River Basin Disaster Resilience and Sustainability by all (RBDRS) has been implemented recently in response to water-related disaster that has been more severe and more frequent due to impact of climate change. In order to implement RBDRS really in society, we make clear the importance of RBDRS on flood control plan, and discuss the current issues. From now, it is necessary to develop RBDRS from the stage of consolidation of measures and collaboration to the stage of integration and coordination in order to solve comprehensively social issues such as flood control, urban development and environment conservation and so on. It is important to visualize the goals and roles of flood control and share them among parties involved. For this purpose, we propose the flood control utilizing the hydraulic structure based on the water balance in basin. We discuss the basin flood control plan that is multilayered from small basins such as branch rivers to large basins such as main river, countermeasure management that implements countermeasures of flood control effectively, water-related disaster risk management that is useful for recognizing vulnerability and evaluating effectiveness of countermeasure about water-related disaster.

 In order to realize effective watershed flood control, it is essential to establish evaluation methods for urban planning measures to reduce flood damage. In this study, the consolidation of elementary schools was assumed in the Kakehashi River basin, including Komatsu City, under a population decline, and urban structure changes in 2050 were estimated. We assumed multiple scenarios that differed in the timing and sequence of consolidations, and the number of households relocated each year within the target area. Flood economic loss under urban structure changes were also estimated based on flood analysis results using future extreme rainfall as input, and the impact of different scenarios on flood disaster mitigation were evaluated. Comparing the results with varying numbers of households relocated per year, useful information was obtained for setting the annual target number of households to be relocated in order to obtain sufficient damage reduction in the target area. It has been shown that comparisons by varying the timing and order of integration may be an effective method for selecting advantageous scenario even when the number of households relocated per year is small and sufficient damage mitigation cannot be achieved.

 There are concerns that flood disasters will become more frequent and larger due to climate change. In Japan, river basin flood management is being promoted by combining various countermeasures. In this study, two flood countermeasures were assumed for the Kakehashi River that flows through Komatsu City, Ishikawa, Japan, and their flood damage mitigation effects were evaluated under multiple scenarios. The targets of the evaluation were temporary flood prevention walls to control flooding in the event of a levee breach, and urban planning measures to encourage relocation from areas at high inundation risk to safer areas. Furthermore, future population decline is taken into account when estimating changes in urban structure due to urban planning measures. When temporally flood prevention wall and urban planning measures were implemented independently, the reduction rate in flood damage were 22.2% and 24.8%, respectively. The reduction rate in flood damage when both measures were implemented was 41.2%. Furthermore, when urban planning measures were designed taking into account changes in inundation distribution due to the installation of temporary flood prevention walls, the damage reduction rate increased to 49.0%. This result shows the importance of appropriately coordination of multiple flood control measures.

 In recent years, due to global climate change, the annual frequency of short-term intense rainfall events has increased. In Japan, large-scale heavy rainfall disasters have occurred frequently, exposing the limitations of current river channel planning. At the same time, biodiversity conservation and environmental considerations in river management are becoming increasingly important. In this study, we investigated the effectiveness of traditional Japanese flood control techniques (lateral levee, ring levee, and two-line levee) using floodplain areas, focusing on basin flood control methods in the Katsura River basin, which suffered extensive flood damage during the 2017 heavy rainfall in northern Kyushu. Numerical simulation was used to verify the effectiveness of traditional Japanese flood control techniques using floodplains. The results confirmed that these traditional techniques help to control flood flows, prevent the spread of damage and significantly increase the time available for evacuation. Floodplain flood control is particularly effective as a flood control measure for ecologically important rivers due to the low impact of channel alteration.

 In this study, we visualized the storage functions of each countermeasure on a sub-basin scale for river basins in Toyama Prefecture and evaluated the effects of the basin flood control countermeasures at each target basin. We focused on seven countermeasures: paddy field dam, agriculture pond, school yard storage, storage of existing dams, preliminary release from dam reservoirs, preliminary release from agricultural dams, and the construction of new dams. Our findings indicate that the existing dam, paddy field dam, and school storage will be effective in the target river basins. Additionally, we evaluated the storage of existing dams in combination with the effects of other countermeasures, examining and comparing the effectiveness of these countermeasures at each basin. As a result, we identified effective countermeasures at the Oyabe, Jinzu, and Sho River basins. The method we used in this study does not involve any numerical experiments or detailed simulations, making it faster and easier way to understand the effectiveness of each countermeasure.

 We conducted a numerical simulation of debris flow in Yagi 3-chome, Asaminami-ku, Hiroshima City, where a devastating sediment disaster occurred in July 2014, to examine the effect of houses and other structures on the flow characteristics of debris flow, as well as the spatial distribution of sediment disaster risk in debris flow inundation areas, and proposed a method for utilizing the numerical simulation of debris flow for safe town planning. Based on the state of housing land in 1974, when housing development began to progress in the upstream of the debris flow valley in Yagi 3-chome, we performed a risk assessment using the results of the numerical simulation of debris flow based on the presence or absence of houses and the location of newly constructed houses, and were able to visually grasp the risk areas due to debris flow. Unlike field surveys, this type of assessment can be completed in a relatively short time, and is therefore considered to be a useful tool in the planning stage of sediment disaster mitigation measures in town planning.

 This study aimed to classify irrigation reservoirs contributing to flood damage reduction and evaluate the effectiveness of classified irrigation reservoirs in flood damage reduction in classA river basins and classB river basins. To evaluate the maximum effect of damage reduction in inundation analysis, the situation was defined in which no water was stored in irrigation reservoirs. The damage reduction effect of all reservoirs in Kako (18.6%), Ashida (13.0%) and Matsuura (12.8%) river systems, and Kagawa (18.1%), Okayama (8.6%) and Hyogo (7.4%) prefectures, was high. For 56 river systems and 24 prefectures, flood control use of irrigation reservoirs that reduce damage to sub-watersheds is expected. On the other hand, for 41 river systems and 14 prefectures, flood control use of irrigation reservoirs that do not reduce damage to sub-watersheds is expected.

 Many climate change-aware assessments have been made for the increase in flood discharge due to climate change. The purpose of this study is to understand the future frequency and trend of climate change in the Shingu River system by the difference between the 20 km resolution with a warming projection d4PDF and the 5 km resolution with dynamical downscaling. In this study, we focused on the following three points. (1) the average rainfall in the catchment area of inland rivers and the main river within the catchment area, (2) the total runoff of the entire catchment area, and (3) the simultaneous occurrence of the main river and its tributary inland rivers. As a result, the rainfall of 5 km resolution is generally close to the real rainfall. In addition, the rainfall and calculated discharge of 20 km resolution tend to be underestimated compared to the 5 km resolution when the above three future experimental values are considered. At the least, it suggests that 5km resolution is appropriate when using future projections in this basin, because this underestimates the simultaneous occurrence of the main river flow and heavy rainfall in the tributary basins.

 We assessed the changes in exposed population due to fluvial flood, pluvial flood, and slope failure across Japan from 2015 to 2100, considering climate and population changes under Shared Socioeconomic Pathways. Exposed population due to flood inundation and inland water inundation was defined as the population living in the area where more than 45 cm of inundation occurred. Exposed population to slope failure was defined as the population living in the area where the probability of slope failure exceeded 80%. In all SSPs, the rates of change in the exposed population, including positive and negative rates, were higher for inland flooding, flood inundation, and slope failure, in that order. Especially in SSPs 5-8.5, the exposed population to internal flooding increased significantly. The results of the evaluation of the rate of change in the exposed population due to climate change and population change alone, respectively, indicated that the effect of population change was more dominant than that of climate change on the exposed population.

 The objective of this study was to estimate the effectiveness of planned inundation as a watershed flood control measure in a watershed with a declining population. First, we modified an existing rainfall-runoff-inundation model to allow the height of the levees to be varied by location and time, with each levee model independent of the other. Second, we applied the model to the Abukuma River watershed, which is experiencing a declining population, for three conditions: (1) a natural condition with no levees, (2) the current condition with continuous levees, and (3) a condition with large-scale overflow levees of 1 to 5 km in length at the lowest points at the Shirakawa, Sukagawa and Fukushima basins. The results showed that the installation of the large overflow levees reduced the peak flow by 2-20%, and it also reduced the risk of water level rise due to backwater effect in the tributaries, and that these effects decreased as one moved downstream. We also found that the spatial arrangement of large overflow levees along the main stream and their flood control effects interacted with each other, and hence the relationship between them was not simple.

 Frequency and magnitude of the flood inundation disasters are increasing due to climate change in Toyama Precture, and adaptation measures should be implemented in the river basins. This study presents a cost-benefit analysis of flood damage and various adaptation measures for the Oyabe, Jinzu, and Joganji Rivers in Toyama Prefecture, using detailed 5 km mesh future climate data. Paddy field dams and vegetation management in the rivers are focused on this study because of the green infrastructure aspects and mitigation effects of the climate change. The evaluation results showed that paddy field dams were effective in the target rivers, especially in the Oyabe River, where the cost-benefit was 12.6 at 50% implementation rate and 14.9 at 100% implementation rate. Also, it was confirmed that the vegetation management in the rivers are useful as the counter measures from cost-benefit analysis. However, the general cost of the vegetation cutting should be reduced by selecting the target areas in the rivers and reducing the other related costs of the vegetation management.

 Shiga Prefecture is the only prefecture in Japan that maintains a "Site safety level" which is inundation estimation map considering both internal and external flood. “Site safety level” is the basic information fo r its watershed flood control policy. However, the current integrated hydrologic model does not address th e Lake Biwa floods. This is because the water level fluctuation of Lake Biwa itself due to the outflow fro m the inflow rivers of Lake Biwa and the Seta River weir operation has not been taken into account. In thi s paper, to solve the above problem, a one-level tank model consisting of inflow into and outflow from the lake was constructed and coupled with an inundation model to develop a model linked to the water level of Lake Biwa. The model was used to create a multi-hazard-level “site safety map". Comparison of the res ults with and without consideration showed a tendency for differences in maximum inundation depth and duration of inundation to be generated for the areas around the lakeshore, the inflow rivers and inland lake s that are directly affected by the water level of Lake Biwa.

 In this study, we quantitatively evaluated the runoff suppression effect of stormwater storage facilities contributing to basin-wide flood management and examined optimal operational methods for these facilities. Focusing on Matsue City, Shimane Prefecture, the study proposes a switching storage operation method, adjusting the storage facility's operation based on a threshold of 6.75 mm/hr, equivalent to the drainage capacity of the urban area, and synchronizing with the internal water level during inundation events. The results for the July 2006 heavy rainfall event show a peak reduction of approximately 2.6% in the total inundation volume and 3.7% in the urban inundation volume, with a 5.5% reduction in the area of abovefloor flooding in urban areas. Furthermore, it was found that the "stormwater storage potential" of the storage facilities contributes to the runoff suppression effect. It is inferred that the runoff suppression effect of the storage facilities can be maximized by performing appropriate operations according to the rainfall pattern and scale.

 Inundation in the Akita city center area, caused by the torrential rainfall in July, 2023 is substantiated through the observation data on the rainfall and water level in rivers, field survey data on the maximum level of trace of inundation water and inundation water depth, and analysis of fixed point camera images. As the result, it is clarified that (1) the inundation in the city center area is significantly affected by the inundation water flowing in from the Myoden underpass, that is, the ratio of total discharge passed the Myoden underpass to the total amount of rainfall in the city center area is significantly large, (2) the inundation water outstandingly flows along low-lying streets of which some are waterways in the past, (3) the scale of the inland inundation exceeds that caused by the prospective rainfall of 150 mm/h (once every 1, 000 years or so), announced by Akita city, and (4) the maximum level of trace of inundation water and water depth in the district of Narayama Nobori machi are not a little affected by the tide. Issues to be examined from the viewpoint of flood control in the city center area are also indicated.

 Flooding in urban areas can cause vehicles to float and drift away, causing collision damage to buildings and altering the inundation area by changing the flow of flood waters.

 In this study, an unstructured mesh inundation model and a numerical model that treats vehicles as a single rectangular element were made to develop a prediction method that takes into account the influence of vehicles in the evaluation of inundation areas. This method was applied to a hypothetical inundation analysis in an actual urban area. Calculated results predicted that the vehicle elements transported by the inundation flow would move, influenced by mutual collisions and collisions with elements representing buildings, and that the formation of agglomerations of multiple elements in some parts of the inundated area, which would obstruct the flood flow and cause changes in the spatial distribution of inundation depth.

 This study conducted rainfall-runoff and flood inundation analyses for the Kuma River basin during the July 2020 heavy rainfall event. Rainfall-runoff, flooding-inundation phenomena, and water balance of tributary watersheds were examined. The findings of this study were summarized as follows. (1) The water storage and balance in tributary basins varied depending on the basin characteristics, mainly land use. (2) Inflow from tributary basins, including the Kawabe River, was about the same as the Kuma River. (3) In the Hitoyoshi-Watari section, which was severely damaged due to inundation inflow from the tributary basins was 1,300 m3/s, corresponding to 16 % of the main river channel. (4) Due to the heavy rainfall pattern of this event, the approximate coincidence of the peak flow occurred between the main river and tributaries flowing into the Ichibu-Hitoyoshi water level monitoring section. (5) To mitigate the flood damage, reducing the flow rate at the Hitoyoshi City area by reducing the inflow from the basin or changing the peak flow occurrence between the main river and tributaries through flood control facilities is necessary.

 Heavy rainfall in July 2020 caused flood inundation and significant damage to many buildings in the upstream and middle basin of the Kuma River. There have been a lot of field research on the upstream area of the Kuma River, but very few on the middle area. This study aims to clarify the damage conditions of various type of buildings due to flood flows in the middle area in which the stenosis is located. The Hy2-3D model with a building sub-grid model to evaluate the hydrodynamic forces on each building was applied to the numerical simulation for river and flood flows in this area. The results showed that the velocity and inundation depth for most of the lost buildings exceeded the washout criteria. There was a statistically significant difference in these hydraulic variables between lost and no-lost buildings. These highlight the need to consider both hydraulic and structural factors in assessing building vulnerability.

 For further progress of the river basin disaster resilience and sustainability by all (RBDRSA) projects, it is necessary to develop a numerical simulation model which can assess quantitatively flood risks and various flood-control measures. For this, this study proposed a coupled model with RRI (Rainfall-Runoff-Inundation) model and planar 2D model (RRI-RF2D) for river and flood flow analysis. In order to check the validity of the present model, we conducted flooding analysis in Takasaki River basin due to Typhoon Yun-yeung in 2023. The results indicated that the RRI-RF2D model can reproduce acceptably longitudinal distribution of peak water level and flooding area along the Takasaki River, showing the fundamental validity of the present model. The result also showed that the flooding water was appreciably storaged near road embankments across the river. This effect may have ensured the lead time of flooding at downstream of the road embankment, and it suggests that road embankments may play an important role as one of the flood-control measures.

 The purpose of this study is to classify open levee structures and evaluate the flood control function and its enhancement. This study firstly classified the shape and geological structures of all open levees existed in Class A river system. Secondly, numerical simulations were conducted by changing the riverbed gradient, the ground gradient, the angle of the second levees, the width of the unbanked section and the ground elevation determined from the existing range of the parameters. Finally, the effectiveness of an overflowing levee at the unbanked section was simulated. From the simulated maximum water levels at the downstream of the open levee, the decrease in the maximum water levels were demonstrated by the open levee and overflowing levee. Due to delay in the timing of the start of flood storage by the overflowing levee, the maximum water levels at the downstream of the open levee becomes lower than that without overflowing levee. In addition, overflowing levee reduces the inundated area in between the river levee and secondary levee. Therefore, rather than closing the open levee, construction of the overflowing levee at the unbanked section is expected to improve flood control safety at the downstream river and in the open levee area where flood inundation occurs on occasion.

 In summer, hypoxia becomes a pressing issue at the sea bottom of the inner Tokyo Bay, causing significant damage to marine resources and the ecosystem. Our previous study uncovered an interesting phenomenon: the dissolved oxygen (DO) recovered rapidly when southwest winds of more than 10 m/s occurred over Tokyo Bay. However, the lack of a quantitative evaluation of this critical wind speed has been challenging. This study, therefore, introduces a novel approach by proposing a new estimation equation for the occurrence of DO recovery, taking into account the influence of wind energy. Our analysis of DO recovery at the sea bottom of the inner bay from 2010 to 2011 led us to develop a unique method for estimating DO recovery rates using wind energy based on the Wedderburn number. We confirmed that a sustained southwest wind of more than 6 m/s for over 20 hours was sufficient to induce DO recovery. Furthermore, our numerical simulations provided additional confirmation of the DO recovery at the sea bottom.

 In order to clarify the characteristics of long-term changes of water temperature in the inner part of the Ariake Sea due to climate change, time series data of water temperature and atmospheric temperature observed from 1973 to 2023 were analyzed. Representative values for each year were calculated by applying an appropriate periodic function to their temperature data. Changes of representative values indicated that the water temperature in the inner part of the Ariake Sea has been increasing in the long term. In addition, there will be a change in the influence from river water since the rate of increase in water temperature are different between coastal zone areas and central parts of the sea. Therefore, the long-term changes of a river water temperature and its river discharge were examined. The results showed that increases of the atmospheric and the river water temperature due to the global warming have led to further increase in coastal water temperatures, and the spatial water temperature distribution in the inner part of the sea has recently become more uniform.

 The behavior of hypoxia in a shallow lake is strongly influenced by wind conditions. This research conducted a 3D hydrodynamic and water quality simulation in Lake Suwa, Japan, to analyze the impact of wind condition setting on hypoxia based on two cases: (1) uniform wind conditions derived from observations at AMeDAS Suwa observatory and (2) non-uniform wind conditions calculated by an atmospheric model. A comparison between the cases showed that considering the spatial distribution of wind enhances horizontal circulation and thermal stratification in the lake. In the southwestern part of the lake, the velocity in Case 1 tended to be larger than that in Case 2 during strong winds, suggesting that the bottom velocity in Case 1 would be overestimated. Additionally, the non-uniform wind increased the frequency and scale of hypoxia compared to the result under uniform wind conditions throughout the calculation period.The result highlights the importance of detailed representation of the wind field in understanding and predicting the behavior of hypoxia in shallow lakes.

 In this study, we examined whether or not agricultural reservoirs remaining in satoyama near cities are suitable as CO₂ sinks from the viewpoint of CO₂ absorption characteristics of the reservoirs by using field surveys and numerical simulations. The results of the field survey indicated that the target reservoirsufficient nutrients necessary for the growth of waterweeds. The ΔDIC, which indicates the degree of photosynthetic activity, showed a large difference in trend between July and October (stratified) based on the relationship between chl.a. Also, The results showed that the target reservoir had sufficient nutrients necessary for the growth of waterweeds. In addition, to understand the carbon dynamics, we verified the validity of the numerical calculations and performed numerical calculations of dissolved inorganic carbon (DIC). The results showed that photosynthesis by algae is active under mild climatic conditions and that CO₂ is absorbed.

 The Karasuhara Reservoir, where abundant phytoplankton proliferate, is a typical eutrophic lake. In mesotrophic lakes, previous studies have shown that aquatic plants utilize photosynthesis to absorb CO₂ from the atmosphere. However, research on CO₂ in eutrophic lakes remains unsolved, particularly regarding the long-term emission and absorption of CO₂. Dissolved inorganic carbon (DIC), which determines the CO₂ flux at the water surface, varies depending on the water quality environment, thus necessitating an understanding of its controlling factors. Therefore, observational data from 2010 to 2018 were used to investigate correlation and attribution (Structural Equation Modeling analysis). Furthermore, a box model was proposed to analyze the factors governing DIC. As a result, DIC's internal production is significantly influenced by a decrease due to photosynthesis and an increase due to the mineralization of dissolved organic carbon.

 We investigated the estimation mehod for mixing depth from the viewpoint of phytoplankton habitat by using water temperature and dissolved oxygen (DO). An automatic water temperature and water quality monitoring system was installed in Yamashiro Pond on Kume Island, Okinawa Prefecture, to measure the spatiotemporal detailed vertical distributions of water temperature and DO. The data obtained every two hours at a resolution of 1 cm depth were used to determine the depth above a certain Thorpe length (L_T) (zm(L_T)) and the depth within a certain difference range of DO relative to the surface (zm(ΔDO)), respectively, as mixed water depths. The relationship between zm(ΔDO) to zm(L_T) was analysed for a period when convection due to surface cooling was the dominant external mixing force. DO difference values, 2-5% had the highest correlation with zm(L_T). The versatility of zm(ΔDO) and its applicability to dam water quality protection as well as future issues are discussed.

 In this study, we investigated the characteristics of vertical distribution of phosphorus in five dam reservoirs in western Japan, under the anoxic condition of bottom waters. As a result, a vertical distribution pattern was sometimes confirmed in which the phosphorus in the bottom layer was relatively smaller than that in the middle layer in all reservoirs. In addition, the observational results in the Tsuruda Dam reservoir suggested that such vertical distribution might be triggered by the elution of phosphorus and Fe²⁺ from the sediment and their resuspension near the bottom. It was also confirmed that released phosphorus from bottom sediments was rarely observed in the Tsuruta Dam reservoir. The main factors behind these results were considered to be due to the relatively high supply of nitrate from upstream and the low amount of labile organic matter in the sedimentary mud, which made it easier to maintain a relatively oxidative environment in the bottom layer and could suppress and delay the phosphorus release from sediment.

 It is important to understand long-term trends in vegetation in semi-arid regions that are extremely sensitive to climate change. In this study, we analyzed spatiotemporal and long-term trends of vegetation, precipitation, and land surface temperature in the Mongolian plateau using satellite observation data. The results showed that the long-term trends of vegetation and precipitation in the target area were increasing, and their characteristics differed by latitudinal zone, elevation, and season. The results also suggest that the vegetation distribution period is shortening around 46°N, but the vegetation is expanding to the south in July and August. Furthermore, surface temperature and air temperature were positively affected by global warming and negatively affected by precipitation and increased vegetation, suggesting that the north-south difference in surface temperature and air temperature in the target area of this study increased with the increase in vegetation and precipitation, with a boundary of about 44°N. These findings highlight the complex interactions between climate change and environmental variables in semi-arid regions, emphasizing the importance of considering both spatial and temporal variations in environmental trend analyses.

 The AMSR satellite microwave radiometer provides single-value soil moisture estimates, but it doesn't show how soil moisture is distributed within its footprint (FP), which could be useful information for users. To address this, we created a soil moisture map with a 100-meter resolution using Sentinel-1 SAR data. We then analyzed how the soil moisture distribution within the FP relates to the AMSR soil moisture estimates.

 Our study found two key points: 1. When the AMSR soil moisture estimate is below 10%, the variation in soil moisture within the FP is small. The difference between the 75th and 25th percentiles is generally less than 4%. 2. When the AMSR soil moisture estimate is 10% or higher, the soil moisture distribution within the FP shows diverse patterns, making it hard to explain the variation uniformly.

 Soil moisture products from AMSR2 and SMOS were validated against in-situ observations for 8.5 years from 2012 to 2020. The error characteristics were analyzed focusing on their relations with soil type and vegetation amount. The correlation under sparsely vegetated conditions remains low at 0.2-0.6, suggesting the importance of improving the radiative transfer model for soil, in addition to that for vegetation.

 While ensemble weather forecasting has been increasingly used in flood forecasting, there are few studies of applied to landslide prediction. In this study, the soil water index was calculated using 200 members rainfall forecasts to predict the risk of landslides for the line-shaped rainband that hit the Kyushu region in July 2020 heavy rainfall. The spatial distribution of the soil water index was visualized, and variations in values and locations were classified and confirmed. The soil water index was calculated at five different points of 1 km² in Ashikita City, where many landslides have occurred. As a result, 19% of the 200 members exceeded the alert level 4, which is equivalent to the evacuation order, a small value compared to the flood forecast where 56% of the members exceeded the previous maximum value. For the prediction of landslides, which are localized phenomena, the results suggest that landslide forecasting does not overlook low probability but high-risk values and takes into account the uncertainty in the location of the rainfall forecast.

 In this study, the pore structure and flow/transport characteristics of dolomite were evaluated by combining the analysis of CT images and numerical modeling. The histogram of the volume of the pore clusters indicated that the relatively smaller clusters are more abundant. In addition, the result of the box-counting method revealed that the pore structure is fractal in 2.76 dimensions. Moreover, by computing semivariograms, it was found that the pore structure is spatially correlated. The variogram data were also fitted to three representative models (Gaussian, exponential, and spherical models). As a result, compared to Gaussian and spherical models, exponential model provided excellent fitting results in terms of the determination coefficient and root mean square error. Numerical modeling of the flow in the constructed pore model showed that the flow velocity field inside the pore exhibits heterogeneity and that dead-end pores exist. The breakthrough curves (BTCs) at the downstream surface obtained from transport modeling exhibited significant tailing as the Peclet number, which indicates the dominance of advection, increased. This was suggested to be due to the effect of dead-end pores.

 The Shimabara Peninsula in Nagasaki Prefecture has been facing a significant issue of groundwater contamination due to nitrate-nitrogen. In response, the prefecture launched the Shimabara Peninsula Nitrogen Loading Reduction Plan in October 2006 and has been implementing various measures to address this problem. This study utilized The MannKendall test to analyze the trend of change in nitrate-nitrogen concentrations based on data disclosed by the prefectural government and water source wells measured by the water-works department of Shimabara City. According to the data collected by the prefecture, it can be said that nitrate-nitrogen pollution in the last eight years has not shown any improvement trend. The factors that led to the reduction of nitrate-nitrogen concentrations in earlier years should be carefully examined. The concentration trends in the source well data of Shimabara City indicate that the situation is not improving and is approaching a serious situation.

 In this study, two types of flume experiments were performed to investigate the momentum transport mechanisms and sediment deposition in the steady wake region behind a submerged vegetation patch. First, PIV measurements for vertical and horizontal planes were utilized to reveal the turbulence and secondary flow structures. Results of the velocity measurements show that the streamwise vegetation patch length affects the development processes of the vertical and horizontal mixing layers and the secondary flow structure. Second, the observation of sediment deposition indicated that fine sediment particles are stably deposited in the steady wake region where both the flow velocity and the turbulence intensity are small.

 After river bridges are constructed, their structures deteriorate over time, and the conditions of the associated river channels change. Consequently, older bridges have suffered damage during recent heavy rainfall events. Numerous studies have been conducted on bridge scour, primarily focusing on flow patterns around the piers and scour depth. However, studies that consider bed topographical features are rare, except for those involving real river scenarios. Therefore, this study aims to qualitatively investigate the phenomenon of pier scour on sandbars using mobile bed flume experiments.

 Specifically, unsteady flow conditions were established to allow for the formation of alternate bars, and the scour phenomena of elliptical piers were investigated. A 360-degree camera was installed inside the pier to monitor changes in the scour bed height during flow. The results revealed variations in scour depth based on the characteristics of the alternate bar deposition and scour zones, with the maximum scour occurring near the walls in the sandbar scour zones. Furthermore, it was observed that the scoured bed elevation was not replenished during the transition from deposition zones to scour zones.

 Local scouring that occurs downstream of a groundsill may affect the stability of the structure. Therefore, prediction of flows and local scouring downstream of such a structure is of great importance in river engineering. In this study, numerical simulation of local scouring downstream of a groundsill is carried out by using a three-dimensional flow model that can predict simultaneously both surface and seepage flows in a generalized curvilinear coordinate system and an equilibrium sediment model. Although the velocity distribution near the riverbed and the location of the dunes behind the scour during the submerged jet is not simulated well, it is shown that the model reproduces the longitudinal distribution of riverbed and water level, vertical velocity distribution during the submerged jet and the backfilling process, and the temporal variation of the maximum scour depth reasonably. In addition, the influence of seepage flows on local scouring is also discussed.

 Secondary flow of the second kind is ubiquitous in real rivers and is considered to coexist with secondary flow of the first kind caused by centrifugal forces. Therefore, Secondary flow of the second kind may have a significant influence on sediment transport patterns, including scouring near riverbanks. However, the importance of Secondary flow of the second kind has not been paid much attention in actual rivers, and they have been neglected. In this study, in order to clarify the importance and role of secondary flow, numerical experiments were conducted in a rough open channel turbulent flow field with a small aspect ratio, where secondary flow is dominant. The results indicate that the secondary flow velocity in a rough-surface open channel is much larger than that in a smooth-surface open channel, and that the motion of particles is greatly influenced by the instantaneous flow velocity.

 Because the bank failure can expand the damage of hinterland rapidly, it is important to develop an embankment with high resilience. The bank failure is generally induced by the scouring; however, previous experimental researches have shown that the scouring can be restrained by laying gravel on a sand layer. However, this fact has not been examined using a numerical simulation. Therefore, in this study, we develop a numerical model which can simulate a scouring process of sand and gravel layer. In the present model, the motion of gravel is tracked using DEM, and the deformation of sand layer is solved using elastoplastic model. In the manuscript, the scouring of both sand layer and sand and gravel layers due to vertical jet is simulated, respectively, and both calculated results are compared. From the comparison regarding the scour depth, it is shown that the present model can provide the same result as the experimental one that the scouring can be restrained in the case of sand and gravel layers.

 In this study, numerical experiments on mixed-size sediment flows were performed by using a numerical simulation of three-dimensional coupled motions of solids and fluids to analyze the factors that have an effect on the vertical segregation associated with the flow. The segregation velocity of each particle size was defined as the difference from the mean velocity in the vertical direction and this index made it possible to consider the progress of segregation quantitatively. The coefficient of variation of particle-size, which is the ratio of the standard deviation of the particle size to the mean particle size, was also obtained as an indicator of the particle size distribution. The effects of the velocity gradient and particle size distribution on the segregation process were analyzed using these parameters. The analysis results show that the larger the coefficient of variation of the particle sizes, the more pronounced the segregation velocity as the velocity gradient increases.

 In recent years, the river structures have been damaged due to the huge floods. 3D flow field around the river structure affects on the sediment transport process (bedload and suspended sediments) directly, which makes strong unsteady and non-equilibrium characteristics. In addition, the suspended sediments also affect on the vortex structure, and it is important to consider the effect of interaction force between fluid and sediment. In this study, a solid-liquid multiphase turbulent flow model (GAL-LES model) is applied to a vertical jet scour experiment to clarify the effect of suspended sediment on the vortex structure. The results showed the applicability of the GAL-LES model to local flow through comparisons of lateral scour shape and maximum scour depth. Furthermore, the results indicated that the large vortex structure has been weaken due to suspended sediments, suggesting that the interaction between sediment transport and vortex structure appears appreciably.

 In the Ichinomiya River basin in Chiba Prefecture, a wide range of flooding occurred due to typhoon Yun-yeung on September 8, 2023. To assess the effectiveness of ongoing flood control measures, we examined the situations of the river flow and the inundation based on the observed data and did the numerical simulation with and without the flood control measures. By comparing the observed data, the rainfall amount in 2023(R5) is higher than one in 2019(R1), but the river water level and the inundation volume of R5 flood are less than those of R1 flood. The simulated results showed that the flood measures are effective in reducing flood damage in R5 flood. These results quantitatively demonstrate the effectiveness of recent flood control efforts in mitigating the R5 flood's impact to the R1 flood.

 A linear rain band occurred due to Typhoon No. 7 on August 15, 2023, in Tottori Prefecture, and damage due to riverbank erosion occurred in the Saji River in the Sendai River basin. On the other hand, a village was not inundated from damage despite concerns about inundation due to overflow. It is possible that the wide valley bottom at the upstream curved part exhibited a water and sand storage effect, which may have prevented inundating damage in the village downstream. In this paper, we considered the wide valley bottom of a rapid river in a mountainous area as a local widening part of the river channel, examined the impact of water storage and sand deposition in the local widening part of downstream, and conducted a two-dimensional riverbed variation analysis to confirm the water storage and sand storage effect of the local widening part. The results showed that the local widening part in the mountainous area has the effect of depositing sediment of a wide range of grain sizes, including large boulders, reducing the amount of sediment discharged downstream and suppressing the rise in water level.

 In recent years, the frequency and amount of heavy rainfall have increased, and the risk of rainwater (internal) inundation has been increasing in urban areas where are less infiltrated areas. In this study, a rainwater inundation simulation was conducted for the eastern part of the urban area in Takamatsu City, Kagawa, Japan, by considering sewerage, drainage pumps and buildings. The results showed that the inundation depth and the flow velocity increased when buildings were considered. In addition, the results also showed that the condition, which was not taking buildings into account, underestimates the risk. It was suggested that the risk of rainwater inundation could be predicted in advance by checking the fullness of sewage pipes.

 We developed a flood simulation model capable of evaluating the flood control function of paddy field dams and proposed a method for mathematically exploring the optimal placement of these dams using mathematical programming. The flood simulation model employs an unstructured grid flood simulation model with added functionality for paddy field dams. Genetic algorithms were used to search for the optimal placement of the dams, with their on(1)/off(0) status represented as a gene sequence in the algorithm. The model was applied to a specific area in the Takatoki River basin, which includes an open levee and multiple paddy fields at the confluence of rivers. The results demonstrate that the genetic algorithm enables an efficient search for configurations that reduce water levels compared to exhaustive search approaches. For rainfall events that do not exceed the storage capacity of the paddy fields, the reduction in water levels due to paddy field dam placement is low, suggesting that the effects can be expected regardless of the placement. On the other hand, for rainfall events that exceed the storage capacity of the paddy fields, the reduction in water levels due to paddy field dam placement becomes significant, indicating the importance of considering the placement of the paddy field dams.

 In this study, we developed a new inexpensive smart drainage gate to replace existing smart paddy field dam equipment. An analytical model of the Makigawa basin in Kahoku-cho, Yamagata Prefecture, was constructed to evaluate the flood mitigation effects by the new smart paddy field dam equipment, and inundation simulations were conducted with the conventional and the new smart paddy field dam under multiple rainfall scenarios. The results showed that the flooded area and flooded volume reduction rate by the new smart paddy field dam were greater than those of a conventional paddy field dam under all rainfall scenarios. It was shown that, in watershed where sluiceway is fully closed to prevent the inflow of river water during heavy rain, the smart paddy field dam, which completely store rainwater up to the standard drainage water level, may have a higher flood mitigation effect than the conventional paddy field dam that store rainwater while allowing run-off.

 The 2D shallow water equation (SWE) is being used in inundation analysis in underground spaces. However, inflow through staircases is estimated by overflow equations and this method ignores the momentum generated by the flow at the staircase. In this study, the results of a staircase flow using the 3D Reynolds-averaged Navier-Stokes (RANS) equation and the 2D SWE were compared, and a formula was created for estimating Manning's roughness coefficient that can correctly reproduce the energy loss of 3D RANS analysis. Using the formula, a flooding analysis was performed in a modelled subway station using the 2D SWE, and an obtained distribution of walking difficulty generally agreed with the one by 3D RANS. It was also found that the 2D SWE model in which the inflow was directly added to the underground space underestimated the walking difficulty and dangerous areas might be overlooked.

 This study applied a flood forecast system to Kyoto Prefecture and evaluated the accuracy for past four flood events. The flood forecast system is based on the RRI model, that is forced by a short-term rainfall forecast produced by JMA. The questionnaire survey showed that municipal governments required flood forecast accuracy for alert level 3 for evacuation and evacuation leading time from 1 to 3 hours. This study shows that the accuracy against alert level 3 for 1-hour, 2-hour and 3-hour lead time, based on 42 gauged stations for 4 flood events, is 0.84, 0.73 and 0.63 respectively. In addition, this study demonstrates that the accuracy of a flood forecast for a small catchment, which is smaller than 100 km2, is 0.78 using evaluation criteria that includes 1-hour lead time. In conclusion, this study shows that the flood forecast has a high potential to predict a water level for a small catchment.

 The increase in heavy rains due to global warming in recent years is causing floods in various places. In Hyogo Prefecture, we operate a real-time flood prediction system for the entire Hyogo prefecture, with the aim of predicting water levels and flooding up to 6 hours in advance, taking into account flooding in small and medium-sized rivers. In this study, with the aim of further improving accuracy, we estimate a background error covariance matrix that takes into account the decline in correlation using four indicators such as river channel distance and straight line distance, and use an optimal interpolation method as a surface water level correction method. Water level data was assimilated. Cross-validation of prediction calculations was performed using the assimilated water level data as the initial calculation value. As a result of verifying the observation points by dividing them into assimilation points and verification points, it was confirmed that the accuracy had improved at the assimilation points and that the accuracy had not decreased at the verification points. It was applied to a real-time flood prediction system.

 We investigated the feasibility of using a flood prediction method that takes into account the runoff and downstream process of rainfall from Today's Earth-Japan to make decisions about evacuating trains parked in a rail depot before flooding occurs. In this study, we verified the accuracy of water levels at water level observation stations near rail depots when using Today's Earth-Japan for 111 past flooding cases. The results showed good capture rates and oversight rates. In terms of lead time, a lead time of more than one day was secured at more than 90% of locations during Typhoon Hagibis in 2019, when flooding occurred.

 Although an early warning system is one of the most effective tools for flood disaster prevention, it is difficult to provide the accurate information predicted due to the uncertainties involved in flood forecasting. In this study, a forecasting experiment was conducted for the Hiwasa River to clarify the forecast uncertainty and accuracy. The results showed that the underprediction of floods was caused by rainfall forecasts, suggesting the effectiveness of wide-area flood forecasting. The forecast uncertainty increased with forecast time. The uncertainty up to three hours ahead was smaller for the typhoons than for the non-typhoons. Furthermore, while the lead time for evacuation is expected to be one hour and thirty minutes, the accuracy rate is 35.7%, which is not high even one hour ahead. Therefore, in order to improve the information literacy for flood forecasting, it is important to provide the forecast information by wide-area and to understand the uncertainties involved in flood forecasting.

 Waterfalls often migrate with formation of upstream cyclic steps. In this study, we hypothesized that the face angle and drop height of waterfall would affect the flow field of upstream channel and alter waterfall migration mechanism. We conducted laboratory experiments with erodible mortar beds to investigate migration rate, flow filed and bedload saltation trajectory from PIV and PTV analyses. Our experimental results suggest that (ⅰ) the face angle changes gradually with erosion, and when the angle exceeds a certain angle, no step is formed upstream, (ⅱ) since the final face angle is determined by the drop height, the final migration rate is dependent on the drop height, (ⅲ) as the face angle becomes steeper, the location of the increase in particle impact velocity moves away from the waterfall lip and the step formation interval increases, (ⅳ) as the drop height increases, the collision frequency decreases due to the increase in saltation distance, making it harder for steps to form.