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The 2011 Tohoku Tsunami caused severe damage to many coastal defenses along the Japanese coastline, including areas where two adjacent upright sea dikes of different heights met. There are many places along the coastline where structures of different heights and shapes meet due to the various design codes. In this study, the authors aim to examine the tsunami flow around such dikes by conducting three-dimensional hydraulic experiments in a tsunami wave basin and numerical simulations using the fluid dynamics model OpenFOAM. Twelve experimental cases were tested by changing the waveforms, and also by keeping the height of one side of the dike model constant while adjusting the dimensions of the other side. The results show that the difference in height between the structures significantly influences the volume of overtopping over the relatively lower dike and the inundation depth and velocity behind the dikes. Furthermore, different characteristics of inundation patterns are observed between the inundation depth and velocity. Hence, this study illustrates the necessity to re-assess the effectiveness of such structures against tsunami events. [ABSTRACT FROM AUTHOR]

A near-field tsunami inundation forecast method (NearTIF) for the Pacific coast of Central America is developed and tested on the basis of the 1992 Nicaragua tsunami earthquake case. The appropriate source model of the 1992 Nicaragua earthquake, estimated using the W-phase inversion with a depth dependent rigidity curve from a previous study, is used as a reference model to test the proposed NearTIF method. The tsunami inundation along the Pacific coast of Nicaragua is computed from the reference model of the 1992 Nicaragua earthquake. The tsunami inundation obtained using the NearTIF method matches the tsunami inundations computed from the reference model. The tsunami heights and the tsunami inundation of the 1992 Nicaragua tsunami surveyed by a previous study are roughly matched by the tsunami inundations obtained using the NearTIF method in this study. Although the computational time for the tsunami inundation from the reference source model is about 95 min, the computational time plus the database search time for the NearTIF method is approximately 2–4 min. The method presented in this study can be used as a near-real time tsunami inundation forecast method for the Pacific coast of Central America. [ABSTRACT FROM AUTHOR]

The aim of this study is to estimate how and what kinds of tsunamis are generated near shorelines in the Japan Sea, using earthquake tsunami sources proposed by the Japanese government. Tsunami propagation based on these sources was first simulated based on the linear Boussinesq theory for Miho Bay, where a historical tsunami inundated the shoreline. The results indicate that tsunami sources not only near the bay but also farther from the bay could generate significant tsunami fluctuations. Thus, the amplification of the latter tsunamis was examined using Fourier analysis. Tsunami waveforms in the bay periodically fluctuate with large amplitudes after the first wave. The Fourier analysis specified these periods to be 13–16 min, consistent with the estimated resonance periods of the bay. Therefore, it can be concluded that one of the factors responsible for the amplification of tsunamis in Miho Bay is the bay-scale resonance. To investigate why resonance occurs during tsunamis, a relationship between the tsunami source geometry and the generated tsunami was examined. This analysis indicates that specific tsunami sources located in critical areas could enhance the resonance in Miho Bay. [ABSTRACT FROM AUTHOR]

Following the 2011 Tohoku earthquake, coastal forests dissipated the energy of the tsunami and trapped flotsam. Although studies have shown how trees dissipate flow energy and are damaged in the process, the factors determining the trapping of flotsam by lines of trees or their capacity to do so have not been quantitatively examined. Quantitative evaluation of these factors may lead to tree-based strategies for suppressing the secondary damage caused by tsunami flotsam. To increase the ability of trees to mitigate damage, their physical limitations need to be identified. Field surveys revealed that integrated clusters of flotsam were caught by trees even if individual floating objects were smaller than the distance between the trees. Hydraulic experiments revealed relations between physical factors and the capture of flotsam. From these relations, we derived a simple formula for evaluating the trapping capacity of lines of trees. The estimates from our model were consistent with the actual results of the 2011 tsunami in Tagajo city, Miyagi prefecture, where trees did not topple under the forces applied by trapped flotsam. [ABSTRACT FROM AUTHOR]

Since the 2011 Great East Japan tsunami, many improvements have been made in both hard and soft solutions for tsunami mitigation. After the 2011 Great East Japan tsunami, a posttsunami survey was conducted along the coast in Miyagi Prefecture, which was one of the most tsunami-affected sites because rapid acceleration of the tsunami currents broke and washed away the trees, resulting in extensive damage to inland houses. In contrast, some of the houses located inland and away from vegetation with a dropping step survived. This shows a possibility that a step combined with the vegetation offers greater tsunami energy reduction by providing additional resistance. Laboratory experiments were conducted to investigate the energy reduction through a compound defense system (vegetation and a backward-facing step). Vegetation with a step (VS) showed greater energy reduction compared to that of only vegetation without a step (OV) due to additional loss by collision with the bed surface. However, the relative energy reduction in OV remained almost constant with the increase in the initial Froude number (F0, where the Froude number is obtained from a model without vegetation in a flume), whereas the relative energy reduction in VS showed a decreasing trend with increasing F0 because the energy reduction due to collision decreases with increase in water depth or F0. [ABSTRACT FROM AUTHOR]

Tsunami fission generates short waves at the front face of tsunami and these short waves may expand the inundation area, increase the hydrodynamic forces exerted on structures, and amplify the damage to coastal communities. It is therefore important to understand to what extent the tsunami fission generates such short wave components under variable conditions for development of better tsunami disaster mitigation strategies and plannings. This study carried out numerical experiments to describe under which conditions the tsunami fission occurs in a bay. The experiments indicated that the amplification of the nearshore tsunami height induced by the narrowing bay geometry and the bay-scale resonance enhance the tsunami fission. Based on the computations, a parametric diagram for the occurrence of tsunami fission was proposed in terms of the tsunami properties, the effect of bay geometry and the bay resonance. The developed diagram was applied to Sendai, Ryori, Yoshihama, and Toni Bays, which were affected by the 2011 Tohoku Tsunami. The proposed diagram consistently explained the occurrence of tsunami fission in Sendai Bay and in Ryori Bay as observed in the video footage. The diagram also suggested that the tsunami fission would have occurred in Yoshihama Bay. A high-resolution numerical computation was performed to simulate the tsunami deformation in the Yoshihama Bay and Toni Bay based on a nonlinear dispersive wave model. The result supported the validity of the diagram in describing the generation of tsunami fission in Yoshihama Bay. [ABSTRACT FROM AUTHOR]

Given the importance of ports to immediate disaster relief operations after a major tsunami, breakwaters are necessary to sustain a minimum level of functionality required to carry out port operations. However, most breakwaters that protect fishery or industrial ports are not designed to withstand a tsunami. For this reason, many of these breakwaters were destroyed by the 2011 Great East Japan Earthquake and Tsunami. However, some breakwaters withstood the tsunami, which facilitated the rapid resumption of port operations. In this study, 67 ports were investigated by using satellite images to identify the percentage of breakwaters that suffered damage due to the 2011 tsunami and how the tsunami height and breakwater width influenced the degree of breakwater damage. Because tsunami heights were not measured at the breakwaters, they were estimated by interpolating the data observed at nearby locations with a Gaussian weighting function at a reasonable accuracy. Possible drawbacks of this simplified method are also discussed because a certain degree of estimation error is inevitable. Breakwaters less than 8 m wide were quantitatively demonstrated to inevitably experience extensive damage when the tsunami height exceeded 14 m. In contrast, breakwaters that were more than 14 m wide, exposed to tsunami heights of less than 6 m, or both were not greatly damaged. An examination of the effectiveness of armor blocks also revealed that their placement could substantially reduce breakwater damage due to a tsunami. The results suggest a simplified method, particularly for assisting local fishery administrators or other authorities concerned, to practically estimate the risk of a port suffering damage because of future tsunamis. The proposed method should contribute to prioritizing the protection of ports in order of their likelihood of failure. [ABSTRACT FROM AUTHOR]

A computational framework for probabilistic tsunami risk assessment due to a mega-thrust subduction earthquake is developed and is applied to the 2011 Tohoku Tsunami from retrospective viewpoints. The uncertain tsunami source characteristics are represented by multiple source inversion models and their stochastic variations that are generated using the spectral analysis and synthesis method. By conducting Monte Carlo tsunami simulation, stochastic inundation depth maps can be developed, which are subsequently integrated with tsunami fragility curves to develop stochastic tsunami risk maps. The stochastic tsunami risk maps display spatial variability of tsunami damage probabilities for a building portfolio, reflecting not only possible tsunami scenarios but also uncertain tsunami resistance of buildings. The numerical results indicate that both stochastic tsunami risk maps and risk curves are affected by the local terrain features, proximity to major tsunami sources, and building characteristics (material type and story number). Consideration of different reference tsunami source models in probabilistic tsunami risk assessment are identified as one of the critical contributors to the overall uncertainty of the tsunami risk predictions. Therefore, in determining critical scenarios for tsunami evacuation and risk mitigation, a wide range of possible tsunami scenarios should be considered in light of the current limited seismological knowledge for the mega-thrust subduction earthquake. [ABSTRACT FROM AUTHOR]

A numerical computation of the 2011 Tohoku earthquake tsunami was performed to identify fundamental features of the tsunami evolution along the coast of Hokkaido, Japan. Edge waves formed at multiple locations where the refracted tsunami focused, governing local surface oscillations and regional variations in tsunami height along the Pacific coast of Hokkaido. The computation reasonably reproduced the distribution of surveyed tsunami height as well as the time records of surface elevation recorded at ports in Hokkaido. The major features of the frequency spectrum for the 2011 Tohoku tsunami were identical to those for the 2003 Tokachi-oki earthquake tsunami; inherent local properties of surface oscillation caused by the passage of edge waves existed, determined by the local bathymetry. [ABSTRACT FROM AUTHOR]

The field survey results obtained by the team of authors in the north of Miyagi Prefecture between April 1 and 6, 2011 are summarized referring to a part of the survey results by other teams of the 2011 Tohoku Earthquake Tsunami Joint Survey Group. The inundation height above sea level was measured using a laser range finder with a reflection prism. The inundation height was generally larger at bay heads, as well as promontory tips, except several points, which were sheltered by a peninsula or had a tapering area. The tsunamis reached inland far away from the sea along valleys and rivers. Not only steel frame buildings but also many reinforced concrete buildings were collapsed in Onagawa Town. According to the numerical results, the first of tsunamis from the west coast and the second of tsunamis from the east coast should come together in Utatsusaki Peninsula, which is consistent with the interviews to survivors. [ABSTRACT FROM AUTHOR]

At 14:46 JST on March 11, 2011 a magnitude 9.0 earthquake (2011 Tohoku Earthquake and Tsunami) occurred off the Pacific Coast of Miyagi Prefecture. This study investigated the extensive changes in beach morphology due to the earthquake and tsunami along the 15 km Northern Sendai Coast using remotely sensed data. The remote sensing analysis on the beach topography and coastal forest demonstrated the following notable characteristics of beach morphological change: erosion of the northern barrier at the mouths of the Nanakitagawa and Natorigawa Rivers; erosion at an old river channel; scour landward of the seawalls in the longshore direction; erosion and deposition in beach areas with detached breakwaters; and deposition in coastal forest areas. Linkage of the deposition in the forest areas with the damage type of coastal forests was observed. The impact of the earthquake and tsunami on the beach morphology was serious; roughly 60% of the study area was degraded by 0.2-0.5 m in elevation mainly due to land subsidence, and a total of 0.4 km2 of beach area was eroded mainly due to erosion of the northern barrier at the mouths of the Nanakitagawa and Natorigawa Rivers. This study explores the geographical changes brought on by a tremendous earthquake and tsunami, which will help to elucidate the mechanisms of coastal forest destruction, beach erosion, and their interaction during tsunami events. [ABSTRACT FROM AUTHOR]

Japan was hit by the tsunami generated by the greatest earthquake in the history of Japan. The authors conducted the post-tsunami field survey in Miyagi prefecture immediately after the event to measure the inundation depth and investigate damaged buildings. Most of the buildings surveyed were wooden houses and could be classified into 4 damage levels. The primary data of inundation depth and damage levels obtained from the field survey were used to create the tsunami fragility curves, which described the degree of structural damage as a function of the hydrodynamic characteristics of tsunami inundation. The developed fragility curves show that damage with more than a 50% chance of occurring is classified as minor damage, moderate damage, major damage and complete damage when the tsunami inundation depth is between 2.5-3.0 m, 3.0-4.0 m and 4.0-4.5 m and greater than 4.5 m, respectively. By comparing the developed tsunami fragility curves, wooden houses in the studied area have higher structural performance than those obtained from historical events in other areas. The new curves suggest that wooden houses will be severely damaged if the inundation depth is greater than 3 m and collapse if the depth is greater than 4 m while wooden houses from historical data can resist only 1-2 m. Moreover, wooden walls in Japan reduce the overall failure of a structural member because a wooden wall is easier to destroy (compared to brick walls in Thailand and Indonesia) and allows the tsunami to pass through, which reduces the pressure at the attacking front of the tsunami. The fragility curves are very important for the loss estimation and reconstruction plans of the city; they are also crucial for reducing the disaster damage from a future tsunami. [ABSTRACT FROM AUTHOR]

At 14:46 on March 11, 2011 (local time), a large earthquake of magnitude Mw 9.0 took place, generating a tsunami that caused severe damage to the east coast of Japan. To comprehensively record tsunami trace heights and impacts along the coastal region, the Tohoku Earthquake Tsunami Joint Survey Group was organized immediately after the event. As part of this group, the authors conducted a field survey in Miyagi and Fukushima Prefectures. The surveyed area can be divided into 2 parts from the point of view of its geographical features: the northern part (a rias coastal area) and the southern part (a coastal plain area). In this paper, the characteristics of the damage due to the tsunami in each area are analyzed by using both the results of the authors' own field survey and the Joint Survey Group. In the rias coastal area, inundation heights were more than 10 m, which resulted in the flooding of the low-lying grounds located at the inner part of the bays. The tsunami wave caused widespread destruction in this area, and coastal buildings (including reinforced concrete buildings) suffered severe damage. In the southern coastal plains, inundation heights were 5-10 m and the tsunami reached a few kilometers inland, though unfortunately there were not enough high locations or buildings for the residents to evacuate. In addition, an extensive line of coastal dikes and forests, which had been placed to protect the wide plains behind them, also suffered extensive damage. From these geographically dependent inundation and destruction patterns, a number of important lessons on how to modify and improve future risk management strategies can be obtained. [ABSTRACT FROM AUTHOR]

The behavior of the 2011 Tohoku earthquake tsunami in Tokyo Bay and its resultant damage to the coasts are studied using a field survey, an analysis of observed tsunami wave profiles and numerical hindcasting using FVCOM and Delft3D-Flow. The field survey reveals that the tsunami caused several fishery ports to overflow, and unexpectedly large inundation and run-up heights of 2.84 m and 2.91 m were recorded in Funabashi Fishery Port and Shin-Futtsu Fishery Port, respectively. Examining the observed tsunami wave profiles, the tsunami wave entered the bay with a maximum height of approximately 0.9 m for the second wave and a period of approximately 60 min, propagating along the major axis of the bay with almost uniform height. Numerical hindcasting results using both the models are fairly consistent with the measurements, revealing that the tsunami wave heights are locally amplified in ports because of wave reflection, water accumulation due to their narrow and enclosed shape, and, to some extent, the harbor resonance. Heavy damage to seaweed farming was also observed, especially around the Futtsu Cape and the northern part of the Banzu tidal flat, which numerical analysis attributes to the appearance of large hydraulic resistance on seaweed net. [ABSTRACT FROM AUTHOR]