It has been clarified that calcium in cement-treated soil is leached by magnesium salts contained in seawater, and the soil characteristics are changed. In this study, unconfined compression test and content analysis were carried out on cement-treated clay with different stabilizer content and curing time that were deteriorated by immersion in Mg solution, and changes in strength characteristics and their mechanisms were discussed. As a result, it was shown that the strength of the deteriorated cement-treated clay was significantly reduced regardless of the initial conditions. The strength of the deteriorated cement-treated soil depends on the amount of cement and the curing time, and the strength after deterioration is proportional to the strength before immersion in Mg solution. The decrease in strength was caused by the transfer of calcium silicate hydrate to magnesium silicate hydrate, which has a relatively low binding force.

 In this study, the effects of loading conditions of overburden pressure during curing of clayey soil mixed with steelmaking slag on the dry density and shear strength development were investigated.

 The results show that the later the final overburden pressure acts, the smaller the consolidation settlement and the smaller the dry density. It was found, however, that at 28 days of age, even at such small dry densities, the shear strength due to solidification was dependent on the magnitude of the final overburden pressure. However, the earlier the final overburden pressure is reached, the earlier the increase in shear strength is expected to occur.

 This paper focuses on the stability of a pile in sand bed under cyclic axial loading for TLP-type floating offshore wind turbines. The emphasis of the experiments was placed on understanding the responses, Stable, Unstable or Metastable, depending on mean and cyclic loading and investigating the degradation of tensile capacity of piles due to the cyclic load history. A range of centrifuge pile-sand tests was performed under a centrifugal acceleration of 50 gravities. It was found that the interaction diagram indicating pile-stability based on the experimental results for TLP-type was consistent with those from previous studies for bottom-mounted-type foundation for offshore wind turbines. The experimental results also show the pile tensile capacity could decrease due to the cyclic loading in Metastable.

 Authors have been developing a technique to make high-strength soil concrete block using high-pressure dehydration apparatus with a radial drainage system. Constant strain rate consolidation tests were conducted for solidified treated soil mixed with dredged clay and blast furnace slag cement class B (BB) for clarifying the relationships between consolidation pressure p and coefficient of consolidation c_v/ permeability k. In addition, permeability k of the materials were estimated from exchangeable capacity of cations measured before and after the consolidation tests. Coefficient of consolidation cv during block manufacturing tests were estimated by parametrically calculating time and settlement curves using radial drainage theory. Following results were obtained from this study. (1) when estimating k using ion exchange capacity, it was found necessary to take into account changes over time in specific surface area due to pore structure and solidification. (2) it was suggested that a c_v greater than 1 MPa of working pressure in the constant-strain consolidation test should be used to estimate the c_v of the block.

 Since the height of the bump, which is caused by oyster beds in the Obuchi river around the mouth of the Obuchi lake affects the tidal water transport, the circulation and mixing of materials in the lake, the relationship between the bump and the water level was investigated by the model reproducibility of the salinity in the brackish Obuchi lake. After conducting a sensitivity analysis with a change in the seafloor depth of the overall model topography (zo m), the maximum likelihood method was applied using observations from 2016 to 2020 to estimate a maximum likelihood value with an error deviation of zo (0.17 ± 0.085 m). The error deviation is comparable to the amplitude of the annual tidal constituent in this area, which indicates that the annual tidal constituent passively affects the salinity in the Obuchi lake. Moreover, using the model, the dilution process of the tracer (virtual dye) was investigated by filling the lake with a uniform concentration of 1 unit m⁻³. The tracer concentration decreased with a constant half-life. The maximum likelihood value of zo lead to the half-life of 13.4 (11.5 - 15.9) days.

 Carbon fixation by marine plants such as eelgrass has been the main focus of research on the blue carbon effect in coastal areas. In this study, we collected organic sediment from the seafloor with excessive organic matter and conducted experiments on the persistent degradation of organic matter by coal ash granules (GCA). The results suggested that the ions leached from GCAs promoted the reduction of COD, but also formed refractory organic carbon.

 Cigu Lagoon is a famous oyster farming lagoon located in the southwest of Taiwan. This study aims to investigate the environmental factors that affect dissolved inorganic carbon (DIC) in the Cigu Lagoon and understand the impact of oyster farming on DIC. First, structural equation modelling (SEM) analysis was conducted to determine the contributions of environmental factors on DIC. The results showed that DIC increased with the increase in inflow, decreased due to phytoplankton photosynthesis, and increased by the oyster biological reaction. Based on the SEM analysis results, a conceptual DIC model was developed to estimate the annual carbon budget of Cigu Lagoon. As a result, oyster cultivation contributes to the increase of carbon in the lagoon, but its effect is small, and the main factor affecting DIC is the carbon sequestration effect of phytoplankton.

 Numerical simulations of regional sediment transports from river basins to coastal zones are important in order to propose comprehensive countermeasures for river and coastal sediment problems. In such simulations, one of important factors is the treatment of boundary conditions of river discharges and sediment discharges. In this study, numerical simulations of regional sediment transports from river to coastal zone by using a regional sediment transport of mud/sands and morphodynamic model by considering tidal and coastal currents : WDM-POM and a river sediment model by using a simple model and observation data : gRSM. In these simulations, several conditions of river discharges were applied and discussions were made especially on differences of coastal sediment movements by comparing between numerical results and observation results of remarkable siltation of Yamakuni River and Nakatsu Intertidal Flat due to North Kyushu Heavy Rain Disaster in 2017. First of all, numerical results by using boundary conditions of river discharges with hydrographs revealed qualitative agreements with observation results in terms of siltation. Furthermore, in conclusions, boundary conditions by using hydrographs were very important and time relationships between the hydrograph and tidal elevations affected severely on sediment movements of the intertidal flat. By using optimal boundary conditions, accurate simulations will give answers of mechanisms and countermeasures of erosion/deposition and siltation.

 Catches of fish in Osaka Bay has declined sharply since the late 1990s. Off the coast of Kobe City, too, there have been poor catches in recent years in the springtime fishery of "Ikanago Ryo" and the seaweed cultivation famous for "Suma Nori". In this study, a social implementation aimed at improving the waterfront space and recovering fishery resources was carried out under the collaboration of four parties: a local environmental conservation activity group, a water right holder, a fishery cooperative, and a research institute. As a result, we confirmed the nutrient supply and biological flocking effects of the container filler. However, as the amount of nutrient elution decreased over time, it was suggested that maintenance work was necessary.

 Microphytobenthos are frequently the dominant primary producers in shallow tidal estuaries, and their photosynthesis is limited by the amount of photosynthetically active radiation (PAR). Most previous studies evaluating the primary production of microphytobenthos were conducted using “in-situ methods, ” which involve placing incubation bottles on tidal flats for certain durations (approximately 1 day). However, as in-situ methods do not reflect the variation in weather and tidal cycle; therefor, they may lead to errors in obtaining a representative value for a certain period (e.g., from month to year). In this study, we conducted field observation and laboratory experiments to estimate the primary production of microphytobenthos based on both P-I curve and mooring systems at two stations on Midori River tidal flats facing Ariake Bay, Japan, in October 2021. The monthly average primary production was compared with that recorded on days when the in-situ method was implemented, which corresponds to spring tide. Finally, we discuss the validity of the primary production estimated using the in-situ method as a representative value. The maximum photosynthetic activity and light saturation recorded in this study were much higher than those previous reported; hence, the microphytobenthos of the Midori River tidal estuary were considered to be adapted to relatively high light conditions. The average photosynthetic activity under high PAR (2493 µmol m⁻²s⁻¹) was 4.80 mgC mgChl-a⁻¹h^–1 lower than that under moderate PAR (1246 µmolm⁻²s⁻¹), indicating photoinhibition at one station. At the two sites where observations were conducted, the error from the average value exceeded by 10% in all except for two of the 16 cases, suggesting that the in-situ method estimates contain errors as representative values for the month.

 The coastal area of the eastern part of Shimabara Peninsula of Nagasaki Prefecture is estimated to be a settling site for larvae of asari clam, Ruditapes philippinarum. However, these settled juveniles are washed away during their growth due to wave influences. Surveys of the settled clams at Takeshima beach, an unused fishing ground located in the center of the coast, have not been previously conducted. In this study, we undertook surveys of the early juvenile clams and the collection of clams by using mesh bags stuffed with gravel along the shoreline. We confirmed the presence of the early juvenile clams at the beach. After 17 months of placement of mesh bags, we caught approximately 200-600 adult clams/m². The result suggests that the establishment of clam fisheries is expected by using mesh bags in the eastern coastal area of Shimabara Peninsula.

 In the tidal flats and the shoals which constructed gradually by using the dredged mud for 8 years, the features of the topographical change under and after construction and the habitat conditions of the organisms such as the short-neck clams were grasped from the existing data and the field survey. According to the result of evaluation of the topographical changes at the construction, some dredged mud moved and discharged before the sand capping. However, after the sand capping, the tidal flats were nearly stable and the habitat of the organisms such as the short-neck clams was maintained. Also, in the shoal construction zone, the eelgrass bed got the early recovery.

 An artificial tidal flat in Okinosu, Tokushima city, Japan, was created as a habitat for the rare species, Cicindela lewisi (C. lewisi). Considering their rarity, adaptive management by sand covering was implemented in July 2021 to expand the area available for the habitat of C. lewisi. This study aimed to determine the impact of the sand covering on benthic animals. The results showed that Callianassa japonica was dominant on the artificial shore, with an expanded range, as compared to that in 2015. Although many bivalves were noted, no adult Ruditapes philippinarum and Laternula marilina were found, suggesting a negative impact of C. japonica. In the sand-covered area, the ground elevation was higher and the grain size of the bottom sediment was larger than in the surrounding areas. Additionally, the dominant species, C. japonica, was eliminated, and Scopimera globose was found, different from other survey points.

 Coral reefs are important water areas from a fisheries because of their propagation function as feeding grounds for fish and nursery grounds for juvenile fish. In regard to the feeding function of hermatic corals, previous studies have indicated that corals are a direct or indirect food source for coral reef fish. However, these findings are often fragmentary information based on observation and analysis of stomach contents for individual organisms under study. Few studies have investigated whether coral-origin-food is a food source that supports higher consumer fishes and what kind of food web structure it forms. Hence, a detailed evaluation of the feeding function of corals has been difficult. In this study, we attempted to estimate the food web of coral reefs by analyzing the stomach contents of coral reef fish and the carbon and nitrogen stable isotope ratios of these fish and their possible food.

 We bred juvenile short-neck clams, Ruditapes philippinarum, which collected in June at the tideland of the lower Hiroshima Yahata River, in land breeding device and discharging them again in autumn in order to avoid the summer when the death rate was high because of the large-scale flood and high temperature. We studied to clarify the land breeding condition that had high survival rate of juvenile clams at the same time. The juvenile clams could be bred to have high survival rate by keeping water temperature 15℃, with little difference of growth rate compared to 18℃. In addition, growth rate was improved by a long time feeding of 12 hours. The clams which discharged in the tideland again in October seemed to survive 40-50% by next May, and it was suggested that the clams could contribute to the reproduction of resources.

 This study was conducted to gather basic information on ways to increase surface area of organism-adhering substrates per unit volume, focused on benthic primary producers, in order to enhance the function of a cold region fishing port as a feeding ground for marine organisms. Test specimens (organism-adhering substrates) were placed under various conditions (e.g., in gaps, in different positions, using different materials) in a port to study the adhering organisms, zooplankton and fish. The results were similar under all conditions, with microalgae and adhering animals found on the surface of specimens. The amount of zooplankton found around specimens exceeded that in the water column. Annelids and arthropods, which are thought to be food for fish, were dominant in the taxa of adhering animals and zooplankton. These results suggest that, by adding adhering substrates and increasing their surface area, primary producers per unit volume of the water column will increase, enhancing the feeding ground function of the benthic ecosystem.

 We developed a new method for estimating the proportion of fertile adults (PFA) in a population of the intertidal gastropod Batillaria attramentaria. Such estimates are necessary to evaluate the function of a habitat as a breeding ground for this species in Tokyo Bay, Japan, and adjacent seas. Using easily obtained morphological indicators as criteria, we constructed a model to estimate maturity and digenetic trematode parasitism to identify snails with mature gonad that are not made sterile by digenetic trematode infestation. The models estimated PFA with moderate accuracy. This method would be useful for evaluating a habitat such as a man-made tidal flat for its function as a breeding ground.

 In Yamaguchi Bay, where horseshoe crabs inhabit, a population survey is conducted every year to determine their population density. The problems of surveying larval horseshoe crab populations are that mudflats are difficult to walk on, require many people, the efficiency of finding individuals varies depending on the skill of the surveyor, and the visual survey area is unstable. The objective of this study was to develop a highly objective method of surveying the density of horseshoe crab larvae by detecting them in images of mudflat surfaces taken by a low-altitude UAV.

 We investigated a method to detect horseshoe crabs on mudflats by taking images of the mudflat surface with a UAV at an altitude of 9 m, calculating the features of the trajectory, and performing machine learning, taking advantage of the fact that the trajectory of horseshoe crabs on mudflats has different characteristics from those of other benthic organisms and reflected light on the mudflats. Although it is possible to identify larval horseshoe crabs on mudflats, the results are not yet satisfactory, and further improvement of identification capability is an issue for the future.

 In Hakata Bay, seabed excavation was carried out to secure sediment during coastal development. In Hakata Bay, the seabed was excavated to secure sediment during coastal development, leaving pits that became a source of anoxic water masses. In order to monitor the recovery of the benthic fauna, benthic organisms have been collected in addition to water quality observation. However, it has been difficult to evaluate the recovery of the ecosystem including the water masses directly above. The environmental DNA metabarcoding survey provided information on the fish fauna, including small fishes and swimmers, and the number of species was larger than the conventional method. Species-specific habitat information provided more detailed information on the distribution of the seafloor and water masses in the estuary and the continuity with the area around the restoration site.

 The present study aimed to clarify the biofouling, shape, and deterioration of artificial substrates that was used to create a habitat for the Japanese spiny lobster, Panulirus japonicus. Substrates A (0.2 mm width, vinylon), B (1.0 mm width, polypropylene), C (1.6 mm width, nylon), and D (4.8 mm width, polypropylene and polyethylene mixture) were placed off Kiire, Kagoshima, on June 10, 2021. Aquatic organisms that adhered to the substrates were identified on July 1, July 28, October 1, and October 27, 2021. Larvae of P. Japonicus were found on substrates B, C, and D. Further, the aquatic organisms that adhered to substrates A, B, C, and D were mostly shellfish, gammaridean Amphipoda, and caprellids, which are bait for P. Japonicus. The breaking strength of fiber measured by tensile testing decreased by 13.2%, 19.4%, 24.9%, and 6.7% for substrates A, B, C, and D, respectively, after soaking in the sea from June 10 to October 27, 2021. Flume tank experiments revealed that substrate D had the highest ratio of projected area from the upstream to still water (0.97) at a flow speed of 0.1 m/s. The ratio of projected area from the side decreased with increasing flow speed for substrates A, B, and D, whereas there was little change for substrate C.

 On 13 August 2021, an underwater volcano, Fukutokuoka-no-ba of the Ogasawara Islands erupted producing a large amount of pumice. Much pumice had arrived around the north part of Okinawa Island since 17 October 2021, causing large damages to the fishery, shipping and tourism. On 28 October 2021, urgent field observation of deposition of pumice was carried out at the Yagaji fishing port located on the north coast of Motobu Peninsula. Video picture of floating pumice around the fishing port was taken using an UAV, as well as the observation of deposition of pumice on the beaches. It was found that a large amount of pumice was transported from the north and deposited on the north side of the fishing port facility, because the movement of pumice was blocked by the fishing port breakwater.

 An underwater volcano, Fukutoku-no-oka, of Ogasawara Islands erupted on 13 August 2021. A large amount of pumice produced by this eruption was transported westward by the ocean current and reached the northwest part of Okinawa Island since 17 October 2021. At the Ogimi coast located in the northwest part of Okinawa Island, much pumice was deposited along the coastline. On 28 October 2021, urgent field observations using an UAV were carried out to measure the distribution of floating and deposited pumice around groins. Beach profile was measured using an RTK-GPS. The maximum thickness of deposited pumice on the beach face was 66 cm. Pumice was first accumulated upcoast of groins and it was transported downcoast by longshore current turning around the tip of the groins.

 A underwater volcano, Fukutoku-no-oka, of Ogasawara Islands erupted on 13 August 2021. A large amount of pumice produced by this eruption was transported westward by the ocean current and reached the north part of Okinawa Island since 17 October 2021. At the Ogimi coast located in the northwest part of Okinawa Island, much pumice was deposited along the coastline burying the white sandy beach composed of coral sand. On 28 October 2021, field observation using a UAV was carried out to measure the distribution of floating and deposited pumice around the detached breakwaters. The thickness of the deposited pumice was measured using an RTK-GPS to be approximately 40 cm. Deposition of pumice was significant between the detached breakwaters because of closed sea.

 An observation method of both beach topography and the distribution of deposited floating debris using an UAV was developed and field observation was carried out at the Onuki coast south of Oarai. Using this method, longitudinal profiles, the distribution and the number of deposition of floating debris, and the change in ground elevation can be simultaneously measured. Field observations were carried out on 29 May and 18 November 2021. Since floating debris is transported on the sea surface, they deposit near the break in slope between the backshore and sand dune during high tide and storm wave condition. The elevation of the deposition of floating debris increased in response to the wave run-up height.

 On the Niigata Port Coast (west coast), a costal erosion countermeasure by the integrated shore protection system was conducted in combination with submerged breakwater, jetty, and beach nourishment. The authors studied the measurement technique which enables them to understand the topographic status after the beach has been nourished in a rational and appropriate manner, while tentatively measuring the coastal landform using Airborne Laser Bathymetry (ALB). ALB can measure the depth from the sea surface up to 8 m below, which is the threshold depth of sediment movement of the relevant sea area. The average of the depth difference at the same point, which was measured by the traditional bathymetric and shoreline survey, was 0.14 m (absolute value), so ALB ensured the same level of accuracy as the former measurement. This result demonstrates the effectiveness of ALB as a monitoring technique to understand surface topographic changes of the relevant nourished beach on a large scale. In addition, as a result of pilot evaluations of beach width soundness, it was found to be able to confirm, in a single approach, the immobile points on both the sea and land sides as well as both the foreshore and backshore slopes, which are necessary for sandy-beach monitoring.

 Total of 8.3 to 16 million people visit approximately 200 major beaches annually in Japan, 700 to 1,400 lost children have been reported recently. It is quite possible that children who are separated from their parents become involved in drowning accidents at beaches, and appropriate countermeasures are required. Although there have been some previous studies on the use of landmarks to guide paths for lost children, there have been no studies that consider the height of children's eye level on beaches where there are few landmarks. In this study, as fundamental research for studying measures against lost children at beaches, we investigated the characteristics of objects that are conspicuous and memorable by children at their eye level through virtual reality (VR) experiments. As a result, differences in the number and duration of fixation were observed between children and adults depending on the difference in their eye level. Subjects of children's eye level paid more attention to objects that were closer or lower, while subjects of adults' eye level gazed at objects that were farther away.

 In Japan, 2,000 to 3,000 drowning accidents occur every season at major bathing beaches. To prevent drowning accidents, beachgoers themselves need to be aware of the dangers of the ocean and avoid them. As a way to do this, bathing beaches provide daily risk levels regarding swimming conditions to beachgoers using beach safety flags in three colors. However, the risk levels are determined subjectively and empirically by lifesavers and beach administrators based on weather and sea conditions, and the characteristics of past drowning accidents are not taken into account. In this study, we examined an objective method of determining the risk levels based on the probability of drowning accidents. As a result, the AI model that can predict the probability of drowning accidents with high accuracy was constructed, and it was suggested that this method can appropriately determine the risk levels and prevent drowning accidents.

 As a simple and economical ground investigation method for seafloor, the authors have developed a novel method which allows sounding to estimate strength and stiffness of the ground and continuous sampling simultaneously. Using a prototype apparatus of improved samplers, two tests were conducted in a soft clay ground. As a result of this demonstration tests, continuous samples of about 7 m were collected that allows detailed observation of each soil layer. From the results of cone penetration tests, visual observation of continuous samples and soil testing of samples, detailed ground stratification can be evaluated.

 The purpose of this study was to verify the feasibility of high-resolution, high-observation frequency, and high-wavelength resolution red tide observations in the Yatsushiro Sea, where red tide damage is serious, using multiple satellite data such as Sentinel-2, GCOM-C, and Himawari-8. To achieve this goal, we analyzed the data on July 15, 2021 under the large-scale Chattonella red tide with simultaneous satellites observation. The results revealed a high correlation between the blue and green two-band model and red tide cell density in the high-resolution Sentinel-2 data. Furthermore, it was found that there is a positive correlation between chlorophyll-a concentration data from Himwari-8 and red tide cell density.

 This study proposes the use of image geometry transformation as a method of water surface refraction correction, which is necessary when the three-dimensional shape of underwater structures is determined using photogrammetry. Three image geometry transformations (depth-dependent affine, depth-independent affine and depth-independent perspective transforms) are proposed to transform images of underwater structures taken from a camera above water into images taken from an apparent camera position calculated by ray tracing considering refraction of light rays at the surface of the water. Accuracy verification experiments using a chessboard method showed that the depth-dependent affine and depth-independent perspective transforms were highly accurate, with u- and v-directional grid spacing errors of about 1% in the case of θ≤10°. The depth-independent perspective transformation was also found to correct trapezoidal distortions. Depth-independent perspective transformation does not require depth information and is therefore considered to be an effective refraction correction method for photogrammetric surveying of underwater structures.

 In ground improvement by chemical grouting, monitoring the penetration of chemicals into the ground in real time enables efficient construction management. Because the chemical solution has an extremely low resistivity, resistivity tomography is considered to be effective as a monitoring method. In this paper, in order to compare resistivity section and the spread of injection chemicals visually, transparent ground models were constructed, and resistivity tomography were carried out while colored low resistivity chemical solution was injected from the two injection holes.

 As a result, it was possible to roughly grasp the situation where the chemical spread in a spherical shape by resistivity. It was possible to capture the situation in which the spheres of the chemical solution injected from each injection holes were sufficiently attached, however it was difficult to capture the time when the two spheres just attached by resistivity tomography.

 The purpose of this study was to develop a simple 3-D groundwater flowmeter to directly measure seawater inflow to the beach, and to clarify the occurrence of vertical seepage during the rising tide and seawater upwelling during the falling tide. A new paper-cylinder type groundwater flow directional anemometer was devised, tested in a laboratory experiment, and used to observe coastal groundwater infiltration in Yamaguchi Bay. As a result, it was possible to measure the three-dimensional groundwater flow directional velocity of groundwater infiltrating into the beach in 15 minutes, and the groundwater recharge and discharge of the beach at the site were clarified.

 To improve the accuracy of wave run-up height observation, morphological noise removal method is applied to the video frame analysis. As a result, it is assured that the morphological noise removal is quite effective to neglect the noise appropriately and automatically which occurres due to the change of background brightness even if the threshold value of binarization is kept small as 10 to prevent the miss detection of maximum height of droplets. Also it is suggested that the method constructed in this study analyze wave run up height accurately compared with visual analysis of video frame. In addition, validity of this method is clarified by comparing the result wave run up height based on the video shot waves from different direction at the same time.

 This study employs the satellite optical and SAR data to investigate the shoreline change at Tanintharyi coast, Myanmar, which is a typical fast-developing region in Southeast Asia. Successful extraction of the shoreline position indicates that satellite remote sensing is a powerful tool to monitor the coastal area, especially in data-insufficient regions. Long-term shoreline change analyzed from the optical satellite data shows that large evolution occurs at the river mouth. Annual shoreline change derived from the Sentinel-1 SAR data shows that the shoreline at the river mouther moves forth and back repeatedly after the construction of the small seaport. Quantitive evaluation of the impact due to the construction of the seaport needs more data such as rainfall and river discharge.

 In a ground improvement work, there are cases in which a design must be changed due to problems such as inconsistency in the position of buried objects and discrepancy in the ground conditions. In these cases, advanced management methods of work progress control are required to properly evaluate the effect of ground improvement. This research examined the applicability of geophysical exploration to visualize the ground improved by chemical grouting. The laboratory model test and the field test at construction site were performed using the P-wave amplitude attenuation tomography. These tests demonstrated that the P-wave amplitude attenuation can recognize the change in ground condition due to chemical grouting. Besides, we discussed the method in which construction information and geophysical exploration result were integrated by BIM/CIM.

 A machine learning model for automatic discrimination and classification of gravel beach components was reconstructed. The classification characteristics and usefulness of machine learning were also examined using the reconstructed trained model. Furthermore, the spatial and temporal variation characteristics of the gravel beach components were investigated using the reconstructed model. The results showed that the reconstructed machine learning model was able to discriminate and classify gravel beach components from ortho-mosaic images.

 Marine pollution of microplastics (plastics less than 5 mm) is attracting worldwide attention. In this study, the maximum number of MPs in the seabed was 97.2 / kg-dry. It was found that the number was high near the estuary and low offshore. The polymer was spatially different. It was confirmed that the number of MPs and the polymer changed with time at all the survey points. This suggests that there may be multiple sedimentary processes in this area.

 In many cities, river water is used for drinking and industrial water. Therefore, when a chemical substance flows into a river, the water supply is stopped while it stays near the water intake of the water purification plant, which adversely affects the lives of the citizens. It is expected that such a situation will occur even during a large earthquake.

 In this study, the chemical substance transport was analyzed and the influence on water intake was examined in Yodo River during a Tsunami attack, using 2DH-3D hybrid model that can analyze the propagation of a tsunami from a wave source in plane two dimensions and the local flow in three dimensions including the density difference. As a result of comparing the behavior of chemical substances under the condition that the salinity of river water is different, the influence on water intake was the largest under the condition that the salinity difference between the upstream side and the downstream side of the barrage was large. It was shown that the behavior of chemical substances is greatly affected by the salinity, and it was found that consideration of salinity is indispensable when predicting the behavior of chemical substances.

 In order to quantify the blue carbon effect, continuous measurement of CO₂ in the field is desired. Redox potential in seawater changes greatly due to the dissolution of CO₂, but it also changes significantly due to dissolved oxygen (DO) consumption. In this study, we investigated the redox reaction associated with the dissolution of CO₂ and the DO consumption due to oxidation of reducing substances. The mechanism of CO₂ dissolution was clarified by (1) dissolving CO₂ in seawater; (2) the relationship between precipitation and pH; (3) the dissolved CO₂ concentration; (4) comparing the experimental results with theoretical values. Furthermore, the redox reaction in a system in which CO₂ and DO compete was clarified through the redox potential. The findings of this study suggested the possibility of measuring dissolved CO₂ concentration by redox potential in the field.

 This study presents a series of the experimental results on the neutralization of leachate at coastal landfill sites by injecting water with ultrafine bubble (UFB) size gases containing CO₂ for stabilization enhancement and global warming countermeasures. Diffusion of neutralization by injecting UFB water into the experimental alkaline water was studied by experiments using a vertical column (3 m high). As a result, when the temperature and density differences between the UFB water and the experimental water were slight, the UFB water diffused upward and neutralized. The results suggest that, when applied to coastal landfill sites, the leachate's pH can be efficiently lowered by determining the injection depth of UFB water by measuring the temperature and density of the target water and UFB water in advance.

 A number of studies have investigated the physical mechanisms of the formation of coral cays through field surveys, laboratory experiments and numerical analysis. These studies show that coral gravels can be naturally accumulated and form coral cays under certain conditions while such natural conditions are limited. To enhance accumulation of coral gravels, authors proposed a gravel trap and showed its effective performance through laboratory experiments.

 This study conducted a field survey to test the performance of the gravel trap in the actual field. The trap was placed around Ballast island located in the north of Iriomote Island, Okinawa. Through the field survey, it was found that the trap with modified layout can effectively accumulate coral gravels transported from various directions. Dominant direction of gravel transport, inferred from accumulated gravels around the trap, reasonably agree with the estimated dominant wave direction around the trap.

 The objective of this study was to quantify the relationships between water surface and sediment CO₂ fluxes on 13 tidal flats in the coastal area of Osaka Bay. The tidal flats are generally composed of sandy sediments, and their chemical properties indicate an aerobic environment. CO₂ fluxes at the water surface of the tidal flats showed a release at 8 of the 13 sites, and CO₂ tended to be released at the sites with strong river influence at inner part of Osaka bay. The daily CO₂ flux in the sediments was higher than that in the water surface, suggesting that CO₂ is released in the sediments as decomposition of organic matter. These results suggest that CO₂ tends to be released from the water surface on tidal flats, and that CO₂ at sediments is released by the decomposition of organic matter accelerated in low tide.

 “Blue carbon”, carbon absorption due to photosynthesis in aquatic ecosystems, has been considered mitigation measures for climate change. Blue carbon-forming seagrass are not only uniformly distributed throughout shallow water areas but also often distributed in patches, creating a high potential for interactions with currents to create complex flow regimes. As a result, carbon capture in a water body is greatly affected, making it difficult to estimate its sequestration accurately. In response to such problems, a submerged aquatic vegetation model (SAV model) that can reproduce the interaction with the flow field in detail has been proposed in the previous studies. This study aims to clarify the influence of SAV colonies on the flow field using the SAV model. The results confirmed the high reproducibility of the SAV model and indicated that the deflection of SAVs might be a significant factor controlling the flow field around the SAV colony.

 In offshore construction projects where turbidity is generated, such as reclamation, it is effective to predict the next day's turbidity dispersion and proceed with the construction. In actual construction projects, the practice of turbidity dispersion forecasting has not progressed due to the lack of a method to measure the distribution plane of turbidity, which serves as the initial value for the simulation. The objective of this study is to develop a method to incorporate the planar distribution of turbidity into the initial values of the analysis in order to ensure the accuracy of turbidity dispersion prediction that can be used in construction projects. In this paper, an estimated turbidity distribution map was created using satellite images capturing turbidity diffusion, and calculations of turbidity diffusion were conducted using this as the initial value. As a result, it was inferred from the satellite data analysis that sedimentation has a significant effect on the process of decreasing turbidity. The diffusion prediction calculation that incorporated the planar distribution as an initial value showed that the planar distribution could be reproduced by incorporating the effect of sedimentation, suggesting the validity of the method.

 A group of pumice from a significant eruption in August 2021 on the Ogasawara Islands, Tokyo, drifted to the coastal areas of the Okinawa/Amami region about 1,400 km away, causing extensive damage to the tourism and fishing industries. Although there have been studies on the drifting of spilled heavy oil and containers, there have been few studies on the accumulation process in port facilities after a large amount of pumice stone drifted into the coastal area. For this reason, the laboratory continues to study the migration characteristics of pumice in a coexisting wind and wave field using a wind tunnel channel.

 In this study, we examined the effects of wind and waves on the migration characteristics of pumice stone groups within a constriction to elucidate the phenomenon of pumice stone burial in port facilities. As a result, the processes of deposition and flow of the pumice stone group depend on the constriction width. Furthermore, the flow through the constriction is changed. In addition, The speed of pumice stone movement can be deduced from the vertical component of the wind.

 Since October 2021, a large amount of pumice drifted to the coasts of Amami and Okinawa, causing serious damage to the fishing and tourism industries. The pumice was generated by the eruption of the Fukutoku-Oka-no-Ba located in the southern part of the Ogasawara Islands, and drifted due to ocean currents and winds. In Japan, located in a volcanic area, pumice disasters may occur in the future, so it is necessary to develop a prediction method of pumice drift. In this study, particle tracking simulations were carried out assuming pumice drift in Okinawa coastal areas. Especially, the reproducibility of pumice berthing and leaving was verified in Oura Bay. Also, the applicability of the simulation as a forecasting tool were examined. As a result, it would be a practical forecasting tool by running a flow simulation of tidal and ocean currents in advance and executing only a particle tracking simulation considering the wind pressure flow.

 It is an important issue for port management to monitor the drifting conditions of marine debris in port waters because many marine debris are easy to drift in the port waters. In this study, we attempted to construct an automatic marine debris detection model using deep learning from sea surface images for the purpose of constructing a simple monitoring method for marine debris and applied the model to sea surface images taken over a long period of time in port areas to monitor marine debris. The sea surface images were taken at an anchorage of Naruo-hama located at the inner part of Osaka Bay. We created teacher images of drifting debris from the sea surface images, verified the detection accuracy of multiple trained models with different combinations of teacher image types and training models, and constructed a model capable of detecting wood and other debris from the sea surface images. The model was applied to images of the sea surface taken between July and December 2020 to examine the inflow and outflow of marine debris and revealed that most of the marine debris was wood.

 Waves and currents are considered as external forces in the design of offshore wind turbine foundations, but there is little observational data on currents in the open ocean, so estimates must be used as reference. The JCOPE2M is an easy-to-use estimator for the Japanese coast, but it is based on daily mean velocities, so it is recommended to use the highly accurate JCOPE-T.

 In this study, the relationship between JCOPE2M and JCOPE-T is summarized, and the possibility of using JCOPE2M as an ocean current for offshore wind power foundation external forces is examined. As a result, it was confirmed that JCOPE2M can be used with appropriate corrections. The vertical profile of currents should be obtained by using the high-resolution JCOPE-T. However, since the effect of the drift current appears in the surface layer, a simple 1/7 power law based on the surface velocity of JCOPE-T is likely to overestimate the vertical profile of currents.

 This study aims to rationalize earthquake and tsunami simulation technology by automatically converting and integrating various data with different description formats (DPP). Specifically, a general-purpose ground model “GridModel” is created using borehole data and numerical elevation models available on the National Land Transport Data Platform, automatically converted into a 3-D finite element model with nodes and boundary conditions, and output as an input file for seismic response analysis software such as FLIP. We have also developed a program that automatically creates input files for T-STOC from the "Cabinet Office Nankai Trough Large Earthquake Model Study Group" data. Furthermore, we have tried to link these automatic input data generation programs to take into account the results of subsidence of the ground surface from seismic response analysis in the tsunami simulation.

 In the seismic design of offshore wind turbines, aeroelastic analysis codes are used for wind and waves, and general-purpose analysis codes are used for earthquakes, and the cross-sectional forces obtained from each analysis are sometimes superimposed to evaluate performance. However, this method does not allow for a precise evaluation of the behavior of offshore wind turbines that are subject to wind even during earthquakes, and may lead to economically disadvantageous designs.

 Therefore, in this study, the effect of wind during an earthquake was evaluated for a monopile supported offshore wind turbine using a coupled analysis in which wind and earthquake act on the wind turbine simultaneously. In order to consider liquefied ground, which is a concern in coastal areas, a modeling method for soil-structure interaction springs in multiple support excitation analysis was investigated.