Coastal Engineering Journal

The 2011 Great East Japan Tsunami exposed many hidden weaknesses in Japan’s tsunami countermeasures. Since then, many improvements have been made in both structural measures (numerical simulations, coastal defense structures, building damage assessment and control forests) and nonstructural measures (warning/observation and evacuation). This review summarizes the lessons and improvements in the five-year time period after the 2011 event. After five years, most of the lessons from the 2011 tsunami have been applied, including more realistic tsunami simulations using very fine grids, methods to strengthen coastal defense structures, building evacuations and coastal forests, improved warning content and key points to improve evacuation measures. Nevertheless, large future challenges remain, such as an advanced simulation technique and system for real-time hazard and risk prediction, implementation of coastal defense structures/multilayer countermeasures and encouraging evacuation. In addition, among papers presented at the coastal engineering conference in Japan, the proportion of tsunami-related research in Japan increased from 15% to 35% because of the 2011 tsunami, and approximately 65–70% of tsunami-related studies involve numerical simulation, coastal structures and building damage. These results show the impact of the 2011 tsunami on coastal engineering related to academic institutions and consulting industries in Japan as well as the interest in each tsunami countermeasure.

The 2011 Tōhoku event showed the massive destruction potential of tsunamis. The Euro-Japan “Risk assessment and design of prevention structures for enhanced tsunami disaster resilience (RAPSODI)” project aimed at using data from the event to evaluate tsunami mitigation strategies and to validate a framework for a quantitative tsunami mortality risk analysis. Coastal structures and mitigation strategies against tsunamis in Europe and Japan are compared. Failure mechanisms of coastal protection structures exposed to tsunamis are discussed based on field data. Knowledge gaps on failure modes of different structures under different tsunami loading conditions are identified. Results of the wave-flume laboratory experiments on rubble mound breakwaters are used to assess their resilience against tsunami impact. For the risk analysis, high-resolution digital elevation data are applied for the inundation modeling. The hazard is represented by the maximum flow depth, the exposure is described by the location of the population, and the mortality is a function of flow depth and building vulnerability. A thorough search for appropriate data on the 2011 Tōhoku tsunami was performed. The results of the 2011 Tōhoku tsunami mortality hindcast for the city of Ishinomaki substantiate that the tsunami mortality risk model can help to identify high-mortality risk areas and the main risk drivers.

The 2011 Tohoku Tsunami is considered to be one of the most tragic events in Japan’s disaster history, and represents an important milestone for the research community regarding the investigation of the characteristics of tsunami inundation. A thorough analysis of tsunami inundation was conducted using numerical modeling, and measurements from a video recorded from the rooftop of a building in Onagawa in Miyagi Prefecture. In this study, we analyze the destruction of buildings using numerical simulations and tsunami fragility functions. Numerical results for the locations at which the tsunami eyewitness video was recorded are compared with measurements. In addition, we considered the effect of the breakwater in Onagawa bay to evaluate its contribution to reducing overland tsunami inundation depths. The results of our simulations show that the maximum inundation depth due to the first incoming wave was over 16 m, and over 500 buildings were washed away with this first wave. This result is consistent with the video data. Further, we found that the breakwater, which was not originally designed against tsunami waves, reduced the maximum tsunami inundation depth at least by 2.0 m in Onagawa town.

In the 2011 Tohoku Earthquake, many structures were destroyed by the tsunami whose magnitudes were much larger than the design level. Tsunami defense structures in coastal areas will need reinforcement in the future, and for that reason it is important to evaluate tsunami behavior and resulting tsunami force. This study analyzes tsunami transformation around a Haramachi thermal power station which suffered serious damage by the tsunami due to the 2011 Tohoku Earthquake and which has complex topography and building layout. A three-dimensional (3D) tsunami simulation is carried out for a surrounding region of the Haramachi thermal power station using boundary conditions of the water surface elevation and flow velocity obtained from a two-dimensional (2D) tsunami simulation by a nonlinear shallow water model. Using these boundary conditions, the computer fluid dynamic model FLUENT is employed to simulate the tsunami behavior around the Haramachi thermal power station. The validity of the predictions is examined by comparison with actual traces of tsunami inundation depths (I.D.).

A numerical sediment transport model (STM) was used to investigate coastal geomorphic changes that resulted from the 2011 Tohoku earthquake tsunami in Rikuzentakata City and Hirota Bay on the southern Sanriku Coast of Japan. The simulation was verified using observed inundation processes and heights, measured topographic changes and sediment deposition. Aerial video footage recorded by the Iwate Prefectural Police was also used. The results show that the numerical model was able to predict the spatial distribution and volume of erosion and deposition in Hirota Bay, as well as sediment transport processes. The effects of sediment transport on tsunami inundation were also investigated. Numerical results revealed that the majority of the sand dunes were eroded by the first wave, especially during the strong return flow of the receding wave. Large flows and sand dune erosions can occur elsewhere if tsunamis inundate a plain with a limited shore-normal width. These events could cause large-scale morphological changes comparable to those that occurred in Rikuzentakata City.

This study investigates the changes in coastal topography of the Rikuzen-Takata Coast in Iwate Prefecture, Japan before and after the 2011 off the Pacific coast of Tohoku Earthquake Tsunami and the effects of coastal structures on these changes. The changes in coastal topography were analyzed using bathymetry data and aerial photographs before and after the tsunami in addition to aerial video during the tsunami. The bathymetry data were obtained from 1989 to 2002 during the construction of three submerged breakwaters and from after the 2011 tsunami until 2013. The aerial photographs were acquired from 1947 to 2015, and the aerial videos were acquired during the tsunami run-up and backwash. The results demonstrated that the coast was eroded mainly due to the tsunami backwash, and the submerged breakwaters trapped the seaward transport of sediment from the coast. Erosion was partly prevented in locations where the seawall was not washed away. The coastal structures had significant effects on the behavior of the coastal tsunami and on sediment transport. We also found that the coast did not recover naturally at the desired speed after the tsunami because the coast had been stable before the tsunami. Coastal restoration five years after the 2011 tsunami are also summarized in the Appendix to illustrate the future reconstruction plan for the study coast.

On 11 March 2011, an exceptionally large tsunami event was triggered by a massive earthquake offshore, the northeast coast of Japan, which affected coastal infrastructure such as seawalls, coastal dikes and breakwaters in the Tohoku region. Such infrastructure was built to protect against the Level 1 tsunamis that previously hit the region, but not for events as significant as the 2011 Tohoku tsunami, which was categorized as a Level 2 tsunami [Shibayama, T., Esteban, M., Nistor, I., Takagi, H., Thao, N. D., Matsumaru, R., Mikami, T., Aranguiz, R., Jayaratne, R. & Ohira, K. [2013] “Classification of tsunami and evacuation areas,” Nat. Hazards 67(2), 365–386]. The failure mechanisms of concrete-armored dikes, breakwaters and seawalls due to Level 2 tsunamis are still not fully understood by researchers and engineers. This paper investigates the failure modes and mechanisms of damaged coastal structures in Miyagi and Fukushima Prefectures, following the authors’ post-disaster field surveys carried out between 2011 and 2013. Six significant failure mechanisms were identified for the coastal dikes and seawalls affected by this tsunami: (1) Leeward toe scour failure, (2) Crown armor failure, (3) Leeward slope armor failure, (4) Seaward toe scour and slope armor failure, (5) Overturning failure, and (6) Parapet wall failure, in which leeward toe scour being recognized as the major failure mechanism in most surveyed locations. The authors also propose a simple practical mathematical model for predicting the scour depth at the leeward toe of the coastal dikes, by considering the effects of the tsunami hydrodynamics, the soil properties and the type of structure. The key advantage of this model is that it depends entirely on quantities that are measurable in the field. Furthermore this model was further refined by conducting a series of hydraulic model experiments aimed to understand the governing factors of the leeward toe scour failure. Finally, based on the results obtained, key recommendations are given for the design of resilient coastal defense structures that can survive a level 2 tsunami event.

This study proposes a new embankment reinforcement using steel sheet piles against tsunami overflow, which has been known as the main cause of the failures of the embankments by the 2011 Tohoku Earthquake Tsunami. Effectiveness of the proposed technique was discussed through a hydraulic experiment. A model of embankment was set in a horizontal open channel, and one or two steel plates are installed into the embankment from the top as vertical walls inside. Temporal variations of the shapes of the embankment and the sheet pile structures were obtained from video images. In most of the cases, the sheet pile structures started to rotate after the erosion of the landward slope of the embankment. However the rotation stopped at about $30^{\circ }$ and $10^{\circ }$ from the initial location with the single- and double-wall cases. Height of the embankment after overflow was less than 20% with no reinforcement, while more than 70% and 95% of the height were kept with the single- and double-wall structures, respectively. The performance of the embankment with the reinforcement was also discussed in terms of tsunami energy reduction with an additional fixed-bed experiment.

This paper presents a practical design method for armor units to cover a rubble mound at the rear side of a caisson breakwater against tsunami overflow. In this method, the overflow depth of tsunami is used to represent the external force. This enables the estimation of the required mass of the armor units to be done more robustly and easily than in the conventional method based on the flow velocity. Hydraulic model experiments were conducted to investigate the armor stability. We found two important factors for armor stability. These were the impingement position of the overflow jet and the harbor-side water level. These effects were taken into account in the method. Numerical analysis on the fluid forces acting on the armor blocks was also conducted to explore the fundamental expression of the new design formula for armor stability. Empirical formulae for the stability estimation were then derived based on the findings from experiments and numerical analysis. The overflow depths of the stability limit corresponding to two failure modes, overturning and sliding, were obtained by two formulae. The stability numbers for each armor unit were determined through the experiments. The estimated results by this method agreed well with the experimental ones.

Bridge superstructures were washed away in the 2011 Tohoku earthquake tsunami. To determine whether and when bridge superstructures are washed away, it is essential to evaluate the temporal change in horizontal and vertical tsunami forces. In this study, tsunami force acting on bridge superstructures and its estimation formulae are investigated from hydraulic model experiments and numerical simulations. From experimental results, it is found that it can be determined from a comparison between the temporal change in horizontal tsunami force and that in static friction force taking into account vertical tsunami force whether and when a bridge superstructure move. Furthermore, a drag force equation with the projection area taking into account water surface elevation and a Morison equation in which inertia force is added to the drag force equation are proposed for horizontal tsunami force, and an estimation formula composed of buoyancy force and downward lift force is proposed for vertical tsunami force. From a comparison with numerical results, it was revealed that the estimation formulae can predict the temporal change in the horizontal and vertical tsunami forces except for short wave period, suggesting that it can be evaluated using the estimation formulae whether and when a bridge superstructures moves.

To design and construct land structures which can withstand earthquake-induced tsunami, it is most essential to evaluate tsunami wave force exerting on the structures quantitatively. This study, therefore, proposes the estimation method of tsunami wave pressure acting on a land structure using inundation depth and horizontal velocity at the front of the structure, which were calculated by a two-dimensional depth-integrated flow model. The comparison between the numerical and experimental results revealed that the proposed method could reasonably reproduce the vertical distribution of the maximum tsunami wave pressure exerting on a land structure.

The relationship between the collision direction angle and the collision force of woody debris and its behavior is investigated through large-scale bore flow and airborne experiments. In the case of the airborne experiment’s oblique collision angle being larger than 20${}^{\circ }$, the oblique collision force is reduced by up to half of the longitudinal collision force. In the case of the bore flow experiment’s initial arrangement along the transverse direction, the wood-log moved with a rotation along the channel floor. Collision forces in the bore flow experiment indicated the trend less than that in the airborne experiment. The effects of water cushioning between the wood-log and vertical wall, and the surrounding water caught by the rotation of debris were considered. Major collision force formulas for collisions in the long axis direction of woody debris are verified through comparison of this study and recent experiments. By considering the reduction in collision energy by the rotational motion of debris after oblique collisions, their collision force formulas are improved to estimate the oblique collision force. The improved formulas can reproduce the reduction of significantly oblique collision forces compared with longitudinal collision forces by the above experiments. The effect of the collision angle’s uncertainty on the estimation of the collision force was investigated as a case study on a probabilistic evaluation of the collision force.

The evacuation behavior observed in Kesennuma City, Tohoku, Japan during the Great East Japan Earthquake and Tsunami and a corresponding evacuation simulation are presented. We first organized the damage and evacuation behaviors observed in the target area using the results of previous studies and official surveys. In addition, we compiled all available multimedia sources recording evacuation behavior during the 2011 event to identify the precise temporal and spatial details of the evacuation behavior. Then, a tsunami evacuation simulation was developed based on all compiled evacuation data, considering the tendencies regarding the use of main roads for evacuation, residents’ shelter preferences, and pedestrian-car interactions. The developed simulation was validated against the compiled evacuation behaviors by inputting the estimated initial conditions of the 2011 event. The traffic scenarios calculated in the simulation closely reproduced the actual traffic flow as observed from the evacuation data. The evacuee populations at several shelters in the simulation also quantitatively reproduced the trend of the real numbers of evacuees reported by Kesennuma City. The results of the simulation exhibited a better capability to estimate the actual evacuation behavior during the 2011 event than that achieved in previous studies.