We conducted a field survey in the coastal areas of Wajima City and Shika Town, Ishikawa Prefecture, located in the western part of the Noto Peninsula, to investigate the effects of the tsunami and ground uplift caused by the 2024 Noto Peninsula Earthquake. The maximum height of the tsunami trace was 4.2 m in the Akasaki district of Shika Town. The inundation area and tsunami damage were localized and mainly limited to the port area or just behind the seawall. The uplift of the ground was most pronounced north of the Saikaifuto district in Shika Town, and the ground rose 3.5 to 4 m at the Kaiso and Kuroshima fishing ports in Wajima City, significantly reducing the water depth in the ports and drying up the sea floor in some areas. On the sandy beach on the south side of the Kaiso fishing port, the shoreline advanced 150 to 250 m, exposing a bar zone that was below the water surface before the earthquake, and the beach width increased significantly. It was estimated from the aerial photographs that the sandy beach expanded over about 250,000 m² due to the uplift of the ground.

 In order to clarify the actual damage caused by the 2024 Noto Peninsula earthquake, we surveyed the damage and the height of tsunami traces in the coastal area of Noto Island, Ishikawa Prefecture. The survey revealed that in the northern part of the area inundated by the tsunami, tsunami traces with a flood height of approximately 2 m were observed at the Hachigasaki Beach. On the other hand, we obtained testimony that Enome Fishing Port was inundated about 0.2 m through interviews. Although the two sites are located only about 1 km apart in the northeastern part of Noto Island, there were differences in the inundation conditions. Therefore, we conducted a tsunami reconstruction analysis to clarify the differences in the inundation process. The Hachigasaki Beach is open to the direction of tsunami incidence, whereas the Enome Fishing Port is surrounded by breakwaters and has a narrow harbor entrance.

 On January 15, 2022, a large-scale eruption occurred at the submarine volcano Hunga Tonga-Hunga Ha’apai near the Tonga Islands, and a significant sea-level change occurred a wide range of Japan’s the Pacific coast area from the night of January 15th to 16th. The Japan Meteorological Agency took the unusual step of issuing a tsunami warning and advisories.

 In order to clarify the actual evacuation behavior of citizens, a questionnaire survey was conducted among residents of municipalities where tsunami warnings and advisories were issued. The results are as follows. 1) It is important to call out to those around you to wake up a sleeping person, and the decision to evacuate requires a lot of talking to the people around you. 2) In evacuation decisions made late at night when there was no earthquake-induced shaking, residents were found to have made evacuation decisions based on information.

 In the shallow methane hydrate (MH) mining project, it is planned to discharge the mud water into a deep-sea depression at the drilling site. In this study, to understand the turbidity during the discharge into the sea, experiments were conducted using a simulated deep-sea bottom mud (chinese kaolin) and deep-sea bottom mud collected from the seafloor of the MH existence site in the Japan Sea to simulate continuous long-term mud injection using a water tank that reproduces a depression. Numerical analysis using a density-flow model was also conducted to understand the behavior of the input mud water.

 As a result, it was confirmed that the density flow spreading tended to be dominant in the case of continuous long-term injection. In addition, it was confirmed that the flow velocity at the time of injection was higher in the mud from the deep-sea bottom at the MH existence site under the same concentration conditions, and that the turbidity due to muddy water tended to be prolonged after the injection. It is suggested that these characteristics are affected by the clay minerals contained in the mud and the organic content in the mud as seen in the strong thermal loss.

 “Blue carbon”, or carbon captured and stored due to photosynthesis activity in SAV (submerged aquatic vegetation) ecosystems, has been attracting attention as a means of mitigating climate change. However, SAV creates complicated interactions with water flows, making it difficult to accurately assess carbon uptake and mass transport. In response to this problem, some studies have examined the interaction between SAV and flow, but the coupling between SAV and flow is insufficient and the branches of SAV are not considered. Therefore, this study aims to develop a coupled hydrodynamic-SAV model that takes branches into account. In addition, the developed model was successfully applied to analyze the results of laboratory experiments, revealing good agreement.

 This study conducted a long-term flow monitoring to investigate effects wind-driven currents influenced by topographic effects in Lake Kasumigaura where tidal currents and geostrophic current do not influence. In addition, a high resolution numerical model was developed to reproduce wind-driven currents in the study area. Observed results show that wind-driven currents tend to be enhanced near the coast leading to intensified kinetic energy. Meanwhile, in the middle of the lake basin, counter currents are generated by enhanced wind-induced currents along the coast, which results in suppressed kinetic energy in the middle of the lake basin. Numerical simulations provide enhanced wind-induced currents along the coast same as the observed results. However, model results in the middle of the lake basin are different from the observations. This inconsistency is supposed to be caused by small-scale sub-grid scale turbulent motions that are not considered in the numerical model.

 This study investigated mass transport processes influenced by the Kuroshio current and coastal topography with a Lagrange particle tracking simulations. Particles are released from the entire area of off the southeast Japan mainland as simulated by the Regional Ocean Modeling System (ROMS). Unlike conventional particle tracking methods that release particles from specific regions, this approach enabled the evaluation of material transport processes throughout the simulated region by initially distributing particles across the entire domain. The released particles diffused strongly along the boundaries of the Kuroshio current and eddy fronts, exhibiting pronounced directional dependence. This directional dependence was observed from coastal areas to the Kuroshio, with the east-west horizontal diffusion coefficients being 5-10 times larger than those in the north-south direction. On the other hand, in the offshore regions, isotropic diffusion occurred with strong seasonal dependence, showing that the horizontal diffusion coefficient in winter is approximately one order higher than in summer.

 Synoptic numerical modeling was conducted to evaluate the open ocean influences on the thermal environment of five aligned embayments (Osaka Bay, Ise Bay, Suruga Bay, Sagami Bay, and Tokyo Bay) along the Pacific coast of Japan. We exclusively focused on the open ocean effects by analyzing the heat flux exchange through each bay's mouth, thereby disregarding contributions from tidal and fluvial influences. The mean heat flux (MHF), predominantly comprising seasonal variations, emerged as a primary driver of seasonal thermal fluctuations, exerting a more significant impact on heat exchange compared to the eddy component. Warm water diverging from the Kuroshio reached the bays east of Sagami Bay through the Oshima West Channel, where the MHF was intensified during the Kuroshio berthing period, correlating with its axis fluctuations. In the Osaka Bay-Suruga Bay region, topographically constrained horizontal circulation mitigated direct intrusion. The direction and magnitude of MHF into and out of the bays varied depending on the degree of closure exhibited by each bay.

 In the previous study, a two-dimensional movable bed experiment was carried out under the condition that bed rocks were buried under the gravel beach, while changing the location of the bed rocks and the thickness of the gravel layer. The critical thickness of the gravel layer in determining whether gravel is deposited on the foreshore or not was determined together with the classification of the types of the deposition on the foreshore. In this study, the mechanism that an equilibrium slope of the beach corresponding to the thickness of the sand layer becomes gentle when a rocky bed is buried under the beach was included in the BG model (a model for predicting 3-D beach changes based on Bagnold’s concept), and a model for predicting longitudinal profile of the beach was proposed, referring the results of the previous experimental study. In the reproduction calculation, when the thickness of the gravel layer is small, it was confirmed that offshore movement of gravel is intensified because the seepage flow is weak and the return flow over the rocky bed is accelerated.

 Taking Shichiri-ga-hama Beach facing Sagami Bay as the study area, beach changes since 2007 were reproduced using the BG model, and the effect of beach nourishment as a method against beach erosion was studied. Since rocky bed extensively distributes beneath the sandy beach, return flow during the wave run-up can be strengthened, accelerating seaward sand transport. Therefore, considering that the return flow causes the decrease in equilibrium slope, such a mechanism was included in the BG model. Moreover, wave direction seasonally changes on this beach, so that such an effect was also considered. As a result, it was concluded that recent shoreline recession near the east end of Shichiri-ga-hama Beach was well reproduced. Furthermore, to recover sand beach in this area, beach nourishment at a rate of 5,000 m³/yr is needed.

 On the Chigasaki coast, Kanagawa Prefecture, beach nourishment has been continuously carried out by putting sand along the shoreline from the hinterland to recover a sandy beach. In this method, beach users cannot reach the shoreline from the cycling road located in the hinterland, because high scarp is continuously formed along the shoreline. Also, the high scarp caused the feeling of pressure to the beach users. To reduce the minus damage, a new method (continuous beach nourishment in a designated area) was proposed, in which beach nourishment is carried out in a designated narrow area and sand is supplied from this area to the nearby areas by longshore sand supply. In this method, beach nourishment area is restricted, so that the longshore length where scarp is formed can be narrowed relative to that in ordinary beach nourishment. The effectiveness of the new method relative to the ordinary method of beach nourishment was clarified by the numerical simulation using the BG model.

 Beach changes on the Shizuoka and Shimizu coasts with a gradually-curving shoreline along Mihono-matsubara sand spit were predicted using the BG model and employing the real coordinate system instead of expanded coordinate systems. On these coasts, many detached breakwaters with short offshore distances and five artificial headlands have been constructed as a measure against erosion. By this calculation, overall beach changes of these coasts associated with the movement of a sand body were reproduced, including the change in shoreline position and longitudinal profiles. The effect of beach nourishment until 39 years from the present was predicted. It was confirmed that beach nourishment at a rate of 8×10⁴ m³/yr is required on the Shimizu coast to maintain or recover a sandy beach.

 A detailed understanding of the sediment transport mechanisms induced by breaking waves can enhance the accuracy of the sediment flux modeling. Process-based numerical simulations can reveal the hydrodynamics of breaking waves, and the authors have investigated the sediment transport under breaking waves from the grain scale using Lagrangian models, which have advantages in application for breaking waves. However, the discussions were limited to a narrow channel, and further discussions were required.

 This study used a DEM-MPS scheme to conduct a three-dimensional sediment transport simulation in an oscillating water tank. After comparatively validating the simulation results from PIV measurements we performed in a corresponding condition, the vortex structures under plunging waves are visualized. Extracted velocity fields with certain spatial scale bands illustrated that vortex filaments with rotating axes perpendicular to the wave crest were aligned and that their scale was a quarter of horizontal roller. We also describe the contribution of the vortex filaments with the pressure fluctuation to the local sediment transport.

 For efficient maintenance dredging of estuary ports, it is important to understand the mechanism of sediment deposition in the port. In this study, the particle properties of suspended sediments transported from Shinano River into the Niigata Port (the West Port District) were investigated by water sampling analysis and in-situ measurements based on laser diffraction and image analysis methods. The suspended sediments were rich in organic matter and consisted mainly of clay and silt fractions. In-situ particle measurements indicated that the cohesive suspended sediments in freshwater were transported into the port as freshwater flocs, and that the floc sizes increased further in seawater. These settling processes with flocculation, caused by the presence of organic matter and contact with seawater, are important physical processes that should be appropriately implemented in predicting sedimentation problems in estuarine ports. In addition, the reliability of the floc sizes obtained from the in-situ measurements is discussed.

 The swash zone experiences constant changes in wave conditions due to tidal changes and the wave run-up and backwash, leading to fluctuations in soil water content. Therefore, the purpose of this study is to elucidate the interactions between soil water content and elevation changes that occur along with these wave conditions. Field surveys were conducted in the swash zone to measure soil water content and ground elevation change using a laser distance meter at the same location, and the relationship between the two was investigated. The results showed that soil water content varied periodically in response to tidal level and wave run-up. Furthermore, each period of soil water content change was divided into time interval, and the relationship with ground elevation change was examined for each interval. It was found that during the rapid increase in soil water content during flood tide, sediment deposition increased with shorter durations of the period, and during the soil water content stabilization at high level around high tide, sediment deposition increased with longer durations of the period.

 In this study, pore water pressure in the coastal sandbar layers was measured alongside waves and sandbar profile changes to explore the correlation between sandbar profile change and fluctuation in pore water pressure. In this analysis, negligible changes in sandbar profiles were compared with erosion phases, and accretion phases. The results indicate that pore water pressure fluctuations in the sandbar decayed more rapidly with increasing depth during erosion and accretion phases compared to the phases with almost negligible change. Additionally, a comparison between the vertical pressure gradient and wave steepness revealed that the pressure gradient tended to fluctuate significantly when wave steepness ranged between 0.02 to 0.04 during erosion and accretion phases. During erosion and accretion phases, the fluctuations in the vertical pressure gradient were dominated by upward pressure propagation, suggesting a possible loosening of compaction within the sand layer.

 Monitoring of the wave run-up heights on beaches becomes a useful tool in the analysis of shore protection. However, there is no method appropriate for long-term monitoring. Wave run-up height increases when a steep profile is formed due to offshore sand transport after the action of storm waves. Thus, the understanding of the topographic changes near the shoreline is important in coastal protection. A method to estimate the change in longitudinal profile below the mean sea level from the wave run-up height was proposed in this method using an image analysis taking the Chigasaki coast as an example. When comparing the wave run-up height under the condition with similar wave conditions, the run-up height decreased by 1.2 m after the condition that the trough was refilled, whereas it increased by 1.0 m when a trough was formed. The wave run-up height obtained by image analysis showed the same characteristics of that estimated from the longitudinal profile of the beach. It was concluded that the present method is effective for estimating the change in longitudinal profile near the shoreline from the wave run-up height.

 Wind-blown sand is a problem on sandy beaches, but there have been few studies on the effect of waves on blown sand on beaches. In this study, experiments were conducted to simulate the sandy beach on the west coast of Niigata, and the purpose was to improve the accuracy of the calculation of the amount of wind-blown sand by numerical calculation on the sandy beach and to establish a calculation method. In the study of only the air, similar results to previous studies were obtained in both experiments and numerical analysis. On the other hand, the amount of wind-blown sand was underestimated in comparison with field data. Therefore, in order to take the gas-liquid phase into account, we conducted an experiment and numerical analysis in which wave generation was performed in addition to the air blow. In the experiment, it was confirmed that the amount of sand dispersed increased compared to the case where only the air was taken into account, suggesting that the accuracy of sand dispersal calculation is improved by taking waves into account. Further evaluation of the mechanism near the shoreline is needed to improve the accuracy of the numerical analysis.

 The anti-scour performance and stability of an annular filter unit designed to effectively prevent scouring around monopile foundations against waves were investigated through 1/60th scaled irregular wave hydraulic model experiments. The experiments were conducted in comparison with conventional round filter units. Five types of scour protection were tested under different wave conditions: no protection, round, annular 3D (where the bag diameter is three times the pile diameter D), annular 2D plus asphalt mat, and annular 2D plus round outer circumference. The results of the seafloor surface image measurements showed that even under the wave conditions that caused scouring when the round filter units were used, the scouring when the annular filter unit was used was significantly reduced. The annular filter unit did not move under the wave conditions where the round filter unit was observed to move, confirming the superiority of the annular filter unit.

 In recent years, Shichiri-ga-hama Beach in Kanagawa Prefecture has eroded. The cause is assumed to be the discharge of sand turning around the Inamura-ga-saki Point that bounds the east end of the beach. To investigate the cause, field observations of the seabed by divers were carried out together with the sampling of bottom material offshore of this headland. Since sand transport around the headland was considered to occur mainly during high waves, wave crest lines offshore of the headland were determined from the satellite images and incident wave direction was measured during storm wave conditions. Numerical simulation of the nearshore current around this headland was also carried out under the oblique wave incidence and the sand discharge from Shichiri-ga-hama Beach was investigated in relation to the extent of oblique wave incidence around the headland. It was found that part of sand composed of Shichiri-ga-hama Beach was transported eastward turning around Inamura-ga-saki Point.

 Marked beach changes occurred since 2012 at Nagama-hama Beach on Kurima Island located in the southwest part of Miyako Island as well as the beach changes at a sand spit extending at the Yonaha Bay mouth. A natural sandy beach composed of coral sand was eroded away at Nagama-hama Beach, and the sand spit, originally extended northward, elongated in the direction of NNE. The beach changes were analyzed using satellite images along with the field observation. On the basis of the observation records at Meteorological Observatory at Miyako Island, the course of typhoons since 2012 and resulting strong wind velocity and direction were investigated, selecting large typhoons affected Miyako Island. It was found that when large typhoon propagated northward in the sea east of Miyako Island, strong wind blew from the WSW or SW direction along the southwest coast of Miyako Island, resulting in strong wind waves from these directions. It was concluded that beach changes were triggered by these strong waves during typhoons.

 Fresh water due to the flood from the rivermouth is transported as the mud water plume, so that sediment transports and morphodynamic changes outside the rivermouth are stongly depended on the salinity at the rivermouth boundary accompanied with wind, wave and river discharge conditions. In this study, by using the WDM-POM which was a quasi-3D sediment transport and morphodynamic change model developed by authors, and the gRSM which was a river sediment calculation model by authors, movements of muddy water plumes were investigated by changing the salinity at the rivermouth boundary. From numerical results, it was revealed that high turbidity zones were concentrated almost in the center of muddy water plumes, and velocity fields were varied especially near fresh/salt water fronts compared to homogenious results. The maximum spatially-mean velocity deviation over the whole numerical domain was about 8.0 cm/s as changing the rivermouth salinity from 0.0 to 30.0 ‰. It was due to the strength of muddy water plume and fresh/salt water front. In conclusions, it was shown that to set the accurate salinity at the rivermouth or to conduct simulations with boudanry conditions of rivers at the far upstream was important in order to calculate the accurate flow field inside the rivermouth to estimate sediment tranports and morphodynamic changes of coastal zones.

 In the area between L-shape groin and No. 4 breakwater on the Shimizu coast, measures against beach erosion which reduces minus impact to the scenic beauty of the coast have been taken. To this purpose, detailed investigations have been carried out on the beach changes in the area between L-shape groin and No. 4 breakwater. On the other hand, beach has been eroded over time despite the beach nourishment downcoast of No. 4 breakwater located at the north end of the coast, which is in contrast with the significant shoreline advance upcoast due to the beach nourishment. Thus, the shoreline recession in this area due to decrease in longshore sand transport approaches to the level not to be acceptable. In this study, while comparing the beach changes in the areas upcoast and downcoast of No. 4 breakwater, erosion downcoast of No. 4 detached breakwater was investigated in detail and future measures against erosion were described. It was concluded that appropriate measure is the continuation of beach nourishment downcoast of No. 4 breakwater.

 Shoreline changes along the coastal zone of Eletan Bay of 29 km width bounded by the Cimanuk River and Cipunagara River deltas in west Indramayu on Java Island were investigated using satellite images together with UAV images and field observations. The results show that sediment supplied from these rivers is mainly deposited around the river deltas with least supply to the coastal zone of the bay. Under the condition, longshore sand transport occurred due to obliquely incident waves caused by easterly trade wind. Since the bed material is composed of fine material with a small equilibrium slope, bed material supplied from the land was not deposited near the shoreline area, resulting in no shoreline advance in response to the shoreline recession. The sand discharged offshore and deposited in the offshore zone.

 On tropical island countries, coral reefs well develop, and coral sand is transported shoreward due to waves for a coral cay to be formed. On these coral cays, the ground elevation is so low that strong impact by sea level rise may occur. On the other hand, the significant impact to the coral cay may be also caused by artificial modification related to the rapid economic growth, resulting in beach erosion of the island. In this study, beach changes triggered by the closure of an inlet located between Kaddhoo and Maandhoo Islands in south Laame Atoll in the Maldives were investigated. After the closure of the inlet, a foreshore was formed by the shoreward sand transport originated from sand deposited on the lagoon side. The rate in the volume of the foreshore on the lagoon side was measured to be 1.8×10⁴ m³/yr between 2018 and 2021. Moreover, it was found that to maintain the sand supply from the ocean to the lagoon side, the inlet of funnel type is needed.

 On Maabaidhoo Island in Laame Atoll in the Maldives, a large amount of sand was supplied from the coral reefs since 2013. On Fonadhoo Island in the same atoll, sand supply from the coral reefs occurred between 2018 and 2022 in the same manner as that on Maabaidhoo Island. This sand movement was investigated using satellite images together with a field observation on the deposition and erosion along the shoreline on July 11, 2023. It was found that a large amount of sand had been supplied from the coral reefs between 2018 and 2022 due to the shoreward sand transport on Fonadhoo Island, resulting in increase in foreshore area. This shoreward sand transport was confirmed from the evidence that many streak patterns on the coral reefs approach to the shoreline over time and connected to the shoreline forming a berm on the foreshore. Moreover, it was found that approximately 13,000 m³ of sand increased during this event.

 Future changes of wave climate due to climate change were estimated and the impact assessments on beach topography of Kochi coast were conducted using wave prediction data and past field data. The results show that energy-averaged mean wave direction would be changed anti-clockwise in the future. We also confirmed that the wave direction has already begun to change anti-clockwise in recent years, and the slight difference in wave direction caused changes in the direction of the longshore sediment transport because the angle of the shoreline differs slightly from place to place, indicating that changes of wave direction have a significant impact on sediment transport. Furthermore, it was found that future changes of wave direction will increase westward sediment transport and beach erosion will be more severe, especially on the east side of the coast.

 To clarify the effect of climate change on beach topography, we analyzed future changes of wave climate in Kochi coast and Tottori coast. We proposed a method to assess the impact of climate change on beach topography separately for typhoon and non-typhoon periods by wave simulation for each of 2, 000 typhoons in the present and future climates using d4PDF, and estimating future changes in wave energy and wave direction together with wave projection data. In Kochi coast, the influence of typhoons is more dominant than that of the non-typhoon period and increases further in the future climate, while in Tottori coast, the influence of typhoons increases to the same degree as that of the non-typhoon period in the future climate, although the influence of the non-typhoon period is significant in the present climate. We also confirmed that future changes in longshore sediment transport will be more strongly influenced by wave direction because the direction and the east-west ratio of sediment transport change due to future changes of wave direction. The probabilistic assessment method proposed in this study could be applicable to other coasts affected by occasional typhoons.

 The mouth of the Sekikawa River, which flows into the Sea of Japan, opens predominantly in the NNW direction, where winter high waves prevail. Consequently, during the winter season, high waves can propagate upstream in the river channel, leading to sediment deposition from the mouth of the river to approximately 1.6 km upstream on the left bank of the Sekikawa River. Previous studies have reported that sedimentation was facilitated by long-period waves. In this study, first, we analyzed data such as aerial photographs, topographic surveys, and river flow rates to understand the geomorphic characteristics formed by river flow. We revealed the formation of a sandbar near 0.7 km upstream. Next, to elucidate the factors influencing sediment deposition from the mouth to 0.7 km upstream, we conducted wave and flow observations within the river channel and topographic surveys during the winter high wave period. It was found that sediment within the river channel from the mouth to approximately 0.4 km upstream primarily moves and deposits due to wind waves, while upstream from 0.4 km, there is potential for sediment transport during the propagation of long-period waves.

 A numerical morphodynamic model for predicting the flushing and recovery of river mouth bar was presented. The numerical model was based on the three-dimensional morphodynamic model. In the river mouth bar flushing process due to a large discharge occurred by Typhoon or seasonal rainy conditions, the increase of water level on the upstream area of the river-mouth bar and river flow velocities were considered. In the recovery process of the river-mouth bar, the sediment transport distribution due to waves over the sand bar was considered. In this study, the applicability of the presented numerical model was examined for the sand bar of the Tenjin River Mouth in Tottori Prefecture. Two flushing cases, which were due to Typhoon1721 with stormy waves in Oct. 2017 and seasonal rainy strong flood in July, 2021, were used. In both cases, the river mouth bar flushing and reformation were reproduced. From comparing with the previous field investigation results, it was confirmed that the numerical model presented in this study gave qualitatively good results.

 The morphology of sandy beach is expected to change significantly due to climate change-induced sea level rise. However, the prediction is limited to consideration of shoreline changes using modified Bruun rules and similar methods. Therefore, in this study, we modified the source term of XBeach Surface Roller Energy (SRE) in order to simulate the planar topographical changes from the shoreline to the wave-breaking region more accurately. The study focused on the Hamamatsu Shinohara coast in Shizuoka over a period of approximately 7 months. The simulation resulted in more accurate changes in the morphology from near the shoreline to the offshore bar than using the default constant coefficient value. In particular, it was confirmed that setting the SRE dissipation term as a variable dependent on water depth is effective in improving the accuracy of morphological change predictions, and this study provided an effective model for developing adaptation measures against beach loss in the future.

 While the impact of longshore sediment transport on beach erosion is significant, many aspects remain unexplored. The present study introduces an experimental setup using a circular wave tank, which allows for the formation of a continuous beach environment without beach edges, and a numerical model that replicates these experiments. Experiments with spiral waves revealed that topographic changes are periodic along the longshore direction, and that this periodicity varies with wave conditions. In the numerical simulations, the method for generating spiral waves using a moving mesh approach to address the challenges posed by cylindrical meshes. The results highlight the critical role of interactions between shoreward and return flows, which generate longshore component velocities that enhance the development of longshore currents.

 Sediment processes at the Tenryu River estuary were analyzed using X-band radar observations, bathymetry data collected by fishing vessels, and numerical calculations using XBeach. At the estuary, we observe two sediment processes: (1) sediment transports from the river to the coastal region, or river terrace, by the floods, and (2) sediment transports from the river terrace to the adjacent coastal area by the waves. We show a synthetic monitoring method to observe these sediment processes. In 2021, several prominent floods transported sediments from the river to the coastal region. We analyze these from the results of X-band radar observations and XBeach calculations. We show the deformation of the river channel, river sandbar, and surface flow velocities during the floods. Then, the deformation processes of the river terrace are derived from the X-band radar observations and depth collected by the fishing vessels. We analyze the refraction of the waves above the river terrace and estimate the depths. From the overall results, we describe qualitatively and quantitatively the sediment processes at the estuary and show the amount of sediments provided by the floods.

 The porosity in the surf zone’s sand layer is known to exhibit variations. However, there are few studies on this, and existing models of sediment transport and topography change do not take into account the spatio-temporal variation of porosity in the sand layer. In this study, experiments were conducted using a tabletop wave generator. Regular waves were generated on the constructed sand slope, and images of the sand layer were taken by an X-ray CT system to measure the spatio-temporal variation of porosity under conditions where the topography was going through constant changes. The recorded images were analyzed to investigate the relationship between the topographic change and the porosity change in the sand layer. As a result, it was confirmed that there were layers with large porosity at the surface during topographic change in both accretion and erosion areas. The thickness of the layers (i.e. with large porosity) were about 4 mm in the accretion area and about 2 mm in the erosion area. The differences in thickness were due to loose sediment deposition and surface scraping in the accretion area and erosion area, respectively, leading to a twofold increase in the former.

 The influence of soil type contained in turbidity currents which cause damages to infrastructure placed on the seafloor has not been fully investigated. In this study, turbidity currents using suspensions of silica flour, kaolinite, and Tokyo Bay dredged clay with different volume concentrations of C = 0.5-11% were observed in an experimental flume, and the average head velocities of turbidity currents u_av were measured. The viscous characteristics of these suspensions were also measured by a viscometer. The values of u_av of the silica turbidity currents without clay minerals increased non-linearly with increasing C. On the other hand, the u_av - C relationship of clay turbidity currents showed an upward convex trend, where the value of u_av reached its maximum at the relevant C equivalent to water contents 10 times higher than their liquid limits. The values of plastic viscosity and yield stress of the clay suspensions changed significantly at these relevant C values. Furthermore, the resulting Reynolds numbers also corresponded to the range for the transition from laminar to turbulent flows.

 At the mouth of the Hojo River drainage located in the central part of Tottori Prefecture, river mouth blockage due to the formation of a sandbar frequently occurs. In this study, on-site experiments were conducted to demonstrate a method of flushing a sandbar by operating a diversion weir, which is a pneumatic rubber-coated fabric weir located 2.2 km upstream from the mouth. Experimental conditions were set up focusing on the upstream water level and flow rate with the opening and closing of well weirs and sluice gates, the water level at the mouth of drainage before overtopping, and the height of the sandbar. Preliminary numerical examinations were also conducted whether sandbar flushing can occur. The initial ratio between the water level and the crown height of the sandbar and the rise of the water level at the river mouth after the operation of the diversion weir are important factors in the possibility of sandbar flushing, therefore these parameters were reflected setting in the initial conditions of the field experiments. The feasibility and usefulness of this method have been confirmed through video recording of the process of sandbar deformation, observations of water level, and topographic surveying before and after the experiment.

 At Ikobe Coast on the Pacific side of the Atsumi Peninsula, dunes began forming in front of the sea cliffs around 2010 and have since developed to a height of over 20 m. This rapid dune development is attributed to increased wind-blown sand resulting from beach expansion due to coastal protection projects. However, alongside dune growth, significant erosion has been observed on the downdrift side of the shoreline, indicating that the formation of large dunes affects longshore sediment transport. This study analyzes the relationship between dune development and changes in the beach and submarine topography using dune topography survey data, bathymetric survey data, and satellite images. The analysis reveals a negative correlation between submarine topographic changes and dune volume changes during the dune development period. Additionally, by calculating the sediment budget within the dune area under the assumption of increased wind-blown sand, it is found that sediment supply to the dunes significantly reduces the longshore sediment transport.

 At the mouth of the Kurobe River flowing into Toyama Bay, alluvial sand from the river discharges into the deep sea through the steep seabed near the mouth along with the discharge of sand into a steep seabed offshore of the Aramata area located immediately south of the mouth. The rest of the sand discharged from the mouth is transported southward and finally discharges into the deep sea in the Ikuji area located south of the Kurobe River. Thus, shore protection of these areas closely relates to the longshore sand movement and offshore discharge of sand. Selecting the Aramata and Ikuji areas as the study area, beach changes were analyzed using the NMB survey data. As a result, it was found in the Ikuji area that 3×10⁴ m³ of sand discharged at a once offshore of the detached breakwater of pile structures, and the seabed became unstable when the seabed slope exceeds 1/1.9, causing the landslide on the seabed.

 A moving bed hydraulic model experiment was conducted to investigate the slope sinking that occurs on the slope behind a seawall on a coastal roads during high waves under wave (non-overtopping)-groundwater level interaction conditions. The flow characteristics in the sand layer were investigated using pore pressure gauges and manometers under four cases of countermeasures (permeable layer) with different widths in the behind of seawalls. Infiltration and exfiltration flow components into the sand layer were calculated using a pore water pressure gauge measurements. These seepage velocity was analyzed using groundwater gradient and Darcy's law based on manometer measurements. The present results indicated that the cross-sectional shape of the buried permeable layer not only drains groundwater effectively but also suppresses the effects of infiltration and exfiltration into the sand layer.

 On the Shimizu coast, it was not confirmed whether or not northeastward longshore sand transport prevailing along this coast can smoothly reach the north end of the coast, because artificial headlands have been installed on the coast. Therefore, 2.75×10⁴ m³ of sand (d₅₀ = 2 cm) excavated from the riverbed of the Abe River was experimentally nourished between artificial headland Nos. 4 and 5 in January 2022, and subsequent movement of the sand was observed by NMB and UAV surveys four times. It was found that nourishment sand was mainly deposited in a zone shallower than -4 m until May 2022, and the sand deposition zone near headland No. 4 moved northeastward by 200 m by September 2022. The entire volume of deposited sand between headland Nos. 4 and 5 until May 2022 was 3.8×10⁴ m³, and the volume of nourishment sand was 74% of all the nourishment sand. Moreover, the nourishment sand was transported while maintaining the characteristic height of beach changes of 8 m. Even though there exist headlands, sand was gradually transported alongshore while passing through the HLs.

 Several small and medium-sized rivers connecting to the Shichiri-Mihama coast are closed due to sand and gravel. The progression of river mouth closure increases the risk of flooding in the river basin, so countermeasures are necessary. In this study, piles were installed in a staggered pattern to prevent river mouth closure. The study focused on the effects of the countermeasures on lowering the height of the berm, advancing the location of the berm to the offshore side, and controlling the amount of sedimentation in the overland area. Hydraulic model experiments were conducted under various wave conditions to clarify the effects of different installation conditions of the countermeasure piles on topographic changes. It was confirmed that the installation of the countermeasure piles suppressed the accumulation of landward-rushing gravels, regardless of the spacing of the countermeasure piles. In particular, under wave conditions with large wave steepness, the effect of the countermeasure piles became more pronounced as the spacing of the countermeasure piles became closer together.

 The Motofuji calvert, located in Fuji Coast (Fuji City, Shizuoka Prefecture), experienced problems with the clogging of its outlets due to coastal drift and high waves. In response to them, hydraulic model experiments were conducted for several years starting from fiscal year 2016, and an unprecedented construction method of " relocation of a jetty and excavation behind calvert outlets " was selected as the optimal solution. After the completion of the construction on-site by fiscal year 2020, simple measurements and photographic documentation using UAVs have been performed to monitor sedimentation at the outlets, refilling of the excavated areas, and changes in berms formed by high waves. The results show that a berm height of T.P.+2.5m could serve as one of the benchmarks for implementing re-excavation behind the calvert outlets.

 Mangroves have tsunami attenuation and coastal erosion prevention effects. Yamamoto et al. (2015) compared erosion experiments with calculations using a planar two-dimensional model and showed that the model was sufficiently accurate with respect to topographic changes, but three-dimensional calculations are required for more detailed studies. Regarding the tsunami attenuation effect of mangroves, Yoshida et al. (2023) demonstrated the validity of numerical simulations by modeling mangroves using the Dupuit-Forchheimer rule, which is computationally inexpensive in three-dimensional calculations. In this study, using the same modeling method as that used by Yoshida et al. (2023), we conducted sediment transport calculations behind mangroves, comparing them with hydraulic model experiments and examining the effects of resistance coefficients. Similarly to the experiments, erosion behind the mangroves was small, while erosion was significant in other areas.

 High-frequency and wide-area coastal monitoring is expected to benefit from the use of Synthetic Aperture Radar (SAR), yet efficient methods for extracting coastlines from high-resolution SAR images have not been established. In this study, we developed a shoreline extraction method using DeepLabv3+, a convolutional neural network, applied to 1-meter resolution X-band SAR images. To explore applicability on coasts with structures such as offshore breakwaters, we created multiple models by varying the combinations of coastal images used for training. For coasts without structures, using SAR images of the specific coast for training enabled coastline extraction with an average error of 3 to 4 meters. Even for coasts with structures, incorporating SAR images of coasts without structures in the training reduced the average error in coastline extraction with a similar accuracy. These results demonstrate the potential of using high-resolution X-band SAR images for coastal observations.

 In big data (BD) methodologies for monitoring seafloor topography over long periods, irregularly spaced spatial data from fishing vessels are allocated onto grid points to generate bathymetric data. Interpolation of large amounts of depth data is necessitated for this purpose. Kriging, a widely used method for terrain data interpolation, requires computational costs to eliminate anomalies, posing a challenge in the BD methodologies dealing with massive data. On the other hand, Support Vector Regression (SVR), a machine learning technique, is suitable for estimating surfaces with minimal influence from anomalies in spatial domains. In this study, we applied SVR to interpolation in the BD methodologies. The demonstration showed that it achieves accuracy equivalent to Kriging while reducing computational time for generating terrain with 50 m grid spacing to approximately 2.5 % of one-point-removed Kriging.

 In 2023, 8500 m³ of sand was excavated at the north end of Ajigaura Beach and the sand was nourished at the south part of the beach for sand back pass. UAV survey was carried out on June 1 before beach nourishment and July 8 and October 10 after the nourishment to investigate beach changes. During the observation, storm waves associated with T2307 hit this area between August 10 and August 15, and nourishment material was transported away. Because UAV survey was discretely carried out, temporal changes in beach topography were not known. Therefore, time-lapse cameras were set on the beach, and photographs were taken at every 1 hour to measure wave run-up and change in foreshore. It was found that nourishment material was quickly transported away during high waves with H_1/3 = 3 m and T_1/3 = 10 s during T2307, and a berm with an elevation of T.P.+2.8 m was formed.

 LiDAR employing a green laser, which exhibits high underwater transparency, has been rapidly adopted for the measurement of coastal bathymetry. The green LiDAR scanner has become lighter and can be mounted to UAVs, thus it is anticipated that it will be used for measuring coastal areas in various environments. However, there are currently no technical guidelines for measuring coastal bathymetry using LiDAR from UAVs. Furthermore, there is a lack of clarity regarding the differences with other techniques such as narrow multibeam sonar using boats (NMB) and aerial laser bathymetry using airplanes (ALB). Here we aimed to comprehensive overview of the characteristics of each measurement method in terms of efficiency, depth limit, point cloud density, and footprint. Additionally, we show technical guidelines for coastal bathymetry using UAV-mounted green LiDAR were shown based on actual cases of the measurements. High-accurate and high-density coastal bathymetry data will substantially provide new insights of coastal morphological changes.

 It is challenging to assess the efficacy of countermeasures against channel burial by siltation through model experiments. Therefore, numerical analysis is a crucial verification method. However, conventional analytical models are low-resolution hydrostatic approximations that cannot accurately depict the topography of the navigation channel and surrounding structures and eddies. Consequently, it is impossible to analyse and verify the effectiveness of anti-siltation measures by installing depressions and submerged dikes around the navigation channel. This study employed a high-resolution vertical two-dimensional non-hydrostatic model with a grid size of approximately 0.25 m to calculate the flow velocity and suspended solids concentration (SS) around the channel and examine the siltation distribution. The results demonstrated that the eddy generated upstream of the channel causes a high concentration of SS to sink obliquely downwards at the outer edge of the clockwise eddy, with a horizontal-to-vertical flow velocity ratio. This accumulation occurs in the low-flow region directly below the eddy. Implementing countermeasures reduced SS deposition in the depression and the layer below the eddy, with a 50-70% reduction in the amount of SS buried in the channel.

 At Toyo Port, silt accumulates in the ship channel after disturbances such as typhoons.In this study, bathymetric survey results, wave and river flow data were analyzed, and the sedimentation process was confirmed by numerical simulation. Data analysis estimated changes in sediment volume from bathymetric measurements since 2018 and showed that there is a high correlation with wave and river discharge. Therefore, it was inferred that the sediment from the river during floods was transported toward the channel while being sedimented and resuspended. In the numerical simulation, the sedimentation in the ship channel caused by Typhoon 1824 was reproduced by siltaion model adopted sigma coordinates. In particular, it was found that wave current generated at the river mouth enhances the sediment transport to the ship channel.

 Localized coastal erosion is occurring around the crest type of artificial reef in the Kaike area of Kaike coast due to high waves caused by typhoons and winter storms. Although emergency beach nourishment measures have been implemented for beach conservation, the situation of resolving continuous maintenance nourishment has not been reached. In this study, we conducted field investigations, including wave and current observations, as well as fixed-point camera surveys, to estimate the mechanisms of topographic change around the artificial reef for preserving the sandy beach in the Kaike Area. The erosion that occurred during the high waves in late January 2024 was estimated to be caused by the rise in water level behind the artificial reef and the failure of the waves entering from the opening of the artificial reef to diminish. Instead, they reached the shore side of the artificial reef and caused erosion of the sandy beach at the opening of the artificial reef.