The well-known destructive tsunami triggered by 2011 great Tohoku earthquake in East Japan has shown that the existing tsunami breakwaters are not enough to prevent big tsunamis. Some breakwaters were completely collapsed by earthquakes or tsunamis. It was found out that, even though in the intact condition from the tsunami attack, the existing tsunami breakwaters were insufficient for sheltering corresponding harbor and coastal areas, typically in Ofunato Bay. In order to solve the tsunami disaster problem, the control function of tsunami breakwaters was revised and improved as compared to the previous one. Especially, the 3-D structure of the opening mouth of tsunami breakwater is a major point of this study.

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In Hokkaido, 180 slipways have been installed along national roads for small fishing boats, and half of these between vertical seawalls. A sight-seeing bus, running on a coastal road, was damaged by a wave that overtopped a slipway between vertical seawalls on September 8, 2004. In this study, the mechanism whereby waves are concentrated such that they overtop the slipway was clarified by numerical analysis and hydraulic experiments using regular waves. Furthermore, wave forces acting on a vehicle running on the coastal road were also examined by hydraulic experiments with irregular waves. Based on the above results, a safety operation system for the site of the accident at the time of storm waves was proposed.

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In this study,the change characteristics of wave dissipation performance of rubble mound seawall under the damage progression of armor layer are investigated using a numerical wave flume and a series of model profiles for damaged armor layer. The reflection coefficient and overtopping rate are used as the performance measures. A number of irregular waves with different wave profiles and wave group properties are generated in the numerical wave flume to obtain many data of the performance measures. Secondly,a neural network model is applied to predict the change of the performance measures with the cumulative damage of armor layer. The three-layered neural network calibrated by the results of the numerical experiments can predict the reflection coefficient with sufficient accuracy and the overtopping rate reasonably well.

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The reduction of wave overtopping and wave force by a porous wave barrier was investigated to clarify hydraulic performance of such a coastal structure. Two-dimensional model tests for a wave barrier mounted on a seawall were carried out to examine variations in regime of wave overtopping, mean overtopping rate and maximum wave force acting on the barrier with changing barrier parameter and incident wave intensity. The test results show that the porous wave barrier can reduce the overtopping rate and wave force in different overtopping regimes effectively. These reduction effects of the barrier are mainly governed by the porosity of barrier and several wave parameters. Their reduction ratios between the presence and absence of the barrier are influenced by variations in the wave run-up height induced by barrier installation.

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Authors have developed a measurement technique using Particle Image Velocimetry(PIV) for the purpose of accurate measurement of individual wave-overtopping volumes in two dimensional wind-wave flume experiments. In order to clarify the characteristics of appearance frequencies of individual wave-overtopping volumes for a vertical seawall, model experiments were conducted generating irregular waves and winds at the same time. The main conclusions are as follows: 1) Frequencies of large wave-overtopping increase as the wind becomes stronger. Probability Density Functions for normalized wave-overtopping volumes, however, are constant and agree with an exponential distribution with the expected value of 1.0. 2) Mean wave-overtopping volume can be estimated from wave overtopping rate.

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Numerical simulations on the wave overtopping phenomena were carried out for double seawalls with permeable bottom by using the numerical model of CADMAS-SURF. The double seawall consists of lower front and higher rear walls and permeable bottom between the two walls. The present paper has proposed the selection procedures for the optimal cross-section of the double seawall. The optimal cross-section can reduce the crown height of the double seawall. Consequently the double seawall is effective on the preservation of landscape.

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A new submerged breakwater, which has several vertical plates on a low mound, was proposed to simultaneously reduce both mean water level rise and transmitted wave heights behind the breakwater. In this paper, numerous two dimensional numerical simulations were performed in order to obtain fundamental knowledge on the effects of the new submerged breakwater. Similar numerical simulations were carried out for an existing large submerged breakwater as well. Wave spectra, significant wave heights and mean water level rises behind the breakwaters were obtained from the numerical results. Comparison of them indicates that the proposed submerged breakwater can effectively reduce the transmitted wave and mean water level rise at the same time.

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In tsunami disasters, many objects such as building, houses, cars and ships move under the tsunami wave forces. It was difficult to calculate wave-rigid body interactions,such are displacement of the body due to wave forces and moving boundary condition of the body to wave field. In this study, a new algorithm for the interaction is developed,and added to numerical code for tsunami wave run-up scheme. In this algorithm, pressure and shear stress for the same direction are summed for translation motion. Rotational torque is also calculated to know rotational angle. Numerical results are confirmed by animation of the results. Results are quite reasonable and satisfactory.

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After a mega earthquake occurred off the Pacific coast of Tohoku region in Japan, some of coastal structures were collapsed owing to the local scouring by overflow from seawalls. In order to understand the characteristics of the scouring behind seawalls due to tsunami, experimental studies and numerical simulations were carried out. Also tsunami energy damping was estimated by artificial trenches. As soon as overflow starting, the local scouring profile reached 80% of the maximum profile in a short time. Turbulent energy and dissipation distribution had effects on development of scouring. A trench can reduce tsunami energy, but the differences of tsunami energy damping due to shapes of the trench is small.

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Flow of tsunami past a section of a concrete girder bridge has been simulated by using a Large Eddy Simulation (LES) technique. The forces and moments on the fixed or simply-supported girder due to the flow were evaluated and the motion of the bridge deck, assumed to be a rigid body, is solved simultaneously with the flow. The force results on the fixed deck agree with previous studies. The initial motion of the bridge deck was found consistent with the implication of the survey of the site of the failed bridge and accounts of the observers and the video images.

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In the Tohoku-Pacific Ocean Earthquake, many bridges were washed away by the tsunami, the wave height of which exceeded the conventional assumption. In the future, reinforcement of tsunami disaster prevention measures will be needed, and the evaluation of tsunami wave force acting on bridges will be very important. In this study, numerical analysis using VOF method was carried out to evaluate a tsunami wave force acting on a bridge superstructure that is generally used with highway bridge. And by comparison with the waterway model experimental result, applicability of the numerical analysis was examined for the evaluation of tsunami wave force on bridge superstructures.

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In this study, the characteristics of tsunami water pressures acting on a parapet and on the ground behind the parapet are clarified by wave flume experiments. The pressures acting on a lower part of the seaward face of the parapet are larger than that on a higher part of the seaward face of the parapet, which is consistent with the previous studies. It is shown that the maximum pressures acting on a lower part of the landward face of the parapet are smaller than that on a higher part of the landward face of the parapet. The influence of flow separation on the pressures acting on the landward face of the parapet is under investigation. As for the ground behind the parapet, when the tsunami wave height is approximately 2 times higher than the crown height of the parapet, the ratio of the measured water pressures to the wave height reaches the maximum value.

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Refuge facilities are required to save human life from a tsunami. Fender piles around a refuge facility could be a countermeasure to protect the facility from tsunami driftage. Hydraulic physical model tests were conducted to investigate tsunami wave force on a refuge facility with fender piles. Although it was found that fender piles had potential of wave force reduction, it was strongly depended on a tsunami wave profile. Visibility of a refuge facility should be high for evacuees. But there is a possibility that fender piles could bring it to deterioration. So, relationship between the layout of fender piles and the visibility was studied qualitatively. It was deduced that straight layout of fender piles would give safety feeling to refugees in a refuge facility and staggered layout would provide it to evacuee approaching toward a refuge facility.

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Hydraulic mechanism of sustained tsunami wave pressure acting on land structures was investigated theoretically. The effect of wave pressure amplification due to wave reflection was estimated based on momentum conservation law. On the other hand, backwater effect was modeled by energy conservation law. The formulas which indicate wave pressure on 2D wall and 3D structure were derived. The theoretical result was compared with the hydraulic model test results and numerical computation results and good agreement was found out.

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Koizumi Bridge, located in the Kesennuma City of Miyagi Prefecture and with its superstructures washing away, is investigated. Based on recorded video, the tsunami runup is analyzed; the shape of tsunami wave impacting Koizumi Bridge is evaluated simultaneously with estimation of the flow velocities. As the consequence, upon a flood-like flow with water level increasing slowly, a 2.5m high breaking wave acted on the bridge with velocity as 3m/s. In accordance with the velocities from Numerical Simulation, the outflow mechanism of Koizumi Bridge is figured out clearly as the greater tsunami impact force than the bridge resistance when velocity is 5m/s. Additionally, it is learned that the increasing of the flow velocity in Koizumi area is caused by the terrain feature.

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In this paper, the tsunami damage determination method of the storage tank is proposed. The tsunami damage determination was performed combining three-dimensional tsunami analysis and structural analysis. First of all, three-dimensional tsunami analysis was conducted and the pressure which acts on a tank was computed. Next, the structural analysis model which can express a tsunami damage situation was created, and the computed wave force was made to act. In addition, the tsunami damage determination was performed from the structural analysis result. From this result, it was possible to confirm the usefulness of the damage determination tsunami of the tank by the proposed method. Finally, it was confirmed that was resisted the tsunami by installing the anchor bolt and the behavior lifted by the amount of solution in the tank is changed.

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After the 2011 Great East Japan Earthquake, it is becoming important to consider the reappearance of tsunami damage by the fluid dynamics numerical analysis. Also, the tsunami three-dimensional simulation is being received much attention. However, practical applications of the three-dimensional fluid numerical analysis in the tsunami simulation have not been investigated yet. Therefore the purpose of this study is to propose the three-dimensional simulation method of a large domain using the method which combined two-dimensional analysis and three-dimensional analysis. Moreover, this method is applied to the Koizumi bridge area which suffered a great deal of damage.

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The wave force and wave pressure acting on a model bridge due to tsunami are measured. The tsunami is generated by two different methods in the experiment. The one is the wave like a bore by dam breaking and the other is the wave like a solitary wave by piston type wave maker. The wave pressure acting on the underside of the model bridge under the non-breaking wave condition is much larger than that under the breaking wave condition. The difference between them results from the air compression at the concavities under the model bridge. There is no significant difference between wave pressures due to the waves by two different methods.

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Measures to mitigate tsunami disaster damage to drainage pump stations in coastal areas were studied by conducting a field survey of drainage pump stations that suffered a great deal of damage from the 2011 Tohoku Pacific Ocean Tsunami, and by performing an experiment with a hydraulic model. The field survey showed that the spatial relationship between the pump house and the water discharge tank greatly affected the level of damage suffered by the drainage pump station from the disaster. The ability of the water discharge tank to dissipate the tsunami energy was examined in a hydraulic model experiment. The result showed that even if the tsunami wave height exceeded the height of the water discharge tank, the wave force in the direction of movement of the tsunami wave was reduced.

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Many coastal structures have been destroyed by the tsunami in the 2011 Tohoku Earthquake and urgent countermeasures against the tsunami are desired. The tsunami evacuation buildings can be one of effective countermeasures and tsunami force and tsunami run-up height on the buildings have been studied using impermeable structures. However, effect of windows and other openings, location of walls may have significant effect on tsunami forces and run-up. This study is intended to investigate effect of configurations of building structure on them. Tsunami force is found to be reduced in the case that impermeable wall inside building is set landward. Moreover, this paper shows that it is possible to estimate the tsunami force on buildings considering openings of the outer wall and configuration of inner walls.

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We have carried out three-dimensional numerical simulations of a tsunami passing through a structure to investigate the characteristics of the hydrodynamic force induced by a tsunami. In the simulation, an idealized tsunami was used, which has a sharp head and a constant flow velocity and a constant flow depth in the upstream region. The results show the water level on the front face of the structure can be estimated by using the specific energy of the upstream region of the structure. In addition, the hydrodynamic force acting on the front face of the structure is approximated as the hydrostatic force with the water level of the front face of the structure. Thus, this algorithm enables us to estimate the hydrodynamic force acting on the front face of the structure by using the specific energy predicted by tsunami inundation simulations.

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Among the various offshore structures which are needed for the development of resources in ice covered seas, columnar structures were selected for the study on the oscillating phenomenon caused by ice floe and the dynamic response caused by earthquake. This research was performed by the model test and numerical analysis. It is aimed to propose the ice load evaluation method for the design of ice resistant structures by considering the dynamic interaction between structure and ice floe. The suitability of using the urea model ice, ratcheting phenomenon, maximum ice load and dynamic behavior of structure are presented in this paper.

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A simple judgment method of moving and drifting possibility of debris due to tsunami is proposed. Using it, relationship between the inundated water depth and the critical tsunami velocity for sliding and stumbling is investigated subject to the debris such as lumber, car, truck, boat and light oil tank at power plant site. Referred to the past major formulae of collision force of debris, a practical estimation method of collision force is improved by considering the elastic and plastic deformations of debris. The present method can reproduce collision force of debris by past experiments with several scale sizes better than the past major formulae.

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In this study, we investigated damaged situation of floating structures in Miyagi Pref. caused by tsunamis in the Great East Japan Earthquake and it made clear that some structures mooring with pile were flowed away. However, structures mooring with chains were never flowed away. we used a case of Izushima, Onagawa-town to examine a mechanism suffered floating structures that mooring with piles and chains. In the case of pile mooring, from the result of examination, it was estimated that piles were left out by tsunami water level rising. In the case of chain mooring, at the result of numerical calculation using by STOC and mooring simulation, it was found that dredging anchor blocks became important factor against outflow.

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The 2011 off the Pacific Coast of Tohoku Earthquake Tsunami at 14:46 JST, 11 March 2011 affected the large areas of eastern and northern part of Japan. Many sea disasters of vessels moored along the terminals such as ship drifting and grounding, and damages to the mooring facilities and equipment were occurred. Recently several reports such as questionnaires of the sea disasters have been reported. Among the sea disasters, there was a successful example of safe mooring harborage of an Aframax tanker (100kDWT class crude oil carrier) during the tsunami. In this study, we have carried out a survey of the mooring harborage by a hearing investigation to the crews, and tried to reproduce the ship behaviors during the tsunami by the numerical simulation methods on ship motions and tsunami to find out the causes of the safe harborage.

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Interaction of surface water waves with a floating platform is discussed considering nonlinearity of fluid motion and flexibility of oscillating structure. The set of nonlinear shallow water equations is applied to water motion interacting with a horizontally very large and elastic thin plate floating on the water surface. Calculation results of the floating thin-plate displacement due to the propagation of a tsunami, in terms of a solitary wave, are compared with the existing experimental data. Surface waves generated by the takeoff or landing of an airplane are simulated, such that when the offshore airport is constructed in shallower water, the slope of runway becomes larger and the complicated oscillation is harder to be damped down because of the reflection of floating thin-plate waves at the ends of the offshore airport.

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When a huge tsunami attacks on a composite breakwater, overtopping flow can scour a caisson mound and a seabed and the resultant growth of scour hole may cause sliding or overturning of a caisson. Conventional design methods, which do not suppose this kind of catastrophic failure, are not the reliable tool to predict a structural resistivity against a huge tsunami attack. While, a numerical wave flume to provide time-dependent solution of Navier-Stokes equation can be a key tool to show a rational answer. In this study, a failure of composite breakwater brought by a scouring of seabed is simulated by an accurate particle method. And a measure to prevent the failure is examined by the numerical wave flume.

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In the initial report on a tsunami disaster in the Great East Japan Earthquake, a scouring at a joint between caissons was listed up as one of the causes of failure of a composite breakwater. Although posterior experiments proved that it was not crucial mechanism, a flow field behind a caisson was not investigated in detail in those experiments. In this study, to clarify a complicated flow field driven by jet from a joint and tsunami overflow, we carry out a numerical simulation by using a numerical wave flume based on an accurate particle method. Especially, we focus on a three-dimensional flow structure around a joint between caissons.

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A three-dimensional fluid-structure analysis model, which consists of a computational fluid dynamics tool “OpenFOAM” and a structural model “SeanFEM”, was proposed in this study to discuss seawall failure mechanism caused by tsunami. The utility of the model was verified by applying it to the failure phenomenon of seawall in Kojirahama Fishing Port broken by the 2011 Tohoku Earthquake Tsunami. It was found from the numerical result that wave pressure rapidly increases after the tsunami hit the seawall. Moreover, the numerical results showed that the seawall turns over toward land in a similar way to the video images of the seawall failure taken at the port during the tsunami attack. Therefore, the model can be said to have a potential for examining the failure mechanism of coastal structures.

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Siding distance of a caisson is sensitive to a few or several extreme high waves in a storm. The high wave in the surf zone, therefore, should be accurately implemented in order to calculate accurate prediction of caisson displacement. This paper analyses impact of random wave height distribution with wave breaking effects on computed sliding distances of breakwaters. Three different distributions of wave heights after wave breaking are applied for the calculation of sliding distance. The numerical results of model breakwaters show that the computed sliding distances of breakwaters vary from 2 to 10 times due to the distributions of wave heights at the most sensitive condition. It is found that frequency distributions of sliding distances depend on wave breaking condition even if expected sliding distances are similar.

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The purpose of our study is to develop numerical simulation to investigate the toughness of tsunami disaster countermeasures. Moving Particle Semi-implicit (MPS) method was used in the simulation. The wall boundary was applied polygon-wall boundary instead of particle-wall boundary. As the result, we suggested that MPS used the polygon-wall boundary is possible to calculate with high accuracy fluid dynamics than particle-wall boundary. Furthermore, in the simulation of tsunami overflowing the breakwater, the presented method was able to reproduce the vortexes generating by tsunami overflowing the breakwater. The values of velocity also were in agreement with that of the model experiments. It was obvious that the roll-up flow accompanied with the vortexes is due to scouring of the mound.

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To prevent the overturning of a caisson on a mound due to tsunami overflow, the effectiveness of the change in the configuration of its landward upper part and the installation of a mooring cable at its seaward side is investigated using a coupled fluid-structure interaction model. Numerical results show that the change to slopes milder than 1/2 whose landward edge is under the still water surface can prevent the overturning of the caisson, suggesting the enhancement of its tenacity. This effect can be assessed using the scale of vortices formed at the landward side of the caisson. Furthermore, the mooring cable would be an effective countermeasure to enhance the tenacity of the caisson. However, the strength of the mooring cable and its attachment to the caisson and the mound needs to be assessed because of the action of large impulsive force.

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This study is an investigation of the mechanism of the washout of bridge girders by tsunamis, performed by conducting 1/10 scale model experiments and numerical calculation for the case of the Numata Overpass. First, the flow experiment conducted. It confirmed that the flow velocity below the bridge girder is higher than the flow velocity above the bridge girder, applying downward force on the bridge girder. Therefore, the result was that the bridge girder was stabilized by increasing resistance to the flow. Next, the solitary wave experiment confirmed that a lift force caused by fluctuation of the water surface washed out the bridge girder. Finally, CADMAS-SURF 3D replicated the experiments numerically and confirmed the validity of the calculations.

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A large number of breakwaters were damaged by the Great East Japan Earthquake on March 11, 2011. Many were destroyed by tsunami overflows. In this research, physical experiments were conducted using various breakwater cross-sections to clarify the exact failure mechanisms caused by tsunami overflows. Using the experimental results, the cause for the breakwaters' collapse can be verified using the safety factor of sliding, overturning and bearing capacity of each breakwater. If the overflows scoured the rubble mound, the safety margin was estimated using a post-scouring cross-section. Scouring by overflows decreases the rubble mound's bearing capacity, increasing the likelihood that breakwater would collapse. When the scouring reaches the bottom of the caisson, it generates a hole that the caisson gradually falls into.

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The measure of floating objects is regarded as important to reduce the second damage caused by tsunami, but there is little knowledge about measures. In this research, the capture effect of containers is examined by hydraulic model experiments using the tsunami barrier. As a result, when pile pitch is smaller than the width of the container, the outflow rate depends on height of the pile and inundation. In addition, the outflow rate extremely tends to rise when the depth of the inundation more than a half of the height of the tsunami barrier. Thus, it is necessary to secure the height of pile of the degree to double of the inundation height in order to get a high capture effect.

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Seawalls play a very important role in protecting Japan's coastline, but they may sometimes be destroyed by high waves. The leaning revetment is a common type of seawall structure. The modes of destruction of the leaning revetments were classified into two types: sliding of the bottom of the revetment bodies in the offshore direction, and joint fracture. However, factors which choose each destruction mode were unknown. In this study, experiments were conducted to inspect destruction processes of leaning revetments and its factors in putting emphasis on behaviors of revetments' bodies. The results of the experiments indicated that blocks in front of revetments might restrain sharp sliding of revetments' bodies, and a joint fracture might occur if an impulsive wave force acted on a revetment in particular conditions.

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In the port to be deep water area toward an opening there are many examples which take over to the breakwater with wave dissipating blocks and extend upright breakwater. In this case a discontinuous part appears and it is just going to be anxious about a stability decline and wave force increase of the wave dissipating blocks. Therefore, this plane experimental study was clarified the stability characteristic of the wave dissipating blocks when a wave was entered on a discontinuous part at angles more than 60 degrees and the wave force characteristic to be acted to a caisson and we were able to obtain the underlying data of the future breakwater design.

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Authors carried out hydraulic experiments to clarify the relationship between tsunami fluid force and tsunami velocity acting on a bridge deck subjected to surging breaker bores, plunging breaker bores, waves whose surface inclination is mild slope, and breaker bores with a bridge deck sets up under the still water level. Experimental results show the mechanism that the height of a bridge deck to a tsunami height with meanings of variation of surface inclination of waves causes a significant difference of wave velocity. In addition, it causes variation of Froude number, and leads differences of horizontal and vertical fluid forces acting on a bridge deck.

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The serious damage of coastal dikes occurred in the tsunami caused by the 2011 off the Pacific coast of Tohoku earthquake. It is important to reinforce the coastal dikes with concrete blocks and to give the function of tenacious structures. In case of coastal dikes, the stability and damage of coastal dikes are affected by the strength of foundation in spillway and the fluid forces acting on concrete blocks armoring on slope just near the crest of dikes in the side of land for the over flow in tsunami. The relationship between the upstream depth and pressure distributions under the concrete blocks on the mound of slope, and the fluid forces and overturning moment acting on a concrete block adjacent to the crest of dike in the side of land. It becomes clear that the concrete block leaves from the crest of dikes in the side of land in the case of overturning moment exceeding the gravity moment of the concrete block in water.

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This study discusses the evaluation method of target safety level of composite breakwaters and breakwaters covered with wave dissipating concrete blocks in view of minimum life cycle cost. Authors evaluated target safety levels of breakwaters under various conditions on waves and disaster loss costs. As the results, it was found that effect of disaster loss cost is smaller than that of wave conditions on target safety level. Target safety level was found to be strongly dependent on wave breaking conditions.

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A simple and high-accuracy stability verification method for armor units covering breakwater rubble mounds against tsunami overflow is presented. The method is based on hydraulic model experiments conducted in a wide range of conditions. Empirical formulae are derived to predict the overflow depth of the stability limit of armor units. The formulae take into account the effect of an impingement position of overflow jet and the effect of water depth by using two parameters. Numerical computation is also carried out aiming at the establishment of a stability analysis method for armor units. The validity of the computation method is confirmed by comparing the measured current field. The stability of armor units is investigated by computing the hydraulic force acting on each armor unit.

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"A pneumatic caisson mooring system", which is completely different from the conventional ship mooring, has been devised: a caisson, which is set at the sea bottom in front of the quay wall, starts to move upward, touches the ship's bottom and continuously lifts the ship upward. After loading and/or unloading, the caisson starts to move downward and, at some point, the ship separates from the caisson and floats freely. In order to investigate the response of the ship hull in these transient states, wave tank experiments were conducted. The markers, attached to the ship hull and the caisson, were recorded on video in motion, and their trajectories were analyzed. It was found that the ship moves upward at almost the same speed as the caisson just before contacting with it, and this always results in the ship's smooth landing on the caisson.

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In order to investigate the instability mechanism of caisson-type composite breakwater under tsunami condition, a scale of 1/100 model experiment was performed in laboratory. Loading tests were also carried out to investigate the reduction of bearing capacity under the existence of seepage flow. From the results of laboratory experiment and theoretical analysis, it was confirmed that the bearing capacity of rubble-mound can considerably decrease due to the tsunami-induced seepage flow. It is concluded that the effect of seepage flow in the rubble-mound should be taken into account when making a design of the caisson-type composite breakwater against tsunami.

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In this research, the stability of the quay is examined by hydraulic model experiments at the time of the back rush of overflow tsunami. As a result, by taking earth pressure in back into consideration, it makes clear that the collapse phenomenon of a quay is a slide due to water level difference between front and rear side of a quay. The value of pressure of front side, which is in contact with the rubble ground, has dispersion. This dispersion has influenced stability of a quay and it is producing variation in failure of a quay. Moreover, in order to abolish the variation in the pressure, when it verified using the front water level, it turned out that stability is verifiable to a safe side. In the range of this experiment, it is possible to verify the stability of a quay by applying this assumption.

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Massive tsunami caused serious dameges to breakwater. There are two reasons for a breakwater destroyed by tsunami. The one is wave pressure, and other is seepage flow. Seepage flow makes a rubble-mound unstable and piping. There are few studies of seepage flow. For this reason, this study aims at analyzing stability of breakwater in terms of seepage flow by using the Discontinuous Deformation Analysis and Shear Strength Reduction. This study analyzed breakwater in Kamaishi. The safety factor is calculated by checking displacement in the rubble-mound blocks. As the result from this study, the following conclusions have been obtained. (1) Safety factor can be calculated by change of the maximum displacement in rubble-mound blocks. (2) Safety factor improved by covering rubble-mound of the port side.

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The strong wave and flow generated by the run-up of Tohoku-Pacific Ocean Earthquake and Tsunami in 2011 has destroyed many structures. Although the destroyed structures are already started to be restored, the details of how these structural damages occurred have not been clarified. This study aims to consider the structural damages by analyzing the external forces which acted on the destroyed revetments. In order to obtain the external forces acting on the revetment, the tsunami numerical simulation using VOF method was conducted. The calculated water level reached over 10m and the flow velocity was 5-10m/s. It was estimated that these strong flow and pressure caused the destruction of the revetment.

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We recently applied a geosynthetic-reinforced soil (GRS) technology to increase the resistance of tsunami barrier to an overflow erosion. In the present study, we performed a series of model tests in the laboratory to evaluate the resistance of previous and new tsunami barriers on the strength against the over-flowing tsunami. We examined dynamical balance of concrete panels covering on the crest and its outflow conditions. The test results indicated that the GRS seawall has a function of tsunami barrier that has a high resistance to erosion due to tsunami overflow. The length of geogrid layers required to prevent outflow of concrete panels was evaluated quantitatively under various tsunami heights.

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Fluid motion around a coastal dyke due to overflowing tsunamis was investigated by conducting hydraulic experiments and analyzing videos recorded during the 2011 Tohoku Earthquake and Tsunami. The results of the hydraulic experiments showed that overflowing tsunamis could be classified into three types, and velocity field around coastal dykes and pressure acting on coastal dykes were different in these three types. This classification can be helpful to consider the mitigation of damage on coastal dykes. The videos showed that the type of overflowing tsunamis around coastal dykes varied with the location, and this type was considered to have an influence on how overflowing tsunamis affected coastal dykes in the 2011 tsunami.

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In this study, a new type breakwater with a submerged circular cylinder array was developed. Laboratory experiments were carried out for investigation on effect of the height of submerged circular cylinder array on wave absorber function. Experimental results were compared with theoretical solutions. For the other type of a new breakwater with rotatable circular cylinder array, the characteristics of wave absorption were investigated.

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The double row submerged breakwater (DRSB) has been adopted in Yoriihama area in Niigata west coast to prevent the nourished sand from leaking through the channel locally scoured behind the submerged breakwater. Field observation for wave and currents around the DRSB was carried out, and the observation results have been compared with the results of physical model experiment conducted in their designing. As the effects of DRSB installation, it is confirmed that the wave transmission ratio of the DRSB satisfies the target value. However, the reduction of water level and current velocity behind the DRSB has not been clearly confirmed.

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