This study aims to determine the sediment volume generated by slope collapses in a river basin based on hydrological and geographical factors. For comprehensive sediment management throughout the river basin, there is the need to estimate the volume of sediment yield from mountain slopes as well as to estimate river channel sediment dynamics. In estimating the volume, one must consider not only the scale of rainfall during flood, but also the basin storage capacity, which is an index of the degree of moisture in the basin. Therefore, the authors quantified the basin storage capacity by using a hydrologic cycle model that simulates long-term spatial distribution of rainfall and runoff before and during a flood. This paper proposes equations for estimating the volume of sediment yield in the Nukabira river basin of Hokkaido by using rainfall, slope angle and basin storage capacity as explanatory factors. As a result, good reproducibility was obtained for the total of sediment volume and for the distribution of sediment.

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Most gravel-bed rivers show the longitudinal sorting on riverbed materials. This phenomenon is due to combination of the selective sorting and the abrasion in bed material transport processes. We developed the stochastic model of bed-load transport with gravel abrasion processes. Applying this model to some ideal rivers which have different concavities in their longitudinal profiles and different geologies in their watersheds, we examined that the longitudinal sorting of riverbed materials was dependent not only on their selective transport according to the hydraulics, but also on their abrasion processes characterized by their watershed geology and lithology.

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For comprehensive management of sediment of inner bays, we attempt to evalaute suspended-solid(SS) source in the watershed of Tamagawa River, one of large rivers flowing into Tokyo Bay. For this purpose, we conducted continuous monitoring of turbidity in four stations and X-ray fluorescence (XRF) analysis of riverbed material obtained at many stations. It is found that the coefficient b in sediment rating curve in low-flow condition is larger than that in high-flow condition in mountain rivers like Akikawa River. The results reveal that 67 % of total SS transports in Tamagawa River is transported from mountain rivers and other of SS is mainly supplied from riverbed materials which are picked up under high-flow conditions. These tendency are also confirmed through the XRF analysis of bed material.

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A coagulation treatment is one of countermeasures for long-term turbid water in a reservoir that occurs after a large inflow discharge or in a large-scale water circulation. The conventional coagulant has a problem of treating withdrawn sludge and the development of natural coagulant which can be deposited in a reservoir is demanded. We have taken allophane for this study and have conducted some investigations for practical use. We used turbid water collected from a reservoir. The investigations have been conducted by means of experiments in settling cylinders. We found that both distribution of coagulant and agitation of turbid water were indispensable for coagulation treatment. Moreover, we found that sufficient duration of distribution brought effective coagulation. Finally, we found that the allophane has limited coagulation performance, which can be estimated by measuring zeta potential.

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In Kurobe sediment flow system, several problems have been caused by change in the sediment balance, such as sedimentation in dam, river bed degradations, beach erosions and etc.. It has been necessary to resolve these problems and improve the sediment transport by the sediment management. In order to promote the sediment management, it is important to study sediment balance within effective grain size ranges and to know a grain size range which causes the change of sediment transport. In this study, the effective grain size ranges is fixed in Kurobe sediment flow system, and the causes of the change of the sediment transport and issues of sediment management are confirmed in view of the sediment balance within the effective grain size ranges.

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The main purpose of this study is to understand the evolution process of river bed in case that the sediment size distribution is extremely wide. In this fundamental experimental study, the bed is composed of three groups of sediment, and one of the grain size is completely different from each other. One is L-particle which cannot move at all in the stream, and constitutes a bed frame. And M-particle or S-particle exists in the space surrounded by L-particles, and moves as bed load or suspended load, respectively. Hiding effect of L-particle on M and S-particles was evaluated, and the vertical structure of bed was also made clear in the evolution process toward a static equilibrium state. In addition to these, it was found that the arrangement of L-particle cannot affect the state of equilibrium state.

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To understand and explain mechanism of the stones moving in streams is required for among natural and artificial environment, such as flushing bypass channel design, debris flows and sediment-laden flow in rivers. This paper deals with three-dimensional computational method for stones moving in streams taking in to account the shape of stones and mechanical interaction between stones and streams. The method is applied to a real scale experiment in concrete channel in which stones moving in streams were measured. In the case of one stone in streams, validation of the model has been checked by the experimental results. And the importance of stones shape for their moving is demonstrated by the analysis. In the case of a cloud of stones in streams, the present model seems to under-estimate the effect of moving stones and to need the improvement of the model.

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In this study, numerical simulations using equilibrium, non-equilibrium models and DEM for sediment- transport were performed for local scour below a weir. First, the coupling technique between the vertical 2D flow model introduced with the FAVOR method and DEM was explained. Next, comparisons with the previous sediment-transport formulas and the DEM model were carried out, and the validity of the DEM model was examined. Finally, simulations by each model were performed for initial local scour process below a weir. Consequently, the DEM model was able to reproduce the local scour process. Moreover, the development speed of the local scour by the non-equilibrium sediment-transport model was slow. Therefore, it was pointed out that the model is not suitable for prediction of this phenomenon.

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This paper presents the fundamental studies on the bed degradation process of bedrock induced by the bedload transport over the bedrock. We performed the laboratory experiments focusing on the dependency of the grain size and the strength of bedrock for the erosion velocity of bedrocks. The experimental results show that the grain size and strength of bedrock can affect to the erosion velocity equation of bedrocks. In addition, the movement of the sediment particles over bedrock which has constant side inclining bed was measured to understand the direction of sediment over the bed. We also carried out the numerical simulation of bedrock erosion process by the sediment transport over bedrock to understand the performance of the model proposed by us. The comparison between laboratory experiment and numerical simulation shows that the proposed model overestimates the effect of local slope for sediment transport direction over the bedrocks.

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In rivers located in mountain areas, a series of steps are often observed to be formed on bedrock. This is called cyclic steps formed by bedrock incision. This study proposes a simplified model for describing the erosion process by abrasion due to bedload, and obtain an equilibrium step shape migrating upstream without changing its shape. It is found that there are three important parameters: the threshold velocity Ut for bedload, the availability function at the Froude critical point pc, and the average slope. Once these three parameters are given, the wavelength L, the erosion rate averaged over one step length En, and the water and bed surface profiles are determined. It is found that qualitative features of steps observed in experiments are well described by the analysis.

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A fundamental character of central bars hasn't been fully understood because there is a large difference of its existence form between natural river and laboratory experiment. For example, "mode degradation of central bars". This study indicates central bars don't degrade mode and keep its form in channel periodically repeats expansion and contraction. We found out conservation condition of central bars is closely related to channel plane form. This means existence form of central bars is never comprehended without combination of idea of area separation of sandbars and effect of channel plane form.

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Simulate computation for the bed topography of a selected reach in the Toshibetsu River was conducted by using the free software RIC-Nays to analyze 2 dimensional river bed evolution of alluvial rivers. Results show good agreement with the observed bed topography, in local bed scouring due to channel bend, alternate bar formation and so on. Next,Blondeaux-Seminara's linear index parameters showing meander growth rate and bar growth rate were considered giving sampled data from the RIC-Nays simulate computation, in order to know the effects of bend-instability and bar-instability in the bed-form development process. From the consideration, it was known that (1) typical meander wavelength (around 2000m) in Toshibetsu River is close to the resonance wavelength found in the linear analysis,therefore the bar-instability is steady and the bed is scoured by the effect of bend; (2) alternate bars grow with around 1200m wavelength (600m bar length) in the whole channel reach, however the effect of bend instability work to suppress meandering of channel with this wave length; (3) BS parameters change the values in each local section of the river channel following the time of calculation process.

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The extension of relative elevation between low-water course and alternating bars is one of the typical figures in gravel-bed river with much vegetation and that makes strong meandering flow and bank erosion on the opposite side of forested bars in several medium-size floods. In this study, such a morphological change in gravel-bed river is investigated from a view point of interaction between wood land expansion and alluvial process of alternating bars. A depth-averaged flow and bed-variation analysis is carried out considering the events of invasion and destruction (wash-out) of riparian vegetation. Results of the simulation model show extension of relative height due to bed degradation in low-water course and deposition in vegetation zones and immobilization of forested bars which can be seen as resent channel characteristics of gravel-bed river.

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This study presents the numerical simulations of initiation and development process of tidal creek network by using two-dimensional morphodynamic model including the seepage flow model, and also discusses the effect of seepage flow for development of tidal creek network. A two dimensional shallow flow equation and two dimensional saturated seepage flow model based on Darcy law were combined to simulate flow field both on and in the bed. The numerical tests with and without seepage flow model were carried out to understand the effect of seepage flow for development of tidal creek in simplified conditions. The result shows that, by including seepage flow model, the channel network tend to become complex one, which has a lot of small scale channels. In addition, the channel width and depth increases by the very shallow flows induced by water supply from inside the bed. This indicates that the seepage flow from inside the bed can have dominant effect for morphodynamics on tidal areas, especially in case of subjecting the dry and wet condition in the bed surface.

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Recently, locally heavy rainfall occurs frequently at highly urbanized area, and causes serious personal accidents, so importance of flood forecasting system is growing in order to reduce damage of inundation. However, flood forecasting that secured lead-time for evacuation is extremely difficult, because the rainfall flows out rapidly. In order to develop a real-time flood forecasting method for locally heavy rainfalls in urban drainage areas, we have developed flood forecasting models with two types of simplified overland surface and have evaluated these models through case studies in an actual basin. As a result, we confirmed that the simplified models had enough flood prediction accuracies in practical use. Furthermore, the computational time decreased by applying simplified models; therefore, the effectiveness of proposed simplifying flood forecasting model was indicated.

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Recently, we often experience a localized torrential rain whose intensity is much larger than the design intensity for drainage network systerm. In such a case, severe damage of inundation is expected especially in urban area. In order to reduce the magnitude of damage, the information obtained by numerical simulation is so effective. Recently, the authors have started the attempt for a real-time forecast of inundation as well as the localized torrential rain. In this paper, two numerical prediction techniques in each field were summarized. Numerical simulation of inundation was newly attempted by using the observed mesh-data of rain, and the discussion about the results of prediction was also conducted.

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In order to understand the state of expected inundation in urban area due to a localized torrential rain, a numerical simulation technique can be extremely useful tool. The first author has kept conducting such a research work, and has developed his own technique. In this study, the effect of runoff process from a garden area in residential area to a road (and vice versa) was considered numerically, and the new submodel of it was added to the original model. And it was applied to the event which occured in the basin of the Myosyoji River in 2005. It was confirmed that the model works fairly well. As a result of this study, it was concluded that the effect of the above process was not negligible in this kind of simulation.

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In the last decade, torrential rain has been increasing and it caused pluvial and fluvial flooding in Japan. These floods caused inundations in urban and rural areas. Intensity of such rainfall was sometimes excesses over the design rainfall of drainage systems and rainwater was running over the road. In these floods, some portion of rainwater intruded into underground spaces and the users faced the danger of underground flooding. In this paper, pluvial flooding was investigated by InfoWorks CS and underground flooding in a mega-underground mall was also discussed on the bases of calculation by shallow flow model with structural mesh. And then, the effects of counter measures for reducing underground inundation were studied with the criteria of safe evacuation obtained by the previous evacuation tests.

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Short-time heavy rainfall disaster occurred in Osaka City on 27 August 2011. Hourly precipitation was 77.5mm which is the maximum record in Osaka. The purpose of this study is to find the vulnerable factors in pluvial flooding in densely urbanized area. Field survey was conducted in Kita-ku, Osaka City on 28 August 2011 and reproductive calculation was carried out with this rainfall. This research indicated the follows. Roads in high-density urbanized district turn into channels by heavy rainfall. The partial low-lying area such as "bowl-shaped" is prone to floods as much as underpasses under highway and railroad bridges. Store employees may fail to evacuate from the under and above stores due to a take action to prevent a flooding. In flood analysis, to improve the results it is necessary to consider the micro topography.

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Water disaster is one of the typical natural hazards and occurs in many places nowadays. Recently, inundation due to not only river flood but also interior runoff has caused serious damages in urban areas. Numerical simulation models, which consist of overland flow model and sewer flow model etc., have been developed for the examination of measures against inundation due to interior runoff. This study examines the modeling of inflow discharge from overland to sewer system and the setting of runoff coefficients. From the results of numerical simulations, it is found that 500mm/h is appropriate as the upper limit value of the inflow discharge. After the validity of our numerical model is established, its application to hard measures such as river bed dredging and soft measures such as inundation prediction is considered. For the soft measure, the inundation prediction system is constructed based on machine learning using the numerical results. Their results show that the numerical models are effectively applied to implementation of hard and soft measures.

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This study is concerned with breach discharge due to dyke breach for complete and submerged outflows in straight and meandering channels. Such breach characteristics as flow direction of breach flows, effective width of breach section were examined experimentally. Breach discharge and inundation flows were predicted by the numerical model that solves simultaneously flood flows in a channel and inundation flows in a flood plain with the dynamic inundation model, which comprises the finite volume method on unstructured grid using FDS technique. It shows that the model is capable of predicting both the breach discharge and inundation flows for complete or submerged outflows with good accuracy.

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The flood damage mitigation effects of flood protection forest against inundation flows, due to dyke breach, were investigated experimentally and numerically. It is found experimentally that dyke breach flows are diverted and diffused by the forest, so that the velocities and hydrodynamic forces of the flows behind the forest are substantially reduced. It is also demonstrated that the dynamic inundation model, which comprises the finite volume method on unstructured grid using FDS technique, is capable of predicting such effects of the forest against the flows with good to reasonable accuracy. Numerical experiments were also carried out to reveal the effects of arrangement of the forest on the flows and houses.

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The Great Mississippi Flood of 1927 was the most destructive river flood in the history of the United States. After the flood in 1927, Mississippi River and Tributaries Project has carried out and changed the engineering policy. One of the biggest projects in MR&T project is the constriction of Birds Point-New Madrid Floodway. In 2011, May, Mississippi River had flooded dramatically and floodway had used which have not been used since 1937. By using floodway smoothly, explosion of levee had executed which was including in Birds Point-New Madrid Floodway operation measure. On our research, we evaluate the effect of the floodway and reproduce the flood by using numerical simulation. By our simulation, floodway has the effect to reduce the water level 1.5m~3m. We also simulate the characteristics of the floodway.

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The Yubari River is a tributary of the Ishikari River, which flows through Central Hokkaido. In 1898, 30 years after the Meiji Governmenlaunched the policy of encouraging migration to Hokkaido, a historically disastrous flood struck the Ishikari River Basin. Of the 112 people killed in this flood throughout the Ishikari River Basin, 82 fatalities were in or near Kuriyama in the Yubari River Basin, which is part of the Ishikari River Basin. No other recorded flood in the Ishikari River Basin has claimed as many victims. In Kuriyama Town, word of this disaster has been passed down in oral tradition and written records. However, because little hydraulic data is available to estimate the scale of the 1898 flood, including the flood discharge, the flood mechanism and damage have not been adequately investigated. This study aims to elucidate the mechanism of this flood through detailed examination of accounts of flood damage that have been orally transmitted for more than a century.

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A flood occurred in Saba River which passes through Hofu City, Yamaguchi due to heavy rainfall on 21st July, 2009. The rain induced slope failure and caused a large quantity of sediment to flow into the river. In this study, the estimation of the flow balance and the sediment balance were conducted by using numerical simulation and the river bed elevation data observed before and after the flood.The numerical simulation for the flow balance in which the inflow discharge from the branch rivers and the overflow from a bank are considered is in good agreement with the observed hydrograph at the Shinbashi station. Although the numerical simulation overestimates the volume of the sediment accumulated in the river, the accumulation properties in the river can be simulated qualitatively.

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This paper examines the flood situation of the Yoshino River in the Meiji era. Using water level records and lateral profile of the river of those days, a numerical simulation was performed for the flood in Meiji 44, and it was compared with the situation of the flood damage described by the newspaper articles. The flood actual condition in Meiji 44 was investigated in general by the numerical analysis. In the Yoshino River of the time where maintenance of a river bank etc. was not progressing, it turned out that the flood often occurred and it had suffered the serious damage in which many bridges and houses were washed away. As mentioned above, by clarifying the situation of the past flood, when considering future flood control plan, it becomes precious data.

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In order to analyze regional changes frequency of torrential rain that defined hourly precipitation of 50 mm, 75 mm and 100 mm, we represented the spatial distributions of frequency of torrential rain. Secondly, we analyzed relationship of human damage and precipitation pattern in past flood and sediment-related disasters. Number of candidate disaster which more than five persons are killed or went missing in is 49 since 1976. We used total precipitation, total precipitation and maximum hourly precipitation during a disaster period as indicators of precipitation pattern.The results of the present research are summarized as follows:(1)From the 1980-1989 decade to the 2000-2009 decade, frequency of torrential rain had increased in Shikoku area, Kyushu area, Tohoku area and Hokuriku area. In particular, in Tohoku and Hokuriku area, torrential rain did not occur about 30 years ago, but in resent years, torrential rain occurs (2)Relationship of human damage and precipitation are studied by means of the statistical hypothesis test. The results show that Kyushu area has a higher resistibility for disasters than Setouchi area.

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Economic loss estimations for flooding in the Iizuka-city area in the Onga river basin were performed using numerical simulator for rainfall-runoff in watershed and flood inundation process in urban area with rivers and sewer network. The rain-fall runoff was simulated by a distributed hydrological model. The flood flow in rivers and inundation flows on the ground were simulated simultaneously by the 2D freesurface flow model. The dynamic network model for free-surface-pressurized flow combined with the Preissmann slot was used for the sewer network. The economic loss estimation based on the simulated inundation area and depth was compared with the reported value. The effects of flood disaster-reduction system were also examined based on the numerical results.

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This paper describes the development of a large-scale distributed rainfall-runoff/flood-inundation simulation (DRR/FI) model. The DRR/FI model can simulate the rainfall-runoff, dike-break and inland flood inundation processes simultaneously in a seamless/integrated manner. Most of the data for the modeling is obtained from the Digital National Land Information (DNLI) of the Ministry of the Land, Infrastructure, Transport and Tourisms. Data processing is carried out with GIS so that the coordinate system of the model is appropriately set up, thus the model can have the interface with other models such as weather, climate, evacuation, vulnerability and financial models. The model validation is carried out with an example for the Yodogawa river catchment (8240 km²). Modeling results can be applied to the insurance business as well as the government's disaster risk management.

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The current evaluations of flood risk are generally based on the index of very heavy and rare rainfall, such as the intensity of 100 years probability precipitation. However, this approach may lead to the underestimation of flood risk because the statistical records suggest that the flood damages can be caused by moderate rainfall events as well. Here we introduce a new method for evaluating flood risk which utilizes the distribution of daily rainfall intensity in order to include the flood damage due to the frequent rainfall into the evaluation. The precipitation intensity of each day is converted to the PEP(Precipitation Exceedence Probability) index which represents how rare the precipitation intensity of the day is compared to the probability of exceedence of the annual maximum daily precipitation. The proposed method suggests that the flood risk caused by less than 30 years probability precipitation is estimated up to 78% of the all flood risk. The proposed method seems robust because the evaluated annual flood damage all over Japan agrees to the actual flood damage recorded on the national statistics.

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Recently, flood disaster due to local heavy rainfall becomes a serious problem. Evacuation plan for flood disaster is necessary to save human life. The flood hazard map show flood area, evacuation places and routes can be confirmed by inhabitations. The timing of evacuation is very important for safe evacuation, but it is not shown in usual hazard map. In this study, the criterion for safe evacuation during flood was considered by the specific force per unit width, and evacuation timing was also discussed. The results are useful to make an evacuation plan.

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This paper describes criteria of caution and warning on evacuation of district in floodplain including small-medium sized rivers. We computed flood flow with inundation and from breach along rivers, in order to focus on depth and velocity in inundation area. As results, municipality alert to residents in almost districts for shelters, however if they lose the timing to evacuate, residents can choose to evacuate more than the second floor in house. Criteria of evacuation in few districts are discussed by accumulated precipitation, hourly precipitation and water surface level in rivers. We estimated the temporal difficulty of evacuation shelter in inundation from results of flood flowing analysis. Evacuation judgment criteria were suggested to be able to change evacuation more than second floor at occurrence of the difficulty to evacuate.

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Heavy rainfall disaster happened in eastern area of Shizuoka prefecture on September 8, 2010. Thirty-one houses were damaged and 108 houses were inundated in Oyama town, Shizuoka prefecture. In this town, although the house was destroyed, human casualties were not happened. Evacuation activity of this town was researched based on a questionary survey. Data were gathered from 874 households in December 2010. Twenty-two percent of respondents evacuated for shelter or another houses. This ratio is comparable as the results of research in past flood damage areas. The town office appealed for evacuation to residents at 11:55. Since the peak of rainfall or river water level was around 16:00, this appeal was successful as a result. However, the residents who took evacuation before 15:00 were only 50%. Seventy-one percent of the respondent has answered that this town has the danger of flood damage, and the ratio is high compared with other areas. However, we cannot say that this evacuation activity was positive.

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In flood hazardous situation, the effect of congestion and obstruction on people's walking speed may be crucial in evacuation process especially in urbanized areas. In order to simulate these situations, node and arc expression of street network is not enough because this kind of one dimensional expression cannot deal with actions of passing and avoiding the other evacuees.In this study, two types of evacuation simulation models which express the streets as a sequence of rectangles are developed and have been tested. One uses the experimental relation between crowd density and walking speed. The other type uses rule-based collision-avoiding actions of evacuees. The performances of those two models have been compared in the several simulation results in actual flood-plain areas in Japan.

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In the Asahi River, riffles and pools had been scaled down in the downstream reach of Asahi Dam due to sediment deposition in Asahi Dam reservoir. To improve this environmental problem, a flushing sediment tunnel was constructed so as to bypass Asahi Dam reservoir. In this study, first, the authors demonstrated the recovery of riffles and pools and the grain size distribution in the downstream reach by using the data of cross-sectional surveying, air photographs and field investigation. Next, authors' two-dimensional model was applied to a series of flood events in the Asahi River. As a result, the flushing mechanism of Asahi Dam reservoir sedimentation, the recovery process of riffles and pools and the change in grain size distribution in the downstream reach were clarified by the model.

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Reservoir sedimentation is one of the most important problems for securing long term achievement of hydropower operation in the future. In order to solve the problem, we have to note that there are two major types of hydropower stations, which are storage type and regulating type. We have already discussed that we have to focus on sediment management in regulating type reservoirs more than storage type ones from the view point of flood disaster prevention in reservoir areas. In this paper, we studied on technical and economical feasibility of hybrid method of draw-down operation and sediment bypass by numerical analysis, and we found that the hybrid method was very advantageous in both ways. We also analyzed the impact of sediment supply to dam downstream river by sediment bypass and found that it was possible to keep the safety of the river channel by some effective excavation and that the improvement of the river environment would be expected.

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To evaluate the impacts of sediment flushing on fish, it is necessary to predict the decrease of the local space of the bed at mountain stream which is considered the refuges. First, we flushed sediment from a dam to pick out the local spaces which are the refuges for fish and the impacts of sediment deposition on the local spaces. Second, we conducted a set of experiments using artificial sediment supplies to the pools to investigate the relationships between the volumes of the sediment deposition on the bed and the local spaces. Finally, we constructed a model that could calculate the variations in the volume of the local space based onthe volume of sediment deposited on the bed. We have found that we can roughly predict the variations of the volume of the local space insediment flushing and the impacts on fish based on our model.

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For the flood disaster prevention and reduction,the elucidation of the physical mechanisms on the breach of flood levee is indispensable. However, since extraordinary danger exists in the observation during flood and the scale of the phenomenon is large, the observation of the breach have been conducted after the flood. In order to clarify the phenomenon on breach of flood levee, a small-scale hydraulic model experiment needs to be carried out. In this research, we tried to reproduce the result of the actual size hydraulics model test on the breach using the small-scale hydraulic model experiment in which the scale was changed. As a result, in order to grasp a phenomenon correctly in a small-scale hydraulic model experiment, it was shown that it is necessary to coincide the square of the ratio of critical friction velocity with a model scale.

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Short fiber reinforced soil mixture have erosion resistance and tenacious performance against the destruction. In previous studies, erosion resistance of low velocity has been largely confirmed. Therefore, in this study, A series of fast channel experiment, outdoor exposure test and etc. was performed in order to examine erosion resistance and applicability to vegetation of short fiber reinforced soil mixture. In channel experiment, the amounts of erosion by high speed flow at the surface of specimens were measured. Amount of cement and short fiber were varied to make the specimens. As a result, the amount of cement and short fiber to be necessary to satisfy the erosion resistance was cleared. In exposure test, grass growth on the specimens, which have placed outside, was observed for two years. Additionally, soil hardness and soil acidity were also measured. As a result, the amount of cement and short fiber to have higher potential of the applicability to vegetation as well as the erosion resistance was revealed.

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In the lower Tone River, channel dredging has been conducted for the river improvement since 1900 (Meiji 33). However, dredging at the Fukawa contraction area was stopped in 1966 (Showa 41) because river bed elevation there dropped below the design bed elevation. But river bed degradation and scouring has gradually progressed until around 1998 (Heisei 10) in the Fukawa contraction area.In this study, we investigated the causes of the river bed degradation and scouring in relation to flood flow, dredging and river improvement works in the upstream and downstream of the Fukawa contraction by analysis of flood flow and river bed variation considering channel widening and dredging over time.Observed data demonstrated that the river bed degradation and scouring had occurred by floods and channel dredging in the upstream and downstream of the Fukawa contraction area. Numerical computation provided a good explanation for mechanics of bed degradation and scouring during floods.

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This paper presented an experimental study on the impact of both relative spur-dyke height and grain size distribution on the bed topography and bed surface composition around an impermeable spur-dyke. The importance of governing parameter, i.e. the relative spur dyke height, was emphasized through quantitative evidences. It was found in the current experiments that the spatial progress pattern of scour-deposition in the non-submerged and nearly non-submerged condition was different compared with full-submerged condition. In the submerged experiments using non-uniform sediment bed, the maximum scour depth decreased with the increased of the standard deviation as well as in non-submerged spur dyke experiments. The flow measurement around the spur dyke revealed the difference of three-dimensional flow structure under the different spur dyke submergences, and the relationship to the variation of the surface bed composition.

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In this study, coherent flow structures and mechanisms of suspended sediment transport around submerged spur-dikes were investigated by performing a flume experiment with Particle Image Velocimetry (PIV) technique for particles of neutral buoyancy and relatively heavy materials. Large Eddy Simulation (LES) and Direct Numerical Calculation (DNS) of particle motion were additionally applied in order to interprete the results by the PIV experiment. The experimental results show that correlation coefficients of the PIV analysis for heavy particles decrease when vorticity around the cross-stream axis increase around a shear layer between over-flows on dikes and stagnant regions in-between dikes. It is numerically explained that the heavy particles were preferentially transported by downward motions which were provided by separation eddies with cross-stream axis from top of dikes.

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By using three-dimensional numerical model, numerical analysis about local scouring around a pier was carried out. The method of the analysis is three-dimensional LES. The main objectives are to obtain the data of innovative prevention measure that is effective for the actual rivers. The prevention measure is to attach or bury the small-scale structures at the front base of the pier, and to change the current and reduce the scouring depth. Before now, many investigations are carried out, and several important results for the abatement of local scouring were clarified. However, they were the effects under the limited conditions in experiments and numerical analysis. In present paper, the small scale structure is attached or buried in front of the bridge pier and the effect of prevention measure to local scouring is considered. The scouring depth that the small scale structure is attached and buried is smaller than it that the small structure is only attached.

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Spur dikes were expected to create new aquatic habitats according to produce local scour and sediment deposition area around them on the movable bed. However, when the spur dikes were set on the rigid bed for creating new aquatic habitats, it was not known the potential ability to accumulate sediment deposition behind spur dikes. In this study, we conducted experimental flume tests and numerical analysis to confirm the characteristics of flow, sediment transport and bed deformation around the spur dikes on the rigid bed. These results showed that the sediment was not deposited behind the spur dikes, because the sheer stress is smaller than the critical sheer stress in the upstream area of the spur dikes, so that all bed materials were transported to the opposite bank side. However, when the flow is under unsteady condition, it is confirmed that the no sediment transport area is reduced because the large transverse water surface gradient is formed in the downstream area of the spur dikes during the increase process of water discharge and would change the direction of sediment transport material.

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Although gravel beds create a good environment for river ecosystem, the reduction of gravel supply from upper stream caused serious bed degradation problem in recent years. Under these circumstances, gravel bed area gradually disappeared and the exposure of soft rock has progressed. Once the soft rock appeared, it is very difficult to recover the gravel bed. Therefore it is very important to establish gravel bed restoration method. In the Tedori River, groins using natural stones were experimentally installed to restore the gravel bar and further river restoration project to install new groins will be carried out. Therefore, in this paper, the effect of the groins is examined using two-dimensional flow analysis. Simulation results show that groins installation has an effect on the restoration of gravel bed.

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Flood mitigation dam (FMD) is a gateless outlet dam designed only for the purpose of flood control which provides long-term and efficient protection against floods. FMD is expected as environmentally friendly, since almost all incoming sediment during flood periods can pass through bottom outlets that designed at the original river bed level and there will be fewer impacts to downstream river environment. However, during the flood discharge retardation, the characteristics of sediment (sand and gravel) outflow rate are changeable and unknown compared with the normal stage. Especially, coarse gravels and stones may be difficult to be transported through bottom outlets because of a gap of tractive forces between upstream and downstream of dams.This paper tried to find these gaps at Masudagawa dam which is the typical FMD in Japan. RFID technology has been used to monitor movement of placed gravels in the reservoir area and clear correlation has been obtained between transported length of each grain sizes and estimated tractive forces.

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It is known that there are two types of meanders; one is free meandering river and the other is incised meander. Formational and movement mechanisms of alluvial rivers are clarified well. However, the formational mechanisms of meandering rivers in lowland and wetland are not understood clearly. We measured the topography and flow velocity by using ADCP and GPS systems on Chiruwatsunai River, which runs through Kushiro Wetland National Park in Hokkaido. This paper shows results of velocity, topography, discharge, conditions of bar, traction or waftage and others. We clarified hydraulic characteristics and geomorphological characteristics of meandering rivers in lowland area and wetland are similar to them of incised meanders.

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In this study, the numerical flow and sediment-transport model including sub-models for bank erosion considering slump block, landing process and cut-offs, is confirmed that it can indicates that the effect of discharge variation and the formation of channel cutoffs are very important factor for the natural process of river meandering. In the steady flow calculation, the shape of meandering is transmitted from upstream to downstream. The wave height and wave length which transmit depend on the discharge. But in steady flow, the meandering shape which can be seen in a natural river cannot create. In the unsteady flow calculation, in which periodic discharge variation is set, the result shows the complicated meandering process which can be seen in a natural river. The results also indicate that the numerical model which includes the bank erosion considering slump blocks, landing process and cut-offs are reasonable.

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Numerical model for gravel transport was developed. The model employs relation for hiding and bed-load transport of mixtures of Ashida and Michiue (1972) and Wilcock and Croew (2003) and links grain size distributions in the bed load, surface layer and subsurface with the gravel transfer function. The model is also based on finite volume method using HLLC (Harten, Lax and van Leer(1983)) numerical flux. The model was applied to experimental data of a bed degradation and aggradation with armouring and downstream fining. It shows that the model has ability to reproduce the process of bed degradation and aggradation with aromouring and downstream fining, but the more improvement is needed to predict reasonable accuracy.

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A network of channels which consists of the Ota River floodway and five branched rivers is formed in the Ota river delta. To estimate flood discharge hydrographs and bed variations in each channel is important for proper river management. Therefore, we developed a numerical model for the flood flow and bed variation analysis using time series of observed water surface profiles with the networked rivers in the Ota river delta. As a result, the unsteady quasi-three dimensional analysis of flood flows and two dimensional analysis of bed variations using time series of observed water surface profiles is found to provide good results for the flood discharge distributions in the networked rivers in the Ota river delta.

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