On the Hamamatsu-shinohara coast, beach nourishment has been carried out between 2006 and 2015 using 9.26×10⁵ m³ of coarse sediment transported from the riverbed of the lower Tenryu River as well as the construction of three detached breakwaters. Beach changes after the beach nourishment were monitored by beach surveys. The shoreline has advanced at a rate of 15.4 m/yr until 2015 in the nourishment zone, and then the shoreline has gradually retreated after the stoppage of beach nourishment. The relationship between the change in foreshore area (ΔA) between +3 m above and 2 m below mean sea level, and the shoreline change (Δy) was investigated. A relationship of ΔA = 4.41Δy was obtained between the variables with R² = 0.93. Coarse sediment was transported alongshore with a characterstic height of beach changes of 4.4 m. Thus, beach nourishment was effective to maintain the shoreline.

 This study discusses the influence of the relation between the bar-trough type seabed variation and the neighboring sediment transport flux on the evolution of local morphology. First, a theoretical analysis is conducted to clarify the essential parameters characterizing the temporal variability. The behavior of the multiple-bar system at the Chirihama Coast, JAPAN, is then examined based on the parametric model developed by Yuhi et al. (2016, 2017). The correspondence between the estimated flux distribution and the characteristics of the multiple-bar behavior at the site is consistent with the theoretical analysis.

 In this paper, the cross-shore model developed by Larson et al. (2016), which evaluates the cross-shore sediment transport between bars and berms, was employed to simulate decadal profile evolution at Hasaki Beach, Japan. The model parameters were calibrated following an optimization process, and the model showed good agreement with the profile data. A comparison of the model parameters of the Hasaki Beach with those of other foreign beaches suggests that the amount of sediment exchange between the bar and the berm is relatively high at the Hasaki Beach and that the bar grows larger while the response is slower. Sensitivity analysis showed that coefficient C_B, which is related to bar volume, highly affects model performance.

 Beach topography quickly responds to the action of storm waves, resulting in erosion of the foreshore with accretion under the calm wave condition after the storm. It was found that the seabed shallower than 2 and 3 m depths was eroded by rapid offshore sand transport during the storm event with the deposition of sand in a zone between 3 and 5 m depths, and then the beach recovered within 1-2 years after the storm, taking the Chigasaki coast as an example. In this study, topographic changes immediately after the storm and subsequent recovery under the calm wave conditions were calculated using the BG model. Given the equilibrium slope of fine sand d₁ and medium-size sand d₂ are 1/120 during high waves, erosion of the beach nourishment embankment and the water depth zone shallower than 4 m were reproduced. Also, the recovery process of the beach to a gentle slope was successfully reproduced.

 "Extreme coastal erosion" causes the shoreline retreat locally and rapidly. It may impair the function of the beach where beach width is unsufficient. If it happens, there is a need for early recovery. In order to design the countermeasures, it is necessary to clarify the process of extreme coastal erosion and the subsequent recovery process. Therefore, after organizing the cases of extreme coastal erosion, the erosion process and recovery process were reproduced by a three-dimensional topographic change model, and an effective measure to accelerate the recovery was evaluated.

 The risk of beach loss due to environmental changes caused by climate change has become important to discuss. It is important to project future changes in morphological change using a numerical model integrating over a long period in order to consider the coastal protection measures in Japan. XBeach is widely used for modeling such numerical simulation, but there are few practical applications in Japan. This study performed a series of ensemble calculations with model coefficients of XBeach changed independently at the Hamamatsu Shinohara coast, and determined the five model coefficients that significantly affected the results of morphological changes there. Then, the study optimized the major model coefficients through sensitivity analysis based on multiple regression analysis. Based on the results, it was confirmed that the optimized result of XBeach is consistent with the field measurement. It was also found that the coefficient of non-linear wave effect: Sk critically impacted on foreshore erosion by changed return flow to offshore.

 Sediment movement is described in eroding Kochi Coast by using topography survey data in recent 25 years. The erosion is due to the sand exploitation from seabed and the decrease in sediment supply from two rivers. In the eastern zone, which is covered by the sediment from the Monobe River, a series of detached breakwaters appeared to stabilize the nearshore zone up to 500m from the seawall. The erosion was significant in the offshore zone with decaying rhythmic topographies. Sediment retention in reservoirs was significant and thus could compensate the erosion in offshore. In the western zone, which is covered by the sediment from the Niyodo River, the coastal sediment volume was found increasing, after the regulation of marine sand extraction. However, the speed of the sediment volume recovery was found to be small and the sediment volume in the downstream zone was found decreasing. Sediment bypassing and extension of headland works appeared to be effective in mitigating further erosion.

 On the Shimizu coast in Suruga Bay, a new-type groin with pile structure had been constructed until March, 2018 as a measure against beach erosion without damaging coastal scenery of the coast. After the construction, storm waves, such as the waves during Typhoon No. 19 in 2019, hit this coast several times and large beach changes occurred. At present, the construction of the second new-type groin is planned. To this purpose, beach changes around the new-type groin measured until November 2019 were reproduced using the BG model and the arrangement of the new groin was studied using the same model.

 Recent beach changes in Hishinuma area of the Chigasaki coast were reproduced using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept), taking the sand supply to this area by eastward longshore sand transport and beach nourishment since 2007 into account. Furthermore, rapid beach changes due to offshore sand transport caused by storm waves were also reproduced. On the basis of the study, the effectiveness of beach nourishment to recover sandy beach in this area was shown.

 It has been pointed out that global warming may lead to coastal erosion. In this study, we tested and evaluated the applicability of the methods proposed in previous studies to advance formation of sandy beaches.The countermeasure structure is permeable and was installed at three locations depending on the tidal zone of action. For comparison, a countermeasure with an impermeable structure was also installed. According to the test results, the sand surface height on a coral reef beach fluctuates by about 10 cm per day at a constant waves. In addition, it was confirmed that the countermeasure contributed to the topographic advance through the accumulation of gravel.

 The development of a coastal sedimentary features are affected by the fluvial and littoral sediment transport system. It is necessary to understand the sediment budget between the fluvial sediment supply and littoral drift to predict the long-term topographical changes due to the coastal conservation. However, the actual state has not been quantitatively clarified. This study aims to investigate the sediment budget at the Teradomari-Nozumi Coast located in the estuary of the diversion channel of the Shinano River. As a result, the total sediment volume of littoral drift in this coastal area is estimated 160 Mm³, and the volume of sediment supply from some small streams are totaled 84 Mm³ .Then, it assumes that there are only two sources of sediment supply to the coast: sediment discharged from small streams and the diversion channel. The fluvial supply by diversion channel is calculated 76 Mm³ as the difference between the volume of littoral sediment (160 Mm³) and the sediment supply from small streams (84 Mm³). Ministry of Land, Infrastructure and Transport’s research pointed out that the sediment discharge rate is 0.7~0.8 Mm³/yr and which in the period between 1922 and 2018 (for 96 years) is 70~80 Mm³. The result of this study is in good agreement with MLIT’s data and indicate the validity of the sediment budget analysis based on wide-area and long-term geomorphological changes.

 The relationship between shoreline change obtained from a network camera in the Shichiri-mihama coast and wave conditions off Owase city was investigated using deep learning. Results showed that offshore wave energy flux is more related to shoreline change than wave runup especially for 90-day data. Furthermore, the generative adversarial network (GAN) reproduced the characteristics of images classified into each label and showed wave conditions in shoreline advance and retreat.

 A machine learning model classifying orth-mosaic images of a gravel beach created from UAV-SfM/MVS survey into "gravel", "drifting", "vegetation", and "block" was constructed and examined in terms of the characteristics and usefulness. From investigation, it was found that the discrimination accuracy can be improved by reducing the size of the filter layer and increasing the number of filter layers. The visualization using Grad-CAM showed that the indiscrimination of “gravel” and “drifting” was caused by a few points of interest. Additionally, the machine with the trained model can work well even for images that are not used for training.

 Continuous beach sediment mapping of the Shimizu coast in Japan has been obtained by a drone, collecting more than 600 images at a fixed height of 10 meters above the mean sea level along a 5 km stretch of the coast. For each image, the bidimensional wavelet transform was applied to determine the sediment size of grains with a diameter larger than 10 mm. Through the application of such a model, the continuous grain-size distribution of the Shimizu coast has been successfully estimated. The obtained grain-size distribution along the study coast shows that diameters tend to decrease in the direction of littoral transport. In addition, the wavelet method results reveal unforeseen and peculiar local trends in the grain-size distribution, which have not been detected by low-density traditional laboratory sieve analyses.

 The objective of this study is to identify the dominant factors that contribute to extreme turbidity along the coast around Matarbari Port, Bangladesh, and to investigate how the observed turbidity depends on the identified factors. Understanding the characteristics of this extreme turbidity is of importance for countermeasures against severe siltation around ports. Among the factors analyzed, the water level in the Meghna River and Karnaphuli River and the local significant wave height showed a moderate correlation with the measured daily average turbidity, while significant wave period, tidal amplitude, and surface current velocity had weak correlations with the measured turbidity in the Matarbari Port area. Satellite data from the Moderate-Resolution Imaging Spectroradiometer (MODIS) was used to capture the spatiotemporal variation in turbidity in Bengal Bay, including the Matarbari Port area. Through analysis, it was found that there is an approximately 30-day time lag between the river discharge from the Meghna River and the high turbidity around Matarbari. Based on this finding, linear regression was performed, and the obtained regression model showed good predictive ability for the observed daily average turbidity only as a function of local significant wave height and water level at the mouth of the Meghna River.

 Hydranlic characteristics of three types of airtificial reefs were evaluated by using field observation data. Attenuation of wave heights and wave periods was comfirmed by all types of airtificial reefs. The wave setup was less than conventional aritificial reefs due to return flow in the structure. Longshore currents are generated behind the reefs depend on incoming wave direction. At the end of the group of airtificial reefs, however, the current direction is biased and it may cause dynamic topography changes.

 The 2D and 3D model experiments of which scale is 1/16 are conducted to investigate the wave and current field around a slit-type permeable breakwater. The results of 3D model experiment are supported by the results of 2D model experiment. And, the changes of wave transmission and reflection ratios on the permeable breakwater due to tidal currents can be explained by considering the interaction of wave and current field, which is reproduced by a serpent-type wave maker and a horizontal current generator. Moreover, while the existence of slit does not amplify the wave pressure acting to both side caissons at least, Goda’s formulae can also estimate a design wave pressure of such a permeable breakwater in safe.

 There are few research cases of stability of high-top artificial reef although there are some previous studies in the case of coated blocks. In the case of high top artificial reef whose crown height is as high as the design high tide level, wave power caused by the impact of waves and flow caused by wave breaking and return flow on the dyke are thought to affect the stability of the dyke. Therefore, it is unclear whether stability can be evaluated by a desk study such as Hudson’s formula because of complicated wave/flow field compared to general breakwaters and artificial reefs.

 In this study, two-dimensional hydraulic model experiment was conducted to analyze the damaged patterns of the high-top artificial reef, which have very few cases of study, and to examine the stability of the dyke stability. The results shows that (1)there is a risk that the stability of the block could be impaired due to the damage on the offshore shoulder of the rubble mound, although the focus is on the required block mass in the desk study of stability of offshore wave-dissipating facilities, (2)although the damage patterns of the high-top artificial reef was damaged by both pushing and pulling waves, it was often damaged by the pulling waves, and necessary mass of the block was larger than the required mass calculated by Hudson’s formula, (3)in case of installing high-top artificial reef by random stacking of blocks, radial type blocks may be more stable than legged shaft type blocks, (4)considering this experiment and previous studies, the most dangerous condition for the stability of artificial reef is when the crown height and the tide level are equivalent.

 Resilient structures for coastal dike against waves higher than the design wave had been proposed by studying failure mechanisms on the seaward and landward of the coastal dike. However, these studies were conducted with hydraulic experiments separately dealing the deformation on the seaward and landward of coastal dike, in which the interaction between the seaward and landward deformations was not considered.

 In this study, the hydraulic experiments on resilient structures on the both seaward and landward of coastal dike against unexpected high waves were carried out. As the results, it was confirmed that the vulnerability against scouring was higher on the seaward than on the landward of the coastal dike, and that the deformations on the seaward and landward of the coastal dike could affect each other.

 A hydraulic model experiment was performed on tsunami-induced hydrodynamic forces acting on armor blocks covering a rubble mound to validate the assumption in the derivation process of the Isbash formula, which is a method used to calculate the required mass of an armor block. The result of pressure distribution around the blocks demonstrated that the hydrodynamic force works on the plane of the armor block locally. Based on the assumptions of the local distribution of lift force, we modified the formula for calculating the required mass of an armor block, which refers to the velocity value to the sixth power or that to the third power. On the other hand, it was unclear whether the action length of the lift force varies with the size of the armor block. This issue is strongly related with the rationality of the modified formula. Furthermore, the relationship of the near-bottom velocity and cross-sectional average velocity (the former is related with the stability of armor blocks more directly, whereas the latter is supposed to be used in the design practice) remains to be investigated in future research.

 Medium-scale experiments and numerical calculations by DEM were performed to investigate the reduction effects of protuberances such as a semi-cylinder attached to the surface of a structure (flat plane) on the collision force of sea ice. An ice collides with the flat plane after collision to the semicylinder to cause split failure. Each collision force of the sea ice to the semi-cylinder and the flat plane following it was significantly decreased compared to a single flat plane. The collision force was found to be minimal when the ratio of ice width (B) and diameter of the semi-cylinder (Φ) (B/Φ) was 3 -4, and the force decreased up to 20% of that to the single flat plane. The numerical calculation results agreed well with the experimental results. As one of measures to reduce collision force caused by brittle materials such as sea ice, the validity of an attachment such as small sized protuberances that could lead to fracture to the surface of a structure were confirmed.

 In this study, the lift pressures acting on a horizontal slab due to traveling and overlapping waves were analyzed using a gas-liquid two-phase numerical wave models, and the pressure reduction effect by adding holes to the slab was investigated. As a result, it was confirmed from the numerical simulation that the pressure caused by the air compression was generated when waves did not reach to the horizontal plate. In addition, it was found that the pressure due to air compression was reduced to some extent by adding an aperture, and the reduction rate depended on the relative clearance and aperture ratio. For future studies, the response of the structure and the multi-directional irregular waves should be investigated to improve the versatility of the practical design.

 This study conducted a series of hydraulic model experiments to evaluate the porosity change due to the cross sectional deformation (settlement) of the wave-dissipating work, and investigated wave overtopping discharge and block stability from hydraulic model experiments applying the clarified porosity change of wave-dissipating work. Experimental results revealed the relationships between the cross sectional change, the crown settlement amount, and wave overtoppimg discharge. The repair criteria were considered and guidline of alowable settlement was proposed.

 Wave dissipating blocks should be designed so that the hydraulic performances can quantitatively be reflected for coastal protection and disaster prevention. In this study, hydraulic model experiments are carried out to evaluate the reflection and transmission coefficients for sloping breakwaters. Numerical simulation with CADMAS-SURF/2D is conducted to reproduce the measured results. The applicability of the Dupuit-Forchheimer law is investigated through the numerical simulation. The optimum values of the coefficients in the Dupuit-Forchheimer law are estimated. When the coefficients proposed in previous studies were used in the numerical simulation, the reproducibility was not sufficient in several conditions. In this study, the reproducibility was improved by adjusting the coefficients according to the incident wave period.

 The detail of how to impose the periodic boundary condition to the Immersed Boundary Method (IBM) with the Physical Virtual Model (PVM) for calculating the external force is described and it is derived to be necessary to ensure five cells as the virtual boundary. The validity of the proposed periodic boundary condition was confirmed by flow calculations for a single sphere.In addition, a porous medium consisting of multiple spheres was placed in the computational domain, and the flow through the porous medium was computed with a unidirectional steady flow under the periodic boundary condition. The calculation results showed good agreement with the macroscopic empirical law on the pressure drop. The distributions of vorticity and shear stress in the pore of the porous media and wake are presented. Finally, the spatial gradients of velocity components, which are the dominant factors of occurrence of shear stress, are discussed.

 Since physical properties such as density are different between the gas and liquid phases, it is common to solve the fundamental equations with tracking the gas-liquid interface and clearly separating the gas and liquid phases. The fluid force acting on a solid which penetrates the gas-liquid interface differs in magnitude between the gas and liquid phase. The fluid force calculated by the Immersed Boundary Method (IBM), which estimates the fluid force acting on the solid surface from physical quantities such as the velocity and pressure of the surrounding fluid, becomes unstable due to this fact. In this study, we proposed a calculation method of the Navir-Stokes equations with the pressure term transformed to φ = p/ρ in the equation of motion to avoid computational problems caused by the pressure discontinuity at the gas-liquid interface and to simplify the computational algorithm. As an example of the application of these methods, flow analysis around an oscillating cylinder that penetrates the gas-liquid interface was performed.

 Flow field behind a breakwater during tsunami overflow was reproduced using DualSPHysics, which is a CFD code based on the SPH method, and the results were verified by comparing with the experimental data. In addition, behavior of tetrapods behind the breakwater against tsunami overflow was also reproduced. As for the flow field, even under uniform condition in the channel width direction, the 2D and 3D calculations showed different results, and the 3D calculation agreed better with the experimental results. In addition, the flow field was significantly affected by the artificial viscosity, and the results were generally good when the artificial viscosity coefficient was set to 0.01. As for the behavior of the armor units, the initial damage, such as the detachment of a single block, was generally reproduced, but the behavior, such as the sliding of the entire slope at a large overflow depth, was not reproduced.

 In this study, we investigated a design of a tsunami resistant breakwater that is sound against the L1 tsunami, while allowing its overflow, and tenacious against the L2 tsunami. We analyzed the changes in overflow water veins due to differences in breakwater specifications using numerical wave channels and proposed a method for classifying the characteristics of overflow water veins according to facility specifications. We also conducted hydraulic model experiments on fixed floors and moving floors to verify the effectiveness of scour countermeasures.

 The numerical analysis could reproduce the formation of tsunami overflow water veins, and confirmed to be useful for selecting the structural weaknesses and countermeasures against tsunami overflow. In the hydraulic model experiment, the effectiveness of the abdominal mound against the L1 tsunami and the necessity for a scour prevention mat against the L2 tsunami which was set by the desk study were also clarified.

 In performance design, it is important to evaluate the failure of a structure quantitatively in order to properly evaluate the required performance of the target structure. One of the methods to evaluate the failure quantitatively is the model test. In this study, the modeling of models focusing on the failure mode is investigated by using a model experiment with a scale ratio of 1/25, which is relatively common in wave-making channel experiments, referring to the large-scale experiment by Kikuchi et al (2011). In addition, the failure mechanism of the caisson - rubble structure is discussed by changing the mound shape. The coefficient of friction due to the sliding of the caisson was investigated, and the difference was found between the large scale experiment and the model experiment. Next, the behavior of the caisson and rubble mound under eccentric loading was investigated, and it was confirmed that the behavior of the caisson and the load-displacement relationship tended to be roughly consistent. The results of the PIV analysis showed that the deformation of the mound was also qualitatively similar. Finally, in the experiment with different mound shapes, the area of deformation in the mound changed with the continuation of loading, and the failure modes were different.

 The prediction and control of wave motions during the installation of a caisson can improve the efficiency of the work. In this study, the oscillation of a caisson is investigated using a solid-gas-liquid three-phase numerical wave tank, which can analyze the movement and deformation of structures and the contact between structures due to fluid motion, and the results of numerical calculations are compared with those of hydraulic model experiments. As a result, the maximum displacement and its tendency were in good agreement with the hydraulic experiment by changing the grid size. The collision between the caisson and the mound was also investigated by changing the water draft, and it was shown that the oscillation decreased with increasing the water draft. The trend is in good agreement with that of Horisawa et al.¹⁾, indicating the importance of coupled analysis between fluid and structural analysis.

 Internal erosion under the action of transmitted waves was reproduced by a large-scale visualization experiment. The effects of hydraulic conditions such as wave period, wave height, and initial water level on the processes of internal erosion and the extent of its influence were investigated. As a result, it was shown that the flow velocity of sand increased when the sea level was lower than the groundwater level because the internal erosion was promoted by the seepage flow toward the sea side. In addition, it was clarified that the scale of expansion of the cavity formed tended to be large and the extent of influence of the internal erosion was widened in the case where internal erosion occurred in a deep point under the groundwater level, regardless of the wave period and wave height. The particle size condition of the filter material for deterrent of internal erosion under the action of transmitted waves was examined by the same series of experiments. As a result, it was verified that the geotechnical two-layer filter kept sufficient deterrent effect against the continuous action of relatively strong waves and backwashes, under conditions where the median particle diameter ratios were less than 25 and the uniformity coefficients of the filter were larger than 3, regardless of the shape of the filter material.

 In the coastal area of Hokkaido, high waves caused by a bomb cyclone in December 2014 caused the road slope behind the seawall to sink. The damage was characterized by the disappearance of the foreshore in front of the revetment and the exposure of the reef, and the discharge of sediment from the bottom of the revetment. In this site, the beach has been fluctuating greatly for a long time, but the influence of beach deformation on the damage to the seawall has not been fully clarified. In this study, the factors that caused the road slope behind the seawall to sink were investigated based on the wave conditions at the time of the disaster. As a result, it was found that the damage may have been caused by waves with a 5-10 year probability, and that the saturation of the ground behind the seawall by overtopping waves and the reduction of the foreshore sediment thickness due to beach deformation may have increased the local hydraulic gradient at the bottom of the seawall during receding waves, thereby contributing to sediment discharge.

 Power generation function is mounted on “flaring flanged diffuser” and was investigated the rate of increase regarding flow speed through the diffuser by PIV method using two-dimensional model. As a result, the rate is 1.4 times in the relevant part of turbine and the increased area is equality inside of the diffuser. Besides, to grasp the effects of the magnetic flux density on the rotation of turbine and generation property, the experiment is conducted by using three-dimensional model. As a result, the generating efficiency is decline in high flow velocity that is caused by the rotating efficiency has peaked in that place and the case of excessive magnet placement, amount of power generation is the faint. Therefore, it’s essential to clarify the optimal arrangement of magnets to generate power effectively. Based on the results of experiments, amount of power generation in Tsugaru Strait is calculated to be 10~100W at the actual place.

 The Japanese technical standard for the offshore bottom-fixed wind turbines basically follows the international standard established by IEC, considering the natural conditions in the Northern Europe. Therefore, it is necessary to consider the oceanographic conditions such as the high waves and swell associated with the typhoon in the Japanese technical standard, appropriately. However, there are few studies on the effects of the ocean waves on the dynamic response of the offshore bottom-fixed wind turbine. In this study, we investigated the characteristics on the dynamic response of the monopile-type offshore bottom-fixed wind turbine in operation under the winds and waves by using the load coupled analysis. As a result, it is found that the wave loads has a great influence for the bending moment on the sea bottom and it is important to consider the influence of the steepen waves due to the near-breaking waves enough in the design of the offshore bottom-fixed wind turbine.

 Some water accident occurred at Makuhari-no-hama Beach in the inner part of Tokyo Bay for four consecutive years. Some coastal physical factor of the accident needs to be understood to prevent occurring the accident. We firstly grasped the occurrence status of water accidents in the inner part of Tokyo Bay. As the result, specifically many water accidents occurred at Sanbanze beach and around Makuhari-no-hama beach. Expressly, many accidents of Makuhari-no-hama occurred after diving from the groin into the sea. We secondary conducted the field survey. We obtained the information of the topographical information, the current information, the tidal information and etc.. Furthermore, we obtained the situation about the resucue operation. Finally, we conducted the numerical simulation of the wave propagation. In this case, firstly, we simulated the wave and the current dynamics in the field survey condition. Secondary, we simulated in many other wave conditions. From the results of the simulations, the wave and the current condition change by the incidence wave directions.

 One of the causes of water accidents that occur near the coast is the rip current. The rip current has the relationship with the wave height, and the current velocity of the rip current occurres corresponds to the wave height. Videlicet, even though when the low height wave occurs, a corresponded low speed current of the rip current occurres in the area, and the water accident caused by this low speed current sometimes occurred. When we play in the water at the coastal area, we move by some moving method, for example, we move by walking in the shallower than a waist depth water area, however, we move by swimming in a deeper than the tip of the foot depth water area. In the other, we move by only hand rowing at the toes barely reached the bottom area. The each moving speed is different, and the rip current speed that each method can be handled is different. Therefore, we measured the moving speed of each method, and grasped the rip current speed that each method can be handled. Secondary, we tried to modify the calculation method about the velocity of the rip current. In this case, only the wave conditions and the depth information are needed. Before modifing the new method, we conducted the some field survey. When we conducted the field survey, the rip current was pigmented by the sea water colorant, and the pigmented current was recorded as the aerial photography by UAV.

 This paper investigated the influence of swells on design waves through the analysis of observed wave data and case studies of actual swell-induced damages of coastal protection structures around fishing port of Japan. As a result, it was found that the effect of swells on the design offshore waves is relatively insignificant on Japan Sea side except a part of Hokuriku region, three major bays in Japan (Tokyo, Osaka and Ise bays), and the Seto Inland Sea. On the Pacific Ocean side, the influence of swell waves was significant. However, it was also found that the influence of swells is partially accounted for in the recently reevaluated design waves obtained through the extreme value statistical analysis based on the wave data including recent swell events. In addition, an attempt was made to reproduce a case of damage caused by long period swell, and it was found that the observed damages can be reasonably explined by the wave and current field in the vicinity of the structure computed by the Boussinesq-type wave model and CADMAS-SURF/2D.

 In this study, the effectiveness of the movable breakwaters for reducing the water level and preventing storm surge was investigated at Tokyo Bay under the worst-case scenario based on an empirical typhoon model. As a result, the water level reduction effect of the movable breakwaters at the harbor was 11.2-42.2 % for each installation and 27.4-45.7 % for multiple installations in the typhoon path where the maximum stormsurge was observed. In addition, it was found that the multiple installation of the system was effective for reducing the water level in the river, while the installation of the system only at the mouth of the river was sufficient to prevent stormsurge. In addition, a comparison the method of raising the levee, conventional strengthen-ing, and the movable breakwaters on the inundation control effect showed that the movable breakwaters was more effective than the method of raising the levee uniformly by 3.0 m in the case that install multiple breakwaters in the harbor and in the river.

 In recent years, recycled materials such as steel slag in marine areas has been used as a replacement material to preserve coastal areas. Molten slag, which is obtained by melting general wastes, has little experience in use at coastal area and does not have hydraulicity such as steel slag, so it can be used for constructing artificial tidal flats. In this study, the performance of the molten slag has been studied through comparative experiments to create artificial tidal flats using different materials such as molten slag, silica sand, and mixtures. The results of the study showed that the pH value in the tank using molten slag was not increased, which was different from that of steel slag. The adsorption of phosphorus was observed, indicating the possibility of using molten slag as a replacement material for constructing artificial tidal flats.

 We calculated the partial pressure of carbon dioxide (pCO₂) of seawater in Osaka Bay using water quality data observed since the 1970's, and analyzed the spatial and temporal variability of pCO₂ in the urban inner bay. As a result of comparing the spatial distribution of pCO₂ averaged by season every decade, a spatial gradient of pCO₂ was formed in both surface layer and bottom layer during the high temperature period. However, the surface layer in the deep part of the bay tended to absorb atmospheric CO₂, while the bottom layer tended to store high concentrations of CO₂. Furthermore, a cluster analysis was conducted based on the time series of pCO₂ data from all the survey sites to classify the sea area of Osaka Bay into four areas which are the inner part of the bay, the center part of the bay, the southern part of the bay, and the mouth of the bay. The increase trend of pCO₂ of surface seawater in the whole Osaka Bay was 2.3 μatm/year, which was slightly larger than that of the surface seawater observed in the northwest Pacific Ocean. Furthermore, the pCO₂ in the inner part of the bay which is affected by urban drainage showed a strong seasonal variation but the trend of increase was different between in the surface layer (1.3 μatm/year) and in the bottom layer (5.8 μatm/year).

 Cigu Lagoon is a famous lagoon for fish agriculture (oyster) and migration birds, where few seagrasses exist. We aim to investigate the environmental factors affecting carbon fluxes in Cigu Lagoon to understand the influence of fishery. SEM analysis was conducted to understand the influence of environmental factors on the air-sea CO₂ flux. The results suggest that the CO₂ flux is directly influenced by wind speed and indirectly by pH due to the influence of oysters. A three-dimensional environmental model, Fantom, was applied to investigate the effect of the wind on DIC at the water surface. It was demonstrated that strong wind enhances the vertical convection in the water column, resulting in high DIC at the water surface due to the mixing with higher DIC adjacent to the sea bottom.

 Regarding the reduction of atmospheric CO₂, the function of CO₂ absorption in coastal areas has been drawing attention. In this study, we developed a numerical model of pCO₂ dynamics considering effects of freshwater inflow based on the results of field observation in the Yatsushiro Sea, to obtain the basic knowledge about CO₂ dynamics in coastal areas. The model was able to reproduce well the tempo-spatial distribution of pCO₂ and the distribution of salinity and water temperature under different stratification conditions. From the results of the hindcast simulation, it was confirmed that the amount of CO₂ absorbed around the estuary fluctuated greatly spatiotemporally with the big flood. Furthermore, the following two effects of the freshwater inflow on the dynamics of CO₂ in the coastal area are confirmed as follows: i) After flooding, CO₂ release can be promoted by the suface freshwater capping; and ii) After mixing of freshwater and seawater, CO₂ absorption can be promoted by photosynthesis due to blooming of phytoplankton in the surface layer.

 This paper presents the results of a field survey and laboratory experiments to evaluate sediment oxygen demand in Osaka Bay, where Total Pollutant Load Control has been conducted in recent years. The measured sediment consumption rates were used to estimate the volume of hypoxic water by a three-dimensional biogeochemical model with data assimilation. The survey results showed that the oxygen consumption rates in 2019 were smaller than the results in 1999. The biogeochemical model simulated seasonal change in the distribution of hypoxia in the head of the bay. Dissolved oxygen was underestimated near the mouth of the Yamato River compared with observations. Estimaed hypoxia volume was 1.75 km³ in mid-summer of 2018, resulting in 27% reduction of the volume by the assimilation.

 This study presents physical structures and mixing processes and thier effect on the oxygen distribution in the inner part of Tokyo Bay. A one-year analysis was conducted using long-term observation data from Chiba light. The ratio of the vertical density gradient and vertical shear, namely the Richardson number(R_i) was negatively correlated with bottom disssolved oxygen (DO), indicating that vertical mixing conditions strongly influence the bottom DO. The bottom DO was also negatively correlated with the horizontal Richardson number(R_ix), which represents the mixing state considering the horizontal gradient, wind, tidal current, and buoyancy. R_ix exceeded the index of stable stratification in 76% hypoxia events. The wind stress largely influenced vertical mixing, rather than tidal currents. R_i and R_ix was significantly correlated, therefore, the horizontal buoyancy gradient and wind stress can be used to explain the vertical mixing and bottom DO conditions in the inner part of Tokyo Bay.

 In 2020 Kyushu floods, the cumulative rainfall exceeded 500 mm in the entire Chikugo River basin. The water level measured by the Senoshita observatory located downstream of the river became the highest water level in the past years. The Chikugo River is one of the sources of freshwater into the Ariake Sea and plays a critical role in the marine environment and benthic ecosystem of the bay. This study evaluated the effect of large-scale effluent on the development of hypoxia (DO less than 3 mg/L) in the bay by numerical simulation of the hydrodynamic model and the ecosystem model. Meantime, we also compared with five other cases to explain the variation of hypoxia due to different flood patterns. The results indicated that historical extreme flood event in 2020 caused long duration of hypoxia lasting more than two months at Sta.B3 in Isahaya Bay and the most severe bottom hypoxia was occurred throughout the central and northern regions of the Ariake Sea. Furthermore, it also showed that the duration of hypoxia in 2020 was longer than in 2006 when the largest hypoxia had occurred since 2000.

 Recently, typhoons induced the remarkable decrease in the sea-salinity, leading to the mass mortality in urchin aquafarms. This study investigated the formation processes of the low-saline water due to the strong rainfall in Hichirippu lagoon, Hokkaido, Japan, using the quadmaran automated vessel to automatically and widely monitor the salinity field around the aquafarm. The observed salinity maps clarified that the low-salinity water was formed by the rainfall peaks and observed in the lagoon head during the long period. After the emergence of low-saline water, the salinity in the lagoon stepwisely decreased in response to the events of the rainfall peaks. The tidal current was predominant in the lagoon mouth, and their residual component was mainly explainded as the estuarine circulation. These contributed to the gradual salinization in the lagoon water after the rainfall peaks. When the rainfall peaked, the flood runoff was predominant as a residual component and decreased the area-averaged salinity in the lagoon corresponding to the freshwater supply from the catchment area.

 The purpose of this study was to reveal the carbon flow from primary producer (phytoplankton) to secondary producers (zooplankton and Xenostrobus securis), transfer efficiency at hyper-nutrient area in the Amagasaki Canal of inner part of Osaka Bay. The primary production rate was higher than that of other water area, while zooplankton biomass and secondary production rate were lower. The production rate of X. securis was about 1/12 of that of phytoplankton, but biomass was about 29 times higher. Transfer efficiency from phytoplankton to zooplankton and X. securis was 0.76% and 7.8%, respectively. These results suggested that X. securis grew efficiently and fixed organic carbon, zooplankton were not functioning properly as intermediaries in the food chain between phytoplankton and fishes.

 We developed a quadruple-nested downscaling oceanic circulation model coupled with a 3D Lagrangian particle tracking model for quantifying short-distance 3D transport of coral larvae in reef areas on the northwest coast of Okinawa main island, Japan. Lagrangian analysis of about 750,000 particles revealed that particles dispersed with time, although they were affected by some enclosed coastal area. Especially most of the particles released from the north side of Motobu Peninsula, located near the center of the model domain, were transported out of the area. The results of the vertical transport analysis showed us significant connectivity between the mesophotic zone (depth of 30 m or more) and shallow water areas.

 The purpose of this study is to estimate the outflow from seagrass beds in Hiroshima Bay and the destination of seagrass after drifting using genetic distance, tidal current simulation, and stable carbon isotope ratio. There was no statistically significant difference in the carbon content of the Zostera marina leaves, which grew on seagrass beds and drifted on the coast. The seagrass distributed in Jigozen had low genetic differentiation from other seagrass beds, suggesting that seagrass exchanges occur. The seagrass drifting from the genetic distance may have originated from various sites rather than from a specific seagrass bed. Seagrass from the three seagrass beds using tidal current simulation drifted along the coastline one month later but then drifted to the central part of Hiroshima Bay three months later. It was found that the carbon origin in the sediment of Hiroshima Bay is more contributed by seaweed, plankton in seawater and the terrestrial origin than the seagrass origin.

 The utilization of environmental DNA(eDNA) is being considered as a new method for monitoring seagrass beds. In field surveys, the amount of eDNA of seagrass was below the minimum limit of quantification by using the conventional method that is often performed in freshwater areas for animals. In this study, we assumed water sample condition with a large amount of suspended solids, which is inevitable for monitoring seagrass beds using eDNA, and organize the requirements for protocolizing the analysis method for collecting a sufficient amount of DNA. We found the relationship between the seasonal change of Zostera japonica and that of the amount of eDNA by using the new analysis method for low-concentration samples. The numerical simulation indicated that suitable time for sampling was different depending on sampling sites. Additionally, these results showed the possibility of applying the eDNA analysis and numerical simulation methods for future field surveys.