To study nonlinear characteristics of freak wave, this research focuses on the spatial evolution process of weakly high-order nonlinear waves propagating over a sloping bottom. We use modified Non-Linear Schrö dinger (NLS) equation considering bathymetry change to conduct a Monte-Carlo simulation, and it gives spatial evolution of kurtosis and skewness of wave surface elevation at different cases. The effect of water depth and initial quasi-resonant interactions on wave evolution have been explored. We also discuss the relationship between kurtosis, skewness and maximum wave height, as well as their performance around critical water depth for four wave resonant interactions. The results indicate that in deep water initial BFI significantly effects kurtosis and occurrence probability of freak wave. From deep to shallow water, transition region around critical water depth makes this effect become complicated.

 For practical use, a numerical wave flume requires the ease of the connection to the other numerical models which calculate waves in a larger area. In the particle-based numerical flume, a traditional wave boundary model with a piston-type wave paddle is widely used. However, it requires an additional tuning procedure to reproduce the desired input waves by the piston motion of the wave paddle. Therefore, it is difficult to apply the model to, in particular, irregular waves. To resolve this problem, a new wave boundary model is developed. The proposed model uses only the wave heights as the input data. Its good performance is shown through benchmarks targeting regular waves and a solitary wave. Moreover, its good conservation of the water levels is shown through the numerical simulations of overtopping waves.

 In this study, the Faraday wave field, created in vertically oscillating cylindrical containers, was investigated to experimentally simulates a flip-through event caused by concentric focusing waves. We found parametric free-surface oscillation featuring transient oscillation modes. When a cavity, formed at a center of the container, is collapsed by convergent concentric waves, the cavity surface is ejected at high vertical acceleration to form the vertical flip-through jet. Capillary waves, generated at the tip of jet, propagate with orientation of the Faraday oscillation mode, which contributes breakup of the flip-through jet.

 This paper experimentally identified evolution process of residual foams on surfactant surfaces via coalescence, clustering and collapsing behaviors. We found the foam size spectrum has two predominant peaks for foams with/without coalescence. The fine foams are agglutinated to form clusters, while larger coalescent foams repeat coalescing to survive for long time in the diffusion process. The clustering process affect life time of the residual foams, which is statistically modeled in this paper.

 In a 2D simulation of a wave generated by a granular collapse, it is unavoidable to calculate a fluid-solid interaction using a drag-force model because it is impossible to solve a pore-water flow faithfully. Unless the behavior of fluid pushed out by a granular solid can be simulated presicely, however, the reproducibility of a water surface profile can be deteriorated. In this study, a particle-based simulation using MPS method is conducted; however, not only a drag-force model but also the apparent volume change is implemented as a fluid-solid interaction to the proposed model. In a simulation of a wave generated by a granular collapse, we examine the effect of the apparent volume change and the influence of two different drag-force models. It is shown that the apparent volume change is indispensable to obtain an accurate water surface profile. The difference of the calculated results by two drag-force models is slight; however, the model considering only a nonlinear term tends to evaluate a drag force more strongly.

 In this paper, an enhanced fully-Lagrangian meshfree implicit structure model and its corresponding FSI solver are presented for simulation of hydroelastic FSI (Fluid-Structure Interaction) problems. The governing equation of structure is the conservation equation of linear momentum along with assumption of linear elasticity. Time integrations of velocity and position are conducted with a high-order implicit time integration scheme, namely the Newmark-β method. The update of rotational matrix is newly enhanced by considering a novel two-step predictor-corrector computational algorithm. The proposed high-order implicit structure model is coupled with a refined version of MPS fluid model through a consistent FSI coupling scheme. Verifications are carried out for both implicit structure model and the corresponding FSI solver by reproducing a set of benchmark tests, including dynamic response of a free oscillating cantilever plate, fast rotation of an elastic beam and dam break with an elastic gate. The results show notable accuracy and robustness of the proposed structure model and FSI solver. In particular, incorporation of implicit structure solver clearly enhances the efficiency and applicability of the corresponding FSI solver.

 In this paper, a 3D fully Lagrangian meshfree numerical solver for modeling of hydroelastic Fluid-Structure Interaction (FSI) phenomena is presented. Enhanced version of a projection-based MPS (Moving Particle Semi-implicit) fluid model is coupled with an MPS-based Newtonian structure model, resulting in 3D MPS-MPS coupled FSI solver. The Newtonian structure model is founded on conservation equations of linear and angular momenta corresponding to a linear elastic solid. The fluid-structure coupling is conducted by incorporating the FSA (Fluid-Structure Acceleration-based) scheme, where fluid-structure interface boundary conditions are precisely satisfied. The performance of the presented FSI solver is validated by conducting several benchmark test cases with analytical and experimental reference solutions. The solver is proven to possess notable robustness without use of any artificial stabilizers that often lead to unphysical numerical dissipations.

 The extreme wave occurrence from normality and its prediction in deep water become getting clear according to many studies on offshore extreme waves. On the other hand, there are a few studies about the extreme wave occurrence in shallow water. In this study, a series of physical experiments with several slope bottoms were conducted to investigate the transient behavior of the high-order nonlinear wave properties and the extreme wave occurrence on the slope, including in the surf zone. As a results, the kurtosis on the slope follows the skewness changing developed by the second-order nonlinear interactions associated with wave shoaling and its behavior depends on the breaker type. Additionally, the dependence of the kurtosis developed by the quasi-four wave interactions in deep water on the extreme wave occurrence becomes weak on the slope.

 In this study, the Boussinesq-type model in Kim et al. (2009)¹⁾ was applied with the incorporation of the specific mangrove effects to simulate wave propagation and hydrodynamics in mangrove forests. Due to the depth-integrated assumption in Boussinesq-type governing equations, the mangrove effect was parameterized by the Morison-type formula (Morison et al. 1950)²⁾ as an additional force term. The force coefficients were determined based on the experimental findings in Chang et al. (2020)³⁾, who conducted modelscale laboratory experiments using 3D-scanned and printed mangrove trees to reproduce the root structure of a typical mature mangrove tree (Rhizophora species) at a reduced scale. Their proposed formulas were used to estimate the drag and inertia coefficients in the numerical simulation. The vertical variation of the mangrove structure due to the prop roots was also addressed in the computations. A preliminary model validation comparing with the experimental data was presented and discussed. Model tests using different configurations were also provided.

 From winter to early spring, water surface oscillations called "ABIKI", which is a kind of meteo-tsunami or seich, occur in the west coast of the Kyushu island in Japan. In particular, around the Port of Nagasaki, many studies have been conducted on the secondary vibration phenomenon, because damages due to the higher tide with seich have been often observed. On the other hand, there is information about inundation damage with any higher water event happened near the coast of the Port of Hakata in March of 2019. Although the potential for disasters due to inundation is considered to be extremely large behind the Hakata port area with the population density of more than 10,000 people / km², only a few studies have been conducted for that. Therefore, in this study, we analyzed the observed data including the water levels in the coastal area and precipitation etc. during the event of inundation damage in order to examine the relationship between the high water events with the occurrence of secondary vibration in the Hakata Bay in March 2019.

 As a result of the tide level data analysis, a maximum tidal level rise of 20cm from astronomical tide level was recorded at 22:09 on March 20^th, which was the highest in the month. Correspondingly, the Tatara River, one of the rivers flowing into the Hakata Bay, recorded the water level rise of more than 30cm. The results of the spectral analysis of the time series of the tide level data and corresponding atmospheric pressure fluctuation suggest that the highest tide level event could be owing to the secondary oscillation during the spring tide. In conclusion, it should be noted that there is a risk of flood damage around the bay due to the overlap of the high tide and the secondary vibration in Hakata Bay.

 In order to construct a monitoring network of ocean long waves off west of Kyushu, we analyzed the sea-level data simultaneously observed at six points in winter 2019. During the observational period, the secondary undulations occurred at Oshima fishing port with the periods of 24.3 minutes and 12.0 minutes and at Makurazaki fishing port with the periods of 16.8 minutes and 12.3 minutes. They became large when the ocean long waves are large off west of Kyushu. In addition, it was suggested that the sea-level fluctuations with the periods of 8-32 minutes at Meshima Island becomes a more useful indicator than those at Fukue and Uji Islands to monitor the long waves that cause the large secondary wave at Oshima Port. Furthermore, the sea-level fluctuations at Uji and Nakano-shima Islands were confirmed to be effective for monitoring the long waves causing to large secondary undulations at Makurazaki Port.

 A two-dimensional XBeach model faces difficulty in updating the bathymetry profile under calm conditions. This study investigates the inaccuracy of predicting the undertow, which is a seaward-directed current affecting the suspended sediment load. Progress was made in applying XBeach to the prediction of the observed undertow in the field in the Hasaki coast of Japan. The observations were performed between May 9 and June 2, 2016. To assess the capability of XBeach to reproduce the undertow under low-wave-energy conditions, the datasets observed during the calm situation (after May 24, 2016) are used, and the parameters sensitive to the undertow prediction are adjusted. The results reveal a time-shift problem that occurs at the time-varying undertow velocity. The comparison becomes better when the undertow is forward-shifted for 3 h but remains underestimated. To improve the XBeach default results, a modification is implemented based on the Stokes drift, using the coefficient of water depth. This approach improves the accuracy of undertow prediction in XBeach from bad to reasonable quality for the average of RMAE value (from 0.77 to 0.48), against the observation.

 In the previous studies, the development of nearshore currents and a rip current in the vicinity of an artificial headland has been observed on the Kashiwakuma coast facing the Pacific Ocean by tracking floats attached with a small GPS and photographing by UAV. The development of a rip current on a natural beach, however, has not yet been observed on this coast. In this study, field observation on the nearshore currents was carried out by tracking floats with a small GPS, and the diffusion of dyed water in the surf zone was measured by UAV on a natural beach of the Kashiwakuma coast. Furthermore, surf zone was observed using a web camera. From the imagery data, close relationship between the formation of a bar and trough topography in the surf zone and the development of a rip current was observed.

 Supply of water use as the agriculture water was formed mainly in the Koyoshi River. The Honjo plain field was known as production area creationg high quality rice. In addition, there were fewer rations than agriculture, but is used as water service and water for industrial use elsewhere. However, salinity intrusion was occurred by drought in the Koyoshi River. Distanve of salinity intrusion was from the river mouth to nearly 10 km. It cannot pump up water at Yurihonjo City Koyoshi distinct by salinity intrusion. Therefore, it was grasping to the situation about going up of the salt water and it is very important to examine a condition of the going up.

 In the study, it was examined salt going up and relations with external force such as river discharge, the wave height, etc., for the Koyoshi River. Koyoshi River was first-grade river in Akita Prefecture. As a result, it was able to confirm that salinity intrusion was frequent in the Koyoshi a clear relationship was not seen about wave height and the salinity intrusion.

 In Ibusuki port coast, a coastal improvement project is in progress applying the idea of integrated shore protection system combining jetties, offshore breakwaters, seawalls and beach nourishment. In the project, the local groundwater environment included “natural sand-steaming hot spring” should be carefully conserved, because that is an important tourism resource in the region. In this study, effects of beach nourishment against groundwater level and temperature have been investigaged by using numerical calculations reproducing phenomena confirmed by field observations in the test site of beach nourishment.An interesting change due to beach nourishment confirmed by the observations was that high temperature reagion expanded both vertically and horizontally below the nourished sand. The present numerical model has successfully reproduced the expansion of the high temperature region by taking a difference of permeability between the nourished sand and the exsiting sand into account. It seems reasonable to conclude that the expansion of hot groundwater region is caused by the covering finer nourishment sand with lower permeability than the in-situ sand, which suppresses the tide-induced upward flows and upward thermal diffusions.

 This study investigated mass transport processes from the Kuroshio toward coastal regions along the southern area of the Japan mainland using a downscaled oceanic numerical simulator (ROMS) and a particle tracking model. Eddy motions are enhanced in the vicinity of the Izu chain islands and coastal regions in summer. By contrast, in winter, enhaced eddy motions appear in south areas of the Kuroshio. The Kuroshio generates several flow branches due to the Izu-Ogasawara Ridge, which results in a cyclonic eddy in Enshu-nada and transport toward Sagami Bay. Mass transport from the Kuroshioo toward coastal regions are mainly caused by two Kuroshio branches toward (1) Enshu-nada and (2) Sagami Bay. The shoreward transport is much higher in summer than that in winter, since the Kurishio axis is located near the coast in summer. Enhanced eddies in winter contribute to vertical mass transport. The tidal forcing significantly promotes mass transpot toward coastal regions.

 This study investigates the generation mechanism and influences on surrounding marine environment of “Whirlpools at Naruto Strait”, which provide us an extraordinary seascape aimed at being registered as a world natural heritage site, by using a state-of-the-art high-resoluttion numerical ocean circulation modeling technique in a quadruple nested configuration. We found that the pronounced pressure gradient force associated with the meridional surface elevation difference is induced by a phase difference of bifurcating two major tidal waves originated from Kitan Strait that eventually results in intensified tidal currents at Naruto Strait. One branch of the tidal waves propagates counterclockwise along Awaji Island through Akashi Strait, whereas the other comes directly from Kii Channel. Consequently, the whirlpool emerges as a large number of submesoscale eddies primarily due to horizontal shear instability of tidal currents energized at the narrow gap topography between two headlands sticking out to the strait. A dipole of overturning vertical circulations appears underneath the whirlpools with convergent downward transport at the strongest tidal current near the center of the strait, leading to highly efficient vertical mixing. Such a three-dimensional nonlinear mixing promotes a time-averaged southeastward mass transport that extracts the water and materials from Harima-nada Sea to Kii Channel.

 Previous studies have shown that siltfences reduce the effect of preventing the spread of pollutants when flow velocity was increased. However, under the present circumstances, it may be necessary to use siltfences even under the condition that flow velocity is high. On the other hand, there is also an example that installation of multiple siltfences can efficiently prevent pollutants. In this study, a hydraulic experiment was conducted for quantitative evaluation of the diffusion prevention effect of pollutants using silefences under high-speed flow.

 As a result, it was found that the pollutants were more trapped by the influence of the first siltfence and the reduction of the puffing amount of the second siltfence. The deformed shape of the second siltfence was reproduced by using the previously proposed estimation formula with appropriate experimental constants. This effect was confirmed in the experiment using plane wave tank.

 Overflow of the seawall can be caused by sea level rise and enforced typhoon because of global warming. However, the method to evaluate the innundation risk simultaneously considering overtopping and overflow hasn’t been established. In this study numerical experiments by CFD toolbox OpenFOAM were conducted to analyze the condition where overtopping and overflow cause at the same time. At first, the validity of the existing model to calculate flow rate of overtopping and overflow has been checked. Next, the simulation suggested that innundation level on land can rise higher than the sea level because of storm surge and waves. Finally the simple formula to estimate the inundation level on land was proposed by considering radiation stress in the sea area.

 There are few previous studies of storm surge simulation using a non-hydrostatic model, and the applicability of the model has not been obvious sufficiently. In this study, we carried out inundation simulations around Nagoya port due to storm surge caused by a potential maximum typhoon using hydrostatic and non-hydrostatic models. It was found that there is little difference between the numerical results obtained from hydrostatic and non-hydrostatic models. Hydrostatic model is, therefore, judged to be suitable for storm surge-induced inundation analysis from the viewpoints of calculation accuracy and efficiency. The difference in velocity field in runup area was also confirmed by the division number of vertical mesh, and the attention should be paid to the influence of vertical mesh division on numerical results.

 Global warming is expected not only to intensify its strength on a landfalling typhoon in Japan, but also to slow its moving speed under the future climate conditions. A slow-moving typhoon is also expected to increase the maximum potential storm surge in the west side of Ise Bay (Port fo Tsu) rather than in the head of Ise Bay (Port of Nagoya), contrary to conventional knowledge. A mechanism of high-tide formation in the west side of Ise Bay is investigated in this study by sensitivity experiments on idealized topogcaphy using an empirical typhoon model and a storm surge model. The idealized sensitivity experiments suggest that the storm surge in the west side of Ise Bay is likely to be intensified by 1) strong easterly in the slow moving typhoon, 2) existence of Shima Peninsula, and 3) spatial distribution of water depth in Ise Bay, due to the convergence of water currents induced by dipole vortex generations in Ise Bay.

 In the global warming scenario A1B, a powerful intensity typhoon causes not only strong wind but also high tide in coastal areas. To improve the accuracy of the storm surge forecast simulation, it is necessary to correctly evaluate the wind drag coefficient, which represents the shear stress acting on the sea surface from the wind. However, it is uncertain whether the bulk formula of the wind drag coefficient based on the past typhoons is suitable for the storm surge caused by a strong typhoon. From this situation, we require the observation data of many typhoons generated by global warming.

 In this study, we study the characteristics of currents under a typhoon, which is comparable to a strong typhoon during warming, based on data obtained from long-term observations over four years at Amitori Bay, Iriomote Island, Okinawa Prefecture. The fixed observation of velocity and waves used a WavesADCP. As a result, it was clarified that the flow velocity and the momentum transport depends on the typhoon path even if the typhoon has a wind speed exceeding 25 m/s.

 Typhoon Hagibis (2019), which had a central pressure of 955 hPa at landfall, caused record-breaking damage mainly by heavy precipitation in the Kanto region, Japan in 2019. As a consequence, Tokyo Bay was fortunate enough to avoid huge storm surge disaster during the passage of Hagibis. Going forward, design storm surge values across Japan is thought to be dynamically estimated by the external force in this record-breaking typhoon. In this study, a high-resolution coupled typhoon-storm surge model is used for track-ensemble experiments on Hagibis to evaluate the maximum possible storm surge fields in Tokyo Bay and Ise Bay. The worst storm surge in Tokyo Bay is 2.32 m, and it is revealed that the record high (2.03 m) could be exceeded if Hagibis made landfall at the time of high tide. In addition, the maximum sea level anomaly in Ise Bay reaches to 3 m if Hagibis moves northward along the coast of Mie Prefecture, and it could exceeds the record high (3.89 m) by Typhoon Vera (1959) known as the Isewan Typhoon.

 Typhoon HAGIBIS caused enormous damage in JAPAN, 2019. The maximun surge height comparable to the typhoon Kitty was observed inTokyo Bay. On the other hand, it is reported that the tropical cyclones will be strengthened and the cource of cyclones will be change. In this study, impact assessment of return period of tropical cyclones on climate change due to changed tropical cyclones has been carried out using d4PDF and GSTM. It was found that the effect of reducing the number of tropical cyclones generated in the Northwestern Pacific greatly affects the long-term return period.

 This study investigated time-scales associated with mixing in inner parts of bay, lakes and reservoirs using a two dimensional (x-z) numerical simulator, SUNTANS. Several conditions of the horizontal scale of calculation area, stratification and wind speed were used for simulations with 𝑊_e ranging 0.1-10. This study employees teo time-scales: (1) time-scale required for a steady state temperature distribution estimated from the slope of the thermocline, 𝑇_s, and (2) time-scale required for complete vertical mixing, 𝑇_mix. 𝑇_mix is computed from change in the potential energy. A quarter of the internal seiche period was shorter than 𝑇_s and 𝑇_mix excepting highly nonliner cases. This implies that the lower limit of the mixing time-scale is determined by a quarter of the internal seiche period for liner conditions. For highly nonlinear cases, the internal Froude number was higher than unity. The distribution of the Richardson number showed that the vertical mixing expands from the leeward toward the windward.

 Typhoon climatology estimated by global climate model is important for impact assessment of climate change. We developed wave-coupled global atmospheric climate model and conducted a set of typhoon simulation with sea surface drag coefficient considering wave age and misalignment of wave-wind direction. A set of typhoon simulation was targeted to the 100 strongest historic typhoon in the Western North Pacific. The general characteristics of 100 simulated typhoons were analyzed and systematic impacts of wave-coupling on typhoon characteristics. As the results, the systematic impacts on typhoon intensity are not seen. On the other hand, it is found that typhoon tracks at mid-latitudes adjacent to Japan in wave-coupled simulations tends to pass about 1◦ east-ward relative to non-coupled simulations. Systematic differences in typhoon track can affect climate change assessment because long-term (climatological) characteristics simulated by global climate model are used for the assessment.

 In this study, a three-dimensional large eddy simulation with a sharp interface model, based on ghost velocity extrapolations to fulfill dynamic boundary conditions at the interface, were performed to identify the interfacial instability to form water waves owing to turbulent wind. We found multiple pairs of helical counter-rotating vortices in the wind axis displace the interface to form extension of longitudinal scars aligned to the spanwise direction at critical wind velocity. Capillary waves are then created under spanwise vortex pairs appearing in the wind boundary layer.

 Whitecaps generated by wave breaking on the ocean surface play an important role in the local interaction across the air-sea interface. Whitecap coverage is defined by the area of whitecaps per the unit ocean surface. It has been recognized as one of the most valuable physical quantities for describing the ocean surface fluxes such as the momentum, heat and carbon dioxide, so that the quantitative evaluation of whitecap coverage becomes significant from viewpoints of coastal and ocean engineering. In this study, a progressive high-precision whitecap extraction model is first built by using the algorithm of deep learning. Compared with a traditional whitecap extraction model based on threshold value, the algorithm is found to solve problems caused by illuminance condition and color change on the ocean surface, and effectively extracts fine whitecaps with complicated structures. Further, through comparisons with previous algorithms such as Automatic Whitecap Extraction (AWE), Iterative Between Class Variance (IBCV) and the whitecap extraction based on fixed threshold value, the present algorithm is demonstrated to be more accurate for identifying whitecaps, and it reduces the amount of evaluation load, and can effectively apply for changeable ocean conditions. The new whitecap extraction technology is used to determine whitecap coverage when shooting digital images under complicated sea surface conditions. Due to the progressive characteristics of this algorithm, it has not only a high precision processing effect on images taken by a fixed camera, but also has the potential to analyze accurately images from a non-fixed camera system, such as an observation ship equipped with camera system, unmanned aerial vehicle and so on.

 It is important to characterize the transition process of wind-driven water surface to be closely connected to the momentum and gas exchanges across the air-sea interface. In the present study, the transition of the wind-driven surface flow was investigated by means of laboratory experiments, which were carried out using a wind-water tunnel, 17m long, 0.6m wide and 0.8m high. The velocity of the wind-driven surface flow, which is a Lagrangian surface velocity consisting of the Eulerian flow velocity and the Stokes drift velocity, was evaluated by measuring the velocity of float disk rafting on the water surface. According to the experimental results, we examined the critical conditions under which the micro-scale breaking and bubble-mixed breaking waves appear on the water surface. The relation of the surface flow velocity with the friction velocity was found to be changed around u^* = 0.3m/s like the relations with the drag coefficient and the Stokes drift velocity at the water surface. Our experimental results also showed the behavior of the wind-driven surface flow velocity to be varied depending on the windsea Reynolds number. The results suggest that the wave breaking controls the wind-driven surface flow.

 The numerical experiments on the sea surface drag coefficient and mixing layer depth (MLD) for historical event of super typhoon Haiyan in 2013 were carried out. A high-resolution air-sea-wave coupled model was used for modeling. The drag coefficient was estimated by two formulas depending on wind speed and wave steepness with the saturation condition under high wind speed. The condition of MLD was climatrogical normal value plus/minus standard deviation in SODA reanalysis data. Histrical event was occurred under positive (thick) condition. The difference of the drag coefficient formulas gave smaller effect on the typhoon intensity (approximately 5 hPa) than the saturation condition under high wind (approximately 45 hPa). The experiment with wave steepness without saturation estimated the results along the best track. Larger MLD developed stronger typhoon, and the difference of standard deviation value of MLD gave 5 hPa. The change of water temperature structure exited by the passing typhoon was composed two layers as the surface layer and the bottom of MLD, and this structure was independent of the MLD.

 Although the many studies reported the quality of wave fields in a coastal area simulated by the third‐generation wave model, there is little information on the difference in the model performance among various sea conditions such as windsea and swell and the multi-modal sea. This study concentrates on the validation of the two wave models, WW3 and SWAN, under each sea condition using coastal wave measurements along the Japan coast. With the data processing of spectral partitioning for the directional wave spectrum, it was confirmed that both the multi-modality on the directional wave spectrum and the fraction of windsea or swell showed the different features in Japan sea coast and Pacific coast, and the features were almost reproducible by either two wave models. By comparing model accuracy under the swell sea conditions on the Pacific coast, it was suggested that the increase in the RMSE of the mean wave period on the Pacific coast was related to the model accuracy under the swell or multi-modal sea conditions.

 In recent years, damages to port and harbour structures caused by swells have been reported. A double-peaked spectrum is observed when wind waves and swells from a remote source coexist. Its importance was suggested in the revised technical standards for port and harbour facilities. The statistical properties of double-peaked spectrum waves on the upright section of composite breakwaters under the action of those waves were investigated by the experiments in wave flume. The design wave periods for calculating the forces are also discussed on the basis of experimental results of Tanimoto et al.(1986). They showed that the wave force can be calculated if the significant wave period of the double-peaked spectrum wave is obtained directly by individual wave analysis. This study examined the calculation method of the significant wave period TS and the significant wave height H_s of double-peaked spectrum waves. This study compared TS and HS calculated by three existing formulas for the double-peaked spectrum wave, and the applicability was examined. In addition, we proposed a modification coefficient of Tanimoto et al.'s formula for a significant wave period and confirmed accuracy improvement.

 The East Japan Typhoon (Typhoon 1915), which crossed Tokyo Bay in September 2019 and caused extensive damages to port and coastal facilities in the bay, was a compact typhoon with a large pressure gradient near the center. In the ports of Yokohama and Chiba, there occurred many damages such as destruction of seawall parapets and inundation by high waves and wave overtopping . In particular, the directional spectra of the estimated waves off the port of Yokohama show two distinct peaks at almost same period but at different directions. As a result, the wave force was enhanced by the unusually large waves, which are thought to have caused the damages at Yokohama. It is assumed that the rise in average sea temperature in the Northwest Pacific Ocean will continue in the future, increasing the like-lihood of typhoons different from before, such as Typhoon 1915.In this study, we analyze the characteristics of the wave field in Tokyo Bay and the damages caused by Typhoon 1915, and point out engineering items necessary for the maintenance of port and coastal structures against the new type of typhoon that is likely to hit in the future.

 In order to observe and estimate highly accurate directional spectra with a multi-layer ultrasonic Doppler-type directional wave meter (DWM), the characteristics of the water particle velocities in the oblique direction at the multi-layers measured with DWM were examined. Furthermore, we proposed a new transfer function based on observation data that can contribute to the improvement of accuracy of the directional spectrum estimation in consideration of their characteristics. In addition, we examined the weighting function of the weighted least squares method applied when solving simultaneous integral equations for estimating the directional spectrum, and proposed an appropriate weighting function that can estimate the directional spectrum with higher stability and reliability. By introducing these proposed methods to the Bayesian directional spectrum estimation method (BDM) and applying BDM to field observation data, we could prove to estimate the directional spectrum with highly accuracy, stability, and reliability.

 Approximate functions such as a linear function or a wave steepness function are often used as estimation methods of the extreme wave period. However, such estimated wave period often lead to underestimation or overestimation. The purpose of this paper is to propose a new estimation method using the wave development coefficient for the extreme wave period and to examine its applicability. The applicability of the proposed model to three stations with different wave characteristics was verified, and it was confirmed were in good agreement with the observed values on all stations. The standard deviation of the wave period was also modeled, and the model was verified that it almost agrees with the observation.

 Wave-current coupled models have been developed to estimate waves and storm surges in coastal areas where wave-current interaction is active. Many of the models are structured-grid models that are difficult to accurately represent complicated coastlines. On the other hand, unstructured-grid models have high reproducibility of coastlines and can perform highly accurate simulations, however that coupling models are few examples of constructing. The main reason is that unstructured-grid models have heavy computational costs, which makes it difficult to perform wide-area and long-term simulations. In this study, we developed a 1-way coupled model from flow to wave by coupling the existing unstructured-grid wave and current models. In the coupled model, different computational grids can be set for wave and current to perform highly accurate simulations while reducing the computational cost.

 Numerical simulation of wind-induced wave in Tokyo Bay under typhoon Faxai (T1915) was carried out and the results are discussed in the paper. Accurate simulation of the wave height distribution in Tokyo bay, especially during the peak period, has been a significant challenge in the past. Majority of the challenge has been posed in finding an accurate wind field to represent the complex gusts under major typhoons, which eventually acts as the energy source for the wave generation. The motivation of the study is to assess the effectiveness of different wind sources of Typhoon Faxai through wave simulation model. In the study GPV wind data released by JMA were used as the main wind source and later wind field was improved through typhoon-bogussing and 4DVar data assimilation. The simulated wave properties (using each of those wind field as the source input) were compared with the wave observation data from locations in Tokyo bay. Results indicate that a significant improvement of peak wave height in Tokyo-bay under typhoon Faxai can be achieved through processing the available wind through 4DVar data assimilation. The results are presented in the paper and improvements and drawbacks of the different wind fields are discussed in the view point of wave simulation.

 Weather chart-based atmospheric pressure data samples over a period of 47 years were gathered to estimate the radius to the typhoon-generated maximum wind speed, so-called typhoon radius R. Any of the 6 kinds of data samples shows that the radius R increases very slowly at first and then increases more rapidly with increasing central pressure of the typhoon P_c, although the relation has been associated with large data scatter. Next, data samples of wind speeds in typhoons over a number of years were analyzed by using Myers's pressure pattern-based gradient wind speed model corrected for the speed of advancement of the typhoon. Using this model, the estimated typhoon radii, similarly to the pressure-based results, show a consistently slow increase followed by a rapidly increasing trend with the central pressure, but the rate becomes increasingly smaller than that of the former case.

 The present study aims to predict nearshore wave heights and periods for one week in advance on the Japanese coast using Group Method of Data Handling with actually distributed three global wave forecast data. The results indicate that the GMDH-based wave height prediction model can improve the prediction up to 60 % in mean square error, while the GMDH-based wave period prediction model can do it up to 70 %. As a result, it is found that the best performing combination of three global data for training the GMDH-based model depends on location.

 The risk of Yorimawari-wave disasters will increase due to the strengthening of extreme events caused by climate change. Statistical evaluation of low-frequency high waves is important and requires long-term wave data. However, it is difficult to reproduce high waves on the coast of Toyama Bay by the numerical wave model. The authors have developed the neural network (NN) model to estimate wave height at the NOWPHAS observation point of Toyama Bay, but there was a problem in its applicability related to the characteristics of low pressure. In this research, we construct a versatile NN model by incorporating multi-time learning of teacher data and stacking ensemble learning. Then, new NN model estimates the wave height and period for the period without observation data and the statistical evaluation of the low-frequency waves are performed. As a result, a general-purpose and highly accurate NN was constructed, and the statistical evaluation of low-frequency waves at NOWPHAS Toyama point revealed that the observation period was insufficient.

 It has been reported that the optimal route can be used to CO₂ during ship navigation, and it is important to predictthe wave field around the ship up to several hours ahead. In order to improve the avvuracy of local and short-term predictions such as the around field around ships, it is necessary to develop a mechanism to correct the prediction using ship observational data. Therefore, in this study, we constructed a data assimilation system that applies EnKF to a wave prediction model SWAN, in order to make wave predictions using real-time observational data obtained from ships. As a result, improving the accuracy of the ship position and usefulness of EnKF were shown by performing data assimilation using ship observational data. Moreover, in order to improve the accuracy in coastal area and the accuracy of the whole sea by increasing the number of observation points, we investigated the accuracy using NOWPHAS wave observational data. Compared with the estimation accuracy before data assimilation, the accuracy was improved about 27 % at the maximum.

 We conducted the prediction in case of the 2019 typhoon Hagibis to examine the accuracy of the surge and waves prediction using global WRF. We obtained the most reliable sustained maximum wind speed and peak value of surge and significant wave height in case that simulation of the typhoon was started approximate 1000 km offshore from Japan coast. The predicted maximum storm surge was underestimated because the propagation speed of typhoon was underestimated and the wind direction was different from the observation. In addition, in the real-time prediction, we have to consider computation time and computation efficiency not only prediction accuracy. However, the appropriate model configuration from this point of view hasn’t been investigated, yet. To achieve the cost-effective storm surge and waves prediction with high accuracy, the relations among prediction accuracy, computation time, and computation load should be generalized. The results of this study presented the best prediction start timing to achieve the cost-effective prediction in case of the 2019 typhoon Hagibis. More prediction cases must be accumulated to generalize the model configulations.

 Wave-induced turbulent kinetic energy (TKE) plays an important role in numerical simulations of upper layer mixing in the ocean. In this study, improvements to a sea surface mixing parameterization have been made from the viewpoint of wave breaking based on observational data. The data shows that wave dissipation energy is more likely to be diffused into the water column when wind and wave directions are in opposite directions. In addition, the impact of the wave-dependent paramterization of ocean mixing on a numerical simulation of typhoon Haiyan (2013) has been analysed using the ocean-atmosphere-wave coupled model (COAWST). The results indicate that differences in sea surface temperature (SST) arise depending on the applied bulk formula used. It is also clarified that this difference originates from short wave radiation and internal heat transport induced by advection and vertical mixing. These differences have an impact even on atmospheric conditions such as wind speed, sea level pressure (SLP), and heat flux. The changes of both heat flux and wind energy are strongly correlated with that of minimum SLP . This study shows that wave conditions have a wide effect on ocean physics and atmospheric conditions; therefore, parameterizations that are wave-dependent are crucial for numerical simulation of sea surface mixing.

 We conducted tsunami propagation simulations using a 3D tsunami model based on OpenFOAM and compared their accuracy with an analytical solution and results by 2D tsunami models. The results showed that the 3D tsunami model is capable to reproduce disperse waves of shorter wavelengths with higher accuracy than the conventional 2D dispersive wave tsunami model. Moreover, for the Nankai Trough giant earthquake simulation, it was suggested that the 3D tsunami model could predict the short-wavelength tsunami caused by a large slip zone near the trench axis with higher accuracy than the 2D model. The computation time required for the tsunami simulation of the Nankai Trough giant earthquake case was faster than real time with the parallel calculation using about 1,000 cores. It was confirmed that a wide-area 3D tsunami propagation simulation that covers the tsunami source region of the Nankai Trough giant earthquake can be practically imple-mented if an appropriate parallel computing environment is available, and is effective to reproduce short-wavelength component of tsunamis.

 This paper investigates the relationship between length of the porous barrier, distance from the barrier to a rectangular structure, and the characteristics of shielding area behind the barrier. For this purpose, systematic numerical simulations are carried out based on high-resolution scheme and quadtree grids. The results deomonstrate that tsunami inundation depth is reduced inside the shielding area. The transverse length of shielding area is related to the lateral length of the barrier regardless of the porosity of the barrier. Numerical simulations show substantial reduction of hydrostatic pressures acting on the structure installed behind the barrier. Hydrostatic pressures acting on the structure approach constant values with the increase of the ratio of detached distance of the structure to the lateral length of the barrier. Tsunami run-up height in front of the structure can be evaluated from the corresponding specific energy in case of no structure.

 In the present study, large-scale nearshore behavior of the 1833 Shonai-Oki tsunami was investigated. Special focus was placed on a tsunami inundation observed at Sakaiminato City located in Miho Bay, western coasts of Japan Sea. First, observation data for the tsunami was carefully investigated, and the results indicated that the inundation was generated due to a wave amplified by a coastal resonance. Resonance characteristics along the coasts were then investigated through a field survey and a numerical experiment. Finally, the amplification mechanism of the tsunami was analyzed considering the estimated resonance characteristics and a tsunami waveform determined using a source model. The results indicated that multi-scale resonances largely enhanced the tsunami height on the shores of Sakaiminato City.

 It has been pointed out that the generation of tsunami edge waves propagating in the shore line on the continental shelf is one of the causes of the spread of the disaster areas of the Hokkaido Tokachi-oki Earthquake Tsunami (2003) and the Tohoku-Pacific Ocean Earthquake Tsunami (2011). In 2017, there was a M5.3 earthquake with an epicenter of the central western of Kagoshima bay where no active faults identified.In this paper, we investigated the relationship between fault parameters and tsunami height for the entire Kagoshima Bay using a two dimensional model assuming a direct earthquake occurring in the bay.Next, we investigated the generation of tsunami height and edge waves in the coastal area by reproducing the detailed 3D topography of the coastal area in the central western part of the bay and using a 3D computational fluid model. As the results, When the source location and the fault parameters were changed, the maximum wave was the first wave at the offshore measurement point in most cases.At the coastal measurement points, there were several points that satisfied the edge wave dispersion relational expression, and there is a possibility that standing edge waves and progressive edge waves were generated in the central coastal area of the bay.

 Few experimental studies have been performed to examine behaviors of huge tsunamis around the breakwater heads due to their required huge scales in experimental setup. A numerical wave basin based on the Navier-Stokes equation is suitable for the detailed study of the complicated flows around the breakwater heads, however, grid-based simulations have, in many cases, difficulty in stably treating violent flows, which causes the numerical instability. On the other hand, the accurate particle methods are expected to perform stable and reliable simulations targeting the violent flows. In this study, we performed a 3D numerical simulation targeting a huge tsunami to examine its violent flow around a breakwater head by combination of Storm Surge and Tsunami Simulator "STOC-ML" and the particle-based numerical wave basin "PARISPHERE". Its good performance is shown through a benchmark, namely a large hydraulic experiment, whcih is newly conducted in this study.

 This study conducted numerical simulations of Nankai-Tonankai earthquake tsunamis using an Adaptive Mesh Refinement (AMR) method. The AMR model dynamically changes spatial and time resolution with each calculation step. First, to validate the numerical model using the AMR method, the simulation results were compared with the fixed resolution model. The results of the AMR model were in good agreement with those of the fixed grid model. Next, sensitivity analysis was performed by changing the refinement criteria of the AMR method to optimize the refinement scheme. The computational efficiency was significantly improved with constraining the refinement level depending on the water depth.

 In the evaluation of tsunami loads on various structures, the hydrostatic pressure (water depth coefficient), which is equivalent to three times of the design inundation depth, is basically applied, and its magnification is reduced according to the surrounding conditions. Although there have been many hydraulic model experiments and numerical analyses on tsunami wave pressure and water depth coefficients acting on buildings, there is no quantitative evaluation of the spatial distribution and stochastic variation of it using numerical analysis. In the present study, we evaluated the spatial uncertainty of the water depth coefficient by applying the spatial mode decomposition method using eigenorthogonal decomposition to the tsunami inundation depth due to the the Sagami trough earthquake. As a result, we found that the standard deviation of the water depth coefficient was about 0.6 in some areas and tended to decrease about 0.5 at about 1 km.