In the middle reach of Tama River, the low channel bed height has been controlled by continuous river improvements, and the river channel has been transformed into the ship-bottom-shaped channel as the low channel width has been increased in response to flood flow. In this study, we evaluated the hydraulic and environmental characteristics of the ship-bottom-shaped channel by flood flow and river bed variation analysis. In the ship-bottom-shaped channel, sandbar bed is moderately disturbed by floods according to the height of sandbar, which dynamically maintains the habitat for herbaceous vegetation and facilitates tree management. In addition, the water surface width widens as the flow rate increases and the increase in bottom velocity becomes slow, resulting in creating continuous low velocity area that can provide refuge for fish during flooding. Thus, the ship-bottom-shaped channel contributes to the conservation of diverse habitats. And the results of this highly accurate analysis provide important information for design and management of the channel in harmony with flood control and river environment.

 Gravel riverbeds are disappearing due to the growth of forests in river sections downstream from dams because of the reduction of tractive force by flood control. In this study, the measures to activate riverbed gravel movement by changing the flood control rule was examined for the Yuda Dam upstream of the Waga River, a right tributary of the Kitakami River, where the gravel riverbed is remarkably forested. First, it was confirmed that the Yuda Dam has sufficient flood control capacity from the statistical analysis of the inflow to the Yuda Dam and the flow rate of the Waga River at the time of flood as well as the capacity analysis of the downstream channel. Next, the increase in discharge from the dam during floods was examined within the range of the flood flow capacity of the Waga River channel. The result suggested that the river discharge with a probability of occurrence of 1/5 can be changed from about 1100 m³/s to 1800 m³/s by increasing in the minimum flood discharge. Finally, we calculated changes in bed tractive force using a shallow water flow model and showed that the above changes in the flood control rule can increase the mobility of 10-cm-diameter pebbles which is dominant in the Waga Riverbed.

 This study aimed to achieve early evacuation by effectively utilizing the issuance of internal flooding hazard information in urban areas to mitigate damages caused by internal flooding. We verified the outflow characteristics of sewer water levels and their predictability. The results revealed that the time difference between peak rainfall intensity and sewer water levels is only about 5-10 minutes. Moreover, it was demonstrated that sewer water levels rise rapidly and may lead to overflowing when reaching full capacity, emphasizing the importance of predicting the time to reach full capacity for each sewer system.Considering the very short evacuation time in small urban catchment areas, we investigated the use of forecasted rainfall to secure evacuation time. The results showed that the rainfall forecast for the next 10 minutes can be utilized effectively. By combining the aforementioned timeframes, we can secure approximately 20 minutes for evacuation. Consequently, we concluded that sewer water level observations are valuable for early evacuation planning.

 In recent years, record-breaking rainfalls and urbanization have caused inundation hazards in many parts of Japan. Hazard maps are used for evacuation in the event of inundation. However, conventional hazard maps generally show only the depth of inundation. Actual evacuation actions are affected by the flow velocity and evacuation distance. In this study, we defined a difficulty of evacuation in the event of internal inundation hazard as an indicator that includes more than just the depth of inundation. Furthermore, this study aimed to visualize and evaluate the difficulty of evacuation by applying it to the target area. Inundation simulations were conducted for several probability years. The difficulty of evacuation was evaluated for parameters related to walking in water, evacuation distance, and roads. The results showed that the difficulty of evacuation was smaller near evacuation sites and larger away from evacuation sites. The difficulty of evacuation varied with water depth and flow velocity for several probability years. The results suggest the possibility of visualization of new evacuation difficulty points.

 In December 2022, the revised Civil Aeronautics Law came into effect, allowing so-called "Level 4 flights, " which are flights that take place outside the visual line of sight of drones in urban areas. However, because unauthorized flights over third-party land are not permitted and social acceptance by residents is required even when permission is obtained, we believe that river space with minimal risk should be used to realize drone logistics in urban areas. On the other hand, since automatic and autonomous flight is a prerequisite for drone logistics, spatial information is necessary. This paper compares 3D point cloud data generated from images taken by drones in a demonstration experiment with various types of spatial information on rivers, and shows that 3D point cloud data is more useful than other types of spatial information.

 Injury risk of accident of falling to water is assessed by using a coupled human-flow simulation. In assuming 10m of falling height, a series of simulations were conducted with different combination of water depth and initial body posture. By applying injury index of automobile accident, concrete injury type and survival rate were evaluated from forces affected both from fluid and collision to bed surface. While both of standing posture and inverted posture show relatively smaller risk for entering to water surface, colliding to bed surface result in high injury risk in the case of shallow water. Especially, colliding with inverted posture result in crucial injury. Furthermore, falling with a prone posture causes lung rupture due to impact to water surface. Therefore, it was found that both of water depth and posture have a large influence on injuries and survivalbility.

 In developing countries where meteorological observation networks are not sufficiently deployed, the provision of accurate satellite rainfall using satellite products is expected not only for disaster prevention, but also to expand the range of application of technology and research requiring observed rainfall. The aim of this study is to propose a new satellite rainfall estimation algorithm HiDREDv2 by using a fully convolutional neural network, on the geostationary meteorological satellite Himawari. In this study, a model suitable for meteorological phenomena is constructed by combining the features of each of the existing models. The accuracy of this model for 6-hour rainfall accumulation was 13.26 for RMSE and 0.69 for FSS. The accuracy of the model was significantly improved over GSMaP for the 4-day accumulated rainfall during the heavy rainfall period due to the heavy rainfall in July 2008.

 In this research, the authors studied how to improve the versatility of the existing soil moisture estimation algorithm developed on the premise of using ALOS/PALSAR. We first showed that the existing algorithm, though originally set to assume that the incident angle is constant, can use the ALOS2/PALSAR2 multi-polarized wave data by adapting it to process changes in the incident angle. The problem is that multi-polarized wave mode observation is highly infrequent. However, with multiple SAR observations and estimated soil moisture maps created based on them, it is technically possible to estimate the ground surface roughness by performing in-verse estimation using the microwave scattering model. Therefore, with the creation of such datasets in mind, we experimentally estimated soil moisture from the Sentinel-1/C-SAR data using the random forest method, a machine learning approach.

 River bank erosion is the major issue along large rivers such as Brahmaputra in Bangladesh. This study is aimed to develop the method for estimating river morphology along river bank to monitor movement of sand bars and erosion near river structures. The method combines satellite imagery, which is used to estimate water extent, and observed water level. Elevation of each pixel is estimated with several water level gauges and merged with an inverse distance weight method to consider slope of water surface. Developed method was applied to Brahmaputra river. This study used Sentinel-1 satellite images with Synthetic Apeture Radar (SAR) and optical satellite Sentinel-2 with MultiSpectral Instrument (MSI), both operated by European Space Agency. By comparison between observed and estimated cross section, the developed method effectively captured shape of river cross section. The results demonstrate that this method is used to detect sand bars movement and scouring around river structures. However when the method is applied to relatively small scale phenonmenum of less than 100 meter horizontally, applicability is affected by several factors including availability of satellite images influenced by cloud cover, observation frequency, and range of water fluctuation. These factors should be considered when practical application is considerd.

 This paper presents an attempt to establish a methodology for visualizing the effect of dam on flood mitigation in an easy to understand way for public. The target river basin is Kumagawa River catchment. Specifically, the Kawabegawa dam which is supposed to be constructed in near future in the catchment was dealt with. The discharges into Ichifusa dam and planned Kawabegawa dam for the rainfall event of July 2020 were first reproduced using a storage function model and radar-raingauge analyzed data. Then, the inflow to Kawabegawa dam was converted to outflow considering its dam operations. Using the inflow/outflow, the effect of Kawabegawa dam on flood mitigation in downstream area was estimated. Herein a shallow water equation model tailored for supercomputer Fugaku was used to carry out large scale computation at downstream to estimate the flood depths. Finally, two worst case scenarios out of 1000 ensemble flood simulations were considered to investigate the effect of Kawabegawa dam at a worst situation. As a result, it was shown that Kawabegawa dam has certain effect on flood mitigation.

 In this study, the effect of reducing peak flows and inundation areas of tributary rivers was evaluated by simulation when storage, infiltration, and inundation flow control measures were implemented in the catchment area of Hitoyoshi city. The introduction of a rice field dam covering 1.4% of the target watershed area, storage within a school ground and park encompassing 0.04% of the target watershed area reduced peak flows in the Yamada, Nishikawauchi, and Omizo Rivers by approximately 1-8% for rainfall events with up to 50-year probability. Moreover, 1.1km long road raising within the Fuku River basin contributed to reducing approximately 51% in peak flow for this river. The introduction of rice paddy dams was estimated to decrease the inundation area during rainfall events with up to 30-year probability. Additionally, rice field dams combined with strategies involving storage within school grounds and parks and road raising reduce the flooded area for rainfall events with up to 50-year probability.

 In addition to global warming progressing on a global scale, serious population decline and aging are progressing in Japan. While the scale of floods is expected to increase due to future climate change, the elderly are vulnerable to disasters, so an increase in the proportion of the elderly is thought to lead to an increase of victims of flood damage. We investigated how the scale of potential human damage would change compared to the present when all of the effects of increasing human damage due to climate change and aging and the effects of reducing human damage due to population decline were taken into account. The research was conducted on the Omono River basin in Akita Prefecture using the RRI model for inundation analysis and the LIFE-Sim model for human damage estimation. As a result, even though the population will decrease significantly in the future projections, it is assumed that the scale of human damage will be greater than the present due to the expansion of the scale of floods and the aging of the population.

 In this study, five open levees were assumed to be constructed along the Tedori River, a first-class river in Ishikawa Prefecture, Japan, and inundation simulations were conducted to evaluate the effect of the open levees on reducing economic losses in events of flooding due to levee breach. The simulation results assuming the planned rainfall showed that the potential flood damage was reduced by more than 80 billion JPY (about 87%) depending on the cross-section where the levee breach occurred. On the other hand, discontinuities are formed in the areas where the open levees are constructed. In some cases, without levee breach, inundation occurred even when the flood was at the level where they could safely flow before the construction of open levees. The simulations assuming L1.5 rainfall (rainfall between the planned rainfall and the maximum expected rainfall) also demonstrated the effectiveness of inundation mitigation by open levee, and damage reduction of about 100 billion JPY (about 82%) could be expected. The maximum expected rainfall caused flooding in a wide area even without levee breach, and inundation would have increased if open levees had been constructed.

 Rice paddy dam initiatives are attracting attention as part of river basin flood control measures. In this study, we quantitatively evaluated the effectiveness of the rice paddy dam introduction in controlling runoff at the dam basin scale, focusing on the upper reaches of the Hiji River basin, including the Uwa Basin (Nomura Dam basin). In the runoff analysis using the storage function method considering the presence or absence of rice paddy dams, it was confirmed that by expanding the basin to the Aza-scale level, a similar approach to the conventional storage function method (with a model parameter k=2.4) could be applied. Furthermore, in the runoff analysis considering factors such as land use ratio in the dam basin, it was possible to reproduce the actual inflow volume to the Nomura Dam. By conducting runoff analysis at the dam basin scale using various probabilistic rainfall events and rainfall patterns, it was possible to identify the conditions under which the introduction of rice paddy dams effectively reduces the peak runoff volume.

 Considering the increasing severity and frequency of water disasters due to the impact of climate change, it is necessary to develop a method that can efficiently and accurately evaluate the flood control effects of rice paddy dams, which are one of the watershed flood control measure. In this study, we proposed a new method that can easily consider the operation of drainage from rice fields, and introduced it into the comprehensive flood analysis model. Furthermore, we applied the method to the Karube River Basin in the Takahashi River system, and conducted numerical analysis for the August 2021 heavy rain assuming normal drainage and paddy dam operation. Analysis results clarified the effects of different drainage operations on the inundation status and depth of the paddy fields, and indicated that it was possible to examine the effects on farm management for each paddy field.

 Inundation simulations were carried out using a compound flood analysis model for considering simultaneous inundations, where rainwater flooding and river flooding (overflow flooding) occurred at the same time. The effect of simultaneous inundations on evacuation was investigated using the unit width specific force M as an indicator. The results showed that during the simultaneous inundations, 1) the area exceeding the limit value of M, which is the criterion for safe evacuation of elderly women, adult women/elderly men, and adult men, expands, 2) evacuation places that cannot be evacuated may occur, and 3) it is necessary to start evacuation earlier, and 4) the duration of evacuation at the evacuation places becomes longer. These results indicate that it is important to consider the simultaneous inundations for safer evacuation during heavy rainfall, since the simultaneous inundations make the risk of evacuation increase.

 A questionnaire survey was conducted to understand the actual situation of young school students, and to seek disaster prevention issues, with the aim of implementing watershed flood control and taking long-term measures to create a safe, resilient and sustainable social environment. The study was conducted in the Natsui River watershed three years after the East Japan Typhoon of 2019. The actual conditions were determined from a simple tabulation of the questionnaires. We conducted a systematic analysis of issues based on the actual conditions using a dimension tree. The main results showed that there was a difference in disaster awareness between upstream and downstream areas, and between junior high school students and elementary school students. In addition, the results of the dimension tree showed that the promotion of common awareness discussion as well as disaster prevention education on campus could be effective in raising awareness of flood risk.

 A new tracking method for precipitation cells which is able to detect precipitation cells in Line-shaped rainbands using high-resolution radar (XRAIN) is developed. This method is applied for Line-shaped rainband on September 2015 and quantify the spatiotemporal characteristics of these cells. Results showed that precipitation cells with a lifespan of more than 20 minutes occur approximately every 10 minutes, exist at intervals of about 10 km, and move northward at speeds of 15 to 25 m/s. About 70% of the cells disappear within 20 to 30 minutes, while the rest continue to develop. This study provides a method for quantifying the spatial characteristics of precipitation cells in Line-shaped rainbands, offering statistical insight into their physical structure.

 The heavy rainfall in July of Reiwa 2 caused significant damage in the vicinity of the Chaya settlement in Watari, Kumamoto. The floodwaters exceeded the levees and rapidly flowed along the railway line, resulting in house collapses in the levee area. In Japan, rivers often have dual-section configurations with low channels and high floodplains to accommodate different flow rates during normal and flood conditions, and the Kuma River is no exception. This study aimed to investigate the flow structure and turbulence characteristics in unsteady flow by installing levees in a dual-section meandering channel. Laboratory experiments using stereo PIV measured water depth and flow velocity in steady and unsteady conditions with and without levees. The results showed that without levees, there was little difference in water level, but with levees, water levels dropped before and after surpassing the levees, and the water level increased in the inner part of the levee. Observations of the main flow velocity distribution revealed faster flow in both the low channel and the high floodplain, correlating with the distribution of house collapses in the Chaya settlement caused by the heavy rainfall in July of Reiwa 2. Additionally, turbulence characteristics showed larger values during the reduced water flow period.

 In mountain rivers, we often see three-dimensional vortex motion caused by large rocks and boulders. To solve these flow, a non-hydrostatic quasi-three dimensional model considering flow equations on boundary surfaces is used. But, there are issues with analysis accuracy. So, we focused on its characteristic evaluation method of vertical velocity distribution by cubic function, not considering boundary condition near the riverbed. To improve this problem, we divided the computational area into two layers, and added the additional boundary conditions. The purpose of this model improvement is to solve complex flow in the vicinity of the riverbed more acuurately. At the same time, we noted the inflection point and set as the boundary surface corresponding to the maximum position of the Reynolds stress distribution. We determined the outline of this new model using the three dimensional open-channel flow with rough bed. As a result, it was able to evaluate backwater regions and complex secondary flow.

 The influence of the kernel representation of the history force in the Maxey–Riley equation on the particle behavior in isotropic turbulence was investigated under a wide range of the particle-to-fluid mass density ratio (from 0 to 10000). The Basset kernel based on the classical theory and the Mei–Adrian kernel, which adequately represents the effects of finite-Reynolds numbers and long-time delays, are compared. It is revealed that the Basset kernel overestimates the history effect, but the extent is only slightly, at least under the present investigation. While the history force is negligible for quite-heavy particles, it plays an essential role for slightly-heavy or light particles. The importance of the history force is more remarkable for light particles. An examination of the probability density function of each term of the Maxey–Riley equation has shown that (i) it is the pressure-gradient force for light particles and the Stokes drag for heavy particles that yield particle clustering and (ii) the presence of the history force partly cancels the contribution of the pressure-gradient force for light particles and weakens the contribution of the Stokes drag for not-so-heavy particles, and hence relaxes the non-uniformity in the particle distribution.

 The present study aims to elucidate the basis of momentum transport in partially vegetated open-channel flows by laboratory experiments. For four cases with different vegetation patch widths, PIV measurements were performed to investigate effects of the patch width on the vertical and horizontal vortex structures. Results demonstrate that large-scale horizontal vortex and secondary flow in a non-vegetated region are developed most remarkably in moderately wide vegetation flow. In addition, repeated measurements by shifting the vertical and spanwise measurement positions enabled us to reveal the secondary flow structure and the dominance of vertical vortex in momentum transport around the lateral edge of a vegetation patch.

 The evaluation of the hydrodynamic forces generated by a permeable resistance is important for the proper prediction of tsunamis run up, since they have significant effect on the resistance and attenuation of breaking bores with different Froude numbers (F_r). In this study, basic experiments were conducted to measure temporal variations of wave heights of the breaking bores through a group of cylinders with the different F_r, and results were compared with one-dimensional calculation results based on a theoretically derived hydrodynamic force evaluation method. The larger F_r is, the larger the attenuation of the wave heights and propagation speed of the breaking bores by the permeable resistance. This is considered to be due to a difference in reflection of the breaking bores caused by presence of the resistance. The hydrodynamic force evaluation method used in this study was able to reproduce the longitudinal distribution of the wave height attenuation regardless of the F_r, indicating that conventional evaluation method for uniformly distributed drag coefficients cannot be used to evaluate the attenuation of the wave height and propagation speed regardless of the F_r.

 When a large flood event occurs, the submerged wood are expected to be transported downstream and damage river structures as gates and weirs. Hence, it is required to predict time-dependent distribution of submerged woods and related numerical simulation. However, the transport characteristics of submerged wood are unknown due to the light specific density and directional shape compared to the bed load material. The existing theory of sediment transport cannot cover them. Therefore, this study conducted hydraulic experiments to obtain a fundamental knowledge. An image analysis with color tracking method was applied to time-series of the yaw angle / horizontal position of the submerged wood model and the traveling distance until contact with side walls. Particularly, we considered influences of wood length and root. The longer wood is transferred more stably. The initial yaw angle and root change the motion patterns and the traveling distance.

 This study incorporates a numerical model for driftwood into OpenFOAM, an open-source computational fluid dynamics library that can handle unstructured grids. Simulations for model validation are conducted for experiments of wood accumulation at a bridge pier and a weir. Numerical results well simulated blocking probability at a bridge pier. Additionally, the simulation explained the behavior of driftwood around a weir to some extent, although further model improvements of water-wood interaction are necessary to accurately simulate the wood blocking probability at a weir. The present numerical model allows for driftwood simulation including flow around bridges and weirs with complex shapes, which will be highly practical tool for risk assessment and countermeasure design against driftwood.

 Long-term field data analysis and numerical simulation were conducted to understand recent morphodynamics and particle size distribution characteristics of the Joganji River. Field data confirmed that sedimentation in the upstream and erosion in the downstream, which was confirmed in the early 2000s, has continued in recent years. The grain size distribution has not changed much, and the average grain size changes greatly around 6 k. Numerical analysis revealed that the low channel path is difficult to move even with 1500 m³/s, resulting in a coarser grain size of the riverbed in the upper reaches. However, the transport limit of bed load material in the lower reaches changed greatly with the flow rate. The discharge which strongly affects the grain size distribution was estimated to be approximately 1500 m³/s, not the annual average maximum discharge(700 m³/s).

 Hydropower is important as a CO₂-free energy source that can maintain stable output in the long run. On the other hand, reservoir sedimentation management is one of the most important issues in ensuring the sustainability of hydropower operation. As sedimentation management in hydropower dams, sediment sluicing operation has been introduced in some dams. However, sufficient knowledge for optimizing the operation has not been obtained. In this study, we summarize effects of sediment sluicing operation obtained so far at the Setoishi Dam, which has been conducted the operation in stages. we also study on the impact of the heavy rainfall in July 2020 that occurred during the oparation. In this paper, we discusse the ratio of flood water depths above river bed and spillway crest elevation, which is an index for easily estimating effects of the operation. we also elucidate reservoir sediment transportation due to the operetion including the heavy rain in July 2020 with numerical analysis and field data.

 Remote sensing technique has enabled to depict the turbidity distribution not only at limited observation points but also throughout watersheds. In this study, turbidity maps were processed based on satellite data information (Sentinel-2). They were used to investigate the turbidity distribution in spatiotemporal manner in the Iwaki River basin (Japan). The characteristics of turbidity in each tributary were different in terms of intensity and duration, which were agreed with the rainfall distribution and geological structures. Turbidity distribution was converted into fine sediment discharge through rainfall-runoff model, and we confirmed the continuity of fine sediment discharge before and after the confluence of the tributaries.

 In this study, a rainfall-sediment-runoff model is proposed, in which a hillslope erosion model is incorporated into a river basin sediment transport model consisting of a distributed rainfall-runoff model and a unit channel sediment transport model in. In detail, the hillslope erosion model is constructed by applying the convection equation of suspended sediment flow containing the erosion-rate and deposition-rate terms of suspended sediment to the hillslope area, which formulates the sediment supply process from hillslopes to the unit channel induced by the gully erosion. The sediment transport processes in the Abira River basin were simulated by the model for the rainfall events from July 16 to August 23, 2022. The predictions of the grain size distributions of riverbed materials at different locations, suspended sediment concentration, and its grain size distributions over time are close to the observed results. The comparison of two cases with and without hillslope erosion shows that the fine sediment supplied from hillslopes gradually changes the conditions of bedload sediment flow and suspended sediment flow throughout the river basin as the flooding progresses. This suggests that hillslope sediment supply due to hillslope erosion plays an important role in influencing the long-term sediment transport process in river basins.

 We performed flume experiments of sediment flows containing two-particle-size and investigated the effect of the dimensionless flow depth which is defined the ratio of flow depth to large particle size on concentration of large particles at the sediment flow's front which are characteristic phenomena in mixedsize sediment flows. The experiments were performed under different conditions of the flow rate, channel gradient, and particle size that are related to changes in dimensionless flow depth. It was found that large particles did not concentrate at the flow's front in the range of the dimensionless flow depth roughly less than 1 in the experiment's results. Result of particle's velocity by PIV analysis implied that in the range of the dimensionless flow depth less than or equal to 1, movement in the flow direction of large particles was suppressed due to decrease in buoyancy caused by protrusion of the particles to the water surface. In the range of the dimensionless flow depth roughly greater than 1, tendencies for the concentration of large particles at the flow's front were more pronounced in conditions in which the supply flow rate was higher, the channel gradient was lower, and particle size ratio was bigger when the channel gradient reduced.

 We investigated the effect of the sand component with a diameter on the order of 0.1 mm (about 0.01 times a debris flow depth, which is the major grain-sized component of a debris flow) on the concentration of coarser particles at the flow’s front. As the experimental results, it was suggested that the sand component in the flow’s interior was contributed to grain size segregation due to the "dynamic sieving" (i.e., the component passed through the interstitial spaces between gravels in the flow’s interior and fell into the lower layers) in the same manner as the other grain sized components. It was also suggested that the concentration of coarser particles at the flow’s front could be estimated by using the ratio of flow depth to average grain size, the dimensionless shear rate, and the uniformity coefficient of the debris flow material, regardless of the content ratio of the sand component in the material.

 In this study, we examined a statistical method for easily predicting the debris flow distance. First, a decision tree was constructed using the presence and absence of debris flow as the response variable and the surrounding topography as the explanatory variable at 99 sites in Yamaguchi Prefecture. As a result, the model was able to classify the presence and absence of debris flow with an accuracy rate of 0.850. The best estimation method for the distance of debris flow using this model was applied to estimate the distance of debris flow in the debris flow simulation. Similarly, when the presence and absence of debris flow in the disaster records of 67 sites in Yamaguchi Prefecture was used as the response variable, the model was able to classify the presence and absence of debris flow in the disaster records with an accuracy of 0.808, indicating good estimation on the distance of the debris flow. Our method is considered to estimate the distance of debris flow to the relatively high-risk area compared to the results of debris flow simulation and the debris flow hazard area.

 In Azuma River basin, the bare slopes formed by the 2018 Hokkaido eastern Iburi earthquake may impact the sediment dynamics of the river channel in the long term due to the sediment supply from the surface layer during rainfall. However, the surface layer of the bare slope in this watershed area is a complex mixture of clay and sand, which makes the prediction by the existing method difficult. In order to simulate the sediment dynamics under the characteristics, this study conducted the following four items; erosion experiment on clay-sand mixture surface employing rainfall supplier, two-dimensional riverbed deformation simulation on the experiment, developing the empirical sediment supply equation based on the analysis of the simulation results, and conducting a long-term sediment runoff simulation for the entire watershed by introducing the sediment supply equation. The long-term simulation successfully reproduces the suspended sediment observation in the downstream area. Additionally, the proposed simulation indicated that early countermeasures on the bare slopes effectively reduce sediment discharge on the multi-year scale.

 Improvements were made in the Radiative Transfer Model (RTM), focusing on the dependency of the dielectric behaviour of wet soils on their water retention characteristics. The methodology for estimating vegetation water content was also modified from the one which is used in the JAXA’s standard algorithm. Comparing with the in-situ measurements in Mongolia, Spain, and Australia, the proposed algorithm in this study was shown to have higher accuracy then JAXA’s product under wet conditions. The appropriate consideration of the vertical profile of soil temperature was shown to be necessary for further improvements.

 To quantitatively assess the changes in water risk due to future climate change, it is necessary to employ a water resource model that adequately incorporates the impacts of human activities on the water cycle. Specifically, for estimating water risk at a high resolution in Japan, where advanced water use practices are widespread, it is important to consider water abstraction from areas distant from rivers and from outside the basin. Therefore, in this study, we developed an irrigation network across Japan and incorporated it into the water resource model to quantitatively evaluate the effects of irrigation on water use and the mitigation of water stress. Water aqueducts for irrigation purposes existed in about 16% of the total number of Japanese land grids. In Japan, approximately 28% of the water flowing through the irrigation network was sourced from river water usage for agriculture, The modeled supply shortage also improved from 25.5 billion m³/year to 16.9 billion m³/year for the 1994 average. In regions such as the paddy fields along the coastal areas of the Japan Sea side, significant mitigation of water stress due to irrigation water from outside the river basin was observed.

 Increasing the spatial resolution of global hydrological models is very challenging. Recent studies have shown the effectiveness of adapting a high-resolution model in a local scale, but without addressing an important issue of global hydrological models, that is water is supplied from the same grid as the water demand over the entire grids. This study addresses the issue by comparing the result of two models: one with explicitly incorporating water distribution system, the other with implicitly expressing water canals, and with and without the assumption of water supply of global hydrological models. The result shows that most of the water could be supplied from different water sources with the assumption of global hydrological models that could consequently mislead the assessment of water risks. The result emphasizes the importance of developing high-resolution models to better understand the water balance, but it needs a tremendous effort mainly for collecting data of water infrastructure.

 When coupling AGCM, OGCM and land model, the grid system of AGCM is generally applied to LSM as it is, but in this case, geometrical characteristics or distribution of vegetation are not considered. Catchment-baed grid is suitable for land model, but it has complex shape and difficult to deal with, and there is problems on the conservation in the remapping. In this study, an algorithm of 1st order conservative remapping algorithm for arbitrary spherical grids including catchment-based grid and suggest the methodology to implement the catchment-based grid to land model in the coupled system with aogcm.

 In order improve the accuracy of urban weather simulations using mesoscale models, we constructed high-resolution database of urban geometric parameters for Japan and the U.S. These countries offer detailed building data across all alnd areas. In addition, we undertook a sensitivity analysis of the urban geometric parameters. This involved inputing them into a one-dimensional offline calculation using an urban canopy model. As a result, it became clear that not just aerothermodynamic parameters, but also building morphologicl parameters such as plan area index and average building height significantly impact weather calculations. This result indicate the necessity for a highly accurate database.

 Rooftop albedo data are necessary to improve the accuracy of weather simulations. In this study, the rooftop albedo of building using all possible methods to understand the uncertainty of albedo measurements, including on-site measurements and remote sensing in Tokyo. As a result, the albedo values calculated by each method were different, and the one by the hyperspectral meter, showed a large discrepancy from the other due to the directivity of the solar reflectance. The remote sensing methods using satellite and aerial images include albedo values other than that of the building materials targeted by the field observation due to the influence of image resolution, which leads to the difference between the albedo values.

 This study aims to create a database of turbulent flow statistics within the urban district based on an urban turbulent boundary layer simulation. The inflow turbulence gradually loses its characteristics while progressing and developing the turbulent boundary layer. Turbulence finally becomes well-adjusted to the local surface conditions that are generally represented by massive obstacles in urban districts. This concept allows for the decomposition of a huge computational domain and a reconstruction of the simulation results of the decomposed simulation pieces. We apply this to create a mean flow database in the urban canopy layer of the Tokyo 23 wards. The simulation domain was decomposed in spanwise and streamwise directions. This resulted in the mean wind speed distributions being generally reconstructable after friction velocity normalization without any additional adjustments. On the other hand, if organized turbulent structures develop and stay for significant time above the building canopy, the mean flow structure is disturbed, and the quality of the reconstruction worsen.

 Urbanization causes river environmental degradation and reduction of biodiversity. The purpose of this study is to reveal the relationship between environmental factors and fish community structure of highly urbanized small and medium sized river. The 71 sites surveyed were classified into three groups based on similarity of fish fauna: tidal, non-tidal, and low species diversity. In the non-tidal area, domestic alien species were selected as indicator species, and in the low species diversity area, many non-native loach species (Misgurnus sp (clade B2)) were observed, which may have an impact on native fish species. On the other hand, the number of carnivorous non-native fish species was extremely low, and the degradation of the physical environment of the stream channel due to urbanization is considered to be the most significant factor in the decline in species diversity. The low species diversity was also associated with low velocity and depth diversity, indicating that the urbanization rate and the degree of concrete coverage of the streambed are the main causes of the environmental degradation.

 In this study, we examined the river biotic integrity scores on different tributary habitat types (natural and artificial tributaries) and relationships between the scores and discharge in the tributaries of the Gonokawa River in Japan using quantitative environmental DNA (eDNA) metabarcoding. The analysis results indicate that the biotic integrity scores in the natural tributary streams tended to be higher than th ose in the irrigation channels. In addition, stream discharge was positively related to species richness based biotic integrity scores . This study indicates that quantitative eDNA metabarcoding is an efficient bio monitoring tool, elucidating the biotic integrity of t he tributaries.

 In this study, we used quantitative environmental DNA metabarcoding to estimate basin-wide fish species diversity in the Takatsu River in Japan. We calculated fish diversity indices at each sampling point and used generalized linear model (GLM) to examine the relationships between the fish diversity indices and environmental factors. Using the GLM, we created fish diversity potential maps of the river. Our results showed that water temperature and river slope were correlated with species richness, while the water depth, river discharge, proportion of paddy fields, and water temperature were correlated with the Simpson diversity index. Species richness had a significant positive relationship with water temperature, and the Simpson diversity index had a significant negative correlation with the river discharge. Additionally, we observed higher species richness in the downstream and higher Simpson diversity index in both upstream and downstream areas. These results indicate that it is possible to estimate the basin-wide spatial distribution of fish diversity.

 We evaluated the function of a tributary as Ayu Plecoglossus altivelis altivelis spawning grounds based on surveys of riverbed materials and spatiotemporal surveys on downstream migration and spawning of the target fish using environmental DNA analysis. The study sites were the mainstream and a tributary Yato River in the lower-Gonokawa River system in Shimane Prefecture, western Japan. Compared to the mainstream, the tributary had higher proportions of small-sized riverbed materials and higher penetration values, both of which are suitable riverbed characteristics for the fish spawning grounds. The downstream migration surveys revealed that the fish uses the tributary as a habitat in the summer and then migrate downstream within a short period in the fall. Multiple surveys on spawning revealed that the fish uses both the mainstream and the tributary as its spawning grounds. Furthermore, the magnitudes of the environmental DNA fluxes (environmental DNA concentrations × discharge) were of the same order between the mainstream and the tributary, especially during the late spawning season, indicating that the tributary is functioning as major spawning grounds of the fish in the Gonokawa River system.

 The aims of this study are to propose suitable habitat criteria for extracting ayu (Plecoglossus altivelis) spawning habitat and to develop a general method for extracting suitable spawning habitat formed by flood disturbance using a planar two-dimensional sediment transport model. The method was applied to the alluvial fan section of the Nagara River in mid central Japan, and its effectiveness and issues were examined. A numerical model that evaluates flow resistance for a wide range of flow conditions in a gravel-bed river was subjected to a runoff that causes riverbed disturbance during the ayu spawning period, and then suitable sites were extracted based on the suitable habitat criteria under normal water flow condition. The proposed habitat criteria identify unstable gravel beds by the combination of two conditions: the grain size distribution of the material in the streambed and the shear stress acting during normal flow, it was able to identify more limited set of riffles suitable for ayu spawning habitat than conventional methods.

 When dams and weirs are constructed in rivers, it becomes more difficult for fish to travel upstream, so constrictive fishways are needed. In order for fish to travel up the fishway, they must be attracted to the entrance of the fishway and pass through it quickly without staying in the fishway or escaping. Typically, fish avoid bubbles. In this study, this property was used to increase fish passage rates.The position of generated bubbles on the pool bottom was changed and also the amount of bubbles were changed between 0 to 10 × 10^-3 m³/(m²s). It was found that the best bubble position is diagonally opposite one of dropping flow and also that this effectiveness is increased with an increase of the bubble volume.

 When eels migrate upstream from the ocean to rivers, dams and weirs act as barriers to their migration, and this has a negative impact on the survival of the species. When eels migrate upstream, no significant frictional force is expected to be generated between the eel, which is covered by a mucous membrane, and the bottom surface. Therefore, Onitsuka et al. devised the installation of rectangular strip roughness with no gaps in the transverse direction. However, the upstream corner of the rectangular strip roughness has a gap between the belly of Anguilla japonica and the bottom surface, resulting in a decrease in frictional force. One way to avoid this problem is to use triangular strip roughness. In this study, the spacing of the triangular strip roughness was set to 0.5 times the total length of Anguilla japonica, and the geometrical conditions suitable for the run were sought by varying triangular strip roughness and the slope of the fish ladder. The results showed that under the conditions of this experiment, a triangular strip roughness gradient of 16.7° and a fish ladder gradient of 10° were the most suitable for the 200mm long Anguilla japonica to migrate upstream.

 One of the reasons for the decrease in the Anguilla japonica population is the straying into the intakes of dams and weirs, and a method using avoidance behavior from sound is used as a preventive measure. Studies have been conducted on the effects of sound on the avoidance characteristics of eels, but no studies have been conducted on Anguilla japonica. In this study, the effects of sound pressure levels and frequencies of 91 to 126 dB re 1 μPa and 20 to 300 Hz, respectively, on the avoidance characteristics of Anguilla japonica were investigated in a circular pool of stationary fluid. As a result, Anguilla japonica tended to avoid sounds with frequencies of 150 to 200 Hz at underwater sound pressure levels between 91 and 126 dB re 1 μPa. In addition, the swimming speed of Anguilla japonica tended to increase in the frequency range of 150 to 200 Hz. Therefore, it is possible that Anguilla japonica avoid sounds with frequencies of 150 to 200 Hz near the water intake, preventing Anguilla japonica from straying into the water.

 The purpose of this study is to propose a flood inundation hazard map for each flood inundation site, taking into account the risk of levees erosion potential, in order to support local residents to recognize the risk of their own residential areas and to promote evacuations for the flood disaster mitigation. Therefore, in this study, flood inundation analysis was conducted for the Oyabe, Sho, Jinzu, Joganji, and Kurobe Rivers in Toyama Prefecture, and the flood inundation probability at each grid was calculated. The results of the analysis show that the probability of inundation in the inner area of the levee, the probability of inundation above floor level, and the probability that horizontal evacuation were affected by the erosion potential of the leeves. Also, it was found that the risk of inundation of the Jinzu River is particularly high and there is a high probability of flooding and the need for horizontal evacuation, especially in the lower reaches of the Jinzu River and low elevation areas. It should be emphasized that the right bank of the Jinzu River needs to be given higher priority than other rivers in terms of erosion countermeasures.

 In the gravel bed rivers, levee disasters caused by erosion occur frequently, regardless of over-flow. Particularly in curved sections of the river channel, we observe disasters where erosion of the backside of the low-water revetment leads to levee erosion. This study focuses on the levee eroison occurring on the outer side of the curved sections of the river channel, where the risk of that is high. We reproduced the levee erosion during flooding through large-scale hydraulic experiments, elucidated the process of erosion disaster progression, and verified the effectiveness of countermeasures based on that disaster process.