A huge deep pool called ’Bentenbuchi’ existed in the Yadagawa river which flows the northern part of Hyogo prefecture until the 1970s. However, the Bentenbuchi pool is filled with sediment and has disappeared as of 2013. In this study, the cause of disappearance of the Bentenbuchi pool and the mechanism of the disappearance were examined. First, from the comparisons of the aerial photographs from the 1970s, it was shown that the Bentenbuchi pool has disappeared after the installation of the Ono weir in the 1980s. Then, 2-dimensional riverbed variation analysis with consideration to the existence or nonexistence of the weir was performed. From the results of analysis, it was specified that the installation of the Ono weir is the cause of disappearance of the pool, and the mechanism was also clarified.

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Topographic changes in the river-mouth bar in the Yuragawa River are continuously activated by river and ocean currents and sea wave. The bar recently shows the imbalance situation of its geometry, it may cause sedimentation problems. In order to avoid the problems, it is important to understand the characteristics of the topographic change in the river-mouth bar, and to propose a control method of the bar geometry. In this study, variations in geometrical properties of the river-mouth bar are analyzed on the basis of the hydrological data (river water discharge and sea wave height). As for the bar control, the effectiveness of spur dikes, which can change the flow direction, for erosion of the bar was evaluated by means of flume experiments. The two-dimensional numerical model (iRIC Project) is also applied to further investigate the effect of spur dike on the bar control.

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Levee breach due to overtopping flows recently occurs in flood events. It is of great importance to understand mechanism of dike failure and predict flows and sediment transport during its process accurately. In this study, numerical simulation of levee breach is carried out using a three-dimensional flow model in porous media and density function methods and a non-equilibrium sediment transport model. A simplified bank erosion model is also incorporated in the simulation. A numerical model is applied to levee breach in the small scale hydraulic experiment. Comparing with the previous experimental results, it is shown that the numerical model can reproduce widening process of bank failure reasonably.

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Aerial photographs of a cobble bar taken by an unmanned aerial vehicle (UAV) are analyzed for the purpose of measuring spatial distribution of cobble sizes before and after a flood occurred in the Ibo River in 2013. The photographs cover the bar area spanning about four hundred meters and the images were analyzed by using a software BASEGRAIN that can measure particle size distributions by a sophisticated image analysis method. A significant change of cobble size distributions due to the flood was made clear by comparing the mean diameter distributions. At the same time, surface flow measurements by an image analysis method, STIV, was conducted during the flood to estimate the shear forces acted on the river bed, which can well explained the movement of cobbles by the flood forces.

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In this paper flume experiments were conducted to study effects of partial removal of a weir on bed morphology. The depth and width of the removed part were systematically changed. The channel formation and bed degradation were focused. It is found that the channel is formed at the center of the flume by partial removal. In comparing the systematic changes of the removal depth and width, it was noted that changes of the removal depth affects in wider area than the removal width. Moreover, the deep channel was confirmed in the channel. The results indicate that partially removed weir can improve sediment movement from the upstream to downstream. Since the various bed shapes can be formed by the partial removal, there is a possibility that can contribute to improve habitats of river organisms, such as riffle-pool structure and a smooth slope by deposition along the bank side.

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The objective of this study is to show the difference in vulnerability for drought among the area with different land-use history. The vulnerability was evaluated by the value of maximum NDVI in pre-drought, drought, and after drought year. Evaluated droughts were occurred in Spain, India, East, and East Africa. Drought event area was defined by the value of Drought index (scPDSI) and Soil Water (GSWP3). There are several trends in cropland, whereas no trends in grassland. In cropland, there is negative trend of maximum NDVI in drought year with decreasing the duration of cropping in India. On the other hand, opposite trend was observed in East Africa. These trends seemed to depend on the soil type which is appropriate for cropping practice.

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Climate change in the tropical Andes is a critical pressure for the sustainable water resource management and ecosystem conservation in the region. Among natural resources possibly affected by climate change, glaciers and wetlands are considered to provide a variety of ecological services and hydrological functions. In this study, for better understanding the ecological and hydrological significance of wetlands (bofedales), we analyzed satellite data (ALOS/AVNIR2) to detect the distribution and form of wetlands in the Tuni Lake catchment, Bolivia, where the water resources in the capital cities strongly depend on. We detected a large area of wetlands mainly located along the stream, especially in less glacierized sub-catchments. Besides, we found that relatively small-scale wetlands were distributed on hillslopes in high elevation (up to 5500 m above sea level), supplying water to downhill mainstreams. Hydrological effects of wetlands are discussed using our monitoring data on flow rate and precipitation, indicating the higher runoff ratio from the Tuni sub-catchment, which has a relatively large cover of wetlands.

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A diversion facility installed at reaches upstream of Pirika Dam to generate downstream flow into the fish passage, and allow the fish to migrate to the sea. However, when the flow rate of the diversion facility exceeds 7.00m³/s, there are concerns that masu salmon smolt have trouble reaching the dam reservoir, because the smolt in the dam reservoir cannot migrate to the sea through the diversion facility. A PIT tag system and radio transmitters for biotelemetry was used to evaluate the diversion facility. It was found that 88.2 % of the smolt were able to enter the diversion facility, and 80.4 % of all smolt were able to access the fish passage. It was also clarified that smolt downstream migration had two peaks per day. During the study period, although the flow rate was in the range between 2.21m³/s and 30.4m³/s, the diversion facility was revealed to have satisfactory functionality for the downstream migration of smolt.

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In this study, we elucidate the habitat of the near threatened goby species Luciogobius pallidus based on the field work. We simulated the fluctuations in river-bed induced by flush floods and fluctuations in salinity induced by tide and regular flow in order to clarify the mechanisms of formation of their habitat. The results of the field work showed that this species inhabit gravelly bottoms and brackish waters with medium salinity. Simulations of the fluctuations in river bed showed that the habitat of this species is formed at the boundary between the areas with coarse and fine sediments after the flush floods. Simulations of the fluctuation in salinity showed that the habitat of this species is exposed to waters with mostly high and/or medium salinity levels.

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Many lakes and wetlands exist in the Arctic. Their storage effects have the potential to improve the surface hydrology simulation and to decrease the surface air temperature bias. There are, however, only few global land surface models that incorporate wetlands and the effects upon climate are not yet well investigated. In the present study, we examine these effects through sensitivity experiments using MATSIRO land surface model with a simple wetland scheme that stores part of snowmelt. First, we conduct offline global land simulations and evaluate the results using multiple observations. The result shows that the river discharge simulation in the major Arctic river basins and the underestimation of evapotranspiration are improved. Further, we conduct a series of on-line AGCM experiments using MIROC5 with climatological monthly SST and sea ice boundary conditions. The result shows that the impact of wetland scheme upon the reduction of near surface air temperature in summer is significant.

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A heavy rainfall from the 12th typhoon in 2011 produced huge sediment in the Kumano river basin. The sediment may induce the severe influences such as the increasing of the reservoir sediment and the river bed aggradation. Therefore, the sediment management in this watershed supposedly becomes more severe. In this study, the sediment runoff model considering the sediment production and supply due to the landslide was applied in order to discuss about the sediment management in the next 100years. To obtain the spatial distribution of the initial grain size distribution, two stages of pre-calculations were em-ployed. As a result of application, the sediment management combining the control of the amount of sediment passing through the dams and the control of the sediment supply from the tributary rivers was turned out to be effective.

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There are some studies of sediment runoff using volume of deposited sediment in large dams. However, quantitative evaluation of sediment runoff is not sufficient because the basin area is large and has various surface geologies. We paid attention to sabo dams. The basin area of sabo dam is small and has almost uniform surface geology. The analysis of its deposition sediment is suited to develop the characteristics of sediment runoff.In this study, we develop the prediction method of the volume of sediment runoff by deposited sediment volume of sabo dams and apply this method to Nagara River basin in Gifu Prefecture. As a result, we could evaluate the sediment runoff by using results of basic surveys under Sediment Disaster Prevention Act.

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The sediment color contains useful information such as geographical features of drainage basins. In this study, to estimate the possibility of the sediment transport between beaches and rivers, statistical analysis was conducted by using sediment color. Study site were 16 beaches and 12 rivers in Toban region, Hyogo prefecture, Japan. Toban coast has been faced to serious shore erosion and has taken strong measures by government. To evaluate the effectiveness of countermeasures, it is necessary to grasp the sediment dynamics. In this study, the principal component analysis was applied for image of the sediment on the surface ground at each investigation site. It is clarified at least 5 primary components are required to reproduce the sediment color composition at each field and the existence of sediment transport which coincides with the offshore current in this region was confirmed.

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The Arase Dam on the Kumagawa River in Kumamoto Pref., Japan, has been subsumed under removal project as the first high dam removal in Japan, - a process slated to influence the downstream sedimentology. During the first stage of the removal, in April 2010, the dam’s gates were opened. In this study, we investigated sediment characteristics on the river and tidal flat and estimated the changes in sediment outflow using a one-dimensional uniform bed load simulation. A remarkable level of sediment coarsening was identified on the surface of the tidal flat. The deposition rate on the tidal flat, which was estimated by vertical profiles of sediment size distribution and bed load flux, corresponded to the opening operation of the dam’s gate. The bed load simulation of the Arase Dam’s reservoir shows that the gate opening could increase the sediment outflow.

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Recent years, slit Sabo dams have been selected for their function to prevent debris flow disaster and conserve continuity of water and sediment flow simultaneously. In order to clarify the effect of a slit Sabo dam on sediment transportation, the authors have researched the characteristics of rainfall-runoff and sedimentation around a slit Sabo dam in a channel in the Tenjin river basin since 2010. The authors had reported that a slit Sabo dam with steel frame had trapped driftwoods which would prevent the sediment runoff to the downstream channel. This paper reports the results of the field observation from 2012 to 2014 and quasi-2D numerical simulation which enables to consider the influence of the driftwoods on the sediment transportation.

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Laboratory experiments were performed on fundamental characteristics of flow pattern and bed topography in the connection area between an upstream large-amplitude meandering channel and a downstream straight channel. It is found that, in the fixed-bed condition, the water depth in the meandering channel drops down locally just in front of the connection point, which is lower than the water depth in the straight channel, and that, as well as along the meandering channel, the lateral location of the maximum main flow velocity swings from side to side along the straight channel to a certain extent downstream from the connection point. It was also observed that, in the movable-bed condition, sandbars are formed in the extent region in the straight channel due to such flow characteristics.

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In rivers in mountain areas, a series of steps are often observed on bedrock. They are thought to be cyclic steps formed due to erosion of bedrock, which should be driven by abrasion due to bedload sediment transport. We demonstrated a series of flume experiments of the formation of cyclic steps on bedrock by abrasion due to bedload transportation using weak mortar as the model bedrock. The shapes of cyclic steps are dependent on the sand feed rate under the water discharge of this experiment, i.e., a short upstream-facing slope and a long downstream facing slope for the lower sand feed rates, whereas long upstream-facing slope and a short and steep downstream facing slope for the higher sand feed rates. This result differ from our previous study, whose water discharge was lower and the shape of the steps was independent from the sand feed rate. In the case of higher discharge, sediment cover affects the location of vortices caused by the hydraulic jumps to make the short and steeper downstream facing slope.

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To elucidate influence of bedrock cover thickness on bar morphology in free bar generating condition, we conducted hydraulic experiments. We changed bed material thickness (less and enough for fully grown bar height) and bedload feed rate (equilibrium state and 30% of equilibrium state). The results showed that bedrock exposures around bar fronts were stretched in the case bedrock cover thickness is thinner than fully grown bar height. But, in the case bedrock cover thickness is same as fully grown bar height, the exposures were not stretched. We assumed that easiness of bedrock exposer by bed material erosion and exposure rate at bar formed time related to the occurrence of exposure stretch. In the combined condition case of enough bed material thickness for bar height growth and 30% of equilibrium bedload feed, the exposure rate was not increased during experiment.

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It is an indispensable subject to understand the formation mechanism of the plane shape of a river channel, when considering maintenance of a river channel, and it serves as the basic point of river improvement or river environment. A meandering channel and a braided stream are generally mentioned as the plane shape of a river channel. Research on these formation mechanisms has been conducted. However, the relationship between sand bars and bank erosion which governs the plane shape of a river channel has not been amply clarified. The hydraulic experiments on bar formation using an erodible bank channel were conducted in this research. When alternate bars were formed, straight channel shifted to the meandering channel. When double row bars were formed, there were a case of shifting to meandering channel and a case of shifting to a gourd type channel. When riverbank erosion starts, migration speed of bars becomes slow and the bars develop to the transverse direction. It was confirmed that the observed wavelength of meandering or a gourd type wavelength is almost double of the stability analysis result.

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In this study, we performed numerical simulations on the change of bedform under an unsteady discharge condition using a vertical two-dimensional flow model for bedform evolutions. In order to clarify the effects of non-equilibrium sediment transport in the bedform response to the flow variability, we compared the bedform response characteristics calculated by non-equilibrium sediment transport model with that calculated by equilibrium sediment transport model. It was found that the non-equilibrium sediment transport model reproduced the increase in bedform wavelength with the increase of discharge and the gradual decrease in wavelength after the discharge decreases, which were similar characteristics observed in experiments.

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Linear stability analysis of the formation of fluvial sand bars with bank erosion is performed with the use of St.Venant shallow water equations and continuity equation, and a process-based bank erosion model. The process-based bank erosion model is derived from the integration of sediment continuity equation in the bank region. The time variation of bank locaion is proportional to the lateral sediment transport rate at the junction between bed and bank regions, which is evaluated by the lateral sediment transport rate in the bank region. and the time variation of the bed elevation at the junction. It is found from the analysis that quasi-meandering with the wavelength slightly shorter than bars takes place with the formation of bars in the range of moderately large aspect ratios.

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A linear stability analysis of interfacial instability induced by turbulent flow on the ice surface is performed by the use of the Reynolds-averaged Navier-Stokes equations incorporated with the mixing length turbulent model, the heat transfer equations in ice and flow,and the Stefan condition to describe the time evolution of the ice surface.It is found from the analysis that, if the outside air temperature is higher than the melting point and the ice bottom temperature is lower than the melting point, antidune-like boundary waves are formed on the ice surface while, if the outside air temperature is lower than the melting point,dune-like boundary waves are formed on theice surface.

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This study is concerned with the dimensionless tractive force τ*S for stable low-water channel of gravel and sand bed alluvial rivers. A non-dimensional parameter α contained in the τ*S -relation, that has been obtained theoretically by the author, is examined with use of existing data of natural Japanese and overseas rivers as well as three Japanese A-class rivers in Kyushu, so that such relationship as α=KI-1/2 is identified and the value of coefficient K is simultaneously estimated. Then, the obtained τ*S-relation compares with τ*S for the above mentioned field data, and the validity of the τ*S-relation is confirmed. In addition, the stability of low-water channel of the Onga River after the flood in 2012, when maximum past flood has occurred, is examined by means of the τ*S-relation, so that river sections required close attention for river managements in the Onga River are identified.

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As one of flood control measures, installation of a fixed weir is scheduled at the bifurcation point between the Asahi River and its distributary, the Hyakken River. Forestation has recently been of concern in these rivers, because it might cause a change of diverging discharge at the weir. Therefore, it is necessary to clarify the effect of vegetation on distributary for river management. This study describes flow simulations for two floods of different scales in these rivers, and demonstrates the relation between vegetation conditions, longitudinal variation of water levels and diverging discharges. Results showed that vegetation growth contributes little to increase of water levels because these rivers were mostly covered by trees whose heights were taller than water levels during these floods. Additionally, simulated water levels are approximately consistent with observations, except for situations when the Asahi River discharges were recorded at less than 2000 m³/s. Such discrepancy is supposedly attributable to low accuracy of the rating curve.

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Vegetation communities are often observed to grow in stripe pattern or patches within many stream channels. In such a situation, hydraulic engineers should examine an appropriate vegetation management system paying attention to flood control and vegetation ecosystem. On the other hand, the cavity zone along riverside is expected to produce native habitats of various ecosystems and water-friendly environment. However, effects of side cavities on flow resistance and turbulent flow structures are ambiguous. In this paper, the influence that arrangement patterns of longitudinal discontinuous vegetation zones gave to the flow resistance in an open-channel was examined experimentally. Results showed that the relationship between flow depth and discharge significantly depended on the arrangement patterns and the reason could be explained by the difference of momentum transport from main flow region to vegetation and cavity zone.

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The influence of the model drag coefficient on prediction accuracy of flows past emergent vegetation canopies by a classical canopy model has been examined. Two types of numerical simulations are carried out: The preliminary analysis based on a two-dimensional immersed boundary method gives an estimate of the spatial distribution of the drag coefficient over the vegetation canopy of the target flow. The estimated drag coefficient is introduced into the main analysis where a three-dimensional large-eddy simulation (LES) together with the canopy model is employed. The prediction accuracy of the main analysis is discussed against the corresponding experimental data and another LES result with a uniform drag coefficient in the canopy. The present LES with the estimated drag coefficient shows an improved prediction especially on stagnation phenomena at the leading edge of the vegetation patches. The improvement, however, is found mainly at the time-averaged flow level only. This is because the estimated drag coefficient is obtained from the time-averaged flow and does not reflect the instantaneous flow features directly.

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Drag in a row of equally-spaced circular cylinders arranged along the centerline of a straight duct is critically examined by a series of high-resolution numerical simulations based on an immersed boundary method. Time series of the pressure gradient required to maintain a constant flow discharge which is an indicator of the macroscopic resistance to the flow are obtained from the numerical experiment.It is revealed that the resistance is maximized when the surfaceto-surface distance between neighboring cylinders is close to the cylinder diameter,irrespective of the bulk and stem Reynolds numbers of the flow. The pressure gradient is found to be roughly scaled with the effective duct width L. and the bulk-mean veloicty Ub. , which take it into account that the actual cross-sectional area of the flume is decreasd by the presence of the cylinders. The present study suggests that the influence of the stem Rynolds number needs to be reflected for a better scaling of the drag.

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Most rivers in Japan have currently accelerated vegetation growth due to human interventions, e.g., dams and weirs. Such accelerated growth resulted in reduced habitat heterogeneity, gravel riverbed, and flood conveying capacity by increased flood water stages, etc. Even though some models were developed to quantitatively analyze herbaceous-plant and woody succession, they have only introduced vegetative growth rates statically as a function of their flood levels but not addressed its dynamic succession processes. Firstly, we developed a numerical model for herbaceous-plant and woody succession that integrates soil nutrient and morphological processes. Secondly, we selected, as a research field, a middle reach of the Sagami River and analyzed spatiotemporal patterns of vegetative succession and morphological processes. Thirdly, we verified and showed applicability of the developed model.

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This study aimed to evaluate the practicability of Plant Community Cluster (PCC), a new spatial unit proposed for riparian vegetation management in our previous study. To verify the PCC, we conducted the demonstration experiment on ability of the PCC in the Chikuma River. First, we examined the PCC could extract the plant community boundaries pointed by the plant ecologist and river engineer. Second, we examined the PCC abstract trend of surface substrate material change. In the results, we clarify the following that; (i) boundaries extracted by the PCC were almost coincided with boundaries of landscapes classified by two river engineers, (ii) PCC had the correlation to trends of surface substrate material change. This result indicated that PCC have the practicability for the riparian vegetation management, and possibility to clarify the mechanism of vegetation dynamics in the river.

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The floods sometimes occur so that the discharge, velocity, flow depth are changed. In such situations, fish behaviors may be changed. However, there is little information on the fish behaviors in flood. In this study, the swimming behaviors of Zacco Platypus (Oikawa) in the increasing discharge flows were recorded with a digital video camera. It was found that swimming speed of Zacco Platypus increased with an increase of velocity. In contrast, the ground speed of Zacco Platypus did not so change, irrespective of increasing discharge. The ground speed and swimming distance of Zacco Platypus did not change, even if the acceleration of velocity is changed. The effects of the acceleration on the swimming behavior of Zacco Platypus was smaller than that of the velocity.

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Ice harvor type fishway has commonly installed in Japan because it is a hybrid type fish ladder which is comprised of weir and orifice. But under severe low discharge and bed degradation condition, only the orifice flow state exists. In this case, fish cannot migrate through the fishways. To operate fishway in such severe flow condition, we proposed to close part of the orifices and put triangular shape blocks on the weir. Function of fishway was investigated by using 3D flow simulation. Numerically obtained flow state in the fishway with ten series of ice harbor type fishway shows in good agreement with the experimental results. And the proposed method can maintain favorable fishway function under low discharge and bed degradation condition.

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Most of fishways installed in Japanese rivers belong to the pool-and-weir fishway type. This fishway is usually made by concrete, so that the color of the pool side-wall and bed are gray. On the other hand, it is pointed out that the fish behavior is affected by the wall color. Unfortunately, the effect of wall color in the fishway on the fish behavior has not been investigated. It has been found that ayu (Plecoglossus Altivelis Altivelis) evade red (5R4/14). In this study, the position where painted red were changed into pool side-wall and bed. It was found that the migration rate increases by painting downstream and left bank sidewall. It’s because ayu evaded painted sidewall and center of fish’s school is located nearby upstream notch.

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Fish sometimes enter into an intake gate of weirs and dams, so that a lot of released larval ayu (Plecoglossus altivelis altivelis) are lost. Some techniques with air bubbles, electricity and transmitted light have been developed to avoid fish from the intake gate. However, a lot of fisheries co-operative associations pointed out that the most of such techniques have no effect. It is qualitatively known that fish avoid the area which is included air bubbles. In this study, the air bubbles are injected from the bottom of channel with changing the flow velocity, and trajectories of fish are recorded. It was found that ayu avoid the bubble area with an increase of air quantity. The swimming speed of ayu is decreased in air bubble area. In addition, the increase of the flow velocity is promoted evasion behavior of ayu.

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In this research, the attentions are focused on the gravel deposit in the pool-and-weir-fishway and fish behavior. The purposes of this study are examined the effect of the run-up ratio (Rr) on gravel deposit and to make clear the fish behavior. Therefore, the experiments were done. As a result, gravel deposit has decreased the area of plunging flow. Then, Rr of Run2 decreased by approximately 60(%) compared to Run1. Because gravel deposited in the pools, fish rested around the pockets of gravel. Moreover, regardless of the possibility of gravel, fish exerted the unique behavior against the flow and fish swam near the sidewall by choice. The flow velocity of the areas where fish swam was approximately 6BL ~8BL (cm/s).

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This research was conducted to make clear the characteristics of run up of fish in the small model full cross fish way with rubble stones. The efficiency of fish way depends on the structure (shape, bottom slope, rubble size), hydraulics (velocity, depth, turbulence).At first, Authors clarified the relation between the run up ratio and flow condition; Namely, 1) as discharge becomes bigger, velocity increases over than the limit of the swimming capability, run up ratio becomes lower consequently, 2) in the case of longer fish way, the run up ratio becomes lower since fish can not keep the maximum swimming speed (burst speed) for so long time.Next, authors consider the improvement the run up ratio by adding pocket or flat space with rubbles damping the velocity on the way of fish way.Finally, a research was conducted the effectivity in the situation that a part of fish ways were broken.

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An artificial tidal flat was created to compensate for the loss of a natural tidal flat in Okinosu, Tokushima City, Japan; however, the ecological conditions of benthic animals in the artificial habitat differ extensively from those of benthic animals in the natural habitat. In particular, the mud snail Batillaria cumingi, which overwhelmingly dominated the natural tidal flat, was rarely found in the created habitat. We confirmed that a "sinking event" occurred only in the artificial tidal flat. The "sinking event" caused B. cumingii to sink into the sediment and decreased the survival rate. Our study suggested that the sinking event was caused by the lack of silt content in the sediment of the artificial habitat.

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Corbicula japonica is important species in the Teshio River such as fishing product. To keep clam production, it is necessary to investigate habitat condition. Therefore, we have to understand the relationship of elevation and salinity for surviving freshwater clam in the Teshio River. We observed abundance of clam, flow velocity and salinity in the Teshio River. And we conducted to measure salt water boundary using flow velocity of Acoustic Doppler Current Profiler (ADCP). Abundance survey indicates freshwater clam lives widely longitudinal, and elevation more than -1.5m. Observation result of salinity showed sea water run up completely two-layer form. Flow velocity near boundary of fresh and salt water showed a large difference. We estimate the time variation of boundary elevation using ADCP result. This estimation result suggests Corbicula japonica lives in freshwater condition, clam can hardly reproduce due to flow and salinity conditions.

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In this study, we classified in river basins of Chugoku District and created a stream classification map by using GIS analysis and model based clustering. They are classified into 8 types. Then, we clarified basin characteristics of class A river based on above noted classification method. The results of clarification of basin characteristics are effective to express the basin characteristics. Furthermore, we found that faunas of genuine freshwater fish can be potentially explained by basin characteristics. From these, stream classification map can be applied for not only elucidation of water quality but also distribution of freshwater species.

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The concepts of stream metabolism were adopted to clarify the impacts of run-of-river hydropower system on the stream ecosystem. We conducted a field survey with logging the continuous data of dissolved oxygen concentration above and below the hydropower dam. In order to estimate the primary production and the community respiration, the practical procedures were employed with collected data of dissolved oxygen concentration according to the Kayaba’s empirical model based on the Odum’s concept. As the results, daily gross production, community respiration and their ratio could be degenerated especially below the dam at low flow conditions. It was also clarified that the availability and limitation of the present method to adopt this kind of environmental impact assessment.

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A nature restoration project has been conducted in the Oku River in the northern part of Okinawa Main Island. The purpose of this project is to restore the habitat of Ryukyu-ayu, which is extinct in the wild in Okinawa Main Island. The removal of drop structure and the restoration of pool by withdrawing the river bank were conducted to prevent the water leak as subsoil water and revive the structure of riffle and pool. The restoration for fish habitat was indicated by the results of investigation on fish and riverbed forms. These river improvements were evaluated in terms of bed deformation using numerical simulation. The results of numerical simulation indicate that the removal of drop structure induces the change of the longitudinal profile into original one, and the restored pool maintains the function as a pool over the next ten years.

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The Yoshino River, Japan, is rich in water resources, but simultaneously possesses high risk of flood disaster. River improvement works to reduce damage from the floods have been conducted which attain increase of the discharge capacity. On the other hand, local sediment deposition and scouring are growing concern by the repeated floods, decreasing of sediment supply and construction of river structure. In order to resolve these problems, it is necessary to understand the effects of natural and artificial impacts on the river channel. In this paper, the channel transition processes were studied in the Middle Yoshino River using aerial photo, morphology data. Natural and artificial impacts on those processes were evaluated from sandbar deformation, fluctuation of bed elevation and volume in low-water channel. As a result, sandbar deformation by the floods and revetment works was confirmed. The floods and dam construction affected fluctuation of river channel with the sediment deposition and scouring.

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This study examined the morphological behavior of the bed in an open channel flow with vegetation effects by means of a depth-averaged two-dimensional numerical model. We simulated the channel morphodynamics under the experimental conditions and characteristics of channel morphology can be reproduced numerically. In order to take into account the effect of vegetation, a vegetation model (germination-growth-death) is also incorporated. The numerical results showed that general characteristics of the bed morphology simulated by the present model are in compatible with the features pointed out in the previous papers. The agreements between the computations and the experiments were generally good. The present computations, not only support the previous empirical and theoretical results on river morphology, but also show that the present computational model is a powerful tool to predict river morphology under effects of vegetation.

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Large-scale groins were constructed in the late 19th century in the downstream basin of the Kiso River. Riverside pools are formed between the groins, providing various ecological systems and favorable river environments. In this study, we analyzed configuration characteristics of scouring formed behind the large-scale groins of the Kiso River. We conducted field observations by setting up ADCP and other observation equipment downstream of the groins, and analyzed the downstream flow in relation to the scale of the flow, etc. through three-dimensional analysis. As a result, we found that a transverse vortex is formed downstream of the groin during flooding events, and that this vortex may have contributed to the generation of the downstream scouring.

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Importance of small-size material on invertebrate dynamics has been pointed out in previous studies. For clarifying the reference diameter that greatly affects the dynamics, invertebrate was sampled on the downstream riffle of Futase Dam. The average body length of the 4 invertebrates are around 4-14mm and related small particle size are supposed around 1.4-4.1 mm. Five methods for sampling large and small gravel-sand particles were tested. Image analysis that represents macro roughness has a possibility not to count small size particles. Combined method that uses appropriate methods for macro and micro roughness has been proposed. The method can represent the small size particle which is related to the drift phenomenon of invertebrates.

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Subsurface water flow in a gravel bar is closely related to the river environment and stream ecological system. In this study, we investigated the characteristics of subsurface water flow focusing on a gravel bar which has various heterogeneous effects, such as permeability and porosity of gravels, and bar shapes, during normal flow stage. A network of wells was established to measure water table elevations in gravel bars of gravel bed rivers in Tokyo, Japan. Permeability and porosity of subsurface layer were surveyed by considering the relationship with "gravel ratio" as a new indicator. The characteristic results suggested are: 1) the subsurface water almost stop flowing in downstream part of gravel bar based on the consideration of the relationship between the gravel ratio and permeability, and 2) the binary particle groups is effective to present permeability and porosity of gravel bed in actual river. These above mechanisms are verified using by experiments and the numerical simulation model. Thus, this suggested that the water exchange characteristics through both of surface and subsurface layer in the gravel bar are restrictive.

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Flood-channel excavation is not only countermeasure against river flooding but also effective operation for floodplain restoration. In the Ibi River, 14 excavation sites varying in relative height to water level were investigated to examine process of microtopgraphy formation after the flood-channel excavation. Deposit rate was lower in the sites with lower relative height. The proportion of water area (i.e. floodplain waterbodies) in the excavation sites was 10-20%. Sediment profiles were also surveyed at 10 locations in two representative sites and were elaborated in relation to microtopographies that were classified into four types: natural-levee, low land behind natural-levee and isolated and connected waterbodies. Moreover, the microtopography formation process and factors were estimated based on the results of the field survey.

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Although watershed of Lake Teganuma have had a lot of radioactive hot spots since 2011, it is unclear when radioactive contamination in urban area, river and lake will be purified. This study attempts to evaluate radioactive cesium supply from the watershed to Lake Teganuma by using a distributed model for SS in watershed and monitoring data for radioactive cesium in watershed, river and lake. The present model is developed based on "SIPHER model" which is a distributed model for water and material cycles. The results indicate that 1)the present model can reproduce accurately temporal variations of river discharge and SS flux, and 2)the supply of radioactive cesium evaluated with the product of the simulated SS and measured cesium were good agreement with the observed data, showing the fundamental validity of the present approach.

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To quantify pollutant inflow load in wet weather and elucidate its influence on water and benthic quality in urban rivers, field surveys and laboratory experiment were conducted. As a result, water qualities of drainage water from the CSO outlets and freeway outfalls showed significant fluctuations according to precipitation variation. The pollutant inflow load in wet weather had a minor impact on the mass balance in the waters. Moreover, the results of stable isotope ratio analysis indicated that the organic matter from CSO outlets and freeway outfalls did not constitute a large portion of sediments in the rivers.

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To evaluate fertilization effect of the Mekong River flood and inundation, we modeled phosphorus transport. Using this model, fertilization effect and effect of inundation on agriculture were evaluated. We obtained the following conclusions; 1) Settling velocity of phosphorus was calibrated to 0.25m/day. 2) Considering wind effect for settling velocity of phosphorus improve the model especially at the edge of flood plain. 3) Fertilization effect was evaluated spatially and quantitatively. Except for some edge areas of flood plain there are fertilization effect. 4) Effect of inundation on agriculture was evaluated spatially and quantitatively. Some areas need application of fertilizer for current production.

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Transport of suspended sediment (SS) and radiocaesium was investigated for every particle size in the Okawa River system. Turbid water was sampled during flood at five stations, and particle-size distribution was measured. SS was collected using three classes of filter: pore size 0.3 μm, 20 μm, and 40 μm, and the concentrations of SS, ¹³⁷Cs, and ¹³⁴Cs were analyzed. The weight of SS caught by each class of filter and the weight of the same classes calculated by the size distribution did not coincide. Gaussian distribution was applied to the clogged particles below the pore size for the filtered sample. Total Cs concentration had a negative correlation with the calculated center diameter (d50); total Cs was approximately 2,000 (Bq/kg) when d50 was 5 μm and it decreased to 1,500 (Bq/kg) when d50 was 50 μm. Furthermore, total Cs was highest at the start of the storm discharge and it decreased toward the peak of flood. It was found that the transport of Cs was affected by both the size of the suspended particles and the phase of the flood discharge.

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