In the Gosegawa and Shingosegawa rivers in Aichi prefecture, environmental groynes are arranged in parallel and in a staggered manner in the river channel to create various flow fields. In this study, we experimentally analyzed the three-dimensional turbulent structure and momentum transport characteristics of an open channel flow with discontinuous vegetation communities in various arrangements in the flow direction, targeting overflow conditions. As a result, it was found that the secondary flow characteristics changed in the flow direction due to the influence of having a non-vegetation area in the flow direction, and the flow was greatly advected at the overflow point. In addition, in staggered and parallel arrangements, it was clarified that momentum transport by advection is superior to turbulence, suggesting that the momentum transport characteristics around the vegetation zone affect the magnitude relationship of the flow resistance characteristics.

 Three-dimensional numerical experiment of turbulent flow over narrow (b/h=1.41) open channel rough surface with roughness elements laid on the bottom were conducted to investigate changes in the flow structure and turbulence structure when suspended loads are added. Main velocity and secondary flow velocity profiles were distorted in the vicinity of the densely arranged roughness elements, but the longitudinal changes were small at higher locations. The secondary flow cells formed due to the imbalance between the Reynolds stress gradient and the pressure gradient overlap with the flow over the roughness elements and the flow that dips into the roughness valley, resulting in a longitudinal change in the secondary flow velocity distribution and a larger secondary flow velocity magnitude than in an open channel smooth surface turbulent field. The presence of suspended particles suppresses the transport of the vertical Reynolds stress due to velocity fluctuations at the bottom corner, which results in a reduction in the magnitude of the secondary flow velocity.

 In this study, the characteristics of the flow field associated with the suction operation in a compound open channel with a suction operation at the tip of flood plain are investigated by measuring the flow velocity and visualization. The fluorescent dye injection method was used for flow visualization, and PTV was used for flow velocity measurement. From the flow visualization, it was found that the spatiotemporal concentration at the tip of the flood plain of the longitudinal vortex structure was changed to intermittent formation by the suction operation. In the average flow velocity distribution, it was clarified that the oblique upward flow disappeared by the suction operation, and the swirling flow paired in the clockwise direction and the unti-clockwise direction also disappeared. In the turbulence intensity distribution, it was found that the value with suction operation was lower than that with no suction operation. In the Reynolds stress distribution, it was clarified that the phase of the distribution is reversed depending on the presence or absence of suction operation. It was speculated that the intermittentness of this longitudinal vortex structure was the cause of the change in the flow field.

 River bridge were mainly damaged by local scour around bridge piers due to flood flow. However, the flow structure and local scour around piers in flood condition remain unknown. In this study, we developed a new flow and riverbed monitoring system around a bridge pier based on a radio-controlled boat with an ADCP and a position identification method. We applied the present system to the field measurement at the Noda Bridge in the Edo River. The result indicated that the present system can efficiently identify the ADCP position and measure the riverbed height and complex three-dimensional velocity distribution around the pier.

 This study is a hydraulic model experiment aimed at improving the economically advantageous reduction system and compacting the reduction section as a reduction gear for the spillway of a hydroelectric power plant. The return-flow type underwater reducer consists of a perforated deflector and endwalls that reduce momentum through complex interactions of flows. This method is mainly applied to underwater force reduction of dam outflow facilities, and has the advantages of less noise and fog generation due to reservoir outflow, but has disadvantages in construction and maintenance of deflectors due to the narrow spacing of the holes. This study conducted hydraulic model experiments on deflectors (slit case, etc.) and compacting force-reducing sections to understand (1) the maximum water level, (2) water level fluctuation, and (3) velocity vectors by image analysis. As a result, this study understands the flow structure and momentum reduction effect, and proposes an improvement system in terms of constructability, cost reduction, etc.

 For free jumps with undeveloped, partially developed, and fully developed inflow conditions (abbreviated UD, PD, and FD jumps) in horizontal rectangular channels, time-averaged velocity distributions were obtained experimentally under a given inflow Froude number and Reynolds number. For the FD jump, the jet entering the jump is lifted upwards and diffuses shortly in comparison with those for the PD and UD jumps. Shear stresses on a channel bed were indirectly measured, and skin friction coefficients for the UD, PD, and FD jumps were shown. The comparison of the present data revealed effects of boundary layer development of supercritical inflows on velocity characteristics in the jumps.

 There is a possibility that a gate which allows the simultaneous flow under and over sluice gate can keep the advantages of these individual flow systems with reducing the weekpoints of them, which imagines the perspective from the view point of water management. But it seems that there is no satisfactory theoretical study which enables the design for this type of structure. The present paper proposes the method of calculating the total discharge as the root of a quadratic equation employing the momentum theorem and the equation for discharge over a weir. The calculation method proposed in this study was verified using the previous experimental data, and good agreement was obtained. Thus the validity of the proposed method is implied.

 Internal waves play a significant role in mass transport, suggesting one of the essential factors controlling the water environment in an enclosed basin. The recent studies demonstrated the importance and effect of a "breather" on flow in a three-layer fluid; e.g. breathers induce resuspension and mass transport due to the wave breaking over a uniform slope. Thus, we aim to investigate oblique breather interactions in a three-layer system to clarify the characteristics of breathers. As a result, a Mach stem was found to occur due to the two oblique breather interactions, which may be associated with a critical angle.

 One of the levee strengthening construction methods that the Ministry of Land, Infrastructure, Transport and Tourism is studying is the surface protected type, which is a construction method that protects the surface of the levee body with a covering material. On the back slope of the embankment, the structure is protected with an erosion prevention sheets, a block, etc., but in order to study the stability of the block, the flow on the back slope at the time of overflow is an supercritical flow, and there is a problem that it is difficult to evaluate the fluid force acting on the block.Therefore, in this study, we changed the shape of the surface protrusion of the block using a small-scale model and measured the fluid force acting on the block by overflow water. As a result, it was confirmed that by providing an inclination in the surface protrusion or reducing the protrusion height, the drag acting on the block is reduced, but the lift force is conversely increased. In addition, using the coefficient obtained from the conventional hydraulic property test, it was shown that the fluid force acting on the block can be larger than the fluid force obtained from the calculation formula of the mechanical design method of the river revetment.

 In recent years, levee-breach due to overflow have occurred frequency. As a countermeasure, overflow-resistant levee are required. It is important to ensure the stability of the back slope protection works against overflow, which is one of the levee reinforcement against overflow. However, the hydrodynamic forces characteristics acting on the protection works in the overflow condition have not clarified. In this study, model experiment were conducted to investigate the relationship between the shape of the protection works and the hydrodynamic forces in the overflow condition, and the vibration characteristics of the hydrodunamic forces in relation to the unevenness.The results show that the flat blocks, the hydrodynamic forces increases significantle due to the unevenness. The large roughness of the protection works does not increase the hydrodynamic forces due to unevenness. However, eve if the absence if landsides, the hydrodynamic forces were found to vibrate signigificantly. In the future, it will be necessary to consider the introduction of an appropriate safety factor for stability evaluation and maintenance to achieve the specified performance.

 In this study, model tests were conducted focusing on the amount of water leakage inside the river levee in relation to the height of the external water level, and a quantitative understanding of the risk of piping failure was attempted based on the relationship between the amount of water leakage and the degree of piping development after correcting for differences in foundation soil characteristics and water level conditions. As a result, it was found that there is a critical range of cumulative leakage from the start of leakage to the piping failure. Furthermore, we introduced the concept of a piping failure curve (a fatigue curve of levee strength against the time of action of the external water level) and presented the basis for exceptions to this concept. The future goal is to clarify the existence of the leakage limit value that causes piping failure and to establish it as a quantitative index that can be applied to actual levees.

 Many levees of small and medium rivers breached due to heavy rain in July 2018 and typhoon No. 19 in October 2019, and eventually enormous inundation damage occurred. In order to strengthen the embankment and lead to early evacuation in small and medium rivers, it is necessary to improve the accuracy of flood inundation analysis and predict the weak points of the embankment in advance. Therefore, in this study, validity of the analysis was examined by a flood simulation when upstream discharge and downstream water level were not available with respect to the Suna river in Okayama city, which was severely inundated due to the bank break. Furthermore, a simple bank breach prediction method was used to evaluate the bank break risk, and the effect of the bank height data on the flood inundation analysis results was examined numerically. As a result of comparing the numerically obtained inundation depth and the observed data, it was shown that the inundation simulation can be performed accurately by the boundary condition setting method proposed in this study. It was clarified that the weak points of the embankment can be extracted by using a simple embankment breach prediction method.

 In Niimi city, debris flow and inland flooding occurred on September 3, 2019 because of 100 mm/h torrential rains. To perform reproducible calculations for such compound disasters, it is important to establish a highly accurate simulation model that combines debris flows and inundation flows. For this study, reproduction simulations for the disaster were conducted using a model that combines a two-dimensional debris flow model and an inundation flow model using one-way coupling method. Analysis results showed that sediment accumulation around the drainage channel not only reduced the drainage flow rate; it also strongly affected inundation water flow conditions, suggesting that sediment accumulation might have increased the local inundation depth.

 The July 2020 torrential rains caused extensive flood damage in the Hitoyoshi and Kuma basins. In the Watari area of Kuma Village, Kuma County, in particular, many houses were damaged by flooding in addition to inundation. In this study, the behavior of downstream flooding, in which the overflowing stream flows together with the river, in the Watari area of Kuma village, was investigated, and the assumed inundation area, including house collapses, was verified. As a result, according to the estimated inundation zones, all areas in the Watari district except for mountainous areas were designated as inundation zones. The most notable area where houses in the village of Chaya in the Watari district were swept away by the flood was the area between the levee and the railroad, which has a large hydrodynamic force. The behavior of the flood flow in the Watari area of the Kuma River was a downstream type of inundation. Calculations without considering the inflow from branch rivers showed that the flooded flow overcame the levee and entered the inner area of the levee from the upstream of the continuous meandering section.

 This study examines a rainfall-runoff and flood-inundation process in the Kuma River basin during the heavy rainfall event in July 2020. A field survey and a numerical simulation of the rainfall-runoff and flood inundation were conducted. The results show the followings. (1) The heavy rainfall exceeded the planned rainfall. (2) Two levees were breached, and 11 bridges were overflowed or flooded. (3) The heavy rainfall area moved from the upper to the middle of the Kuma River basin. And then from the northwest of the middle to the southeast of the upper basin. (4) In the area upstream from Watari to the confluence of the Maegawa River, the main river overflows. And the flooded area expanded from the narrowed area to the upstream side. In the Hitoyoshi urban area, backwater caused flooding in the branch rivers. And then the main river overflowed, expanding the inundation area. The model can now capture the behavior of rainwater at the watershed level.

 Before instrumental meteorological data are available, old diaries are an important source of daily atmospheric conditions. Data assimilation is a powerful technique for achieving reconstructions because it optimally combines observations with climate models. Although descriptive weather records can be approximately converted into cloud cover, it is difficult to assimilate cloud cover because of the non-Gaussianity of its error distribution. The objective is to reconstruct daily historical weather over Japan by cloud cover data assimilation using a Gaussian transformation method. This study conducts an experiment in the winter of 1995 using visually observed weather conditions recorded by Japan Meteorological Agency (JMA). Gaussian transformation shows great potential to improve cloud cover assimilation in actual reconstructions, especially for humidity fields in the lower troposphere and temperature fields in the middle and upper troposphere. This study demonstrates the possibility of reconstructing high-temporal-resolution atmospheric conditions before the 20th century with diaries data.

 Anthropogenic water vapor emissions became an important component of the urban atmosphere with the process of urbanization, and one of the main sources of anthropogenic water vapor emissions is the fossil fuel combustion-derived vapor (CDV). Due to the extremely low d-excess value of CDV, stable hydrogen and oxygen isotope can be a promising method to partition CDV from other natural sources. Considering the limitation of in-situ observation, this study explored the possibility to use IsoRSM, an isotopic-enable regional spectral model to simulate the emission situation of CDV in the urban area. A one-month experiment in Salt Lake City and a one-year experiment in Beijing were made respectively. The simulating result indicated that the addition of CDV could significantly decrease the d-excess of water vapor, especially when the boundary layer was stable. The simulation with CDV emission fitted better with the time series of in-situ observation than the simulation without CDV in Salt Lake City. Furthermore, the addition of CDV also resulted in an obvious negative correlation between vapor d-excess and specific humidity. High d-excess in the winter season in Beijing might be caused by other factors such as strong kinetic fractionation in evaporation caused by the hard winter monsoon. In the simulations, the CDV fraction in Salt Lake City could reach more than 13%. The results indicated that the anthropogenic emission could not be ignored particularly in studies on the urban atmospheric hydrological cycle.

 Atmospheric Water Generation (AWG), which is the technology to obtain water by artificially condensing water vapor in the atmosphere, has attracted its attention in recent years due to its advantages such as not requiring liquid water sources like oceans, rivers, and lakes. Although much research about AWG has been conducted, its impact on the surrounding environment due to drying the atmosphere has been neglected. In this study, the process of AWG was modeled and incorporated into a regional climate model, and simulations were conducted. The effect of implementing large-scale AWG was examined at three sites with different climates in Tokyo, Riyadh, and Delhi, and found that there was a decrease in precipitation and temperature around the implemented area. These are thought to be due to desiccation caused by the removal of water vapor by AWG, and increased evaporation from the ground surface caused by that, respectively. It was also found that approximately 10 ~ 20 % of the water obtained by AWG was supplied by the increased evaporation from the ground surface at the site, while the rest by water vapor inflow from the surrounding atmosphere.

 This study improves the urban roughness length modelling especially targeting densely-packed highrise building areas. An effective ground level was conceptually designed, considering that anything below this level has neglectable influence on the mean wind statistics within the logarithmic region. This effective ground level is modelled based on a function generated from the available parameters of building geometry, parameters taken from urban turbulence datasets produced from large eddy simulations. The urban geometrical parameters were updated as substitutional inputs for the conventional urban roughness models. The results of the model output of a conventional urban roughness model with the updated input parameters can improve the estimation of the roughness length of densely packed highrise building areas.

 Global hydrological models used in higher resolution require representation of water transfer through aqueducts. In this study, we tried to evaluate water demand and supply more accurately by incorporating aqueducts in Indus Basin which are formed by many canals and links uniting rivers. To represent these aqueducts, we used aqueducts transfer scheme of H08, one of global hydrological models. In order to use this scheme, we collected information from literatures and maps and created the data showing water sources for each water-demanding area. Besides, we considered the effects of links by assuming barrages which are located upstream from the water-demanding area and indirectly supplying water there are also treated as water sources. Because of this, the evaluation of demand and supply became more realistic. The difference between the observed and simulated amount of water withdrawal considering aqueducts became one third of that without aqueducts expression. Also, irrigated areas where water supply hadn’t been represented realistically decreased.

 We used globe thermometers with different diameters (4 mm and 12 mm) for Globe Anemo-radiometer(GAR) to improve the measurment accuracy of wind speed while securing that of shortwave radiation. In addition, the possibility of a novel wind speed measurement method of GAR which based on an empirical method without heat balance analysis. The combination of different diameter globe thermometers reduced the power consumption of GAR and increased the accuracy of wind speed measurement but that of shortwave radiation decreased. An empirical wind speed measurement of 4 mm globe showed usefulness.

 In recent years, heat-related illnesses have been increasing due to urban heat island effect, and the development of a high-density meteorological observation network and assessment of heat stress on the human body are indispensable for citizens’ QOL. In this research, we developed an IoT device that can measure five meteorological elements (temperature, humidity, wind speed, shortwave radiation, and longwave radiation) and can construct a high-density meteorological observation network through wireless communication. The measurement accuracy of shortwave radiation, longwave radiation, air temperature, and wind speed of six prototypes was verified, and the average RMSE of each meteorological element was 19.1 [W/m²], 3.86 [W/m²], 0.389 [°C], and 0.374 [m/s]. The results show that outdoor meteorological observations can be made with practical accuracy. In addition, WBGT was calculated for outdoor meteorological observations, suggesting that heterogeneity of local microclimate is formed even within a small area due to differences in environmental conditions.

 Roof albedo data of individual buildings are necessary to improve the accuracy of weather simulations. In this study, a multispectral camera capable of 5-band measurements was mounted on a helicopter, and albedo was estimated from the obtained aerial images. For comparison, albedo estimation was also applied for 16-band WorldView-3 images. As a result, detailed roof albedo data was successfully obtained from the aerial images, but had a tendency to be lower than albedo data from WorldView-3. This is probably attributed to the smaller number of observation bands of the camera and the influence of shadows from the taller buildings as the images were taken in winter.

 We conducted 1-month long observation with Cloud Image Velocimetry (CIV) at Kagurazaka, Tokyo. Cumulus and cirrocumulus, whose boundary has clear brightness values, are found to be highly applicable in CIV. On the other hand, stratus which has unclear boundary was found to be difficult to apply CIV. Wind speeds observed with CIV was compared with rader velocity obtained by Ka-band cloud radar, and reasonable correlation were found between them.

 A drawdown of water level in the Panama Canal due to accompanying drought becomes serious is-sue not only for Panama but also for Japan as it leads increase in toll surcharges for merchant ships. This study developed a Water and Energy Budget-based Rainfall-Runoff-Inundation (WEB-RRI) model in the Panama Canal and investigated the mitigation measure against a drawdown of water level accom-panying 2015 drought. We recognized that a water level difference of 39 cm, which is the difference between drought 2015 and 2014, can only be reduced to 30 cm, and And the mitigation effect is small even if new dam will be constructed. Therefore, not only the construction of new dam but also desalination of sea water etc., which proposed by the Panama Canal Authority should be considered. Furthermore, we also recommend to consider “inter-basin water transport” from wider area to function the canal without any interruption under climate change and to keeps the 2011 high water level.

 Rainfall-runoff simulations were conducted by using an existing detailed hillslope flow model and a kinematic wave model. Investigations on the simulated hydraulic quantities revealed that soil moisture profile over a cross-section greatly influenced water flows in the slope downward direction and that the hydraulic gradient in the slope downward direction was deemed to be approximately the same as the slope gradient. Based on the above results, we developed a vertical quasi-two-dimensional subsurface flow model by approximating the hydraulic gradient in the slope downward direction with the slope gradient in the vertical two-dimensional Richards’ equation. The developed model was tested on a single slope against existing models. The runoff discharge and soil moisture simulated by the developed model matched well with those of the detailed model, which implies importance of considering soil moisture profile in rainfall-runoff modelings. The developed quasi-two-dimensional model reduced computation time to less than half that of the detailed model for most cases assumed in this study.

 Disasters such as floods and fires happen regularly in humid tropical region such as Indonesia. Reliable hydrological models that can be applied in the humid tropics to assess disaster risks are essential. This study presents the results of field campaign in the humid tropics and assesses the applicability of the Rainfall-Runoff-Inundation (RRI) model with its recently developed storage-discharge relationship to reproduce discharge and soil storage distributions in the humid tropics. Results show that the tropical soils have higher water retention capacity with higher saturated soil water content and pore distribution index compared to typical soils in a temperate region. The contrasting simulation of tropical and temperate soil parameters in the RRI model showed clear difference in discharge and soil storage distribution depending on the selections. The advancement in the RRI model which uses soil parameters directly show potential for the future use.

 The middle Arakawa River has large flood plains and many cross-levees, which can delay the timing of a peak flow rate during a flood event. However, the delay of the peak flow rate obtained from the actual data of flow rate in the 2019 Typhoon Hagibis was large, and the phenomenon could not be explained only by the effect of the cross-levees. In this study, a simulation method that combined two-dimensional flood flow and one-dimensional or quasi-two-dimensional infiltration analysis was developed to investigate the effect of infiltration into flood plains during floods on the flow rate and water level in the Arakawa River. It was shown that the accuracy of the shape of the water level hydrograph (water level rise rate) is improved in this simulation method considering the horizontal infiltration not only from the upper surface of the flood plain but also from the low-water channel to the flood plains. The accuracy of the maximum water level around the First Arakawa Regulating Pond was not high. However, this study demonstrated that the infiltration into flood plains is important to be considered for accurately calculating the time to begin the overflow to the regulating reservoir.

 In this study, the behavior of pore air during air injection in isotropic and laminar anisotropic non-uniform media was investigated by using two-phase flow analysis and a self-similar spatial distribution model. The results showed that the range of influence of the injected air was similar to the homogeneous field in the isotropic non-uniform field, but in the layered anisotropic non-uniform field, the range of influence in the horizontal direction varied greatly depending on the permeability distribution, and there was a possibility that the recovery of contaminated air in the unsaturated zone would be insufficient. In addition, the tendency is stronger as the permeability of the ground increases.

 In this study, we conducted single-borehole dilution tests in a limestone site to estimate the parameters such as the hydraulic conductivity, pore velocity and dispersion coefficients. Two analytical solutions associated with the variation of concentration were linked with observed breakthrough curves to identify the parameters. Swarm intelligence techniques including particle swarm optimization (PSO) and cuckoo search algorithm (CKA) were applied as an inverse approach and demonstrated these efficient ability to solve parameter estimation problem. We found that in the site hydraulic conductivity decreases with the elevation, and estimated parameters varied according to an individual observation hole. Also, subsequent solute transport simulations were in reasonable agreement with observed transitional concentration data. It was revealed that the slope of the breakthrough curves was dependent on the well diameter in the site of concern.

 The purpose of this study is to propose a risk map for each inundation point (risk at the site), taking into account the risk assessment of levees, in order to support local residents to recognize the risk of their own residential areas and to promote evacuations. In this study, flood inundation analysis was conducted for multiple scenarios of the Jinzu River and the Joganji River, which run through Toyama Prefecture, in order to calculate the risk at the site. The external forces are evaluated in multiple stages using the runoff magnitude of each river in each probability year. The results of the analysis show that the risk of inundation of the Kamizugawa River is particularly high. Also, it was found that 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 necessity of horizontal evacuation in the Joganji River was due to the location of the assumed bank erosion points.

 Severe water-related disasters occur in Japan almost yearly due to typhoons and frontal rains. Particularly in recent years, the scale and frequency of torrential rains have been increasing due to global warming. The purpose of this study is to evaluate the damage cost reduction effects of several counter measures based on flood inundation simulations. The flood inundation simulations considering the climate change impacts were conducted at the Oyabe, Sho, Jinzu and Joganji Rivers in Toyama Prefecture. We then evaluated the effects of vegetation management in the river channel and rice paddy dams in the basins on the flood damage cost. From the results of analysis, it was found that the impact of vegetation management was a decrease in the flood damage cost throughout the rivers in Toyama Prefecture. The rice paddy dam was found to be effective in the Oyabe River, which has a large proportion of paddy fields in the basin.

 The paper proposed a framework to mitigate the automobile damages due to flooding at large-scale carparking yard. First of all, the micro elevation of the car parking is measured using both 3D backpack type camera and 3D scanner; then processed to 1m resolution elevation data. Two prepared 1m resolution elevation data by camera and scanner were compared each other and with 5m resolution data by Geospatial Authorty of Japan (GSI). Then, two 1m resolution elevation data were merged to the 1m resolution data resampled from GSI 5m resolution data. Finally, the inundation simulation by a nesting method was applied to the two 1m resolution elevation data. As a result, inundation depth with the elevation data by 3D scanner showed relatively better performance than that by 3D camera. Likewise, the inundation depth can be reduced if the cars are placed on the high elevation area idenfitied by the 3D scanner.

 In this study, the accuracy of the inundation analytical model was evaluated by comparing the obtained results and actual values (inundation area, actual water level, and temporal changes of water level obtained from photos and video) on the inundation due to dike break of the Chikuma River in 2019. In addition, a 2-meter grid was used for the inundation analysis to represent the influence of buildings in the area near the dike break site. Comparison of the analysis results with and without buildings showed the importance of considering buildings in the inundation analysis.

 The flume experiments considering actual building arrangement and floodwater infiltration into buildings were carried out, observing the propagation process of inundation flow and measuring the water depth at several points. These measurement data were compared with the simulation results of three different numerical models: Unstructured mesh model, MLIT model, and Lumped building inundation model. It was found that the Lumped building inundation model, newly developed here, is an excellent model, which can reproduce the building inundation with time lag, and simulation cost is efficient with the similar coarse mesh system to the MLIT model, while the Unstructured mesh model has some difficulties in applying to an actual area for its finer mesh system.

 A new policy, River Basin Disaster Resilience and Sustainability by All, has been attracting attention. In this study, we evaluate the "rainwater storage potential, " which represents the maximum storage capacity of each area, in order to expand the rainwater storage function. First, the flood situation in Matsue City during the July 2006 heavy rainfall was reproduced using an inundation flow analysis. Then, the flood mitigation of the storage facility was evaluated by using the inundation analysis model that takes the storage function into account. As a result, inundation depths were reduced especially in the areas with large inundation damage, confirming the effectiveness of the storage facilities. The evaluation of the storage potential in Matsue City suggests that about 70% of the storage capacity can be applied to reduce the flood damage caused by the July 2006 heavy rainfall.

 In recent years, the frequency and amount of short duration torrential rains have been increasing due to the climate change. When torrential rains occur in urban areas, these areas hold the possibility of being inundated not only above the ground but also underground. In this study, the risk of the mega underground space in Osaka was evaluated by analyzing the inundation of aboveground and underground spaces in the event of pluvial flooding caused by assumed maximum rainfall, and it was analyzed how the difference of rainfall patterns affects the safe evacuation from underground spaces. In addition, it was also examined if installing flood boards at the entrances of the underground space with early inflow improve the success rate of evacuation. The analysis results show that the evacuation success rate varies with the rainfall patterns and that the evacuation success rate improves even when flood boards are installed only at the entrances which floodwaters flow in quickly.

 In Japan, there are many pool-type fishways that have two migration routes such as notch and orifice. However, the conditions of use and the appropriate placement of orifice for Anguilla japonica are unknown. In this study, the vertical position of orifice was varied from 0 to 0.25m from the bottom of the pool, and the flow velocity in the orifice was varied from 0 to 7(1/s) to investigate the migration characteristics of Anguilla japonica. The results showed that Anguilla japonica could migrate under all conditions within the experimental range and the migration rate was not affected by the vertical position of the orifice, while it increased as flow velocity increases. This is because when Anguilla japonica perceive a high-speed flow, it swims toward the upstream orifice due to positive rheotaxis.

 Ecosystem changes associated with dam sedimentation countermeasures such as sediment replenishment and flashing have been reported worldwide. However, little is known about the impacts of sediment sluicing, which have been rarely implemented. In this study, we evaluated the short- and mid- term effects of sediment sluicing on the benthic invertebrates along the Mimi River in Miyazaki Prefecture, Japan. The number of net-spinners consistently decreased in the short term as a result of sluicing. We observed increases in gliders at the site downstream of the lowermost dam in a midterm scale. A non-metric multidimensional scaling method was used to compare the similarity of community structure over time, and a consistent pattern of structural change was observed at the sites affected by sluicing. Therefore, it was suggested that the short-term response to sediment sluicing, rather than to flooding disturbance, could be evaluated in terms of taxa similarity.

 In this study, we compared the fish biodiversity patterns of the two different river basins (the Takatsu and Takahashi rivers) in Western Japan, using quantitative environmental DNA metabarcoding. Our results show that the alpha diversity (species richness and Simpson’s diversity index) of the Takatsu River tended to be higher than that of the Takahashi River. The beta diversity of the Takatsu River also tended to be higher than that of the Takahashi River, suggesting that the Takatsu River could have more variety of fish assemblages along the longitudinal gradient. Finally, on the gamma diversity, the species richness of the Takahashi River was higher than that of the Takatsu River, but Simpson’s diversity index of the Takatsu River was slightly higher than that of the Takahashi River.

 We aimed to grasp the relationship between saline-intrusion behavior and the drifting dynamics of ayu larvae in the Gonokawa River using a three-dimensional fluid simulator. Both measured and computed results revealed that the salinity ranged from 1 to 7 PSU on the surface of the estuary, while it was about 30 PSU near the bottom of the estuary. Therefore, we inferred that the bottom of the estuary is not suitable for the habitat of the larvae soon after hatching. Particle-tracking simulations showed that about 90% of the larvae were suspended near the surface and drifted to the sea in one night. On the other hand, the remaining 10% of the larvae were trapped in the saline layer near the estuary bed and transported back upstream by a tidal excursion in the model simulating the behavior of larvae in the daytime. They could not reach the sea, so about 10% of the larvae were expected to grow in the brackish water in the early stage, where feed plankton are less abundant than in the sea. These results showed that this method could be applied to other rivers to identify if the flow blockage is a cause of the depletion of drifting ayu larvae.

 Effects of Egeria densa on aquatic ecosystems are major concerns in Japan. It is important to efficiently monitor the distribution and abundances of E. densa at large spatial scale and examine the factors affecting the abundance of the species and manage the overgrowth of the invasive aquatic macrophyte in riverine systems. Using environmental DNA(eDNA) analyses, we examined the distribution and abundance of E. densa and the factors that could affect the overgrowth of the target species in the five river basins in Western Japan. Our results indicated that the abundance of E. densa varied among the river basins, and the riverbed slope and low water temperature time were significantly and negatively related to the eDNA of E. densa in the study areas. Our results also indicated that the relationships between the eDNA concentrations and environmental factors varied among the river basins.

 An estimation method of vegetation density in the forests of three bamboo species was proposed to detect treetops using local maximum filtering (LMF) with a circular window under a high-resolution digital surface model via UAV images. At the highest case of estimation accuracy, the bamboo forest density results revealed that the method underestimated only about 10% in any species. The estimation accuracy depended on the window size (WS), a parameter of LMF. Thus, the application of optimal WS according to the canopy size of bamboo was paramount to obtain highly accurate results.

 To know the dispersal and recruitment characteristics of pioneer riparian vegetation that first invade bare sandbars after floods, seeds (particles) transported by the flow were investigated using laboratory flume experiments and numerical analysis. Seed dispersal was observed throughout the entire channel during the flood in both alternate sandbars and braided channels. In the case of alternate sandbars, seed settlement occurred locally at the edge of sandbars, during the period of discharge recession. On the other hand, in the case of the braided channel, even under high flow conditions, the main channel shifts and diverges on a small scale, resulting in advance and retreat at the sandbar edge. Therefore, unlike alternate sandbars, particles were widely settled on the braided bars. The dispersal and settlement of buoyancy seeds are strongly controlled by hydrograph shape, channel morphodynamics during floods, and seed dispersal timing rather than seeds characteristics itself (size and specific gravity, etc.), under the conditions of this experiment and numerical analysis.

 In recent years, climate change has resulted in frequent damage due to excessive sediment inflow. However, the effects of changes in upstream and downstream gradients and associated changes in sediment supply in river channels are still unknown. In this study, we attempted to understand the channel deformation in the place where the gradient and sediment supply change by using the movable bed experiments and numerical simulations. As a result, (1) in a place where the amount of sediment supply is large, although the range of influence is limited, the left and right riverbanks are widely eroded because the alluvial fan-shaped sediment accumulation promotes the branching of the channel. (2) In a place where the sediment supply decreases, the erosion of one bank is predominant and the channel meanders because the branching of the channel is suppressed as the riverbed lowers. (3) When the sediment supply is close to equilibrium, the channel is divided appropriately and the left and right banks are eroded almost evenly, so the maximum amplitude of the channel is smaller than the above two fields.

 We have studied the role of non-equilibrium sediment transport of bed load on bed variations of the braided channels in the middle and lower reaches due to flood flows in the Abe River, which is composed of a wide grain-size distribution of bed materials including gravels. This paper focuses on the role of non-equilibrium sediment transport of suspended load which is greatly affected by the three-dimensional flood flows on bed variation. Our calculations showed that the amount of suspended load is relatively large in the flood descending period, and the tendency for bed load and suspended load to accumulate on the sandbars resulted in a large specific height difference between the sandbars and channels. We also confirmed that secondary flows develop at the side edges of sandbars due to the flow mixing between the flow over the sandbars and the flow in the channels promote the deposition of suspended load at the side edges of sandbars, and it causes the lateral movement of the sandbar and reduction of the width of the channel.

 In the Satsunai River of the Tokachi River system, in order to eliminate the forestation that progresses due to the flow conditions and the stabilization of the river channel, a medium-scale flash release is being carried out in late June in order to eliminate the progress of forestation due to the stabilization of the river channel by causing alternating mainstream changes in bifurcated channels from 2012. However, the cause of the alternating mainstream change in bifurcated channel has not been clarified. Therefore, in this study, for the purpose of examining the relationship between this phenomenon and the discharge and the shape of the bifurcated channels, these relationships are investigated using aerial photographs taken about twice a year. As a result, it was found that the larger discharge, the more likely it is that the alternating mainstream changes will occur and that the ease of this phenomenon differs depending on the shape of bifurcated channels.

 This study analyzes the effect of suspended sediment transport on the bed evolution of a suspended load dominated meandering river by using a two-dimensional (2D) numerical model. The numerical simulations were performed under steady and unsteady flow conditions using bathymetric data and in situ sediment samples. The erosion rate was calculated using a turbulent entrainment equation for fine particles. The results showed active changes in the riverbed when suspended sediment was considered in terms of changes in the riverbed profiles and cross sections. For the steady case, erosion was predominant in areas near the inflection points due to topographic features (narrower width and cross-sectional area) which led to active sediment erosion together with a phase-shifting in deposition patterns. Complex changes were observed for the case of unsteady flow that are caused partly by the non-equilibrium nature of suspended sediment.

 We often see bank erosion progress with meander flows in rivers on a large scale during decreasing discharge. However, it remains unclear how the decreasing discharge affects the meanders and the bank erosion. In this study, we performed hydraulic experiments, under varying discharge conditions, on the bank erosion due to the meander originating by alternate bars. The bank erosion rate increased about twice when the discharge was reduced compared with the case conducted under a steady condition maintaining the maximum discharge. The measurements of the plane flow velocity distribution in the experiments showed that the positional relationship between the flow driven to near the outer bank by the alternate bars and the bank line shape affects the bank erosion near the apex and the meander evolution. We have also observed that the change in the migration rate of bars in response to the decreasing discharge triggered the meander evolution.

 We performed three cases of three-dimensional numerical simulations of uniform sediment flows of single-sized particles in different slopes by using a solid-fluid multiphase simulation model and demonstrated that the sediment volume concentrations of the simulation results were able to agree with those obtained from the equation of the equilibrium sediment volume concentration. We also obtained spectrum of flow velocities at different heights by fourier analysis from the simulation results and investigated turbulent structure of sediment flows. The velocity spectrum showed that the peak of velocity spectrum was appeared near the free surface in the highly-concentrated sediment flows in the great slope channel. It was also demonstrated that the velocity spectrums in different scale was closer to each other near the free surface in highly-concentrated sediment flows compared with those in the low concentration sediment flows.