The tension in elastic mooring lines is always present, which allows for stable positioning of a floating structure. But it has never been applied to VLFS over 500 m. In addition, there is no description of mooring design by elastic mooring line in technical standards and guidelines of mooring design.

 Therefore, in this study, a VLFS model moored by an elastic mooring system consisting of multiple elastic mooring lines was prepared, and the relationship between the restoring force characteristics of the elastic mooring system and the motion response characteristics of the VLFS was grasped. And, the length of the synthetic fiber rope that exhibits the most restoring performance as an elastic mooring system for each length of elastic mooring system was clarified by systematic calculation. The purpose of this study is to show a series of procedures that contribute to the design of elastic mooring systems that can select mooring system specifications from the applicable range of elongation rate of rubber rope, and number of rubber ropes that satisfies the allowable oscillation amplitude of VLFS.

 Container handling simulation of container terminals is a technology to reproduce the movement of container handling machines and containers in a yard on a computer, and to use it for planning the layout of container terminals and evaluating and studying measures to improve their operation. In this container handling simulation model, parameters such as the loading/unloading time of RTG (Rubber Tired Gantry Crane) are generally calculated from the catalog values. However, in actual loading and unloading operations, the catalog cycle time deviates from the actual cycle time because efficient operation trajectory are taken depending on the container stacking situation of stacking bays. In this study, we compared the daily report data of RTGs obtained at container terminals and the results of work observation using camera images installed on RTGs, and studied a method for estimating the container handling time of RTGs using the daily report data. The estimated work time were aggregated for each storage coordinate in the bay, and their characteristics were analyzed. The estimated work time for each storage coordinate were used as input parameters in a container simulation model, and the reproducibility of simulation model was confirmed.

 Active sonar is useful for monitoring poachers and terrorists lurking in and around harbors, however its detectable range is highly dependent on the propagation characteristics of high-frequency sound waves in extremely shallow waters. Offshore wind turbines installed in coastal areas can be a source of noise in the sea and affect the acoustic environment, the extent of which depends largely on the propagation behavior. In this paper, the propagation characteristics of high-frequency sound waves in extremely shallow water are studied by conducting experiments over a distance of several hundred meters in a bay with water depths shallower than 20 m, where the seafloor topography varies; these results are also compared with a numerical analysis using BELLHOP, which is a simulation method based on the modified ray tracing method using Gaussian beams. Considerable agreement between the two is observed as long as the acoustic properties of the marine sedimentary layers required as input parameters for the calculations are properly configured, so as to be expected in its application to the prediction of the acoustic environment in extremely shallow waters.

 It is said that Teizan Canal, a kind of excavated topography, had the effect of mitigating tsunami damage by reducing tsunami inundation depth and tsunami flow velocity. This study examines the effect of excavated topography on reducing tsunami hydrodynamic forces acting on a structure using a three-dimensional numerical model (Cadmas-Surf/3D). It was shown that the tsunami inundation depth in front of the structure and the tsunami hydrodynamic force acting on the structure are reduced by the existence of the excavated topography. This reduction effect is related to the depth and width of the excavated topography, and the effect of the depth contributes more strongly to the reduction.

 Wave forecasts are used in various fields, but short-term wave forecasts for 1 to 10 days are often used for decision-making. For example, in coastal or offshore construction, the accuracy and precision of wave forecasts significantly impact the construction work's operation and efficiency, as waves can prevent the use of work vessels. On the other hand, medium-term forecasts are also important for construction planning, and monthly average wave height forecasts using ensemble wave height forecasts up to several months ahead based on statistical wave height estimation models have been attracting attention. This study examined a method for estimating operation rates by comparing predicted monthly mean wave heights with observed waves in historical data. As a result, it is possible to estimate the operation rate (frequency of occurrence of waves less than 1.0 m in height) to some extent from the predicted monthly mean wave heights and historical observations. However, the estimation of the operation rate is difficult when the predicted wave height deviates extremely from the observed monthly mean wave height or when the prediction accuracy is less accurate. This is because it depends on the information of past wave arrivals.

 Cavity collapse caused by internal erosion of backfilling sand has occurred frequently at the back sides of coastal structures such as seawalls and quaywalls worldwide. Sea sands used as the backfilling sand may contain a large number of fragment of shells and coral, and their particle shapes may differ significantly from that of silica sand, but there has not been sufficient investigation into the effect of sand particle shape on a series of processes of internal erosion, cavity formation and collapse. In this study, we focused on sea sand and silica sand with the same grain size distribution as the backfilling sand, and investigated in detail the processes of internal erosion, cavity formation and collapse under actions of waves, rainfalls and overtopping waves by conducting prototype-scale experiments. The results demonstrated that the difference in suction caused by the difference in the particle shapes of the backfilling sands had a strong effect on the series of processes of internal erosion, cavity formation and collapse.

 Data on sediment grain size and foreshore slope were obtained from 297 sandy beaches in Japan. The former was calculated from images, the latter was measured from the shoreline to the runup point using an inclinometer. The beaches could be classified into four area: Pacific Ocean side, Japan Sea side, inner bay and Okinawa. The frequency of grain size had two peaks at 0.3mm and 1.5mm, and the foreshore slope between 0.08 and 0.16. The dynamic relationships between sediment grain size and foreshore slope was calculated based on the constructal law with the modified coefficient from laboratory experiments. The amount of shoreline retreat in each area was estimated based on Bruun law and the obtained results were comparable to previously reported values.

 In order to clarify the damage process of aquaculture facilities caused by the 2022 Tonga volcanic eruption and tsunami, we conducted a field survey to understand the actual damage in the Sanriku coastal area and simulated the drift of aquaculture facilities in Yamada Bay. The tsunami that hit Yamada Bay repeatedly pushed and pulled within the bay, and the aquaculture facilities moved back and forth along with the current. The distribution of the tsunami water level and current velocity revealed the locations and trends where both were particularly large. To quantitatively evaluate the relationship between tsunami and damage to aquaculture facilities, the total distance traveled by aquaculture facilities was newly defined. A positive correlation was found between the velocity and the total distance traveled, indicating that the effect of the tsunami was larger than that of the water level. This result is consistent with previous findings on tsunami damage to aquaculture facilities. This result suggests that there may be areas in the bay that are at high risk of damage to aquaculture products even in the case of small tsunamis.

 When a volcanic eruption occurs, volcanic debris with pumice falls throughout the surrounding area. In coastal areas, including rivers, large amounts of pumice drift on the sea surface for extended periods causing various problems. However, there have been few studies on the migration process of pumice in shallow water due to the difficulty of quantifying its physical properties and reproducing the process because of its long-term suspension. In this paper, (1) the trajectory of pumice in a wave field and (2) the flow around a structure are investigated using a big wind tunnel tank. As a result, it was found that the trajectory of pumice swarms due to waves is suppressed or enhanced depending on the location. Moreover, the velocity of pumice movement and flow rate around structures are greatly affected by wind velocity.

 Liquefaction prediction and assessment of mixed sandy soil with fines is made from modified N-values based on fines content F_c and the plasticity index I_P. In this study, we conducted prototype-scale standard penetration tests and a series of undrained cyclic torsional shear tests, by focusing on the influence of changes in the plasticity, grain size and fraction of fines on the liquefaction prediction and evaluated the correlation between the modified N-value and the liquefaction strength due to the changes in these physical properties of the ground. The results show that the fines content as well as the plasticity/non-plasticity mixing ratio of mixed sandy soil with fines up to F_c=50% affect the accuracy of liquefaction prediction, and the risk of liquefaction is underestimated in cases with F_c>15% and where the plasticity index is less than 10 in the mixed sandy soil with fines.

 In recent years, UAVs, which have been used for many observations, are equipped with an RGB camera as standard equipment. To obtain the NDVI of the normalized vegetation index, a near-infrared camera that can acquire NIR data must be separately installed on the UAV, or a UAV equipped with a multispectral camera must be used. It would be effective to estimate the distribution area of algal beds from the vegetation index using a normal onboard camera. In this study, we calculated the NDVI using the bands that can be obtained with a near-infrared camera, and compared the NDVI with various indices calculated with each band of the RGB camera for eelgrass beds and green algae beds to investigate the possibility of using the NDVI as a vegetation index for estimating the distribution area of seaweed beds. The results showed that the existing vegetation indices are not suitable for estimating the distribution area of algal beds.

 As a result, the existing vegetation indices did not correlate with NDVI and could not be used for estimating the distribution of seaweed beds. However, the new modified vegetation index proposed in this study, fRGBVI, correlates well with NDVI, indicating the possibility that fRGBVI can be used to estimate the distribution area of seaweed beds.

 In this study, topographic change analyses were conducted for the Sendaigawa River mouth with Tottori Port located in the eastern part of Tottori Prefecture, Japan, with various ocean current conditions to clarify the sedimentation mechanism at the port during floods. A two-dimensional shallow-water flow model with a high-order accurate advection solution was used in the analyses, and the target flood was the July 2018 torrential rainfall flood. The results of the analyses indicate that for a flood discharge of 2,000 m³s^–1 class, regardless of the current conditions, sedimentation may occur in the Sendai Passage, but not large-scale sedimentation in the port. In the case of a flood discharge of 3,000 m³s^–1 class, it was found that the flow situations differ greatly depending on slight differences in the current conditions. In the case of weak currents, the possibility of sedimentation from the channel to the port was shown regardless of the direction of the currents. On the other hand, in the case of strong currents, the sedimentation rate in the port may decrease, although the factor varies depending on the direction of the currents.

 This study identified factors affecting stability of breakwaters, based on Goda’s formula, against increasing wave height expected in future ocean climate. We found increasing risks of structural damages to the breakwaters in the cases of (1) relatively small design wave height given in deep water (e.g., breakwaters in bays) and (2) structures located in shallow water with steep bed slopes (e.g., breakwaters built around remote islands). These high-risk scenarios have been verified through comparisons with the cases of damages observed in the past high-wave disasters. We also proposed reasonable adaptation methods to manage structural stability for high waves expected in future coasts. It was estimated that it will be possible to reduce costs by approximately 15% compared to conventional construction methods.

 For the purpose of constructing a tenacious breakwater, a structure in which steel pipe piles are driven in rows into the ground on the inner side of a gravity breakwater and filled with soil between the caisson and the steel pipe piles has been proposed. The most important thing for this structure is that the piles remain sound even if the caisson is severely deformed. To achieve this, it is necessary to be able to estimate the magnitude and location of the bending moment generated in the piles appropriately. In this study, external forces acting on the piles from outside the harbor were investigated, and their estimation and simplification were attempted. The adequacy of the estimated external force distribution was evaluated by comparing the calculated bending moments and deflections with the experimental values obtained from model horizontal loading experiment. As a result, the external force distribution outside the port was replaced by a triangular shape, and the bending moment generated in the pile was expressed concisely by setting the apex position of the triangular shape as the fill height, the ground surface position, and the first zero depth of deflection of the pile, l_y1.

 In Japan, which has a four-surface ocean ring, many ports and harbors are constructed with wave dissipating blocks to reduce wave reflection and wave overtopping. However, it is difficult to quantitatively determine the current shape of wave dissipating blocks by conventional measurement methods such as visual inspection because of the danger of activities directly above the blocks. Therefore, studies have been conducted using non-contact measurement methods, such as using point cloud data acquired by UAV surveying to evaluate the tops of wave dissipating blocks. However, there are only a few cases of application to slopes due to their characteristics of being continuous in air and water. Therefore, in this study, an evaluation method for slopes, including underwater areas of wave dissipating blocks, was investigated using point cloud data acquired by UAV surveying. As a result, it was shown that an planar evaluation using the highest value is useful for both the slope and the top edge, and it is expected to be utilized as a method to grasp the current status of wave dissipating blocks and to quickly grasp the situation at the time of damage.

 The evaluation of errors in tropical cyclone forecasts is necessary to assess the uncertainty that exists in tropical cyclone disaster forecasting and to make decisions on mitigation actions. We analyzed the "Typhoon Information" materials published by the Japan Meteorological Agency from 2015 to 2020 to examine the statistical characteristics of forecast errors in tropical cyclone characteristics such as route, central pressure, and storm radius. The distribution of forecast error varied from region to region. The results also indicate that there is a bias in the forecast. The standard deviation of the forecast error was found to be as large as the mean. The route error increases relatively monotonically with lead time, while the central pressure and storm radius change less as lead time generally exceeds 30 hours. Due to the large variability of the individual forecast route errors, there is no direct correlation with the radius of the forecast circle, but a certain correlation is shown for the average statistics.

 With the growing interest in digital transformation (DX) of ports and harbors, the introduction of digital twins (DT) has been attracting attention in the field of external chassis (external C/Y) at container terminals. For making more accurate DT models, it is necessary to understand and reflect on the detailed dynamics of external C/Y. However, it is difficult to obtain detailed information about external C/Y such as the travel route and waiting time from gate to loading or unloading operation and the congestion in front of the gate from only the cargo handling records and gate reception records stored in the terminal. In this paper, we investigate a method of analyzing the dynamics of external C/Y behind and inside the container terminals (CT) by using GPS devices. In the analysis of the hinterland of the CT area, we obtained detailed travel routes, distances, and times, including the origin and destination of the hinterland of CT area transportation. Furthermore, the analysis of inside CT could visualize the detailed operational route of external C/Y in CT and the arrival/departure time of each defined area. Then, it revealed that more than 40% of total staying time occurred at the stagnation area and the roads in front of the gate, which is the big problem in terms of efficiency and decarbonization of CT. The accumulation of such data is expected not only to clarify the current issues of CT but also to lead to the mitigation of uncertainty in arrival time for external C/Y, improved cargo handling efficiency, and the construction of more accurate DT models of CT.

 The tsunami waves caused by the eruption of the Hunga-Tonga-Hunga-Ha’apai volcano on 15 January 2022 triggered a tsunami warning for coastal regions in Japan on 16 January. Questionnaire and interview surveys for fisheries in the Iwate coastal area and municipal department in charge of disaster prevention were conducted to search for responses to tsunami warnings and evacuation behavior. The number of fisheries evacuated offshore by fishing boats is fewer than those of evacuees offshore at the time of the tsunami generated by the 2011 Great East Japan Earthquake. The reasons for the difference are as follows. 1) The predicted height of the tsunami was smaller. 2) Because the tsunami warning was issued at midnight in winter, there was a high risk for evacuation to offshore by small fishing boats. The survey results revealed the problems of evacuation of fisheries and fishing boats against tsunamis, such as deciding if to escape to higher ground or evacuate offshore by fishing boats and information transmission in out-of-service areas for mobile phones and small fishing boats without fishing radio equipment. Preparing a hazard map for each coastal region showing tsunami risks at sea, installing high-performance speakers for disaster-prevention administration radios or revolving lights to indicate the danger of tsunamis, and evacuation planning of fishing boats against distinct tsunamis are required to solve the problems.

 Burden of the diver, and environmental pollution is a problem in examination of steel materials thickness using ultrasonic thickness meter. ultrasonic thickness meter has to scrape off the surface of steel materials for the measurement, but in high-sensitivity magnetic sensor; is not necessary, and can expect a solution to the problem of ultrasonic thickness meter.

 As a result of having tried it in a fishing port offering open, there were few differences in ultrasonic thickness meter and high-sensitivity magnetic sensor. However, it is caused by the fact that conditions to measure are different. Therefore high-sensitivity magnetic sensor is usable in the measurement.

 At the seawall of Port K under steep seabed conditions, the vertical wall was subsided due to the collision of boulders activated by the waves, and the pavement behind the seawall was damaged due to the boulders with overtopping waves. In this study, hydraulic model tests with irregular waves in a scale of 1/50 were conducted to clarify the characteristics of boulders activated by wave action that caused such damages. The relationship between equivalent offshore wave height and boulder run-up height was obtained by changing the diameter and specific gravity of a sphere model imitating boulders. Since the run-up height of the sphere has a correlation with the run-up height of the overtopping water mass, the relationship between the ratio of the two and the equivalent offshore wave height was shown. The collision forces of a sphere rolled by waves were measured by dynamometer, and the relationship between the irregular representative value and the equivalent offshore wave force was obtained. Furthermore, for the wave conditions at the time of the disaster, the amount of subsidence was calculated by obtaining the impact energy from the impact force of the sphere.

 It is required seismically strengthened quaywalls, which function as crucial infrastructure for emergency goods transportation, are designed to enable the berthing of vessels even after a large earthquake disaster. To achieve this objective, the relative displacement of the quaywalls in the normal direction is necessarily controlled under a certain tolerance level following an earthquake. However, the existing seismic design approach primarily focuses on residual displacement in a single representative crosssection, thus not directly assessing the relative displacement in the direction of the quaywalls. This study proposes a method for developing a ground response reduced order model with a minimum experimental design, which can be applied in design process in practice. Using the reduced order model, this paper also presents a probabilistic method for evaluating the relative displacement in the normal direction to the quaywalls. Finally, the effectiveness of the proposed method is verified with an example of application to an actual structure.

 Few experimental studies have been conducted on the application of stone gabions as armor units on the sea side of a composite breakwater, leaving the stability and design of such coastal structures largely unclarified. In this study, a 1/40 scaled irregular wave experiment was performed using stone gabions as armor units. The dimensionless average slip amount D’, a value representing the average slip amount for each stone gabion model in the direction of the mound slope, was calculated. This value, when at 0.15, was considered as the significant wave height for design purposes, and was substituted into Hudson's formula, a designated mass calculation formula for a composite breakwater. As described above, N_s, the stability number is an index of stability performance, was calculated. Compared with N_s when filter units were used as armor units, N_s was found to be relatively large when d/h, the ratio of the water depth above the mound top edge to the water depth of in front of breakwaters, was 0.6 in some cases, but was comparable when d/h was 0.4. The findings from this study confirm the stability of armor units on the sea side of a composite breakwater, indicating no issues under the examined conditions.

 The foot protection bag such as Filter Units (FU) is the most common countermeasure for the reduction of the local scour around the monopile of the offshore windfarm facilities. However, it takes a lot of time to set all the FU around the monopile. In this study, collar-type countermeasures were suggested and investigated the reduction of local scour through the movable bed experiments. As an experimental result, the reduction of local scour was not enough only with the collar-type countermeasure. Meanwhile, the reduction of local scour by using both the collar-type countermeasure and FU was enough and almost the same as the reduction of local scour only with FU. As a result of 3D numerical simulation under the fixed bed condition of the experiments, the reason why the reduction of local scour was not enough only with the collar-type countermeasure was the large velocities vicinity of the collar-type countermeasure.

 There is serious concern that the wave-overtopping disasters will increase due to the global warming accompanied by the sea level rise and intense typhoons in the near future. As a countermeasure, we proposed a low-cost and simple mooring type wave overtopping prevention by utilizing a cylindrical body for the vertical seawalls. The both ends of the cylindrical body are moored using rope to the front face of the seawalls. This mooring allows that the cylindrical body moves following the wave motion in front of the seawall. Hence this method has several merits, e.g., low cost, ease of construction and maintenance, and so on. By conducting model experiments, Nakatani et al. (2019) confirmed that the proposed method can efficiently reduce the wave-overtopping. In addition, the tension acting on the mooring ropes, which is one of the important factors for the design of the prevention method, was investigated. Furthermore, numerical simulations were performed to estimate the reduction of wave-overtopping rate by this method. The numerical model was “DualSPHysics” which is based on the Smoothed Particle Hydrodynamics (SPH) model. The numerical simulations at that time, however could not reproduce the mooring ropes, and were simple with a fixed cylindrical body.

 In this study, we retry to reproduce the behavior of the proposed wave-overtopping countermeasure including the mooring ropes by the numerical simulation, and re-examined whether the numerical simulation is useful for evaluating the mooring type overtopping countermeasure.

 As a result, the numerical simulation sufficiently reproduced the motion of the moored cylindrical body preventing the wave overtopping, and the tension acting on the mooring ropes was reasonably evaluated. Although there is still room for improvement in the numerical model, the numerical simulation can be a useful tool to investigate the performance of the proposed method in detail.

 The method was applied to the actual gravity quay wall at the Port of Kawasaki for the purpose of social implementation of the deepening method using plastic grout. In addition, impurity materials removal method was developed to expand the application of the deepening increase method. The method using a water jet was efficient. Plastic grout was injected under the gravity quay wall, and it was confirmed that the method could be applied to the actual structure.

 With climate change, existing breakwaters are directly affected by rising mean sea level, increasing storm surge level and wave height, which is likely to increase the risk of damage. In order to evaluate whether the stability and other performance of the structure can be maintained under changing load conditions in the future, it is necessary to verify the stability of breakwaters under a wide range of load conditions, and the number of structures subjected to such verification is large. The workload required for stability verification is therefore very large. In addition, when there is a wide range of assumed changes in future load conditions, it may be reasonable to narrow down the assumed range and make a decision after obtaining more information, rather than making a decision at a certain point in time. Therefore, this study proposes a practical improvement design method for existing breakwaters using the response surface method and real option analysis to efficiently evaluate the performance of breakwaters under changing load conditions with uncertainty, and to evaluate the necessity of countermeasures and to select the countermeasure method.

 Blue carbon, the carbon absorbed by photosynthetic activity in aquatic ecosystems, has been attracting attention as a means of mitigating climate change. Since submerged aquatic vegetation (SAV), which forms Blue Carbon, often exists patchy at a small scale compared to the topography length scale of the shallow coastal waters and lakes, the flexible movement of SAV causes complex interactions with water flow. As a result, the carbon sequestration rate is significantly affected, making accurate estimates difficult. Therefore, it is necessary to understand the water flows and mass transport when the SAV patch is part of the space. In response to such problems, a detailed SAV model that can reproduce the interaction with the water flows has been constructed in previous studies. This study aims to evaluate the influences of the SAV patch on carbon flux. The results indicate that the occupancy of an SAV patch and the deflection are important factors affecting carbon sequestration.

 On July 28 in 2018, fifteen vehicles slid due to wave overtopping at a coastal road on the steep foreshore conditions. In this research, hydraulic model tests with irregular waves were conducted on a scale of 1/50 to reproduce the wave overtopping on the block mound seawall with 1/7 seabed slope for the wave and tidal condition when the accident occurred. The wave overtopping rate at the accident was 2×10^-2 m³/s/m, and the necessary height of wave splash barrier was found to be 6.6m for the wave height condition of the accident. The characteristics of wave force on the wave splash barrier was measured by six load cells for wave height H_o’=1～10m. Based on the test results, the design wave pressure distribution was formulated as the function of the highest wave height H_max, and the wave force reduction effect was disclosed for different block width conditions. The applicability of the formula was also confirmed for 1/30 seabed slope conditions.

 We measured the wave field around the mouth of Oii River, Yoshida City, Shizuoka Prefecture, with an X-band radar during stormy condition. Fourier transform is applied to the sequence of images, and the phase distribution map of the wave field is processed from the result. From this map, we determined the local wave lengths and directions, which depict the refraction process of the waves approaching the shore. We finally estimated the depth distribution of the area using the linear dispersion relationship with the wave length and frequency of the Fourier transform. The validity of the depth estimation is verified with survey results, which showed reasonable agreement. The proposed method in this study is more flexible than other methods, which use fast Fourier transform (FFT) regarding the choice of data length and frequency.

 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 risks of the bathing beach 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. We have created an AI model that can predict the probability of drowning accidents with high accuracy and developed a new method for objectively judging swimming conditions based on the prediction results. On the other hand, the actual application of this method to be used at beaches requires the continuous collection of 53 features and the versatility of the AI model. In this study, we investigated a creation of the AI model that can predict the probability of drowning accidents with a small number of features and its versatility. As a result, even the AI model using 10 features with high importance was able to predict with high accuracy, and its versatility was also demonstrated.

 To estimate design waves of harbor facilities for a quasi-deepwater wave, the inverse estimation which may require some repeated calculations of wave transformation to infer a design deepwater wave should be conducted. To reduce the repeated times of calculations, to set a required calculation domain, and to predict an appropriate directional spreading parameter of design deepwater wave before performing the calculations, we have proposed a simple inverse estimation method for a deepwater wave by using several diagrams with a quasi-deepwater wave. However, its applicability has been not enough for estimating the deepwater wave with wave diffraction on a real coastal topography.

 In this study, we modify the procedure of inverse estimation on dealing wave diffraction with the expanded diagrams of both wave refraction coefficients and angles and wave diffraction coefficient and angles, and make it possible to estimate the deepwater wave without repeated calculations. Moreover, it is confirmed that an approximate values of deepwater wave property can be estimated by using the modified method and they realize to reduce the times of repeated calculations of wave transformation on both a model and a real coastal topography.

 The applicability of the modified calcia soil to the environmental restoration of a former sea gravel extraction site was investigated. However, there is no quantitative evaluation of the turbidity of the modified soil, and conventional construction methods are difficult because the target area is deep and has high velocity currents.

 The strength characteristics of the modified soil were analyzed based on the results of mixing tests, and the effective free lime rate, Le, was used to evaluate the early onset of strength based on the 24-hour curing data. The turbidity generation intensity was calculated based on hydraulic model experiments, and a vane shear force of 2 kN/m² was set as the strength that can suppress turbidity. Furthermore, we analyzed the results for an arbitrary curing time and proposed a method of preparing the modified soil according to the conditions at the time of on-site construction. These results indicate that the modified soil can be applied by bottom-opening barges.

 Converter steelmaking slag is a reforming material with component and particle size adjustment, and improved dredged soil, which is a mixture of the reforming material and dredged soil generated by harbor works, develops strength by solidification. The improved dredged soil, which effectively combines natural and man-made materials, has been used effectively in environmental restoration projects. On the other hand, there are deepened sea areas caused by sand mining scattered in Seto Inland Sea, and these areas have not been restored naturally. In this study, we conducted field experiment where there are few past result of dumping sediment, to determine the feasibility and effectiveness of using the improved dredged soil in environmental restoration project (raising of sea bottom). At the same time, we conducted various field survey to understand turbidity diffusion etc.

 While a shallow water wave hindcasting become popular, a quasi-deepwater wave is frequently adopted as a design offshore wave instead of an original deepwater wave or a pseudo-deepwater wave. For the quasi-deepwater wave, a design deepwater wave should be inversely estimated with an appropriate directional spreading parameter to calculate design waves on harbor facilities in the same time. So, in this study, the validity of the properties of deep and shallow water waves inversely estimated with the quasi-deepwater wave condition by repeated calculations of wave transformation is verified with the results of extreme value statistical analysis on pre-existing wave hindcasting data which has been used to estimate the quasi-deepwater waves.

 As the results, the directional spreading parameter of design deepwater wave should be set carefully because it can affect not only the properties of design deepwater wave inversely estimated with the quasideepwater wave condition but also the properties of design wave of harbor facilities calculated with the design deepwater wave condition.

 In the evaluation of the effects of tsunami-borne debris, it is important to ensure both rationality and efficiency as well as the required safety. Currently, no evaluation procedure has been proposed to include these perspectives. In this study, a comprehensive evaluation procedure that can be applied to both deterministic and probabilistic evaluations of the effects of tsunami-borne debris is proposed. A screening method is also proposed to improve the rationality of the evaluation procedure. The proposed evaluation procedure consists of three phases, each of which incorporates the concept of qualitative and quantitative screening. In each phase, screening is conducted using screening indicators. In this way, it is possible to conduct rational screening of the effects of tsunami-borne debris while ensuring the safety that the target of the evaluation should have. The proposed method contributes to efficient assessment through the reduction of resources required for the assessment.

 In accordance with the revision of the Flood Prevention Law in 2015, prefectural governments are obliged to prepare maps of the expected inundation zones for the largest storm surges, and the "Guidance for Preparing Storm Surge Inundation Zone Maps" and "Technical Data" have been compiled to provide technical support. However, the guide does not include methods for calculating overtopping flow rates that take reef topography into account, or other analysis methods that reflect the unique characteristics of each region.In this study, we proposed a method that combines the "design depth formula" and the "Goda formula for estimating overtopping flow rate" as a simple method for calculating the overtopping flow rate on reef coasts. In addition, we quantitatively evaluated the necessity of mesh size setting to account for wave set-up and the effect of warm water eddies as considerations when validating the storm surge estimation model. As a result, we were able to appropriately evaluate a method that reflects regional characteristics for estimating the maximum expected storm surge along the coast of the Ryukyu Islands.

 Most of the microplastics (MPs) that flow into the sea are thought to be deposited on the seafloor, and their existence has been reported in seafloor sediments in various sea areas. MP is also included in dredged soil generated in large quantities in port development projects. In this study, we developed a standard method for monitoring MPs in marine sediments and applied the method to dredged soil collected from different ports. This report is the first to compare the distribution density, size composition and polymer composition of MPs covering various ports in Japan.

 In marine construction projects where turbidity is generated by reclamation, dredging, etc., it is necessary to monitor the turbidity from the perspective of environmental preservation. In a general monitoring, the turbidity and SS are measured at monitoring points, but these point measurements cannot adequately identify the actual turbidity, which is spread over a wide area. In this study, a method to quantify the planar distribution of turbidity from aerial images is considered, for the purpose of monitoring turbidity in marine construction projects and predicting its diffusion. In this paper, aerial photographs of turbidity generated in a marine construction was acquired. Then, a method to evaluate the turbidity by the color difference was examined, where the relative value of turbidity between turbid and non-turbid water was calculated using the RGB values of the images. The results showed that the method is capable of indicating the boundary between non-turbid water and low-turbid water among the planar distribution.

 Offshore wind power generation facilities are being developed in Japan in order to realize a decarbonized society. However, there are many uncertainties regarding the behavior of these facilities during earthquakes due to their limited construction experience and lack of experience with seismic damage.

 In this study, model vibration experiments were conducted on a 5 MW monopile offshore wind turbine to understand its behavior during a large-scale earthquake. The model was modeled down to the seafloor surface and subjected to vibration using multiple seismic waves. As a result, it was confirmed that the vibration mode differs depending on the seismic wave. In order to properly evaluate the behavior similar to that of the actual structure, a model ground was created, and the model was modeled to the middle of the soil and subjected to vibration by seismic waves. As a result, it was found that the seismic behavior during and after excitation was different when the model ground was considered, due to the effect of amplification by the seismic motion in the ground.

 During the construction of rubble mounds, geotextiles on the seabed sometimes have trouble of their lifting due to waves. Therefore, an appropriate method is required to calculate the minimum mass of weights to prevent the lifting of the geotextile. In this study, hydraulic experiments and numerical simulations were conducted for the wave stability of geotextiles on the seabed to investigate the effect of the permeability: one type is the impermeable geotextile and four types are the permeable geotextiles. The experimental results showed that the higher the permeability of the geotextiles, the higher the critical wave height for the geotextile stability against wave uplift force. Furthermore, the numerical simulations revealed that the overturning moment for the geotextiles can be approximately reproduced by considering the drag force of the Darcy law estimated from the coefficient of permeability of the geotextile.

 In the Kyushu region, drifted debris released into the sea by heavy rains adversely affects the environment, fisheries, and coastal conservation facilities of the sea. The deposited driftwoods destroy the rich ecosystem, and fishing boats may not be able to go fishing due to concerns about collisions with driftwoods. The driftwoods drifting on the coast hinder the function of wave-dissipating blocks and may cause flowing into the land due to high waves. The Kyushu Regional Development Bureau recovers driftwoods using vessels relying on sightings. However, the collection is taking an enormous amount of time.

 Therefore, in this study, we developed a driftwood drift prediction model to improve the efficiency of collecting driftwood. The driftwood drift prediction model is based on a particle tracking model that tracks the movement of water particles in Finite-Volume Community Ocean Model version 3.2, which is the unstructured-grid model. The unstructured mesh allows the model to reproduce the topography in detail. Additionally, the model is improved by adding buoyancy, wind drag, landfall, and surfacing to the water particle motion to predict driftwood drift in the study. Moreover, the model has been made more sophisticated with the addition of inverse analysis and external data loading functions for improvement of computational speed.

 The simulation for the period of heavy rains in July 2020 and Typhoon Nanmadol in 2022 was conducted. As a result, we were able to explain the collection records in general, and estimated the origin rivers and drift paths of driftwood that drifted to each recovery site in both the heavy rain events. The results of the inverse analysis revealed that the driftwood washed ashore in Isahaya Bay after the July 2020 torrential rainfall was most likely discharged from the second-class rivers located in the bay and in the southern part of the bay.

 The promotion of the blue carbon increase can be considered as an effective measure to achieve the government’s target of carbon neutrality by 2050 which is absorbed by marine organisms such as marine plants. Generally, seaweed grows on the surface of concrete blocks installed in the coastal area depending on the nature of the block installation location. The installation state of blocks and the seaweed attachment state on blocks are different depending on concrete block types. In this study, an estimation method for CO₂ absorption of blocks as well as marine structures such as breakwaters was investigated based on the concrete block shape characteristics, and their installation states. For the flat-type armor blocks (type A), it was proposed to use the total upward-facing surface area of the block for the calculation. However, regarding the wave dissipating blocks (type B), the actual seaweed attachment surface area due to the unevenness of the random piling installation of blocks is larger than the predetermined surface area calculated from the crest width, slope length, and length of the breakwater. Therefore, a calculation method for the seaweed attachment surface area that applies the "ratio of random piling installation" was examined and a range of values between 2.0 and 2.5 was proposed as appropriate values. The amount of CO₂ absorbed by seaweed was calculated following the previous studies. The result obtained for the armor block type A was similar to the results achieved in the previous studies. Although the results of block type B and the whole body of the breakwater were smaller than the results obtained in previous studies, the calculation results were almost the same and larger when the "ratio of random piling installation" was applied.

 Offshore wind power generation is being introduced mainly in Europe and the Japanese government is also preparing to start an offshore wind power generation project. Among them, the coastal areas of Murakami City and Tainai City in Niigata Prefecture were designated as the promotion zone in September 2022, and procedures are currently underway to select a project operator. According to NEDO, the average annual offshore wind speed within the promotion area in Niigata Prefecture is expected to be stable. However, the offshore winds increase in winter and calm down in summer, furthermore, the southern part of the sea area faces a plain, while the northern part faces a hilly area and may be strongly affected by the topography. Therefore, it is necessary to understand the differences in power generation capacity due to seasonal changes and differences in construction sites. In this study, the characteristics of offshore winds were determined for different seasons and different locations. The offshore wind information was obtained from the Meso Spectral Model which is supplied by the Japan Meteorological Agency, and five years of ground surface data were converted to 100 m above ground level at the windmill location.

 In numerical models considering wave-current coexistence field, bottom shear stress is generally calculated from representative values of the wave and current fields. However, the actual bottom flow velocity fluctuates instantaneously from moment to moment, and the velocity within a given time period is represented by a probability distribution. In this study, we improved the evaluation method of bottom shear stress by using an erosion calculation formula based on an evaluation method that takes into account the probability distribution of bottom flow velocity fluctuations. Furthermore, the improved model was used in a reproduction of a previous typhoon. Compared to the average model, in which bottom shear stress is uniquely determined from representative values of wave and current, the stochastic model reproduces the observed values for SS more accurately by taking into account the ever-changing bottom current velocity. In addition, it was confirmed that the stochastic model is more accurately able to account for the effects of wave during high-wave conditions, while the average model is bottom shear stress is dominated by the average flow velocity.

 In the 2011 off the Pacific coast of Tohoku earthquake, tsunami caused movement damage such as drifting of oil storage tanks. It is reported that about 70% of the oil storage tanks damaged by the tsunami are small storage tanks of less than 500kL, and tsunami safety measures are extremely important to prevent and mitigate these tsunami damage. As tsunami countermeasures for small storage tanks, the authors have proposed two sliding/drifting countermeasures using CFRP, which are excellent in durability (corrosion resistance), workability, and strength. In this study, the full-scale steel tank was installed on the RC slab to conduct the large-scale tsunami hydraulic experiment to confirm the effectiveness of the two proposed countermeasures, and the CFD analysis was conducted for the experiment.

 The presence of sand ripples on the seafloor indicates the occurrence of sediment movement. And that greatly affects the amount of sediment movement there. In this study, the shape change of sand ripples and their movement on a tidal flat were observed by continuous image shooting of sand ripples and measuring waves and currents. The movement of sand ripples in one direction was confirmed by analyzing the sand ripple images. It was also found that at the observation points of this study, waves and water depth were the main factors for the generation of sand ripple movement, and wave conditions controlled the sand ripple length. In estimating the amount of sediment movement in the presence of sand ripples, a quantitative estimation of the amount of sediment movement was not achieved in this study. However, the possibility of the quantitative estimation by setting an appropriate observation range was confirmed. Furthermore, we recognized the importance of considering the presence of ripples in estimating sediment movement in the sediment transport formula.

 Rocky shore scorch is an issue in Japan's coastal environment. The restoration of seaweed beds is expected to improve the fishing environment and blue carbon. This paper examines the application of steel Slag Hydration matrix for marine use and examines the applicability of seaweed bed creation blocks more effectively by improving the materials and structures used. The results of these tests are shown in this report. The results of the pH change, bioavailability confirmation, and leaching tests indicate that the steel Slag Hydration matrix has a very low environmental impact and is applicable to marine applications. The porous type is more effective in creating seaweed beds and fishing reef blocks.

 The study of wave run-up on sandy beaches is important not only for the beach management, but also for the functional evaluation of shore protection facilities such as seawalls and for disaster prevention in the backshore area. It has been carried out by hydraulic model experiments, numerical models and field observations, but there has been no research method to quantify the time series in wave run-up height on the beach from images taken at the site. In this study, we proposed a method to quantify wave run-up and investigate the temporal variation of maximum run-up height using image analysis for image data taken from January 1 to December 31, 2021 at Wakasa-Wada coast in Japan as an example. As a result, the method was able to calculate the maximum wave run-up height with an error of 0.49 m in the horizontal direction and 0.04 m in the vertical direction in case of a wave height of 2 m, and with an error of 0.49 m in the horizontal direction and 0.02 m in the vertical direction in case of that of 3 m. The results for wave heights of 2 m and 3 m were compared, and it was found that the method could be applied with higher accuracy when wave heights were higher.

 A new method for easy and automatic detecting shoreline changes was proposed using Landsat satellite image. The accuracy of detecting shoreline was analyzed by comparing to high-resolution satellite images. Indonesia is the largest island country which consists of many large and small islands. There have been no cases of quantitative evaluation of long-term shoreline changes until now. Therefore, this method was applied to major islands in Indonesia to identify long-term shoreline. A macro and quantitative risk assessment of coastal erosion based on beach use was carried out and it is found that the results showed that the risk of northern coast of Java was highest in Indonesia.

 The purpose of this research is to develop a method for effective utilization of discarded seaweed. Discarded seaweed was used as a raw material for recycled paper. The test paper, which was a mixture of paper pack with seaweed 30 %, had the highest strength in 8 test papers. This method has found a new way to use seaweed, leading to a reduction in the amount of waste.