Satellite products are expected to play an important role not only in countermeasures against floods but also in public welfare, especially in developing countries. The purpose of this study is to propose a new method to estimate X-band MP radar reflection factors from the infrared band brightness temperature of the Himawari standard data. The proposed method is as follows (1) Cloud area determination using a large amount of grid data. (2) Redistribution by probabilistic matching method. (3) Estimate radar reflection factors using an optimized mathematical model. The accuracy of the estimated radar reflection factors was examined by calculating the precipitation intensity using the Z-R equation. The results showed that the root mean square error (RMSE) of the precipitation intensity calculated from the radar reflectivity estimated using the 3.9-μm wavelength range was 5.44 mm/h and the mean absolute error (MAE) was 2.82 mm/h compared to the precipitation intensity estimated from the GPV data of the synthetic radar. These results indicate that the estimability of precipitation using the 3.9-μm wavelength range of the Himawari Standard Data is relatively higher than that using other wavelength ranges.

 This study aims to develop a method to evaluate flood disturbances acting on each river segment in a river system from basin-scale. A flood disturbance evaluation index based on dimensionless scavenging forces for representative grain sizes is proposed. The rainfall-runoff-inundation model (RRI model) was extended to calculate this index for the entire river basin. The method was applied to the evaluation of flood disturbance during the 2020 runoff period in the Nagara River basin of the Kiso River system, and compared with the evaluation method using specific discharge as an indicator reported in previous studies. The proposed indices are capable of discussing the substantial disturbance occurring in the river channel from sediment hydraulics perspective. Although the usefulness of the indices was recognized, many issues were identified that need to be resolved regarding the calculation method.

 In this study, Actual mass movements in the Granite Zone (Abukuma highland during Hagibis) were verified environmental conditions. In particular, we focused on the chemical composition that may induce the degree of geological weathering. The mass movement cases were databased from satellite imagery (Sentinel-2B). Spatial information based on numerical geographic information and chemical analysis of runoff were used for data on environmental conditions. The results show that the relationship between cumulative precipitation and slope gradient can indicate a large or small collapse rate. However, the chemical analysis of elemental runoff indicated that the amount of Ca from feldspar and Mg from biotite, which are leached from the granite during weathering, were also involved. The amount of these elements released from the granite varies depending on the geology and topographic position of the granite.

 Suspended Sediment (SS) in rivers negatively affects water quality, leading to increase in the costs of water treatment, loss of reservoir storage capacity and a decrease in fishery and crop productivity. In order to reduce the amount of SS in the river, it is essential to understand the mechanisms involved in sediment transport and the source of the SS in the river catchment area. Oromushi river basin in Hokkaido, which is predominantly characterized by high amounts of SS, was used as the case study. The basin was divided into 13 land groups with 18 sampling stations. The geochemical composition (Na₂O, SiO₂, Al₂O₃, CaO, Fe₂O, MgO, P₂O₅, K₂O and TiO₂) of the soil samples collected from each land group and the deposited sediment at the downstream end was determined by XRF analysis. The detected geochemical components from each land group were analyzed using Structural Equation Modelling (SEM). The results suggested that most of the SS in the river originated from the agricultural field. It is mainly because the vegetation cover is cleared to create land for sowing crops, consequently exposing the soil surface to the effects of raindrop splash. Moreover, our data suggested that SEM analysis gives more reliable data than other methods.

 There have been growing concerns about the effects of climate changes on water quality and river ecosystems due to increased river temperature. It is necessary to predict river temperature at multiple points in a river basin from future air temperature. In this study, we developed river temperature prediction model based on air temperature using deep learning, and predicted river temperature using future air temperature from +4K warming experiment of d4PDF. The results showed the accuracy of water temperature was high at multiple points on the current climate. The prediction of future river temperature showed that the future change in river temperature in summer was smaller compared to other seasons, but the river temperature at the multiple points in the basin increased by 6℃. The results suggest that the changes of daily mean river temperature may lead to the habitat changes of fishes and the overgrowth of invasive aquatic plants.

 Studies on river water temperature have often focused on time scales longer than daily mean, and little is known about daily changes. A two-dimensional heat balance model is developed to investigate daily changes in water temperature in rivers for summer. In a regulated section, water temperature is mainly controlled by solar radiation and advection. Diurnal changes become more apparent along the river flow because of the influence of solar radiation. It reaches the maximum temperature later by a half of the time required for advection. Deep pools prevent downstream water temperature from rising.

 A deep learning model was constructed to continuously detect floating debris based on river surface images captured by an existing fixed-point camera. As a result, a relatively high detection accuracy (F-measure) of 0.79 was obtained for the number of pieces of debris detected, confirming the usefulness of the model. Using the developed model, we analyzed the transport characteristics of floating debris in the Onchi River in Osaka, and found that a first flush phenomenon was observed in the early stage of flooding, and that approximately 90 % of the total number of debris in a particular flood event was transported during the rising period. An estimation formula for the number of pieces of floating debris was constructed, taking into account the amount of precipitation and the number of preceding sunny days, and the annual transport was calculated. The contribution of rainy and sunny days to the annual transport was comparable in the river. It was considered that the installation of oil fences during normal river conditions, the implementation of riverbed cleanup activities prior to flooding, and the development of techniques to collect floating debris during flooding would be effective measures to reduce the amount of floating debris.

 In order to understand the behavior of garbage in rivers, which is the origin of marine debris, it is important to understand the transport process of garbage production - release - deposition - resuspension. However, most research has focused on the amount of discharge or drift, and management authorities inspect riverbanks and have organized data about the amount of collected garbage. This study conducted field surveys of garbage in the Yamato River, where water quality has been improving in recent years but the deposition and littering of garbage remain a stubborn issue, in order to understand the state of garbage scattered on the riverbank. Patrol records indicate a large amount of garbage drifted 5.0 km from the river mouth and its downstream, indicating that the deposition of garbage is similar to the sediment deposition characteristics. In the field survey, 1m square quadrats were set up and 31 samples of man-made litter were collected at 7 locations. The weight, number of pieces, and coverage of garbage were measured and their relationship was analyzed. The weight per 1% coverage was estimated to be about 30g/m², and the weight of garbage larger than 5cm on the short side was estimated to be about 12 g per piece.

 During floods, we have observed that bank erosion occurs not at the peak of flow discharge but during the process of decreasing flow. In this study, we perform a weakly unsteady stability analysis of fluvial sandbars under slowly varying flow conditions, assuming that the rate of flow variation is low compared to the rate of bed deformation. Assuming that the rate of flow variation is sufficiently low compared to the rate of bed deformation, the two different time scales, 𝑡 for bed deformation and 𝑇 for flow variation, are introduced. We perform a weakly unsteady stability analysis using a multiple-scale expansion method with these two different time scales to determine the growth rate of disturbances under the variation of flow discharge. The analysis finds that the bed becomes more unstable when flow discharge is below the peak rather than at the peak. The instability intensifies when the flow decreases rather than when the flow increases.

 Instability analysis has contributed greatly to the estimations of the conditions for the occurrence of alternate bars, equilibrium wave heights, and equilibrium wavelengths. Although the validity of instability analysis has been discussed in many previous studies, there has not been sufficient discussion based on the bottom migrating speed, which is a fundamental element of the analysis. One of the reasons for this is the absence of a method for quantifying the bottom migrating speed during the generation and development of alternate bars. In our previous study, we attempted to quantify the bottom migrating speed during the generation and development of alternate bars, and confirmed its validity through flume experiments. In this study, the validity of the instability analysis was verified by comparing the above-mentioned bottom migrating speed with the migrating speed of the instability analysis. As a result, the migrating speed for each wave number from the instability analysis correspond well with the spatial distribution of the migrating speed of the bars, suggesting that the analysis is valid from the viewpoint of the migrating speed of the bars.

 Although many studies have been conducted on sand bars, the origin of sand bars and the mechanism of their development are still unknown. In this study, we measured the formation process of sandbars with the crossing patterns that occur from flat beds in the very early stage with high spatial resolution and high frequency. We quantified the migrating speed of the sand waves during this process based on the measurement. The results show that the migrating speed of the sand waves at the initial stage have a spatial distribution and that their transverse component is non-negligible in relation to the deformation of the sand waves. It was found that with changes in migrating speed, the high wavenumber sand waves gradually merged into the low wavenumber sand waves that are generally recognized as sandbars. These results suggest that, in the experimental cases conducted in this study, oblique crossing pattern is the origin of the sand bars.

 In this study, we developed the Quasi-3D flow and bed variation analysis method that can calculate bed load and suspended load integrally by calculating the continuous equations and the equation of motion of bed load and suspended load, respectively. And the developed method was applied to the Ishikari River flood in August 1981 and compared with the conventional equilibrium model. As a result, in the meandering reaches, the flood flows became the compound meandering channel flow which was a maximum velocity filament passed the inner bank side around the peak discharge. The calculation results demonstrated the secondary flows opposite to the normal secondary flows in curved channels occurred and the secondary flows transported the suspended sediments toward near the water surface. Finally, our study indicated the conventional model was effective for explaining the riverbed profiles after the flood.

 In this paper, we examine the horizontal and vertical movement velocities and concentration distributions of particles by making full use of particle tracking with a high-speed camera and PIV. As a result, it was found that the horizontal moving velocity had the similar distribution as the conventional theory, but the vertical moving velocity did not match the settling terminal velocity of the particles, and the velocity of upward and downward motion were about the same magnitude. In addition, it was clarified that the concentration of moving particles became one smooth distribution and continuously with the riverbed concentration. Furthermore, by paying attention to the phenomenon on the ripple, we were able to visualize and clarify how the influence of the systematic vortex structure of the water flow appears on the trajectory of the particles.

 To protect river infrastructures (especially, river bridge piers) from the huge flood disaster, it is important to accurately calculate and predict the sediment transport process. The characteristic of flow field around the pier has unsteady and non-equilibrium. Therefore, understanding the sediment transport process of local scouring is a limit to the equilibrium sediment transport equations. In this study, we applied a high-concentration solid-liquid multiphase turbulence model (GAL-LES) to the sediment transport process near the riverbed, and one-way analysis was carried out for the purpose of to verify the effectivity and validity of this model. As a result, it was suggested that GAL-LES model can explain the sediment transport process of a high-concentration fluidized bed in comparison with DEM.

 This study discusses the merits and challenges of using satellite data by investigating how well the number of disaster victims estimated from satellite images matches the actual number of evacuees in the case of the flood disaster caused by Typhoon Ulysses, which hit the Philippines in November 2020. In this case, the Sentinel-2 satellite captured optical images covering most of the two target basins in the following morning after the typhoon had left the islands, making it possible to confirm the inundation status for 80% of the basins’ municipalities. We analyzed the data to compare the number of victims estimated from the satellite images and the actual number of evacuees and revealed a large discrepancy between them. We investigate how well satellite images predict the presence or absence of potential victims and found that both optical and SAR images have a possibility to provide reliable inundation status to determine whether there are potential victims.

 We calculated the rainfall index of the landslide disaster caused by heavy rain events of July 2018, and confirmed that the rainfall index of the landslide disaster in the affected area before the occurrence of the said disaster was lower than a threshold value. Assuming that the said disaster was caused by a prolonged period of rainfall that did not exceed the threshold value, we examined the risk assessment methodology based on the second rainfall index of landslide disaster, which included the second tank for the soil water index. In this activity, the optimal threshold value for the second rainfall index of landslide disaster was examined, which should enable issuance of landslide risk assessment-based warnings before the occurrence of a landslide disaster and prevents the increase in misguidances (i.e. the warning is issued but no landslide disaster occurs). This examination was carried out by altering the threshold values and checking the timing of threshold exceedances and the changes in the number of points of occurrence included in the respective ranges of judgment, as well as the range of misguidances.

 Numerical simulations of debris flows were performed for several debris flows that occurred in Umegou Danchi, Yanohigashi, Hiroshima in July 2018, to study the flow characteristics of debris flows and to evaluate the established sediment disaster hazard areas, etc. The areas of high maximum stress acting on the houses in the residential areas were in good agreement with the distribution of the destroyed houses. The results showed that even outside of the sediment disaster special hazard areas, there are areas where houses may be destroyed. In addition, we investigated the spatial distribution of the inundation area and flow depth of the debris flow that flowed into the residential area, depending on the timing of occurrence of multiple landslides and the difference in the number of landslides. When considering evacuation plans, it was shown that it is important to consider the scale and timing of the sediment flow that flows into the residential areas, and to consider the amount of sediment runoff from multiple mountain streams.

 In recent years, the increase in short-term heavy rainfall caused inundation hazards in various places. In the event of an inundation hazard, evacuation by walking in a stream is sometimes conducted. Hazard maps with only the inundation depth as an index of external force, which are currently used in many municipalities, do not adequately express the risk of evacuation involving walking in a stream. Therefore, the purpose of this study was to create a new difficulty of evacuation and express it as a difficulty of evacuation map. In the experiment, we conducted a walking experiment in actual rivers and an experimental flume with the target age groups in their 20s and 70s. Furthermore, precipitation simulation was performed for the target area. Based on the obtained results, we indexed and visualized the difficulty of evacuation, including the rate of decrease in walking speed to the evacuation center. As a result, it was found that the difficulty of evacuation map created in this study can be used for selecting the route to the evacuation center and selecting the most suitable evacuation center from multiple evacuation centers.

 In recent years, the frequency of torrential rains has increased due to abnormal weather conditions. Torrential rains are extremely difficult to predict and have caused extensive damage in many areas. The objective of this study is to construct a new real-time evacuation route search system that does not involve flooded areas by using a functional form that takes into account the evacuation time and the direction of spread of flooded areas and takes the width between the flooded areas. The number of evacuees was analyzed by comparing the proposed model with the conventional model, which does not take the functional form into account, with the one that does. The results showed that the proposed model has an improvement rate of about 16.5(%) compared to the conventional model, and that it peaks when the value of the coefficient cd is varied.

 As damage from torrential rains and typhoons increases in various parts of Japan, it is necessary for residents to properly perceive risk of disasters in order to encourage voluntary evacuation actions. In this study, using VR content that allows residents to visually experience flood situation, we examined the transformation of residents' disaster risk perception and disaster prevention awareness through workshops in which residents participated. As a result, residents became aware of storm surge inundation assumptions in a manner consistent with the assumptions in the hazard maps. In addition, it was suggested that the timing of actions such as preparation of items and information related to evacuation and protection of household goods may become earlier and the evacuation routes and locations were changed to avoid flood damage. The results suggest the possibility of improving the awareness of disaster prevention.

 This study conducted a questionnaire survey of local disaster prevention leaders after attending a MyTimeLine seminar to analyze the effectiveness of the My-TimeLine seminar in contributing to the development of My-TimeLine Leaders. The results of this study are summarized as follows. 1) 50% (56 respondents) responded positively to the behavioral intention, "I want to be a person who supports the consideration of My-TimeLine in the community (My-TimeLine Leader)”. 2）In the model of this study, the path coefficient of attitude toward behavior was significant for behavioral intention, but the path coefficients of subjective norm and sense of control over behavior were not significant. In other words, the model of this study was not as hypothesized, and only attitude toward behavior was found to affect behavioral intention.

 This study conducted a questionnaire survey after the My-TimeLine seminar was conducted by a non-professional instructor to evaluate changes in knowledge and awareness of heavy rainfall and flood disasters and evacuation intentions compared to those of the professional instructor. The results of this study are summarized as follows. 1) The My-TimeLine seminar conducted in compliance with the reference materials, can be expected to be generally equally effective in terms of understanding of timing of actions and evacuation intentions, even for non- professionals. 2) On the other hand, the effect sizes of the My-TimeLine seminar conducted by non- professionals were smaller than those conducted by experts in terms of understanding flood risk, understanding information available during a flood event, and risk perception.

 Bubble plume is used as countermeasure for preserving water quality of eutrophicated reservoirs. Spreading radius of the bubble plume impinging on the free surface is valuable information for design of the facility. We conducted field experiments for obtaining the surface radius length data due to bubble plume in small reservoir and analyzed with data set of previous researches. It is found that the sum of air injector depth (H) and the spreading radius length (R) is equal to be about four times of Bubble Plume Length (l_BP). The results are versatile in that they can also be used to estimate the maximum plume height (h_P) that occurs when the stratification intensity of the ambient water is greater than a certain level relative to the bubble buoyancy, which has been treated separately in previous studies. The engineering application of the obtained findings in reservoir water quality protection is discussed.

 This study evaluated a mass-conserving model (WindNinja) and deep learning model to produce a non-uniform wind field over a meandering reservoir using wind data from AMeDAS and the in-situ wind data at certain locations over the surface. Results showed that WindNinja can estimate wind magnitude and direction for the stations near the input AMeDAS station. On the other hand, stations that are far from the input data location yielded less accurate results. Moreover, the wind estimate using various cases of deep learning models showed better results than the WindNinja simulation. The prediction of wind magnitude using deep learning model is less affected by the type of input and output parameters. On the contrary, the prediction accuracy for wind direction significantly changes for each case. Finally, deep learning models that utilize the results from the WindNinja simulation were also considered and yielded the most accurate wind prediction. The overall results of this study proved that an acceptably accurate non-uniform wind field can be generated using a mass-conserving wind model and deep learning model despite the limitation in the amount of available data.

 Surface wind is generally considered one of the major forces in shallow lakes as well as large water bodies, as it induces energy that forms horizontal currents over the water surface. Wind is typically inhomogeneous in space, which instantaneously induces distinctive surface movements compared to that of homogeneous wind. In this study, the Weather Research and Forecasting coupled with the Large Eddy Simulation (WRF-LES) model was employed in an attempt to simulate realistic wind over the shallow Lake Kasumigaura of Japan. Based on the modeled data, an integrated wind non-uniformity analysis, which directly derives the coefficient of variance (C_v) for wind speed and direction in various spatial scales using 2D mean filter technique, was developed. With this analysis, the time variation of the mean C_{v_spd} and C_{v_dir} were obtained to classify wind speed and wind direction non-uniformity levels. Finally, each representative snapshot of wind cases (spatially homogeneous and spatially inhomogeneous) was chosen based on the corresponding value of wind speed to simulate and develop an understanding of the effects of non-uniformity on mass transport in the lake using a numerical tracking analysis of floating particle. Although different levels of inhomogeneous wind field illustrated their effects on surface particle transport, the effects on this lake environment were relatively minor.

 The effect of wind on the salinity of the connected brackish lakes, Lakes Shinji and Nakaumi, was evaluated over the recent ten years. The water quality of the lakes was predicted by reproducing the present conditions and reducing the wind velocity to 50% of the present conditions. The reduced wind velocity led to a decrease in the surface mixing in the two lakes. In particular, the significant increases in the salinity of the lower layers of both lakes were predicted. Moreover, a multiple regression analysis was conducted on the amount of water exchange among two lakes and the sea. The results showed that the effect of wind-induced flow by strong winds on water exchange was limited in time and that the gravidity-induced flow by the change in water level due to the blowing effect was more important than the wind-induced flow.

 Lake Jusan is a brackish lake located in the northwestern part of Aomori Prefecture in Japan. The lake is a major fishery ground of Corbicula japonica, which is a popular resource of inland fishery for Japanese. The larvae of the bivalve in the early stage of development is planktonic, so they float in the water right after spawning.Therefore, study of dispersion processes of the planktonic larvae in the lake can contribute to conservation of the fishery resources. A combined model to simulate hydrodynamics in the lake and behavior of planktonic larvae was developed in this study. The floating larvae of bivalve have capability of vertical migration according to salinity preference, which was considered in our developed model. The model was applied to the actual condition of Lake Jusan, and connectivity of populations between the regions in the lake was analyzed from dispersion and settlement of the larvae.The results suggested that the settling rate of planktonic larvae increased due to salinity selection behavior, and that the eastern part of Lake Jusanko was the source of larvae and supplies larvae to the entire lake.

 "jewelry ice", which is appeared in winter on Otsu Coast near the Tokachi River mouth in Hokkaido, has been attracting attention as a new tourist resource. On the other hand, the period of the appearance of "jewelry ice" is not fully clear. The objective of this study is to develop a method for estimating the period of appearance of jewelry ice. Image analysis was performed using images taken by a fixed-point camera to calculate the area of deposition. The appearance phenomena are classified into five categories: formation, destruction, transport, deposition, and melting. The validity of the calculation results was confirmed by comparing the area of jewelry ice deposited by image analysis and the amount deposited by calculated values.

 The effect of tidal variability on the changes in phytoplankton abundance and their size distribution in the tide-dominated Chikugo River estuary was investigated in 2021. The estuary changed from a stratified condition during neap tide to well-mixed during spring tide and salinity intruded until upstream. The estuarine water was clear in the neap tide, which changed to turbid water gradually, and the estuarine turbidity maximum (ETM) zone was developed at 8-14 km in the spring tide with a maximum turbidity of ⁓900 FTU and salinity of 0.2-17. There are 159 phytoplankton species divided into 39 groups belonging to seven categories. Among them, diatoms were found as a major category and their abundance showed a similar trend to total phytoplankton abundance with 79-86% (high tide) and 76-98% (low tide). The abundance of diatoms were found the highest at the intermediate tide in both low and high tide periods. From a spatial perspective, diatoms densities decreased in the middle estuary because of the effect of ETM development. The colonial groups with branching and straight forms were dominated. The cell size of Skeletonema costatum and Nitzschia acicularis increased from neap to intermediate tides and maximum sizes were observed at the spring tide. This study concludes that intermediate tide between neap and spring tides plays an important role in diatom growth. The changes in cell size and shape are essential to retaining the phytoplankton diversity and abundance besides water mixing and ETM formation in the Chikugo River estuary.

 Sediment deposition in the inter-floodplain of a river channel in lowland rivers contain a large amount of fine-grained sediments called “wash load”. The main objective of this study is to validate a prediction model for wash-road deposition that can take into account different sediment concentrations and flow regimes in different rivers, and to understand the effects of ground surface conditions on wash-road deposition at excavated sites along the Ibi and Nagara Rivers in the Kiso River system through field experiments and field surveys. We conducted fixed-point surveys over four outflow periods in the Ibi River, field experiments with four panels of different ground cover conditions, and a total of 67 sites in three excavation sites in the Ibi and Nagara Rivers to determine the amount of sediment deposition and the percentage of wash roads during each study period. The model reproduction and validation results showed that the model performed well under the forested condition. On the other hand, the validation of the model under conditions where only herbaceous vegetation covered the ground surface remained an issue. The model performs well enough to predict the amount of sediment deposited after excavation.

 It is important to take effective and efficient measures to prevent and mitigate damage caused by sediment and flood damages. In recent years, there have been many cases of fine-grained sediments with a grain size of 2 mm or less flowing down and depositing in areas to be protected, such as the Oya-okawa River in the heavy rains in July 2018. While it is required to examine how much such fine-grained sediment contributes to the damage caused by sediment and flood damages, the effect of damage prevention and mitigation by Sabo facilities is not quantitatively clear. It is also important from the viewpoint of comprehensive sediment management in watersheds to quantitatively organize the capturing effect of fine-grained sediment by Sabo dam. In this study, a one-dimensional riverbed variation calculation was performed on the downstream side of the Pekerebetsu River No. 2 Sabo dam, which captured a large amount of fine-grained sediment in the August 2016 flood, and the capturing effect of the Sabo dam was verified. As a result, it was clarified that the Sabo dam has a certain degree of damage reduction effect.

 Maintenance of eco-friendly physical structures according to the situation after their construction is indispensable in order for the structures to continue to exert the biological conservation function intended at the time of design. In this study, "mud removal" that dredges excessive sediment in fish nests in an agricultural drainage canal was focused, and a method for evaluating dredge efficiency was presented. The two-dimensional hydrodynamic and morphodynamic model was applied to an actual agricultural drainage canal section with fish nests and a fish pool, and the sedimentation tendency in the target section was reproduced. Comparing the reduction rates of dredge effect by assuming multiple patterns of mud removal in specific fish nests on the left bank leads to evaluating the sustainability of the dredge effect associated with the amount of dredged soil. This method could provide useful information for adaptive management of fish nests, such as selection of mud removal area where the dredge effect is likely to be sustained and determination of the amount of dredged soil.

 When huge amounts of sediment are produced in upstream areas during heavy rainfall events, they can be transported to downstream and contribute to flood damage in downstream areas. In this study, a sediment discharge model and two-dimensional flood flow analysis were used to calculate floods with large amounts of suspended sediment in the Shira River in Kumamoto Prefecture, which is located in the upper reaches of the Aso Caldera, to investigate the effects of large amounts of suspended sediment deposition in the river channel on flood inundation. The results of the sediment discharge calculations show that, due to the difference of bed slope between the upper reaches of the Shira River and the Kurokawa River, a large amount of suspended sediment flows into the lower reaches of the Shira River before the peak flow is reached. According to the results of the 2D flood flow computation, these sediments are deposited in areas where the sediment transport capacity of the river channel is relatively small, that contributes to the inundation.

 This study investigates the basin scale sediment transport processes in the Sangu river in Bangladesh by using a rainfall-sediment runoff (RSR) model. One year numerical simulations are conducted with two different sediment transport formulas and the results were compared with observed sediment size distribution and suspended sediment discharge. As a result of computation, a clear longitudinal sediment sorting is shown in one case, while that is not shown in the other case, and the difference between two cases is explained by the difference in the erosion rate of suspended sediment formula. The sediment sorting significantly affects the results of the basin scale sediment transport analysis. Thus, we conclude that in a river dominated by suspended sediment such as the Sangu River, it is very important whether the erosion rate of suspended sediment and associated transport processes can evaluate the sorting process.

 In this study, an experimental investigation was conducted to determine the future response of the river channel. The condition of a stable channel occurrence was used as a criterion. The followings were obtained from the experimental data. (1) As a river channel moves toward a state of dynamic equilibrium, it approaches a stable channel. (2) In near-natural channel conditions, the cross-sectional geometry has an average K-value. (3) The cross-sectional geometry has a K-value near the boundary of the stable channel condition when river width or bed level is restricted by revetments or bed breaching. (4) The cross-sectional geometry tends to widen and lower the riverbed when the K-value is greater and less than the stable channel condition. (5) An extensive flow discharge history determines the cross-sectional geometry of a river channel. The cross-sectional shape can be predicted using appropriate channel-forming flow discharge.

 In order to increase the spawning bed area of Ayu in the Heidan area of the Asahi River tidal zone, a portion of a sandbar around a natural spawning ground was excavated and a new artificial spawning ground (sub-current channel) was constructed under normal flow conditions in a project. For this study, field observations and numerical calculations were conducted to examine the suitability of the hydraulic environment of the artificial spawning area creation. Additionally, we examined the impacts of the construction on the existing natural spawning habitat. Results showed that the suitable spawning area was an area where there was no salinity increase at any time, and that the river bed grain size, water depth during spawning time, and current velocity were within the suitable values proposed in previous works. Results also presented that the spawning density in the constructed area might be more dependent on the suitability of the grain size than on the flow regime. Furthermore, the results of high-resolution 2D horizontal calculations showed that the creation of the new spawning area did not significantly alter the hydraulic environment of the existing natural spawning grounds, and contributed to the increase in spawning bed area, which was the original purpose of the project.

 Our study aimed to develop a simple particle size estimation method for tidal flats and to verify its applicability for sediment classification of tidal flats. We conducted 1) the survey of the particle size distribution of tidal flats and photographing of the surface layer at 51 points located at the mouth of the Saba River in Yamaguchi Prefecture and 2) the estimation of the particle size by image analysis using machine learning from the surface photographs. The results showed that the tidal flat surfaces with mud content of less than 20% and those with mud content of 20% or greater could be discriminated from the images with an accuracy of approximately 73%. Next, tidal flat images were classified into four categories of mud contents using machine learning; less than 15%, 15% to 25%, 25% to 35%, and 35% or more. The percentages of correct images in each category were 72%, 48%, 45%, and 75%, respectively. These results suggest that the machine learning image analysis used in this study can improve the accuracy of the discrimination when the partition of the mud content is set to be coarser.

 The effects of tidal and river forcing on the estuarine turbidity maximum (ETM) in monsoon-affected Asian estuaries are rarely reported. Hence, the present study aims to investigate the seasonal and tidal variations in ETM in the monsoon-affected macrotidal Tanintharyi River estuary (TRE), Myanmar. The vertical profile of salinity and turbidity are measured for three years (2017-2019) from the river mouth to the 40 km upstream during both tidal (neap-spring) and seasonal time scales. The salinity intrusion reaches maximum distance (33-37 km) during dry periods and minimum (0.6-16 km) during monsoon periods. The mixing condition changed from stratified to partially mixed conditions during monsoon periods, whereas it varied from partially mixed to well-mixed conditions during dry periods in a neap-spring tidal cycle. The ETM was found in the lower salinity region during the monsoon season and in the higher salinity region during the dry season. Further, it developed under partially mixed conditions during the monsoon season and under well-mixed conditions during the other seasons. The location of ETM moved upstream with a decrease in the tidal range, whereas it moved downstream with a rapid increase in river flow. The magnitude of ETM increased with an increase in tidal range for all the seasons, whereas an increase in river flow magnifies the ETM only during the monsoon season. Hence, the tidal forcing is the most dominant factor, followed by the river flow that affects the ETM dynamics in the TRE.

 A granulated coal ash (GCA) eluting ions such as Ca²⁺, SiO₂, OH⁻ were covered on riverbank tidal flats. In this study, polymerization process of organic matter owing to GCA was verified. Sediment properties were investigated for 27 months after GCA capping. An increase in pH caused increase in coarse particles and decrease in fine particles. The granulation process was due to ions eluted from GCA, which caused the binding of fine particles and complex reaction of organic matter. The granulation of organic matter corresponded to the recalcitrance property, indicating that the amount of coarse particles can be used as an indicator of the recalcitrance of organic matter. The polymerization process was also found in the lower part of the GCA capping layer.

 Mercury pollution has attracted more attention in marine environment since the outbreak of Minamata Disease in Japan more than 6 decades ago. Although the highly total mercury has been dredged by the government, many experiments for mercury measurement were carried out and indicated that trace mercury was still in Minamata Bay, and some were even transported along with the sediments from the bay to the nearby the Yatsushiro Sea. Hence, it is necessary to develop a numerical model for simulating the mercury migration. In this study, a numerical model was established in Delft3D based on in-situ measurement, where the transport simulation of bottom sediment coupled with the internal diffusion technique was applicated. It was used to simulate the transport of trace mercury from Minamata Bay to the Yatsushiro Sea which was assumed to primarily depend on sediment migration. From the modeling result, it could be concluded that mercury spread from Minamata Bay to the seas around Minamata Bay and gradually spread to the entire Yatsushiro Sea. The mercury transport was mainly divided into two patterns, that is, southwest migration and northeast one. Meanwhile, the trace mercury trended to deposit along the seacoast, which was mainly considered due to the terrain effect.

 Clarification of sediment transport in intertidal flats is very important for conservation of intertidal flat and future prediction. Furthermore, since the river basins is closely related to the quality and weight of sand supply, it is important to clarify the regional sediment transport. In this study, we studied Nakatsu Intertidal Flat. Nakatsu Intertidal Flat is sandy and muddy tidal flat. However, in recent years, there is concern about it has erosion and siltation. Moreover, salient siltation of Nakatsu Intertidal Flat was observed from spring to autumn 2017 from field observations by the previous authors. In this study, we targeted North Kyushu Heavy Rain Disaster in July 2017 and verified siltation mechanism of Nakatsu Intertidal Flat from numerical simulations of sediment discharge from the river and regional sediment transport of coastal zone and river. As a result, we obtained spatial distribution of suspended mud and settling velocity of suspended mud corresponding to tidal and river discharge. Therefore, we were able to qualitative reproduction of mud transport in intertidal flats.

 It has been pointed out that intertidal tidal flats play a major role in preserving the water quality and biological environment of inner bays, and conservation of existing intertidal flats and creation of artificial intertidal flats have been conducted. In the Nakatsu intertidal flat in Oita Prefecture, a decrease in the catch of clams has been a problem in recent years. In this study, focused on the muddiness of the intertidal flat, which is one of the causes of the decrease in the catch of clams., and conducted a qualitative reconstruction of the Nakatsu intetrtidal flat by field observations and wide-area sediment dynamics in the coastal areaby using WDM-POM. But there is a problem of not being able to reproduce the muddiness at the shoreline. This paper presents numerical simulations of the formation of a mud area the shoreline of an intertidal flat, and presents numerical simulations of mud dynamics on intertidal mudflats with flat topography and constant gradient. The numerical results suggest that tidal fluctuation and tidal flat topography have an influence on mud agglomeration and sedimentation.

 In flow observation using the STIV method, a standard value of 0.85 is used as the correction factor when calculating flow rate from surface velocity. However, the standard value may not be applicable because the vertical velocity distribution does not follow the logarithmic law depending on the cross-sectional shape of the river channel and vegetation distribution around the measurement line. In this study, two-dimensional and three-dimensional models were used to simulate a real flood event in the vicinity of Nakahara Bridge, a discharge monitoring station of the Asahikawa River in Okayama Prefecture, and the effects of hypothetical changes in channel shape and vegetation distribution on the correction factor were investigated. As a result, it was found that the rerouting coefficients downstream of the sandbars were significantly reduced when the sandbars were developed upstream of the Nakahara Bridge and the vegetation was dense. On the other hand, it was shown that even if a sandbar develops, it is possible to use the standard value of the rehabilitation factor by appropriately managing the vegetation on the sandbar by cutting.

 The main cause of channel fluctuation in alluvial rivers is thought to be the meandering streams caused by the geometry of the alternating sandbars. However, most of the previous studies have focused on the geometry of the bottom surface, and there is a lack of findings on the correspondence with the hydraulics of flowing water, which is the source of sediment transport. In this study, model experiments and numerical analysis of an actual river were conducted for conditions in which alternating sandbars induce flow deflection. The results showed that the development of the bottom shape on alternating sandbars corresponds to the flow deflection. The possibility of occurrence increases rapidly when the wave height-to-depth ratio, which is the bottom wave height divided by the water depth, exceeds 1.5, suggesting that the possibility of occurrence of flow channel evolutions can be estimated.

 In a combined channel system where the upstream part of the irrigation canal is an open channel and the downstream part is a pipeline, a buffer pond must be installed at the contact point between the two. In order to effectively utilize the buffer pond, it is necessary to improve the water storage monitoring and the water supply operation according to the water use by the consumers.Therefore, in this study, we presented a method to prevent: surplus water when the water demand is low; and capacity shortage when the demand is high by predicting water usage, operating a water diversion control gate, and adjusting the inflow into the pond. For the prediction of water usage, we used an autoregressive(AR) model which can usefully predict future values of time series even with a limited number of data. Considering the operation rules of water usage from past data, we introduced them into the AR model. Simulation results showed that the surplus water and the water shortage of the buffer pond can be reduced by sending water according to the amount of water used.

 This paper presents a screening method for predicting flood-induced bank erosion around bridges. In August 2016, four typhoons hit Hokkaido and a number of bridges were damaged. For efficiently extracting vulnerable bridges, multiple regression analysis is applied to predict the erosion width of the channel using stream parameters such as basin area, bed-slope, channel curvature, precipitation and soil properties. The results show that four geological conditions vary considerably in erosion width, indicating that the parameters are unsuitable for screening. However, without soil properties of targeted sections, predicted normalized erosion width shows good agreement with the measured one when applied to a wide range of basin area. This method can be an effective measure to efficiently extract the potentially dangerous bridges.

 In recent years, the development of 3D data collected by airborne lidar survey and airborne bathymetric lidar survey have been promoted, and 3D data of various mesh sizes have become available. We examined the applicability to these hydraulic analysis and examined the index for selecting the mesh size corresponding to the required accuracy regardless of the river scale. In ALB survey, river bottom data as well as river topo data may be missing due to the influence of vegetation, water depth, and water quality. And the gap between the interpolated topography and the actual topography becomes large, especially in mountain rivers with severe undulations. Therefore, we focused on topographical features, examined the effect of missing ALB data on the calculated water level in hydraulic analysis, and clarified the mechanism.

 In the non-contact method of observing river surface velocities, the flow rate is generally estimated by an area-velocity method with a constant surface velocity coefficient. In practice, however, the surface velocity coefficient is expected to vary depending on the river geometry and other factors. A method using a vertical velocity distribution equation based on the concept of the maximum entropy method has been proposed as a flow estimation method that can take this point into account. However, this method requires a measurement of the cross-sectional velocity distribution at least once to set the entropy parameter M, which is a problem for application to a new location. In this study, we examined the recently proposed method of automatic estimation of M using the observed data of STIV, an image measurement method. The results showed that M were reasonably estimated for several rivers, and that the accuracy of flow estimation was improved in cases where a constant surface velocity coefficient, which is usually used, is not appropriate.

 The purpose of this study is to propose several measures necessary to improve the rate of flood version of the Business Continuity Plans (BCP). Japanese companies have generally developed BCPs and disaster prevention plans only for earthquakes. Therefore, the rate of developing BCPs for water-related disasters is very low. In this study, we conducted a questionnaire survey of companies in Toyama, Ishikawa, Fukui, and Niigata prefectures to identify issues to be addressed when developing a BCP. As a result of the analysis, it was found that there are several challenges in developing BCPs, such as lack of manpower, time, knowledge, and benefit. Then, to solve these issues, we calculated the amount of damage caused by floods in Toyama Prefecture and simulated the amount of damage and B/C that could be mitigated by developing a BCP. These B/C maps may be a good tool for the company to judge the benefit of BCPs.

 Three months long ensemble inflow prediction was newly introduced to the operation support system for a single hydroelectric dam developed by Koike et.al.(2021). Inflow predictions; 39 hours ensemble and three months ensemble, were confirmed to have meaningful accuracy and adopted to reservoir operations. The system showed that the reservoir’s authority could expect 6.0% increase of hydro power generation annually.