The estimated height of the tsunami wave in the southern coastal area of Kyushu due to the Nankai trough has been examined by the Kagoshima prefectural government and Miyazaki prefectural government and certain knowledge. However, the time fluctuations of the tsunami water level along the coasts of Miyazaki prefecture, which has shallow topography, and the coasts of Osumi peninsula and Tanegashima in Kagoshima prefecture, have not been sufficiently investigated.

 This study focuses on water level fluctuations when a wide range of active faults move. (1) The relationship between the maximum water level arrival time of the tsunami and the coastal topography in the coastal area was examined. (2) The relationship between the tsunami water level along the Tanegashima coast due to the reflected waves from the Osumi peninsula was clarified. In this study, we found the following. (1) In coastal areas with shallow terrain, the maximum water level is reached in the first incident wave, but in the case of bay terrain, it was found that the maximum arrival time of the tsunami occurs later than the first wave of tsunami. (2) As a result of arranging the main appearance spectra of water level fluctuations along the coast of Shibushi bay, it was confirmed that the water level rises in different major period in the inner part of the bay and the mouth of the bay that depended resonance period of the bay. (3) It was found that there are most of the eastern coast area of Tanegashima occure the maximum water level from the first wave from tsunami, but the western coast area occure the maximum water level that was affected that some reflected waves from the Osumi peninsula.

 In this study, as an initial step for instantaneous tsunami arrival time prediction after an earthquake, we used images of the initial water level obtained from numerical tsunami simulations for the Nankai Trough to predict the tsunami arrival time using machine learning. The arrival times were pre-processed and colored every 3 minutes and every 2 minutes for training. The accuracy of the tsunami arrival time prediction was improved by increasing the number of training data, and the error of the tsunami arrival time was within 3 minutes in some places. In addition, we compared the prediction accuracy of A town and B city in Mie prefecture to confirm the difference. Although the inundation area is overestimated and the arrival time is currently overestimated or underestimated, the arrival time of the tsunami is generally predicted well by machine learning based on the initial water level information.

 When a tsunami strikes, not only structures but also many people are swept away, leading to an increase in the number of missing and dead people. It is important to predict the behavior of drifted debris and the extent of drift after a tsunami in order to protect facilities and save lives. However, the drift behavior is highly variable. In this study, the effect of the initial position on the drifting behavior was experimentally investigated. The results showed that the degree of variability increases with the drifting distance, and the relation between the distance and the variability perpendicular to the drifting direction in this experiment was presented. In the numerical calculation, it was confirmed that the drift direction was in good agreement with the average of the experimental values, although there was some variability.

 The routes of drifting objects to the Okinawa and Amami Islands are studied from the cases that drifting fishing boats and spilled oil caused by tanker accidents were reached around the Okinawa and Amami Islands. Observation data from drifting marine meteorological buoy robot (buoy, hereafter) provided by the Japan Meteorological Agency were analyzed. Buoys subjected to southward winds tend to move clockwise from the Kuroshio region to the northern part of the Amami Islands or southward along the western side of the Okinawa and Amami Islands. The tendency of the buoy movement corresponds to the above cases. Buoys that started around the Izu-Ogasawara Ridge tend to show less variability in their trajectories that they moved around the Amami Islands than those that they moved around the Okinawa Islands.

 In the Ariake Sea, driftwood drifting due to heavy rains and typhoons adversely affects the environment, fisheries and facilities. The deposited driftwood negatively impacts the rich ecosystem, and fishing boats may not be able to go fishing for several months due to concerns about collisions with the driftwood. The driftwood on the coast hinders the function of wave-dissipating blocks and causes water to flow into the land due to storm surges. The Kyushu Regional Development Bureau (KRDB) collects driftwood using vessels relying on sightings, however, in the Ariake Sea where the current is fast, the driftwood often can not be found. Therefore, in this study, I developed an unstructured-grid drift prediction simulation model considering the influence that the drag from wind has on driftwood for the purpose of efficient recovery of driftwood. The particle tracking module of FVCOM, which is the unstructured-grid model, was improved to consider the buoyancy of driftwood, the drag from wind, and changes in the projected area ratio depending on the rotation of driftwood. The simulation for the period of heavy rains in July 2018 and Typhoon Kong-rey in 2018 was conducted. We estimated the times and locations of the driftwood outflow from the rivers by comparing the simulated driftwood drifting locations with the actual driftwood recovery data provided by the KRDB. During that typhoon period, the difference of 3.2 km occurred 48 hours after the release of driftwood by the drag from wind. Thus, considering the drag from wind against driftwood is an indispensable factor in developing a model for accurately predicting positions of driftwood.

 The accuracy of drift model is mostly compared with basic experiments using simple conditon, and the applicability of the model to actual topography is not clearly. In addition, since the drift model uses the simulation results conducted by the fluid model, it is necessary to verify the accuracy of the fluid model itself in space and time. In this study, the accuracy of the fluid model was verified using time series data of wave height and spatio-temporal data of inundation extent and surface velocities obtained from a series of experiments using a coastal city model, and then the accuracy of the drifting model was verified by using flow model whose accuracy was verfied. As a result, it was found that the simulation results by flow model was a good agreement with experimental results, but the accuracy of the drift model varied due to the influence of contact with structures on land and seafloor. In particular, localized contact with the seafloor in the complicated bathymetry is a factor that prevents the model from reproducing the experimental results. This results indicates the need of improvement of drift model.

 Waves from Typhoon Hagibis were observed by observation equipment installed near the embankment of the Suruga Coast. Five ultrasonic wave gages, three capacitance wave gages, and a far-infrared camera were used for continuous observation at 0.3-second intervals.

 As a result of Typhoon Hagibis, sixty waves reached the vicinity of the embankment during the observation period, and these waves caused vertical beach erosion of about 20-50 cm. The erosion process was classified into four stages according to the erosion speed. From the images taken by the far-infrared camera, the average flow velocity during uprush and down-rush of the measured section was determined and compared with the amount of drift sand, which was found to be correlated with the cube of the flow velocity during the rapid erosion stage.

 In lakes and the ocean, pycnocline is formed due to solar radiation and inflow into the coastal area, on which internal waves are generated by external forces, such as wind stress and tide. Internal waves propagate and break over a topographical slope, resulting in the long-term transport of nutrients, methane, carbon, hypoxia, and other water quality components and the deterioration of the ecological system. Internal solitary waves are revealed to have considerable kinetic energy and affect mass transport. Among internal solitary waves, "breather" is found to exist in a three-layer fluid, but the mechanism of the breather breaking over a slope remains unsolved. Thus, this study aims to clarify how breathers deform over the topographical change and break on a uniform slope by using a three-dimensional numerical model. We revealed that the classification for an internal solitary wave breaking is successfully applied to categorize the breaking type of breathers over a slope.

 Turbidity currents in growth process during long-distance are analyzed using OpenFOAM. The Smagorinsky model of LES is used as the turbulence model. Numerical results show that the turbidity currents during the acceleration and growth does not depend on the fluid supply from the rear, forms a self-acceleration cycle only by the flow at the front and flows long distances. In addition, results are shown that the thermal shape turbidity currents extends its own length with the flow and grows into a plume, which sustains the flow for a long time.

 Data analysis of nearshore observations and ocean reanalysis datasets was carried out to understand seasonal variability of the current in the offshore region of surf zone at Ishikawa Coast, Japan. The reanalysis dataset (FORP-JPN02) showed that large-scale northeastern current traveled on the continental shelf along the Kaetsu Coast and the seasonal variability of the current velocity was influenced by the Tsushima Warm Current. The northeast current was also dominant in the offshore region (15 m deep) of surf zone. The current velocity decreased in September and October due to combined effects of increasing winds and waves in the southwest direction and temporal decreasing of the large-scale current in autumn.

 In order to investigate the mechanism of the spatio-temporal differences in the M₂ tidal amplification factor due to the decrease of the bay length in the Ariake Sea. The results show that (1) the amplification factor increases in the whole area due to the decrease in energy dissipation caused by the decrease in the bay length, (2) the amplification factor changes less in response to the change in the tidal force due to the decrease in nonlinearity caused by the weakening of the tidal current, and (3) the resonant tide decreases due to the decrease in the eigen period. (3) the decrease in resonance tides due to the decrease in the eigen period, which is less visible at the mouth of the bay where the oscillation nodes are, but has a greater effect at the head of the bay. The energy dissipation at the mouth of the bay, where strong tidal currents occur and the effect of bay length change is large, plays a particularly important role in these phenomena.

 Through continuous observations conducted during upwelling event in the inner part of Tokyo Bay in 2020, a remarkable two peaks of turbidity was observed just above and below the intermediate anoxic layer. Formation mechanism of the two peaks of turbidity, which was coincide with higher elemental sulpfur concentration, was discussed by comparing TS diagram of data set in the navigation channel obtained between the short period. By employing area wide monitoring-post data sets, intrusion of the oceanic water into the lower layer of the Tokyo Bay and successive continuous blow of northerly wind were attributed to the cause of the upwelling event and formation of blue tide.

 This study investigated material transport and diffusion processes in Sagami Bay and Suruga Bay using the ROMS model with a 3-D Lagrange particle tracking model. Particles that flowed out from Sagami Bay were mainly transported to the north-east ward caused by the Kuroshio Current. For Suruga Bay, particles were mainly outflowed on the west side of the bay mouth and the out flowed particles were transported to the south-west ward due to a cyclonic eddies located off Suruga Bay. Vertical advective transport due to submesoscale eddies largely contributes to diffusive processes and particle transport toward offshore areas. Diffusions caused by the vertical advective term are much greater than that by horizontal diffusion (10^-1 m²/s) implemented by a random walk model. Transport from the inner part toward out of Sagami Bay is much higher for that of Suruga Bay when the vertical advective transport is considered. Tides suppressed south-west (north-east) ward transport in Sagami Bay (Suruga Bay) and enhanced south-west ward transport in Suruga Bay.

 Off the east coast of Japan mainland is called as the mixed water region formed by various water masses, such as the Kuroshio Water(KW), Oyashio Water(OW), and Tsugaru Warm Current(TWC). The mixed water region is known to largely influence marine environments and ecosystems in the North Pacific Ocean. This study investigated seasonal variations of water masses off the east coast of Japan mainland using the Regional Ocean Modeling System (ROMS). We developed a water mass classification method based on the standardized TS-diagram. The newly developed method allowed us to detect water masses in surface layers and coastal oceans, which are not simply detected by a conventional TS analysis. The seasonal variation of TWC influences the distribution and southern boundary of OW. TWC and KW are mixed off Kashima-nada accompanied by eddies in winter and spring season.

 We developed a data assimilation (DA) system aiming to improve the accuracy of the ocean numerical simulation model for the Seto Inland Sea (SIS). The Ensemble Kalman Filter (EnKF) was coupled with ROMS, leading to a new ROMS-EnKF DA system. Observing System Simulation Experiments (OSSEs) were conducted to evaluate the performance of the ROMS-EnKF system in a hypothetical scenario if a high-precision high-resolution coastal sea-surface temperature (SST) data from the latest GCOM-C satellite is available for DA. The GCOM-C SST data is demonstrated to be highly promising for better realization of the SIS circulations. On the other hand, a non-trivial accuracy deterioration occurs in the Bungo Channel, which can eventually be remedied by introducing virtual thermistor chain observations. We further argued practical and economic efficiency associated with mooring deployment designs.

 In this study, we proposed a method for generating synthetic data for ocean color remote sensing that takes into account the complex optical properties of coastal areas. In order to calculate the synthetic data, radiative transfer simulations were carried out based on inherent optical properties (IOPs), and measured water quality data collected in Tokyo Bay, and the effectiveness of the radiative transfer model was confirmed. In addition, the IOPs and water quality items were categorized by hierarchical clustering, and the model was incorporated into a bio-optical model to develop the synthetic data set that can represent phytoplankton species changes, colored dissolved organic matter (CDOM) from terrestrial and internal production sources, and differences in optical properties between inorganic suspended solids and detritus. The synthetic data of Rrs and IOPs were compared with the measured data and applied to the existing IOPs estimation algorithm, and it was confirmed that the synthetic data were generally in accordance with reality.

 In this study, a barred beach profile was constructed and installed in a small-scale laboratory wave flume. The beach profile was composed of a fixed profile containing a sandpit in the middle part of the flume. Different wave conditions were provided to evaluate the effect of sediment compaction on the vertical pore water pressure and sediment suspension. The pressure gradients were achieved by finite difference of measurements from an array of transducers. The magnitude of pore water pressure, pressure gradient, and sediment concentration with low and high sediment compaction were compared. In general, the magnitude of pressure gradients within the sediment layers with high compaction were larger compared to the low compaction conditions. Similarly, the recorded pore water pressure and sediment concentration were examined, and different behaviors such as the differences in the magnitude of the pore water pressure and pressure gradients due to different sediment compaction levels under almost the same hydrodynamic conditions were observed. The average of peak pressure gradients within the sediment layers under high compaction conditions was 1.4 times larger than the low compaction conditions. These results indicate the necessity of a detailed investigation of sediment’s influence on pore water pressure gradient and sediment concentration.

 Development of beach cusps were analyzed by aerial photographs and intensive field surveys. Beach cusps were found be less affected by coastal structures and beach properties. Frequent field surveys were conducted for 27 days in 5 months by using a RTK-UAV. Dynamics of beach cusps were analyzed by using high-resolution topography DEM with a resolution of 1cm. Cusps were found to be developed in a narrow beach band between 1m and 2m above sea level. The shape of cusps was highly variable and appeared to be dependent on the run-up level of swash waves. The decay of cusps was observed when the run-up level was larger than 2m. Steady run-up actions in the range from 1m to 2m tended to develop break-up cusps and double-row cusps.

 The similarity rule of scour amount and tsunami action time is derived from the continuous equation of topographical change. The above similarity rule is applied to scour behind the three-sided coastal embankment due to tsunami overflow in large-scale and small-scale gravitational field experiments and 20G, 40G centrifugal force field experiments. As a result, the ratio of the proto-type scale conversion values of the maximum scouring depth of the model to the maximum scouring depth of the proto-type generally corresponded to the correction coefficient 1 / HS of the scour depth by the above similarity rule when the similarity ratio of the friction coefficient was 1. According to the settling velocity similarity law (Dean Number), at a scale of about 1 / 20～1 / 40, if the sand particle size of the model is set to about 0.1 mm compared to the proto-type sand particle size of 0.3 mm, the correction coefficient 1 / HS of the scour depth is within 0.8～1.0, and the scouring amount can be made almost obey the Froude’s similarity rule.

 Nine onshore-directed dominant sediment transport events were extracted from the 17-year beach profile data observed at the Hasaki coast in Japan. By using these data, the performance of the sediment transport model XBeach for these events was investigated. First, the spatial distribution of wave height was compared with observed data, and the appropriate wave breaking model and its parameters were selected. After confirming the wave propagation, the model performance of profile changes was investigated. In this analysis, the sensitivity analysis of the wave nonlinearity parameter (f_ua), which strongly influences cross-shore sediment transport, was conducted. The results revealed that some events could simulate with good accuracy using optimized f_ua. However, some were not improved even if the f_ua was changed. Although we tried to estimate the value of f_ua using some parameters, e.g., wave energy flux, we could not find a good relationship between them. However, we found a positive correlation between sediment volume change in the nearshore zone and f_ua.

 Long-term change in windblown sand was estimated on the basis of the changes in shoreline and width of sand dune using aerial photographs. Based on the total volume of windblown sand necessary for the development of sand dunes in the geological time scale at the Enshunada coast, the rate of windblown sand toward inland was calibrated as well as the determination of longshore distribution of landward windblown sand. Along this coast, longshore component of windblown sand was estimated to be 1.9×10⁴ m³/yr at maximum, which accounts for 25% of all the longshore sand transport.

 Setting paralell contours as the initial topography concurently with the installation of a model island of Enoshima Island, topographic changes were calculated using the BG model, given the wave field determined by the angular spreading mothod. As the result, a cuspate foreland behind Enoshima Island was reproduced. Given this topography as initial topography, beach changes after the construction of facilities, such as Shonan Port and Katase fishing port, were predicted, taking the effect of dredging of seabed material ino account. It was concluded that the shoreline recession on Katase-nishihama Beach was triggered by the dredging around this fishing port. Furthermore, sand has been transported inland owing to windblown sand by SSW wind in winter in this area. This sand movemet was also predicted by numerical model. Finally, it was found that beach nourishment of 2×10⁴ m³/yr is necessary as a measure to recover sandy beach on Katase-nishihama Beach.

 On a coast, which is composed of sediments with mixed grain sizes and is affected by strong winds, fine sand and medium-size sand are selectively transported to the back of the coast as windblown sand, leaving coarse sand and gravel on the foreshore. In this study, we proposed a model for predicting beach changes on coasts composed of sand of mixed grain size while taking not only effects due to waves but also windblown sand into account. To verify the validity of the model, a model calculation was carried out on a coast with a wave-shelter zone produced by the construction of an offshore breakwater as well as the application to the Inage coast. The calculation results were in good agreement with the field condition, and the effectiveness of the model was confirmed.

 The local topography around sand fences where were installed along to the shore and installed normal to that in limited length in the on-offshore direction was surveyed by using a highly accurate 3D laser scanner. High resolution data drew clear local terrain and enabled to calculate sand volume with a high accuracy. The sand transport rate by wind was estimated by two methods. One was calculated from the accumulated sand volume within the unit length on both, the longshore and the on-offshore sand fences. Other was calculated from the accumulated sand volume at the conner where both sand fences crossed. Local terrains generated around sand fences, such as accumulation form around the opening with 1 m in width provided as passages to the beach, local scour generated at the foot of fence, local scour and accumulation at the seaside end of on-offshore sand fence, and accumulation forms around an opening of 5m that no sand fence installs between the landside end of the on-offshore fence and the longshore sand fence were examined and discussed.

 In this study, to consider the relationship between sea salt spray quantities and environmental conditions, actual field measurement of sea salt spray was carried out along the Japan sea of Hokuriku and Tohoku area. From observation results of sea salt spray, the sea salt spray quantities are affected by the coastal geographical conditions and the coastal waves conditions. By calculating coastal distribution of waves conditions in a coastal zone of Japan sea, the relationship between sea salt spray quantities and waves conditions can determine.

 Artificial reefs with a crown width of 70 m and a crown depth of 1.3 m were constructed with an opening of 40 m width on the Agarie coast in Nago City, Okinawa, to reduce wave overtopping. Three impermeable groins of 100 m length were additionally constructed concurrently with beach nourishment using sand of median diameter of 0.6 mm. After the construction, beach was eroded due to storm waves associated with typhoons. Beach changes of this beach were investigated using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’ concept). It was concluded that the sand discharge of this coast was triggered by the outflow of nourishment sand through the openings of artificial reefs with 3 m depth.

 A movable bed experiment with a model scale of 1/50 was carried out to investigate the reduction of sand deposition around an opening of the Numakawa-daini floodway which was transformed into a setback type floodway. The sand deposition at the opening of the floodway under the storm wave condition is caused not only by shoreward sand transport but also by longshore sand transport turning around the backyard of the floodway owing to obliquly incident run-up waves. To reduce the sand deposition at the opening due to the latter case, the effect of construction of a vertical wall between the foreshore and backshore was investigated, changing the height of vertical wall. The construction of a vertical wall was effective to reduce sand deposition.

 At Mihono-matsubara sand spit located at the north end of the Shimizu coast, beach has been eroded due to the imbalance in longshore sand transport and a large amount of sediment discharges offshore through steep seabed. In this study, the changes in cross-sectional areas of the eroded and accreted areas near the tip of the sand spit were calculated on the basis of the bathymetric survey data and the rate of offshore and longshore sand transport on a steep coast was evaluated and compared each other. The offshore discharge of the sediment was estimated to be 6×10⁴ m³/yr between 1998 and 2010, and it increased up to 7.5×10⁴ m³/yr. Furthermore, it was found that 8.7×10⁴ m³/yr of sediment has discharged in the entire area of the sand spit.

 In order to reproduced the geomorphic changes at the mouth of Shirakwa River based on the advection diffusion equation, it is necessary to confirm the validity of the parameters including the analytical solution. In this study, a straight channel was installed upstream of an open channel, and the channel widened rapidly downstream to make a delta area. Experiments were conducted in which the flow rate and bed slope were varied by supplying a constant amount of sediment. The delta shapes were calculated by image analysis. The "reduced complexity model" (RCMs), which can consider the physical coarse process of sediment transport, was also used. The planar shape of the delta can be characterized by the Froude number using the water level in the channel and the tide level at the mouth of the estuary. The scale of the local diffusion coefficient was confirmed by experiments. In the calculation, the tip of the clinoform layer in the on-off-shore direction was calculated properly, but the longshore direction needs to be examined.

 In our previous study, we found that wind waves are obliquely incident at an angle greater than 45° to the shoreline of a sandbar located south of the mouth of the Nagase River in Lake Inawashiro. Along the shoreline of the northern shore of the lake, sand spits have been formed owing to high-angle wave instability. These sand spits still continue to develop. To investigate the deformation process, field observations were carried out on April 15 and July 30 in 2017, as well as the observation on September 2, 2019. Lakeshore changes were reproduced using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept). As a result, the moving velcity of the sand body predicted in this study coincided with 68 m/yr observed between 2020 and 2015 in the previous sutudy.

 Beach changes along a sandy beach of 13.5 km length between Otsu fishing port and Takado Point in northern Ibaraki Prefecture were investigated based on aerial photographs. A long offshore breakwater was constructed at Otsu fishing port at the north end of this area, forming a large wave-shelter zone west of the breakwater and causing deposition of sand. In contrast, sandy beach in front of the seawall was eroded in the southern part of the area, causing instability of the seawall. It was found that even though the coastline length is long as 13.5 km, topographic changes due to the formation of a wave-shelter zone reach the vicinity of Takado Point at the south end of the study area.

 Field experiment of flooding through an opening of No. 1 floodway that filled with sand was carried out at Numakawa-daini Floodway, while maintaining the highest water level of the tank to be +4.6 m above MSL. Beach topography was measured using a GPS-RTK and UAV before and after the flushing. The change in water level inside the water tank and at the opening of the floodway was also measured. As a result, the water discharge from the opening of approximately 1 m height was capable, when the water level of the water tank is set to be +4.6 m above MSL and 21 m3 of sand was discharged from inside the conduit by the current through the floodway. Furthermore, a v-shape channel excavated next to the floodway was effective in forming discharge channel.

 Sediment transports on intertidal flats have not been cleared enough even in recent years because of a lack of regonal view points and the poor accuracy of amounts of sediment discharges from rivers. Regional sediment transport from mountains to coastal zones through rivers was an important research subject today. In this study, field observations were conducted at the Nakatsu Intertidal Flat and the Yamakuni River which have serious problems of fisheries and sediments, and observation results revealed long-term tendency of erosion/deposition and remarkable siltation due to a flood from the Yamakuni River in 2017, which was caused by the North Kyushu Heavy Rain Disaster. This siltation was recovered until the next spring. However, the long-term siltation from summer to next spring was suggested serious damages on Ruditapes Philippinarums especially in winter. Numerical simulations by using WDM-POM which was a resional sediment transport and morphodynamic change model due to mud and sand considering tides and wind waves and gRSM which was a calculation model of river sediment discharge by using a convenient model and field data, were also conducted in order to verify the siltation mechanism in 2017. Numerical results were well agree with observation results qualitatively and revealed that the muddy water from rivermouth was transported mainly by tidal residual currents toward the Nakatsu Port in the east direction almost during a day. Numerical silmulation considering wind waves in winter and tides was also conducted and numerical results showed that the mud transport toward the Nakatsu Port was exaggerated by coastal currents due to NW wind waves accompanied with tidal residual currents. These strong currents were suggested to recover the siltation in 2017 through the winter. In the future work, quantitaively clarification will be done and future forecast of sediement movements and countermeasures for the siltation will be investigated.

 In the Ariake Sea, the events of heavy rainfall have caused a large amount of river water and sediment to flow into the sea in recent years, resulting in salinity decrease and sediment accumulation. Additionally, a lot of bivalves such as clams and pen shells die. This study developed a sediment transport simulation model with the unstructured-grid system in the Ariake Sea and performed the simulation during the past heavy rain period. As a result, in the Chikugo and Kikuchi Rivers, sedimentation of 40 cm or more occurred near the estuary, and sedimentation of 5 cm reached a maximum of 3.7 and 1.0 km, respectively. The dependence of the sedimentation area on the timing of the outflow of water at the mouth of the Kikuchi River was investigated. The results showed that the sedimentation trend was different when the outflow of water was at high tide and low tide, with the sedimentation distribution from the mouth of the river spreading westward and southeastward, respectively.

 Numerical simulation of morphology change at the Tenryu river mouth during a large flood was conducted to clarify the effects of artificial modification of the sand spit shape on the morphology change and the water level variation during floods around the river mouth. The measured topography and five kinds of modified topography were used in the simulation. It was found that significant water level gradient was developed along the sand spit inside the river mouth during the flood. It was thought that the difference of the elevation between the water surface and the crest of the sand spit at each cross-shore section was an important factor of the occurrence of the collapse of the sand spit. Artificial modification of the sand spit shape to promote the collapse of the sand spit away from its opening, where the water level inside the river mouth was higher, was found to be more efficient to decrease the flooding risk at the river mouth than the modification around the opening.

 In the mouth of the Hojo-River floodway that flows through the central part of Tottori prefecture, blockages occur frequently. River managers excavate sandbars to prevent flooding of internal and external water due to blockages. At that time, they judge the state of sandbar by checking the CCTV camera image. However, there are problems such as lack of personnel required for continuous monitoring and variation in judgment results.

 In this study, we develop a model that enables quantification and automation of the judgment using deep learning of artificial intelligence with advanced image analysis ability. In addition, we confirmed the usefulness of the model through field demonstration experiments aimed at improving sandbar monitoring. The model developed in this study may be applicable to rivers where CCTV cameras are installed in the direction of the river mouth.

 This study investigated the relationship between topographic changes of Oi River mouth and a recent erosional trend of Suruga Coast. Landsat image analyses of morphological features of the river mouth suggested that the transition of the equilibrium state occurred around 1994 and 2010. Until around 1994, sedimentation continued due to the prevention of longshore sediment transport by Oi River Port breakwater located near the eastern side of the river mouth. On the other hand, the river mouth sandspit turned eastward after 2010 when the river channel reached a new equilibrium state due to the sedimentation. The deltaic bar in front of the river mouth, which was the starting point of longshore sediment transport, was also moving in the same way, and the eastward longshore sediment transport has increased further since 2010. As a result, an increasing amount of sediments fell into the submarine canyon at the northeastern end of Suruga Coast, and the sediment volume in the entire Suruga Coast tended to decrease since 2010.

 The river mouth sand bar in the Kumano River, which is located in the heavy rainfall and typhoon hit areas, causes the discharge capacity to decrease and the water level to rise. Therefore, it is important to establish an effective and efficient management method. In this study, in order to understanding the morphology changes and factors of the sandbar, the topography surveys using UAV and RTK-GPS for the sandbar and the sea bottom surveys around the river mouth were carried out. In addition, a numerical analysis of wave, flow, and topographical changes associated with the sandbar recovery process was performed. From the survey results, we clarified the seasonal morphology changes, the characteristics on the left and right sandbar, and the influence of the wave direction. Furthermore, we clarified the sediment transport around the sandbar from the terrace including the adjacent sandy shore during high waves. Finally, the BG model was developed, and examined its applicability to the erosion of the terrace, the sedimentation around the sandbar, and the drifting sediment transport for each wave direction.

 River mouth sandbar grows in Omono River in Akita prefecture. River mouth sandbar topography was affected by combined forces, such as river discharge, ocean waves. River mouth sandbar interrupts salt and wave intrusion and affects the navigation of cruisers and fishing vessels. Therefore, study on sandbar on Omono River was carried out. However, field investigation was conducted more frequently to understand the change of sandbar in detail. In this study, field investigation using UAV was conducted. Then, sandbar change process was analyzed calculating sandbar area and river mouth width from reconstructed topography by the SfM technique. Furthermore, the investigation of relationship between sandbar area and river mouth width and river discharge, cross-shore, and longshore waves was carried out.

 As a result, the following results were found out from this study. The relationship between sandbar area and river mouth width and river discharge, cross-shore, and longshore waves were cleared. The seasonal cycle was discovered. Moreover, recovery of sandbar was cleared partly. It was found out that sandbar area was changed -500-+400 m² per day and river mouth width was changed -2-+2 m per day.

 The 2011 Great East Japan Earthquake and tsunami caused severe damages in coastal areas along Miyagi Prefecture. In the Naruse River mouth, the sand spit was flushed away by the tsunami and restored artificially by the government. However, the sand spit was washed away again by the Hagibis Typhoon in early October, 2020. The morphology of the river mouth after the typhoon resembled the situation immediately after the 2011 tsunami. By using satellite and aerial images and river bathymetry datasets, this study indicates the intrusion of sediment into the river mouth caused an alteration to the long-term sediment budget in the adjacent Nobiru Coast in the future. The outcomes of this study are useful for the government authorities to manage the long-term coastal and riverine morphological changes in this area in the near future.

 The Volta River mouth, located in Ghana, West Africa possesses a significantly elongated estuary sandspit. Rapid development of the sandspit causes the estuary water levels to rise during floods, resulting in flood related disasters. In developing countries, acquisition of detailed data on such events is limited which hinders extensive research on mitigation measures. In this study, satellite images were acquired after 1984 to quantitatively examine the morphology of the sandspit at the river mouth. From the study, it was observed that the sandspit elongated at a constant speed (approximately 580 m/year) before the construction of a groyne system. In addition, there were periodic breaching of the sandspit resulting in a change of river mouth position. Based on results obtained for the sandspit growth rate and incorporating the depth of closure and berm height values from previous studies, the amount of littoral drift was estimated. The value obtained was one to two order larger than the values reported in other parts of the world including that of Japan.

 Slope failure hydraulic model experiment based on a case study of slope subsidence behind a seawall was conducted. The wave characteristics used in the experiments were estimated using SWAN, and the offshore wave conditions were clarified into two types: the development and the attenuation phase. In the experiments was measured the equilibrium slope of the foreshore and the damaged slope behind the seawall. The results showed that the arc shape damage of slope was observed by wave overtopping in the case of wave development phase. And, in the case of wave attenuation phase, the subsidence shape damage of slope was caused by run-up wave and exfiltration flow from groundwater in the backshore. As a result, this study was suggested the characteristics of the two types damage at different wave conditions.

 In order to ensure the long-term stability of the monopile foundations, scouring measures are required for an anchored offshore wind farm. It is recommended that the scouring measures should be applied to the area twice the width of the monopile diameter D. However, the construction load in water depths exceeding 20 m is large. However, it is important to understand the appropriate range of the countermeasure because of the large construction load in the water depth of over 20m.

 In this study, localized scouring at the base of the monopile and scouring countermeasures using rock bags were investigated by moving bed experiments.

 For the scour protection against local scouring phenomena in the foundation of monopile offshore wind farms, there are still many quantitative studies on the necessary scour protection area. In addition, it is necessary to consider the scale effect in hydraulic model experiments dealing with geomaterials. In this study, we conducted experiments at two different scales, 1/65 and 1/130, compared to the previous large-scale experiment at 1/13 scale. From the experiments, the appropriate similarity law for the geomaterials including the scour protection works was confirmed, and the effectiveness of the scour protection works was discussed. The scour depths of each experiment were compared based on the similarity of the sedimentation rates, and the results showed that the experiments were reproducible. It was also found that a scour protection radius of about 1.5 times the diameter of the monopile was sufficient to counter the effects of the flow alone.

 At the opening of the submerged breakwaters, deep scouring may be observed by strong offshore currents. The scouring has not been sufficiently clarified. A lot of studies have been carried out using three-dimensional morphological models. However, the reproduction of topographic changes near the submerged breakwater is not sufficient. In this study, the scouring of submerged breakwaters opening were investigated using the Uradome beach in Tottori . In order to improve the accuracy of the numerical model, we have developed a new three-dimensional morphological model by using a one-equation turbulence model to the quasi three-dimensional current model. As a result, the scouring process was clarified from the field observation results. From the results in one-equation turbulence model, rip currents were generated strong in the scour point. The scouring was found to progress when the top of water depth at submerged breakwaters became low in the process of scouring. The numerical model was improved in accuracy.

 For the purpose of improving the conventional numerical model of predicting sand deposition in small harbors, field investigations were conducted focusing on long-period waves in addition to winter wind waves. In the field survey, the occurrence of long-period waves in the harbor was investigated. The results of the filed investigations were used to analyze the flow due to long-period waves around the harbor, and then the calculations of topographic changes around the harbor were attempted. The small harbor in the target area has a mixed topography of sandy zone, reef zone and breakwaters. Therefore, the influence of the reef zone into the calculation of bottom topography changes were adapted.

 As a result, drifting sand in the reef zone was suppressed, and erosion due to the influence of waves and currents behind the breakwater developed. Sand deposition around the harbor mouth and at the back of the harbor was reproduced, and the results were closer to the actual phenomena.

 The applicability of James' method, which can calculate the overfill ratio of beach nourishment in consideration of the grain size of borrow sand and native sand, to beach nourishment in the coast of Japan has not been verified. In this study, we verified the applicability of the James’ method to the Kineicho construction area in the Niigata coast and the Hamamatsu Shinohara coast in Shizuoka prefecture, where beach nourishment was carried out, and conducted sensitivity analysis on the overfill ratio of the amount of beach nourishment with respect to the particle size distribution of the borrow sand. The results showed that the James’ method can be used in setting the amount of beach nourishment in the coast where changes in soil volume due to longshore sediment transport can be ignored, and that attention should be paid to the rapid increase of the overfill ratio in the case that the standard deviation of the grain size of borrow sand is smaller than that of native sand.

 We carried out numerical simulations on waves and sediment transport aroud the Patimban coast, where is located in the northwestern part of Java Island, Indonesia, to investigate characteristics of waves driven by land and sea breezes and its effects on sediment transport aroud the study area. The simulated wave fields were strongly affected by diurnal variation of the sea surface winds due to the sea breezes. The simulated bed level changes at the depth of less than 10 m were different under only the land breeze, or both the land and sea breezes. The phase relationship between diurnal variations of the tidal currents and waves have an important roles on not only erosion and horizontal transport pattern of the suspended sediments.

 Beach nourishments are implemented in many areas for the countermeasure against coastal erosion. Regarding the investigation of beach nourishment, laboratory experiments focusing on the particle size, mixing ratio, and cross-shore location of beach nourishments have been conducted. However, it cannot say that it is sufficient to examine how much the initial compaction degree of nourishment area contributes to the topography changes of the nourished area. Therefore, we conducted laboratory experiments to investigate the effect of the initial compaction of the beach nourishment on the erosion (topographical change) of the nourished area due to waves. The results revealed that the lower degree of compaction of the beach nourishment, the more significant erosion (topographical change) occurred immediately after wave generation, and the difference in erosion rate between the compacted and loose nourished sand was about twice. Moreover, when the wave overtopped the beach nourishment area, the influence of the compaction degree of the nourished area on the erosion volume is smaller than when it does not overtop the nourished area. On the other hand, when the waves did not overtop the beach nourishment area, it was found that even if the topography shape after the experiment was the same, the degree of compaction of the remaining topography was different.

 Beach nourishment using 8×10⁴ m³ of gravel has been carried out at Akiya (Okuzure-hamada) area between 2007 and 2014. After the nourishment, a wide beach was recovered, but a steep beach was formed as well as the formation of high berm of 3 m above mean sea level. Since the relative height between the crown of the seawall and gravel beach was reduced, part of gravel was transported to the backyard of the seawall. Part of gravel was also transported into the sandy beach across a L-shape groin, damaging use of sandy beach as a bathing beach. Therefore, beach changes between 2005 and 2019 were reproduced using the BG model, and future beach changes were predicted. Measures preventing gravel transportation into the adjacent sandy beach and wave overtopping over the seawall were proposed.