits.     

Socotra Island,Yemen: field survey of the 2004 Indian Ocean tsunami

The tsunami of 26th December 2004 severely affected Yemen’s Socotra Island with a death at a distance of 4,600 km from the epicenter of the Magnitude 9.0 earthquake. Yemen allowed a detailed assessment of the far-field impact of a tsunami in the main propagation direction. The UNESCO mission surveyed 12 impacted towns on the north and south shores covering from the east to the west tip of Socotra. The international team members were on the ground in Yemen from 11 to 19 October 2006. The team measured tsunami run-up heights and inundation distances based on the location of watermarks on buildings and eyewitness accounts. Maximum run-up heights were typically on the order of 2–6 m. Each measurement was located by means of global positioning systems (GPS) and photographed. Numerous eyewitness interviews were recorded on video. The tsunami impact on Socotra is compared with other locations along the shores of the Indian Ocean.     

Occurrence of soft sediment deformation at Dive Agar beach, west coast of India: possible record of the Indian Ocean tsunami (2004)

We describe here a sequence of soft sediment deformation (SSD) structures at Dive Agar beach near Srivardhan in the west coast of India. The ~120-cm-thick sediment package is represented by a basal undeformed sand (layer A) sharply cut by ~30-cm-thick intermixed beach sand and terrigenous sand (layer B1) followed by complex load structures and convolutions (8?C15?cm) within a coarse sandy layer (B2). The layer B2 is scoured by terrigenous sand (layer C1) which is capped with a silty mud layer (C2). The entire sequence (B2?CC1?CC2) is intruded by sand dykes originating from the lower layer B1. This sediment package is further overlain by a heavy mineral reach marine sand (layer D) with liquefactions long axes inclined southward as a result of forceful long-shore drift. The profile ends up with coarse-grained, poorly sorted sand including angular clasts of terrigenous outwash deposits indicating return of distal inundations. Intense deformation (liquefaction) is restricted to the heavy mineral-rich marine and the intermixed sands (layers B2 and D), whereas the terrigenous sand layers show scoured bases with oscillatory and pebbly tops. The presence of complex load structures injecting into the underlying layers, the top-truncated sand dykes, macro-thrust faults, scouring, and inclusion of coral fragments can explain it as a record of tsunami in the west coast. Occurrence of un-decayed consumer plastic material within the deformed layers suggests it as one of the most recent tsunami events (i.e., 2004 IOT), the only reported event after 1945 in the west coast. Alternative marine and terrigenous sands are characteristic of tsunami run-up and backwash deposits, while the dimensions of SSDs may be related to the <2?m magnitude (height) of the 2004 IOT at Dive Agar.     

The post-depositional changes of the onshore 2004 tsunami deposits on the Andaman Sea coast of Thailand

The Indian Ocean tsunami flooded the coastal zone of the Andaman Sea and left tsunami deposits with a thickness of a few millimetres to tens of centimetres over a roughly one-kilometre-wide tsunami inundation zone. The preservation potential and the post-depositional changes of the onshore tsunami deposits in the coastal plain setting, under conditions of a tropical climate with high seasonal rainfall, were assessed by reinvestigating trenches located along 13 shore-perpendicular transects; the trenches were documented shortly after the tsunami and after 1, 2, 3 and 4 years. The tsunami deposits were found preserved after 4 years at only half of the studied sites. In about 30% of the sites, the tsunami deposits were not preserved due to human activity; in a further 20% of the sites, the thin tsunami deposits were eroded or not recognised due to new soil formation. The most significant changes took place during the first rainy season when the relief of the tsunami deposits was levelled; moderate sediment redeposition took place, and fine surface sediments were washed away, which frequently left a residual layer of coarse sand and gravel. The fast recovery of new plant cover stabilised the tsunami deposits and protected them against further remobilisation during the subsequent years. After five rainy seasons, tsunami deposits with a thickness of at least a few centimetres were relatively well preserved; however, their internal structures were often significantly blurred by roots and animal bioturbation. Moreover, soil formation within the deposits caused alterations, and in the case of thin layers, it was not possible to recognise them anymore. Tsunami boulders were only slightly weathered but not moved. Among the various factors influencing the preservation potential, the thickness of the original tsunami deposits is the most important. A comparison between the first post-tsunami survey and the preserved record suggests that tsunamis with a run-up smaller than three metres are not likely to be preserved; for larger tsunamis, only about 50% of their inundation area is likely to be presented by the preserved extent of the tsunami deposits. Any modelling of paleotsunamis from their deposits must take into account post-depositional changes.     

Numerical modeling of the morphological change in Lhok Nga, west Banda Aceh, during the 2004 Indian Ocean tsunami: understanding tsunami deposits using a forward modeling method

A coupled hydrostatic and morph-dynamic model COMCOT-SED was used to investigate the morphological change in Lhok Nga bay during the 2004 Indian Ocean tsunami, and the coupled model predicted the thickness of tsunami deposits in agreement with the measured ones. The relationship between the characteristics of tsunami deposit and flow hydrodynamics was discussed in details. Phenomena such as landward thinning in deposit thickness, landward fining in grain size, and fining upwards in grain size are commonly used to identify tsunami deposits and were examined in this case study. We also discussed the effects of sediment supplies and the constraints that can be put on the earthquake parameters using the information derived from tsunami deposits. This study shows that the model presented in this paper is capable of simulating extreme tsunami events (tsunami wave height ~30?m) in a large domain and that forward models of tsunami sediment transport can be a promising tool to help tsunami geologists understand tsunami deposits.     

A study on pre- and post-tsunami shallow deposits off SE coast of India from the 2004 Indian Ocean tsunami: a geochemical approach

A study was made on samples from one core collected immediately after the December 2004 Asian tsunami to know the geochemical nature of the offshore tsunami sediments. The core sample was analyzed for sediment grain size, CaCO₃, organic carbon (OC) and major elements (SiO₂, TiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O, K₂O, P₂O₅, MnO). The results indicate that the core sample can be divided into two parts: (1) upper 0–25 cm, interpreted to be deposited after tsunami (AT), and (2) lower 25–45 cm, interpreted as before tsunami (BT) as evidenced by the sandy nature AT with fluctuating CaCO₃ contents. The AT part is devoid of OC suggesting that the sediment could have been transported to deeper regions along with the finer particles. Major elements such as SiO₂, TiO₂, CaO indicate high values than the other elements in AT part than in the BT part. The BT part contains Al rich alluvium mud associated with finer mud and organic particles. An analysis of the correlation matrix indicates the possible source of elements and transport of heavy minerals in the AT part than the BT part. The overall results suggest that the sediments could possibly have two different origins.     

The 2004 Sumatra earthquake and Indian Ocean tsunami: What happened and why?

The December 26, 2004 Sumatra earthquake and the tsunami that followed killed over 300,000 people. In this paper, we analyze and discuss the geologic causes for this earthquake, the mechanisms that generated it, and follow up with a discussion on ways to prevent this type of disaster in the future.     

Sheltered coastal environments as archives of paleo-tsunami deposits: Observations from the 2004 Indian Ocean tsunami

The 2004 earthquake left several traces of coseismic land deformation and tsunami deposits, both on the islands along the plate boundary and distant shores of the Indian Ocean rim countries. Researchers are now exploring these sites to develop a chronology of past events. Where the coastal regions are also inundated by storm surges, there is an additional challenge to discriminate between the deposits formed by these two processes. Paleo-tsunami research relies largely on finding deposits where preservation potential is high and storm surge origin can be excluded. During the past decade of our work along the Andaman and Nicobar Islands and the east coast of India, we have observed that the 2004 tsunami deposits are best preserved in lagoons, inland streams and also on elevated terraces. Chronological evidence for older events obtained from such sites is better correlated with those from Thailand, Sri Lanka and Indonesia, reiterating their usefulness in tsunami geology studies.     

Impact of the 2004 Indian Ocean tsunami along the Tamil Nadu coastline: field survey review and numerical simulations

The 2004 Indian Ocean tsunami had a significant impact on the Tamil Nadu coast in India. In this paper, a range of field survey data collected by different survey teams available in literature have been analyzed and compiled to serve as a basis for validation of numerical tsunami simulations. The individual field surveys reveal a significant scatter in the run-up data between the different teams, which point out that the uncertainty in these data must be taken into account when using them for validation. The inundation of the 2004 Indian Ocean tsunami is simulated for the coastlines of Chennai and Nagapattinam based on high-resolution topography. Different spatially uniform Manning friction as well as heterogeneous friction maps is used. Overall, the simulation results showed a good agreement with the field observations, but there are also some observed spatial variability in the goodness of the fit between the data and simulations. In some areas, clear discrepancies are found. The results obtained using detailed land use maps including spatially variable friction are not significantly more accurate than those employing spatially constant values. For most areas, parameters indicating relatively low friction provided best match with the observations. This may also suggest that the inundation is often strongly governed by local variations in topography.     

A probabilistic tsunami hazard assessment for the Makran subduction zone at the northwestern Indian Ocean

A probabilistic tsunami hazard assessment is performed for the Makran subduction zone (MSZ) at the northwestern Indian Ocean employing a combination of probability evaluation of offshore earthquake occurrence and numerical modeling of resulting tsunamis. In our method, we extend the Kijko and Sellevoll’s (1992) probabilistic analysis from earthquakes to tsunamis. The results suggest that the southern coasts of Iran and Pakistan, as well as Muscat, Oman are the most vulnerable areas among those studied. The probability of having tsunami waves exceeding 5 m over a 50-year period in these coasts is estimated as 17.5%. For moderate tsunamis, this probability is estimated as high as 45%. We recommend the application of this method as a fresh approach for doing probabilistic hazard assessment for tsunamis. Finally, we emphasize that given the lack of sufficient information on the mechanism of large earthquake generation in the MSZ, and inadequate data on Makran’s paleo and historical earthquakes, this study can be regarded as the first generation of PTHA for this region and more studies should be done in the future.     

Preliminary estimation of the tsunami hazards associated with the Makran subduction zone at the northwestern Indian Ocean

We present a preliminary estimation of tsunami hazard associated with the Makran subduction zone (MSZ) at the northwestern Indian Ocean. Makran is one of the two main tsunamigenic zones in the Indian Ocean, which has produced some tsunamis in the past. Northwestern Indian Ocean remains one of the least studied regions in the world in terms of tsunami hazard assessment. Hence, a scenario-based method is employed to provide an estimation of tsunami hazard in this region for the first time. The numerical modeling of tsunami is verified using historical observations of the 1945 Makran tsunami. Then, a number of tsunamis each resulting from a 1945-type earthquake (M _w 8.1) and spaced evenly along the MSZ are simulated. The results indicate that by moving a 1945-type earthquake along the MSZ, the southern coasts of Iran and Pakistan will experience the largest waves with heights of between 5 and 7 m, depending on the location of the source. The tsunami will reach a height of about 5 m and 2 m in northern coast of Oman and eastern coast of the United Arab Emirates, respectively.     

Contamination of tsunami sediments in a coastal zone inundated by the 26 December 2004 tsunami in Thailand

Tsunami sediments deposited in a coastal zone of Thailand by the 26 December 2004 tsunami wave were sampled within 50 days after the event. All surface and ground waters in tsunami- inundated zone revealed significant salinity at that time. The tsunami sediments, composed mainly of fine to medium sand, contain significantly elevated contents of salts (Na⁺, K⁺, Ca⁺², Mg⁺², Cl and SO ₄ ⁻² ) in water-soluble fraction, and of Cd, Cu, Zn, Pb in the bioavailable fraction and As in the exchangeable fraction in relation to the reference sample. The origin of contaminants is marine, as well as litho- and anthropogenic. The salts and Pb, Zn and Cu reveal high correlation to each other and to the mean grain size (pore size and porosity). Serious environmental hazard exists in that region because, due to gentle morphology, there is a risk of migration of the contaminants into ground waters and food chain.     

Aftershock sequences of two great Sumatran earthquakes of 2004 and 2005 and simulation of the minor tsunami generated on September 12, 2007 in the Indian Ocean and its effect

We present the seismic energy, strain energy, frequency–magnitude relation (b-value) and decay rate of aftershocks (p-value) for the aftershock sequences of the Andaman–Sumatra earthquakes of December 26, 2004 (M _w 9.3) and March 28, 2005 (M _w 8.7). The energy released in aftershocks of 2004 and 2005 earthquake was 0.135 and 0.365% of the energy of the respective mainshocks, while the strain release in aftershocks was 39 and 71% for the two earthquakes, respectively. The b-value and p-value indicate normal value of about 1. All these parameters are in normal range and indicate normal stress patterns and mechanical properties of the medium. Only the strain release in aftershocks was considerable. The fourth largest earthquake in this region since 2004 occurred in September 2007 off the southern coast of Island of Sumatra, generating a relatively minor tsunami as indicated by sea level gauges. The maximum wave amplitude as registered by the Padang, tide gauge, north of the earthquake epicenter was about 60 cm. TUNAMI-N2 model was used to investigate ability of the model to capture the minor tsunami and its effect on the eastern Indian Coast. A close comparison of the observed and simulated tsunami generation, propagation and wave height at tide gauge locations showed that the model was able to capture the minor tsunami phases. The directivity map shows that the maximum tsunami energy was in the southwest direction from the strike of the fault. Since the path of the tsunami for Indian coastlines is oblique, there were no impacts along the Indian coastlines except near the coast of epicentral region.     

Monthly-mean wind stress along the coast of the north Indian Ocean

Monthly-mean wind stress and its longshore and offshore components have been computed using the bulk aerodynamic method for each of a string of 36 two-degree-latitude by two-degree-longitude squares along the coast of the north Indian Ocean. The data source for the computation is the sixty-year mean resultant winds of Hastenrath and Lamb. The main features exhibited by the components, taking the longshore components as positive (negative) when the Ekman transport is away from (towards) the coast, are: (1) Along the coasts of Somalia and Arabia, the magnitude of the wind stress is among the highest in the north Indian Ocean, and its direction is generally parallel to the coastline. This results in a longshore component which is large (as high as 2·5 dyne/cm²) and positive during the southwest monsoon, and weaker (less than 0·6 dyne/cm²) and negative during the northeast monsoon. (2) Though weak (less than 0·2 dyne/cm²) during the northeast monsoon, the monthly-mean longshore component along the west coast of India remains positive throughout the year. The magnitude of the offshore component during the southwest monsoon is much larger than that of the longshore component. (3) The behaviour of the wind stress components along the east coast of India is similar to that along the Somalia-Arabia coast, but the magnitudes are much smaller.     

Numerical simulation of the Indian monsoon

A review of current work in India on modelling experiments is presented. The experiments are designed to simulate different features of the Indian summer monsoon. The first part of the paper summarises the basic structure of a model, and the problems of computational design. Different grid structures and transformation of the vertical coordinate to include mountains are discussed. Experiments are suggested for minimising truncation errors by using different reference atmospheres, and by normal mode initialisation. A brief account is provided of the use of finite elements for numerical models. The latter half of the paper deals with different physical processes in the atmosphere. They are (i) the radiation balance of the atmosphere, (ii) clouds and precipitation, (iii) frictional effects, and (iv) sea surface temperature. The results of models developed in India are compared with those obtained by general circulation models. It is shown that present models succeed in simulating certain features of the monsoon fairly well, but further work is needed to simulate other aspects, such as, rainfall.