Establishing flood risk evaluation method corresponding to intensification of rainfall caused by climate change is an urgent issue in Japan. This method can be constructed by introduction of confidence interval for design rainfall using large ensemble simulation dataset which can estimate an atmospheric field in the time of global warming. On the other hand, the traditional calculation method of design rainfall adopted past observed data and did not consider the effect of climate change. In this research, the method to update confidence interval is constructed by using bayesian statistical theory which can update estimated quantity by each getting of new observed data. By using this bayesian update method, design rainfall and its confidence interval which consider the effect of past observed data and climate change can be derived.

 We developed an artificial neural network to estimate flow regime for ungauged basins in Japan. The network estimated 5 flow regime characteristics including runoff height of annual (Q_year), 95 days (Q_95d), 185 days (Q_185d), 275 days (Q_275d) and 355 days (Q_355d) by using 25 basin characteristics including climate, land use, geology and topography. The accuracy showed the best in Q_year (R² = 0.76) and the worst in Q_355d (R² = 0.45). To increase learning data and revise basin characteristics regarding land use, geology and topography had a possibility for improvement of the accuracy. We also found that the accuracy of Q_355d had the highest sensitivity for increasing learning data.

 In Japan, a combination of quadratic curves is used to create stage-discharge rating curves. The breakpoints for the relationship between the water level H and the square root of the flow √Q are judged visually, and polygonal lines are applied by the least square method. As a result, there are problems such as individual differences in the shape of the polygonal line, and the great effort required to connect the broken lines. By applying a simple broken point selection method and connecting broken lines, using the AIC method for optimizing the number of lines, a polygonal line with a better degree of matching than that by the conventional visual judgement method can be mathematically obtained and automatically drawn. This new method represents a saving of labor and improvement of accuracy when drawing rating curves.

 In this study, we conducted a flume experiment and a numerical simulation for driftwood dynamics on an alternating bar considering the root wad effect and bed deformation. For numerical simulation, we used Nays2DH with driftwood dynamics model. We then analyzed the relationship between the bed morphology and the wood deposition patterns. The experimental results showed diverse patterns of morphodynamics and motions of driftwood, such as pushing motion, scraping erosion, piping erosion. However, the present driftwood dynamics model could not reproduce partially the angle of driftwood deposition (most of the wood pieces laterally deposited against flow direction in the experiment) because the wood collision is underestimated in the simulation. In addition, the driftwood model does not consider the pushing and scraping motion of wood pieces, and diverse sediment transports such as scraping erosion and piping erosion. It, therefore, should be considered in further study.

 Flooding with a discharge of approximately 4500 m³/s occurred in the lower Asahi River in 2018 because of the Heavy Rain Event of July 2018. This flooding engendered wash-out situation of vegetation such as woody and herbaceous species and lodging situation of bamboo forest. Earlier works on the critical wash-out condition commonly assumed that the related properties of flow resistance due to vegetation and non-dimensional tractive force can be estimated by flood simulations without considering the wash-out situation of vegetation. However, such treatment cannot be applied reasonably to the case in the lower Asahi River flood in July 2018.

 In order to examine numerically the wash-out situation of vegetation during the flood, this study proposed a flood simulation model coupling two processes: (1) vegetation wash-out attributable to local vegetation height and non-dimensional tractive force under local flood flow, and (2) change of flood flows attributable to decreased drag force induced by vegetation wash-out. We analyzed data of topo-bathymetry, land-use type, and vegetation attributes, using ALB data. Results show that our model improves reproducibility of the wash-out situation after the flood, compared with earlier models that incorporated uniformly the critical non-dimensional tractive force, not considering the vegetation height distribution.

 Many kinds of studies regarding lateral mixing in compound channels with vegetation have been conducted. However, the effect of the flood plain height on the suspended sediment deposition has not been clarified. In this study, 12 cases experiments have been conducted by varying (1)height of flood plain, (2)the existence of vegetation and (3)concentration of suspended sediment in order to clarify the mechanism of suspended sediment deposition. The results show that suspended sediment deposition to the flood plain is defined by horizontal vortex, concentration gradient of sediment from river channel to flood plain and section size of momentum exchange. When vegetation invades to flood plain, sediment deposition is promoted instantly with reduction of flow velocity in vegetation area.

 In this study, the mechanisms that sustain the vegetated microtopography on the riverbanks were investigated by using the results of topography survey where the SfM-MVS techniques applied and those of particle distribution analyses of the deposited sediment. In particular, the mechanisms that the relatively corse sand sediment was deposited around the vegetated regions were studied. One of simplified methods to estimate the averaged flow velocity around the submerged vegetated regions was applied in order to examine the hydraulic condition that determine the transition between the suspended sediment and the bed load transport. The results of numerical calculation and the analyses of the observed particle distributions showed that the regions where the coarse sand sediment was deposited were under the specific hydraulic conditions in which the vegetated flow cannot transport the sand particles as the suspended sediment.

 This study aims to predict the potential initial recruitment zone and its coverage rate of riparian vegetation at the recruitment zone. An initial recruitment model was proposed with the considerations of inundation frequency, sediment transport and spring flood. The parameter used in the simulation is calibrated by the UAV monitoring and aerial photograph analysis results at the target river. Then the model was validated by comparing with the field survey results. The simulation results showed that the initial recruitment zone locates along the shoreline with the line shape, and the coverage rate of vegetation at the recruitment zone presents the decreasing trend from the internal boundary to the external boundary. The comparison of simulation and field survey results showed that the initial recruitment model can predict the initial recruitment zone well and approximately express the coverage rate of vegetation. This indicated that hydrological variables are significant for initial recruitment of riparian vegetation.

 A response of Phragmites japonica to flood disturbances was observed and reproducibility of its plant on growth characteristic by a vegetation dynamics model was examined. Three observation sites were set at sandbars in rivers, where the relative heights of the sandbars to the water surfaces were different; which means disturbance frequency by floods are different. The results indicated that a group where the plants that lived more than three years existed at middle relative height, and had the largest number and the longest extension of runners per individual. In relation to the vegetation dynamics model, it could reproduce growth trends of the plant in terms of its volume of leaves and stems and length of runners on a numerical simulation for the site with large relative height well. In addition, it was confirmed that the model is versatile on its scheme, and its applicability to various sites.

 Haze from Peat fire is serious transboundary problem in Indonesia and neighboring countries. In this study, CO₂ flux and peat fire risk reduction effect by water level controlling in drainage canal were evaluated. Firstly, meteorological data, ground water level and CO₂ flux data were monitored at Kepau Baru village in TebingTinggi Island, Riau Province. And then, ground water level and CO₂ flux estimation model were developed and applied. The calculated ground water level showed good agreement with observed data (NS>0.5). The accuracy of calculated CO₂ flux was 2.5 (tC/ha/year) for short term observed data and 5.5 (tC/ha/year) for long term observed data. By using the proposed model, the mitigation effect of water level controlling in drainage canal was estimated. In 100cm dam up case, CO₂ flux was reduced in 17.2%, and thickness of unsaturated peat layer was reduced 46.4%.

 This study on the nitrate-nitrogen removal rate in ponded soil aims to derive and verify the nitrate-nitrogen removal equation, to reveal the influences of temperature and evaporation on the nitrate-nitrogen removal rate. The influences of temperature and evaporation were examined using a paddy field model in which a mixed solution of nitrate-nitrogen and methanol was ponded 4 cm deep on the 2 cm of Andosols.

 The following facts are noted. Firstly, the derived equation considering percolation and evaporation can simulate the change of nitrate-nitrogen concentration accurately, compared to the previous equation. Secondly, the nitrate-nitrogen removal coefficient can be likely to be maximum around 30 degrees Celsius, not increasing with water temperature. In addition, the amount of evaporation rate can affect the nitrate-nitrogen removal coefficient.

 The current status of concrete revetment was investigated in the upper river basin in the central and northern parts of Kyushu, as basic information on river ecosystem conservation. The ratio of concrete revetment reached nearly 20% per riverbank distance. The ratio of concrete revetment was high in rivers with low elevation and small slope. In these rivers, the ratio of concrete revetment was further increased by arranging the roads along the rivers and changing the surrounding land. The rivers flowing through pelitic schist had a high ratio of revetment because the rivers had low elevations, small slopes, and often had roads near the rivers. Considering the conservation of the ecosystem in the basin, it is thought that upper rivers with low elevation and small slope and rivers in pelitic schist have a high degree of modification, and it is necessary to focus on conservation and restoration in the future.

 This paper tried to analyze factors of river temperature formation by using field observation and heat budget analysis in the middle and lower parts of an urban river. The river analyzed was the first-class river Tamagawa, flowing in the Tokyo metropolitan area. Treated water with almost constant temperature flowed into the Tamagawa river from several water reclamation centers. Observation results indicated that the influence of advective heat flux from the treated water greatly affected the formation of river temperatures in the middle reach of the Tamagawa River. In the lower reach, on the other hand, the river temperature flowed downstream with a nearly constant temperature. It was also confirmed from the hrat budget analysis that the river temperature in the middle reache was more susceptible to the treated water than that in the downstream. Furthermore, the influence from the treated waste water could be larger in autumn than in summer.

 Hypoxia occurs in the Higashi-Yokobori and Dotonbori Rivers where two floodgates are located at upstream and downstream edges. Although high DO water mass are imported from outside via the upstream gate, the improvement effect of DO in the rivers has not been clarified. On the other hand, although the seawater with low DO intrudes to the bottom layer via the downstream gate, its behaviour has not been clearly understood. In this study, we conducted water quality surveys to clarify the water quality variation by importing and exporting water and to evaluate the effect of improving DO environment in the rivers. The results showed that high DO water mass could be imported with a probability of 75% or more if the operation was conducted the appropriate time, and that seawater intruded the bottom layer during the gate operation for ship navigation. However, even if high DO water mass is always imported, it is difficult to suppress hypoxia because the oxygen consumption rate is large in the rivers.

 In recent years, the enormous prosperity of an alien aquatic weed Egeria densa has been regarded as a serious problem in many rivers in Chugoku district. Therefore, it is necessary to physically elucidate the growth mechanisms of this species for the development of effective and efficient control system of this species in rivers. Here, we conducted the flume experiments to understand the relationship of the initial growth of the fragment trapped on gravel riverbed to different hydraulic and riverbed condition using the quadruple channels. As a result, it is revealed that the growth of the fragment of Egeria densa tends to be more restrained in the conditions of constant high velocity and fluctuating velocity than constant low velocity. These results can be applicable to natural rivers, indicating that the control of flow rates is effective for restraint of Egeria densa.

 We had the largest flood flow in the observation history of Kawamoto Station in Gonokawa River in Hiroshima prefecture in July 2018. This study surveyed the cover degree of nonnative aquatic weed (Egeria densa) in the river segment (～38km) from Haji Dam to the three tributary junction section in Gonokawa River before and after the flood event, using unmanned aerial vehicles. Although the cover degree has significantly decreased after the flood in the whole area, the aquatic weed remained in certain locations. We conducted the reproduction simulation of the peak flow and examined relationships of the washout rate of the aquatic weed to dimensionless tractive force and flow velocity. Our results showed that the dimensionless tractive force was positively related to the washout rate, indicating that the cover degree could significantly decrease at sites with high dimensionless tractive force during flood.

 Seagrass distribution in a lagoon located near the Sea of Okhotsk was estimated from satellite data, and varidated using DIC and its variation obtained by field observation data. Seagrass distribution was made by converted data from averaged NDVI of satellite Sentinel-2 to seagrass vegetation density. As a result of calculation of DIC model, since field observation data at 17 points in/out of seagrass vegetation area for a period of from July to August in 2018 were successfully represented, seagrass distribution using satellite NDVI was well verified. Bias of calculation results were appeared due to seagrass existence, however, the estimation insufficiency of vegetation under the water and the influence of hydraulic resistance of seagrass to tidal current and water quality were likely associated.

 The purpose of this study is to obtain the knowledge for establishing the Unionid mussel （Nodularia douglasiae nipponensis） survey method using environmental DNA (eDNA) detection. First of all, the design of the primer probe of the mussel was carried out. Next, laboratory experiments were conducted to evaluate the abundance/biomass of the mussels. As a result, a positive correlation was found between the population of the mussels and the eDNA concentration. In addition, we collected the number of the Unionid mussel collected by hand, the eDNA detection rate and the eDNA concentration in agricultural waterways and small rivers. The survey method using the eDNA detection suggested that it is possible to evaluate the presence or absence of the mussels, and it is possible to identify places with a large number of habitats so that important places for conservation can be identified.

 We evaluated the elucidation of river sediment dynamics and the impact on the ecosystem by sediment sluicing using UAV, SfM/MVS and environmental DNA for the Mimikawa River with the first three consecutive dams in Miyazaki prefecture. We could grasp the topography change caused by sediment sluicing in the water area where UAV photogrammetry is not applicable, using ArcGIS and the refraction correction factor. It was revealed that as the sediment transportation in the downstream of the dam was activated by sediment sluicing, depositional tendency near the dam and erosional tendency downstream from the dam have occurred. Our results suggest that the biomass of Ayu could have increased in the river because the environmental DNA concentration of Ayu in the downstream has increased with the activation of riverbed fluctuation by sediment sluicing.

 The purpose of this research is to clarify the characteristics of hydraulic and water quality in the lower Chao Phraya River by using field observation and ADCP results. We have been conducting periodic water quality observations, water quality analysis and ADCP observations for more than two years since August 2016. In addition to the results, we clarified the characteristics of hydraulic and water quality by using automatic water quality and water level monitoring data by Thai governments together.

 As a result, according to the water level change of the rainy season and the dry season, EC increased in February (dry season), which showed high values from 80 km to 100 km from the river mouth. According to the ADCP observation results, it is found that the backflow phenomenon occurs when the water level rises, and the forward flow phenomenon occurs when the water level downs in all layers. At these times, the backflow phenomenon caused rise in the riverbed layer of EC and forward phenomenon flow caused th same EC value in all layers.

 Accurate prediction of climate change impacts on nutrient loading from watershed is important to discuss adaptive strategies in coastal management. The present study developed a distributed river runoff model for Seto Inland Sea basin to evaluate water and nutrient discharge. Using this model and disclosed observations, we investigated the trends in total nitrogen (TN) concentration and loading in 21 major rivers for the decade 2006–2015. The observed TN concentration had a significant decreasing trend in almost all rivers, which was consistent with the simulation. By contrast, increase trends were found in TN loading and freshwater discharge from all river basins in the simulation. The results suggest that the significant trends in TN concentration and loading are caused by precipitation increase for the period.

 Floating scum (sludge) combined with sewer overflow has been observed after rainfall in urban tidal rivers. An early solution for this is required from the viewpoint of the urban landscape, stench, and public health issues. We constructed a scum-observation system comprising flow and water quality meters and fixed-point cameras to investigate the flow and water quality characteristics and the behavior of scum in the Hirano River.

 Our results demonstrated that not only the diurnal and half-diurnal tides but also the tidal components of one-fifth to one-sixth days of the natural vibrations of the Osaka Bay and the Kii Channel had a negligible influence on river flow. The scum appeared throughout the year and was often observed for a few days after rainfall. It was observed that while the scum was transported based on tidal movements, the effects of human factors such as ship navigations also strongly influenced the behavior of the scum in the river.

 In order to clarify the transport process of plastic debris from land, the accumulation of macro-, meso- and micro-plastic debris on river banks were investigated in the Arakawa River. The plastic debris were accumulated at the downstream site comparing with the upstream sites. The amounts of meso- and micro-plastics on the higher river bank was more than that near the river edge, indicating that the degradation of plastic debris could progress on the river bank. In addition, the degradation level of plastic debris collected from the river banks was understood by measuring a ratio of absorbance at carbonyl and methylene groups. The degradation level was high in order of micro-, meso- and macro-plastics, indicating that the degradation level of smaller plastic debris was high.

 We found double peaks in a vertical profile of microplastic concentration in rivers by conducting the field surveys for observing the vertical distribution of microplastic concentration in the Edo River. The first and second peaks appeared in the surface and bottom layer, respectively. The vertical profile was dependent on the specific gravity of microplastics, and the specific gravity found in bottom layer was significantly greater than that found in the surface layer. Furthermore, the settling velocity of microplastics was larger than their rising velocity according to their specific gravity. These indicate that the microplastics having greater specific gravity can concentrate regardless of vertical mixing in a river.

 In rapid flow rivers, it has been reported that, under high-speed flow conditions, an anti-dunes becomes three-dimensional and triangular wave sequence, in which water surface waves become steep, are generated. There are concerns about the influence of this surface wave sequence on bridges and river structures once thy are generated, but the conditions of occurrence have not been elucidated. Soliton resonance is one of the phenomena that generate large amplitude water waves like triangular wave sequence, and it is known that the amplification factor is up to 4 according to Miles (1977). Therefore, in this research, we considered that the triangular wave sequence is generated by soliton resonance, and conducted a shallow flow experiment with sparcely distributed strip roughness. As a result, it was confirmed that the experimental results and the theoretical solution of Miles were in good agreement, and it is found that triangular wave sequence was generated by soliton resonance.

 In channels with supercritical flow conditions, a large triangle-shape water surface wave forms because the water surface over antidunes is in phase with the bed. The presence of the triangle-shape water surface wave causes the deviation of local flow velocity from the average velocity field, and often results in destabilizing revetment blocks. In this study, we measured the bed topography of the 3D antidune, the surface profile of the triangle-shape wave and the spatial distribution of the flow velocity for the purpose of collecting basic data of block design. The experimental results show that 3D antidune has a crescent moon shape drawing an arc down-stream. The crossing streams caused by the 3D antidune rise the water surface elevation near the crest of the antidunes and generate the traianlge-shape wave. In addition, the local flow velocity increases 20 percent from the average flow velocity.

 We constructed a check gate that combined an underflow gate with a overflow weir and conducted laboratory experiments to evaluate the effects of the check gate structure on water flow behavior. We used the coefficient of discharge C in the discharge formula as an index. The experimental results showed that for the underflow gate, C was approximately constant and changed little in response to changes in the ratio of the vertical gate opening d to the distance w between the underflow gate and overflow weir for d/w from 0.12 to 0.14. In the case of the weir, however, C increased from 1.17 to 2.02 within the same d/w range. This result can be explained by the fact that the total head, which influences the overall water flow velocity, was increased by increased flow through the underflow gate.

 This paper deals with the comparison of the characteristics of the lateral overflow between the straight channel and the curved channel. We conducted the hydraulic experiments using the same hydraulic conditions and the same lateral weir configurations for both channels.

 The results obtained from this study are as follows; 1) The water levels for the curved channel are lower than those for the straight channel. 2) For the case that the relative weir height is 0 the deference of the velocity distribution between the straight channel and the curved channel is small whatever the relative weir length. 3) It is conformed that there is the dead water zone on the opposite side of the lateral weir for both channels. The wider the dead water zone is the longer relative weir length is. The dead water zone of the straight channel is wider than that of the curved channel. 4) The ratio of the over flow rate to the inflow rate for the curved channel is higher than that for the straight channel.

 Chikugo river has many cut-off channels nowadays but there are many old meandering parts still existing. All inflowing tributaries into Chikugo River do not connect directly to main stream of present Chikugo River but flow into the old meandering parts of this river. These reasons has never been clarified until now. In this study, cut-off channels and old meandering parts with inflowing tributaries are analyzed hydraulically in the both cases where a tide is effective or not effective. As a results, not only the cut-off channels but also the old large meanderings are found to be considered for water use and flood control.

 To understand the transport characteristics of particulate biological spices in the estuary environment, a particle tracking module was developed and combined with 3-D hydrodynamic simulator. This methodology was applied to the transport of eggs of Coilia nasus during from spawning to hatch in the Chikugo River Estuary. The egg was modeled to reproduce the effect of buoyancy against ambient water and the tracking path was corrected to avoid trapping by step-like bottom represented by z-coordinate. The effect of buoyancy on the transport distance was investigated by comparing the movements of the modeled particles with those of neutral particles. We found the buoyancy plays an important role in keeping the eggs in the suitable environment for the hatch. The survival rate of the eggs was also estimated in terms of the distance from the river mouth using accumulated salinity value. It was found that predicted location of high survival rate is reasonably corresponding to the location which is pointed out as the spawning location in previous surveys.

 In the case which shear stress is added to the free surface of turbulent open-channel flow, we perform numerical experiments by using DNS and the standard k-ε model to investigate how the turbulent field responds to the surface shear stress. The velocity-shear profile of the main flow varies with the positive or negative magnitude of the shear stress, so that the vertical distributions of turbulent quantities change obviously depending on the positive or negative sign of the stress. The results of DNS in the case of the negative shear stress show that the main flow velocity near the half water depth increases compared to the case without the shear stress though the standard k-ε model cannot simulate such a behavior. The scaling relation of the surface divergence based on Taylor microscale are found to be universally established regardless of the positive or negative sign of the stress, whereas the scaling relation with the turbulence energy dissipation rate breaks down. This means that the mechanism of the scalar transport changes by the sign of the surface shear stress.

 In the present study, a turbulent flow structure through vertically double layer vegetation (DLV) as well as single layer vegetation (SLV) under steady subcritical flow condition of inland tsunami in an open channel was investigated computationally by incorporating a three-dimensional (3D) Reynolds stress turbulence model, using FLUENT. The numerical model showed the ability to capture inflection in velocity profiles close to the top of short vegetation height, and suggested the modeling of inflection over this mixing layer region of flow. The production of turbulent kinetic energy and dissipation of turbulent energy through the vegetation region was observed to be comparatively very high for DLV flow as compared to that of SLV flow, which suggested that incorporating short vegetation within tall vegetation significantly increases turbulence in the flow and dissipation of turbulent energy. The 3D simulation data of velocity and shear stress around the mixing layer was also used to derive important information that could be used in 2D modeling of flows through DLV.

 In order to propose an optimal arrangement of pile dikes in considering suspended sediment and deposition, it is necessary to investigate flow structures in a downstream region of a pile dike. In this study, flow structures and deceleration rate were investigated experimentally for one array of pile dike with different diameter and interval distance of cylinders. Downstream flow structures were affected by the diameter and interval distance of piles. Passing flow rate through the pile dike was increased as the diameter increases. The deceleration rate behind the pile dike became larger with a decrease of permeability but it reduced with an increase of the diameter even in the same permeability. The behavior of the deceleration rate was well expressed by using relative interval distance.

 In this study, in order to improve the disaster prevention performance of the Ganlock bank protection which made practical use of the retaining wall construction method as the river bank protection method, the roughness coefficient of the new Ganlock bank protection which improved the old Ganlock bank protection, the stable estimation against the hydrodynamic force, the characteristics of the flow velocity distribution and the inner flow structure were examined by the hydraulic model experiment and the dynamic model. As a result, it was revealed that the Manning’s roughness coefficient of the new Ganlock bank protection is 0.041, which is larger than that of the old Ganlock bank protection. From the calculation of the dynamical model, the critical velocity for drag and lift was calculated. In the vicinity of the new Ganlock bank protection, a large scale secondary flow was formed as compared with the old Ganlock bank protection, and the effect of reducing the primary mean velocity around the bank protection was recognized. Furthermore, a large and characteristic streameise vorical structure was formed around the new Ganlock bank protection and it was inferred that it contributes to the reduction of the primarry mean velocity and the generation of the secondary flow.

 The large filamentous algae Cladophora glomerata often grows on coarse and stabilized (armoring) riverbeds. However, the habitat environment of the algae has been poorly understood. In this study, we investigated the effect of riverbed environment especially bed material size distribution on occurrence of the algae for building a simple assessment method of its colonization risk. A relationship between river environment variables and presence/absence data of the algae at the up and downstream reaches of the Koshido Dam in the Yahagi River, Japan was modelled with logistic regression analysis. It showed that the occurrence of the algae was mainly explained by only the valuables of riverbed material in this study, suggesting that the risk of the algae could be evaluated by the riverbed material size.

 For sustainable riverine environmental management, it is nessesary to understand distribution of stream organisms along environmental gradients within a river catchment. Here this study investigated inveretebrate community responses to narrow enveironment and food source gradients in the Omaru River catchment based on comprehensive sampling of stream macroinvertabrates and a machine learning technique. Sampling of macroinvertebrates and attached algae was carried out at the 33 upstream and 17 downstream sites from 1st to 15th November, 2018. As a result, chlorophyll a displayed significant positive correlation with the browser abundance in the downstream area (r = 0.524, p = 0.031), but not with the scraper abundance. Random forests revealed that while variable importance and predictive ability of chlorophyll a remained less, canopy cover slightly improved model accuracy for abundance prediction of invertebrate community.

 Environmental protection is regarded important along with flood control and water utilization in river management. Understanding the riverine ecosystem is necessary for such river management. In this study, we conducted monitoring on aquatic organisms in Takatsu River and tried to clarify the seasonal variations in biomass associated with the life history events for the dominant organisms (fishes, benthic invertebrates and attached algae). Furthermore, we examined the relationship between the intensity of disturbance (flow velocity and dimensionless tractive force) at the peak of the floods and the changes in biomass before and after floods. As a result, it was possible to capture the seasonal variations of biomass associated with the life history events such as the emergence and spawning of each organism. Moreover, a clear negative relationship between biomass and intensity of disturbance was found in four of nine taxonomical groups. The values of the intensity that make the biomass halved or disappeared were different among the groups.

 In this study, we constructed a distribution prediction model of Oncorhynchus masou ishikawae. We tried to predict the changes in the distribution area at the time of water temperature change at Saba river and Oze river in Chugoku district using the summer distribution information obtained by environmental DNA analysis and the water temperature data obtained from water temperature logs. The results of the distribution prediction model showed that the average water temperature in summer daytimes was a major factor influencing the distribution of this species; the results of scenario analysis at the time of water temperature fluctuation siggest that the potential habitat will decrease significantly as the water temperature rises and will increase significantly as the water temperature decreases.

 The spawning bed for Ayu in rivers has been conventionally inspected by human eyes. Consequently, the previous method cannot be applied for consideration of the potential of a limited spawning habitat, especially in rivers with larger targeted areas, steep bed sections, and environmentally degraded sections.

 This study investigated the potential of the spawning habitat for Ayu at three riffle regions of the lower Asahi River where the river forestation has prevented the renewal of bed materials under flooding. For that purpose, we examined the relation among the spawning behavior, the hydraulic characteristics at riffle, and the environmental DNA (eDNA) concentration of Ayu in water at both day and night, in conformity to the downstream migration of Ayu in autumn.

 Results demonstrated that we can observe the spawning bed at the riffle where the characteristics are preferable and the concentration drastically increases from day to night. Results also showed that we might locally find the spawning bed even at the riffle where the eDNA concentration increases from day to night, though the average characteristics are not suitable for spawning.

 Influence of river water temperature on nutrient reduction rate with decomposition of anadromous fishes was experimentally investigated. Focusing on pink salmon Oncorhynchus gorbuscha which has the largest resource of anadromous fishes in east Hokkaido, experiments on reduction rate with decomposition under temperature corresponding to river water temperature from October to November, run-up season, were conducted. Part of fishes, male/female, river/distilled water, and water temperature, were considered as experimental conditions. As a result, the larger reduction rates of NH_4-N and PO_4-P are, the higher water temperature are. Part of fishes and sex of pink salmon also greatly influence reduction rate. After 50 years from 1968, the reduction rate of NH_4-N increases approximately 40 % by estimating of variations in water temperature and discharge.

 This study assessed microhabitat conditions of Lefua echigonia using field observed ecohydraulic data and a data-driven habitat model. A series of monthly field surveys were conducted in a spring-fed small urban stream in Tokyo, Japan. Random forests (RF), a predictive machine learning method, was applied as a binary classification tool to analyze the relationship between physical habitat conditions and the presence/absence of L. echigonia. From the RF-based habitat model developed, variable importance and response curves were extracted for a deeper understanding of the ecology of target species.As a result, variable importance suggested the importance of water velocity for presence/absence modelling. Response cures illustrated the important instream habitat conditions such as shallow water with low water velocity, and high water velocity with a large percent coverage of large-sized gravels. The habitat information obtained in this study can be used to identify potential habitats for L. echigonia.

 It is estimated that the number of eels (Anguilla japonica) is decreased by the worse of river condition, thus fish ladder for eels is necessary in the river. Grid and zigzag type are known as how to put bunch of brushes and round protrusions on European fish ladder for eels. However, it is not cleared that which type is better and it is guessed that there is no studies about it. This study was made to evaluate effects on migration rate of eels under the condition that how to put on protrusions and discharge in fish ladder are changed. It was found that migration rate of eels and averaged migrating speed of eels keep advancing in zigzag type than grid type in this experimental range. Therefore, it is assumed that migration rate of eels improves in zigzag type.

 The number of eels is decreased these days. As one of the factors about that, intake facilities and drop works prevent eels from migrating. It is reported that mesh mat is useful for migration of eels. However, there is no research on the proper geometric form for migration of eels. This study is made to evaluate migrating characteristics of eels under the condition that density of mesh mat and discharge in fish ladder are changed. As a result, it was found that migration success rate and void entry rate increased and average migration speed decreased with increacing density of mesh mat regardless of a change of the discharge.

 During flood stages, river bed degradation and local scouring below drop structures might be occured. This might be caused by the formation of either plunging flow or hydraulic jump with a surface roller. Because, the main flow is located near a riverbed, and continues along the bed far downstream. In this study, a new energy dissipator was proposed as the installation of sloping apron with large roughness elements which were consisted in either stacked boulders or concrete blocks with a hemisphere shape. By installing small rougness slope (i.e., 1/10 and 1/20 slopes), it is easy to form a surface jet flow without a plunging. The experimental investigation reveals that the formation of surface jet flow might be effective as an energy dissipator below low drop structure from velocity distribution and maximum velocity decay. Especially, in the case of 1/20 slope, the range of tailwater elevation for the formation of plunging flow below abrupt drop can be recovered by the formation of surface jet flow.

 A demonstration experiment of a V-shaped portable fishway was conducted in rivers. Results of the experiment confirmed that the fishway is effective for the upstream mingraiton of eight species of diadromous aquatic animals. In addition, the flow velocity mesurement under multiple flow conditions clarified that the gentle flow field was formed along the waterside of the fishway side walls, where the aquatic animals ascended through, irrespective of the installation angle of the fishway or changes in discharge.

 To generalize the mitigation effect of tsunami energy through multiple defense system, it is necessary to develop a highly accurate numerical simulation model of tsunami inundation. Especially, the accuracy of simulation against overflow from embankment is important because it greatly affects the inundation area and breaking situation of houses.

 In this study, one-dimensional non-linear long wave equation model was improved, and the accuracy of the improved model was investigated by comparing with previous study and flume experiment. As a result, the accuracy of water depth on the embankment can be improved by using QUICK scheme and devising the inundation-front calculation method. In addition, when the ratio of grid size to top-width of embankment is less than 1, simulated overflow rate is highly accurate. Moreover, when the elevation of top and toe of embankment was set adequately, simulated overflow rate and simulated flow depth behind embankment is highly accurate even when the ratio of grid size to top-width of embankment is 2. This study also suggested that the accuracy of the time series of flow depth and velocity behind embankment is high even in the unsteady flow.

 This study conducted two dimensional URANS of flow over drop with trench observed in existing model experiment. We applied the second-order nonlinear k-ε model as a turbulent flow model and analyzed water surface fluctuation by the VOF method. Simulated flow regime over drop changed from super critical flow without jump, undular jump, oscillating jump, to submerged jet, same as experimental results, with increase in length of the trench, at constant discharge condition. Changing process of flow regime during oscillating jump and the period of fluctuation almost accorded with experimental results. We found that URANS with the second-order nonlinear k- model and the VOF method had possibility to simulate the unsteady flow over drop with trench, although simulated trench lengths range as emergence boundary of oscillating jump was narrower than experimental range.

 The Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) has been reformulated to simulate turbulent free-surface flows with high turbulence and large fluctution of the free surface. The commonly encountered stability problem associated with the particle method has been treated by the diffusion and dissipation terms that naturally arise in the particle-interpolated continuity and momentum equations rather than artficial damping terms. The effects of the solid boundaries have been incorporated by formulating the equations of motion separately from the particles away from solid boundaries by using the wall law similarities for turbulent flows over smooth and rough surfaces as well as laminar flow depending on the Reynolds number. The proposed method is compared with the existing SPH method and the experiment and the results are shown to agree with experiments without the stability problem.

 Gravel bed rivers are formed by large particles as gravels and cobbles, and other large particles move on the gravel bed. Therefore, mechanism of interactions between flow and moving large particles should be evaluated. A three dimensional numerical experiment about turbulent flow including rolling and saltating spheres over permeable fixed rough bed was conducted. Turbulent structures of permeable rough bed and effects of moving spheres on the main flow were examined. It is proved that slow flow of low positions is transported to higher positions with saltating particles, then longitudinal velocity of high positions considerably becomes small. So saltating particles have an influence such as the ejection for the main flow. In this condition, turbulence intensities of flows including moving particles become larger than those of clear flow.

 Computation of the Basset history force depending on the time history of the relative acceleration between the particle and the fluid at the particle location is very expensive, and hence, in past studies, either the whole part or some fraction of the history has been neglected. Here the effects of the Basset history force on preferential concentration of small particles in a homogeneous isotropic turbulence under a wide range of mass density ratio have been investigated by direct simulations of the Navier-Stokes equations. The Basset force is numerically approximated by the method of van Hinsberg et al. (J. Comput. Phys. 230 (2011) 1465). While the method is found to be fast and accurate, the results obtained from the traditional method taking account of the contributions only over the latest period of a time scale t_win shows a high dependence on t_win. It is clearly revealed that the presence of the Basset force weakens the level of preferential concentration, especially under the conditions of the mass density ratio of around 1.5 − 10.