To understand saltwater movement in the coastal confined aquifers, we performed laboratory experiments on saltwater in a confined aquifer and numerical experiments on effects of cut-off wall height and transversal dispersivity on the repulsion of saltwater wedge. Experimental results confirmed that saltwater wedge showed repulsion and numerical results showed that most effective wall height for repulsion was saltwater height at the location where the cut-off wall was installed. In the residual saltwater wedge, circulation flow occurred and fresh water discharge flowing along outer part of this circulation transported salt and finally flushed this out. Because that the direction of saltwater wedge to fresh water flow is transverse; the appropriate transversal dispersivity should be given when the saltwater removal with cut-off wall is numerically evaluated.

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Arsenic contamination of groundwater has been rising as a critical issue to be solved. In this study, one-dimensional transport model for arsenic behavior with microbiological redox mechanism was developed. This model defined five reductive-bacterial groups and two oxidative-bacterial groups to model reductive sequence and oxidizing process. Microbial growth was assumed to follow Monod type kinetics. The numerical simulation model includes the chemical species in the bio-, mobile- and matrix phases. This model can describe the mass transport, bacteria mediated bio-chemical reducing processes, iron precipitation and Fe-As adsorption and desorption process. The simulation results can reproduce the results of soil column experiment, and validity of the model was confirmed.

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Laboratory dye tracer experiments with a pulse source were conducted in homogeneous and stratified porous formations under saturated unidirectional flow conditions. A new methodology using spatial moment analysis linked with image processing of a dye tracer behavior was developed to estimate macrodispersivities both in longitudinal and lateral directions. A calibration relation between the dye concentration and the corresponding pixel brightness was established to quantify the spatial distribution of dye tracer. Both dispersivities asymptotically approached a certain value and exhibited a dependency on the difference of hydraulic conductivities associated with the stratification, which played a significant role in the evolution of transverse dispersion. Laboratory study was extended by a random walk particle tracking approach to reconstruct the estimated dispersivity transition, demonstrating a good agreement between the experimental and numerical results not only in homogeneous but in stratified porous media of concern.

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To improve short term prediction accuracy of rainfall, especially for orographic rainfall over mountain areas, a physical approach and a stochastic approach were introduced into a radar image extrapolation method with translation model. In the physical approach, radar observed rain field is separated into orographic and non-orographic rain fields by solving physically-based equations. In the stochastic approach, a bias-modified prediction field was calculated from a lot of spatial error distributions of the pre-predicted rain fields. In order to improve the separation accuracy of the physically-based orographic model, vertical wind velocity W is updated so that the physically-based procedure could re-predict the rain filed which meets to the bias-modified deterministic prediction field. As a result, one-hour prediction accuracy especially over mountain areas were much improved. In the future, more accurate predictions can be expected by improving the W-updating procedure.

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The efficient use of riparian groundwater has been proposed to improve the flow regime of an urban river. In case of a tidal river, it seems that dynamic interactions arise between stream and aquifer. However, its actual condition and mechanism of the water exchange have not been clarified precisely. This research consists of two parts. First part is the numerical simulation on response process of riparian aquifer to a tidal stream. Governing factors on the groundwater behavior and the water exchange process were evaluated by a hydraulic model. Moreover, second part is the field observation on behavior of unconfined groundwater and piezometric head under riverbed in the Horikawa River Basin in Nagoya. The observed data of groundwater are reproduced by the simulation model, and the cyclical repetition of infiltration and exfiltration through the riverbed were verified based on the piezometric head behavior in response to the tidal fluctuation.

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The authors analyzed the hydraulic features of the Daijuu Weir and neighboring levee of the Yoshino River in the historical perspective, as the structures are important factors of flood control for the river. Firstly, the authors analyzed the precious data of water level observations around the Weir started by De Rijke, a Dutch engineer in 1883. Then using a river survey map surveyed in 1901, they restored the status of levees of the Yoshino River before modern improvement works. Thirdly they analyzed a relationship between the Yoshino River Levee and the Daijuu Weir based on the newspaper articles of the fifty years of period from 1878 through 1927. As the conclusion they reevaluated the flood and flood control plan of the Yoshino River around the Daijuu Weir.

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Flood control administration is facing the turning point of policy. The new directions are the following: 1)a shift towards policies that "reduce dependency on dams," 2)addressing issues associated with climate change, including excess flooding from localized torrential downpours, 3)proper maintenance, intensive use and improvement of existing infrastructure, 4)advanced flood control using advanced technologies from various fields, and 5)alteration of zoning toward creating a region that permits flooding in cooperation with the administration in charge of land use. In this study, I comprehensively analyze the history of flood control administration in Japan toward proposing permanent flood control measures, including those that provide for altering the land use of floodplains. The study aims to support the development of flood control principles for next-generation.

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This study evaluated waterborne infectious diseases risk and incidence rate around Phonm Penh in Cambodia. We use the hydraulic flood simulation, coliform bacterium diffusion model, dose-response model and outpatient data for quantitative analysis. The results obtained are as follows; 1. The incidence (incidence rate) of diarrhea as water borne diseases risk is 0.14 million people (9%) in the inundation area. 2. The residents in the inundation area are exposed up to 4 times as high risk as daily mean calculated by the integrated model combined in the regional scale. 3.The infectious disease risk due to floods and inundation indicated is effective as an element to explain the risk. The scenario explains 34% number of patient estimated by the outpatient data.

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In this paper, an equation for describing the cumulative area distribution for river basins is theoretically derived by using a link magnitude law of drainage area and the bifurcation structures of channel networks obtained from Horton’s laws. Analyzed here are 109 large river basins in Japan, which have a wide variety of features in size, geology and climate. The GIS analysis of DEMs for the 109 basins shows that the Shreve’s link magnitude law of drainage areas has a universal scaling property in all of the 109 river basins with the same constant slope. As for the cumulative area distribution, previous researches in GIS analysis and theoretical examination have revealed that it has a power law distribution with a constant slope in lower stream orders. The equation derived here can represent the cumulative area distribution over the whole river basins including the portion of the power law distribution.

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When total supply is smaller than total demand, it is difficult to apply the paddy irrigation water distribution rule. The gap must be narrowed by decreasing demand. Historically, the upstream served rule, rotation schedule, or central schedule weight to irrigated area was adopted. This paper proposes the hypothesis that these rules are dependent on social justice, a hypothesis called the "Society-Justice-Water Distribution Rule Hypothesis". Justice, which means a balance of efficiency and equity of distribution, is discussed under the political philosophy of utilitarianism, liberalism (Rawls), libertarianism, and communitarianism. The upstream served rule can be derived from libertarianism. The rotation schedule and central schedule can be derived from communitarianism. Liberalism can provide arranged schedule to adjust supply and demand based on "the Difference Principle". The authors conclude that to achieve efficiency and equity, liberalism may provide the best solution after modernization.

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In this study, we examined the factor that community activity gave flood response and flood measure by questionnaire survey for Arasaki area. Firstly, we analyzed the influence that factor to give community activity and community activity gave flood response and flood measure by chi square test. Secondly, we perform the type of the respondent by quantification method 3 and cluster analysis, and analyzed a factor to give flood response and flood measure. As results, firstly, gender or age, the residence affected community activity. Secondly, the person that community activity was active participated in activities for flood disaster reduction and tended to expect flood measures in neighborhood inhabitants.

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We suggest to construct small scale dry dams in a river instead of a large scale dam in order to prevent a flood disaster and preserve a natural environment. In recent years, a flood control dam without a slide gate in spillway, known as a ’dry dam’, has been reviewed, planned and built in some sites. In this study, the effects of some slide gates in a bottom spillway which can be opened and closed automatically depending on hydraulic pressure called ’pressure gate’ for a dry dam are examined. In addition, width of a crest spillway gate is discussed to reduce rapid increase of flow discharge in the situation of overflowing. It is demonstrated that a dry dam has an additional ability to reduce damage from a flood surge. Small scale dry dams must be regarded as a flood retarding basin fitting to a natural environment. Therefore, construction of small scale dry dams is one of the powerful options to adapt a natural environment and global warming.

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For the comprehensive flood control measures for alluvial fans, it is required to understand the effects of natural levees and micro-topography on reducing flood damages. We investigate the role of natural levees in the flood control measures for the alluvial fan of the Ara River by mapping historical community development and using the hazard maps.As a result, it is clarified that many communities have been developed on natural levees, and that natural levees are resistant to spreading of flood waters. The above indicates the significance of researches on natural levees and micro-topography as control measures on alluvial fans.

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The river environment management measures of today have been originated from some pioneering river works conducted to meet the request from the residents around 1970-1990 when the environmental issues were the serious social concerns. Establishing a framework for the successful river environment management measures will provide the useful indicators toward the adequate river management under the changeable socio-economic conditions of the future. This paper proposes the framework and verifies its adaptability to some pioneering river works in Japan and abroad. As a result, it was revealed that the directions of river environment management measures were influenced by the backgrounds of the times and the regional characteristics. And showing the purposes and the effects clearly was a good incentive to promote the river works. Especially, by establishing the partnership between the river authorities and the residents through close communication, the ownership of the rivers as the properties and symbols of the regions was fostered, which led to the continuous utilization of the rivers by the communities.

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In Japan, the Flood Control Plan is designed by the Basic Flood Discharge (BFD), which is the hydrograph at a designated point in river. Also, flood control facilities should be designed based on the BFD requirement. The Flood Control Plan method of fulfilling BFD has been used since the Meiji-era (1868-1912), and the BFD has been continuously updated according to the situations of social, economical or technical development for each era. In this study, we investigate the historical transition of the revisions of BFD in all Class A rivers from the 1st Flood Defense Plan (1910) to the revised River Law (1997), and classify the revisions into 5 elements by the revised reason of BFD. We discuss the characteristics of the historical transition of BFD revisions and revised reason with the historical background for each era.

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The purpose of this study is to realize effective and efficient countermeasures against reservoir sedimentation in the near future. This paper shows the case study of JPOWER’s Itoshiro Dam regarding analysis of soil production area in the relevant dam’s basin. The authors are trying to clarify the major soil production area causing reservoir sedimentation by geological expertise and Egashira Model.

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Reservoir sedimentation is one of the most important problems for securing long term achievement of water power operation in the future. In order to solve the problem, we have to note that there are two major types of water power stations, which are storage type and regulating type, respectively. We have already discussed that we have to focus on sedimentation 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 study on technical and economical feasibility of sediment sluicing management by numerical analysis, and we found that it is very much advantageous in both way. We also analyze the relation between trap efficiency of reservoir sediment and proportion of spillway gate size and dam height, and get some outcomes which are useful for dam gate remodeling.

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The Kurobe river has large amount of sediment transport, and the river channel on the Kurobe river alluvial fan and the Shimoniikawa coast that ranges to the river mouth have been formed by these sediment transport. In such the river and the coast, if the sediment transport decrease or it is interrupted by structures, excessive erosion and excessive sedimentation occur. Then problems on the management of the river and the coast are caused. In this study, the sediment transport in the Kurobe sediment flow system, composed of the Kurobe river and the Shimoniikawa coast, were understood, and issues of the sediment management were confirmed.

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Debris flow is a natural disaster common in mountainous areas in Japan and China. It has great destructive force because of its high density and speed, and often causes great losses to human life and the economy. Therefore, countermeasures such as sabo facilities are very essential to reduce the damages. In this study, we applied GUI equipped debris flow simulator KANAKO to evaluate the effects of the debris flow countermeasures in Hongqiao Gully, Sichuan Province, China. The results show that the full dams which were constructed in 2001 still have the function for controlling debris flow to a certain degree. Because the trapped debris flow materials decrease the gully bed slope, and debris flow velocity decreases consequently. Through the comprehensive control of debris flow including trapping, drainage and deposition works, the simulation results indicate that the effects of the countermeasures in 2001 and 2006 are very good. The countermeasures can ensure residents’ life and property on the alluvial fan.

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The influence of snow cover and topographical properties on freeze and thaw action on bare slopes is important to consider the amount of sediment production caused by bedrock weathering in winter season. In order to estimate the freeze and thaw intensity, the heat conductivity analysis in a ground is conducted considering heat balance on a ground surface. In this model, the meteorological data generally observed are used as input data. The temporal change of snow cover on bare slopes is calculated by previously proposed model. DEM data of Gifu Prefecture is used for the calculation and the relation between freeze and thaw intensity and snow cover or topographical properties is discussed. As a result, the influence of snow cover on freeze and thaw properties is large and cannot be ignored. In addition, the combination of elevation, slope aspect and slope gradient produce variable differences in the amount of freeze and thaw intensity.

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In August 2009, the Typhoon Morakot hit Taiwan and caused an extraordinary amount of rainfall. Due to the heavy rainfall, a large number of floods and sediment-related disasters occurred all over the island. In Shaolin Village, Kaohsiung County, a huge landslide occurred around 6 am August 9, destroyed the village completely and killed more than 500 people. After the landslide, authors visited the landslide site and investigated the landslide scour to collect information on factors affecting landslide occurrence such as exposed bedrock and soil layer conditions. GIS analysis using DEM data were also conducted to determine the sliding domain. According to the site investigation and GIS analysis, rainwater infiltration analysis and slope stability analysis were conducted. Results of the analysis suggested that the landslide domain, about 1,200 m long, 500 m wide and 80 m deep was collapsed by multi phased manner.

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Bed material load is divided into bed load and suspended load. In the condition in which bed load and suspended load exist simultaneously, there occurs active interaction, transition from saltation to suspension, and vice versa. For this reason, transition rate is inevitable for stochastic model for bed material load transport. However, general mathematical expression for transition rate between two phases was not obtained, and hence applicability of stochastic model was not sufficiently examined. In this paper, we derived the expression of transition rate and added it to transport model. The proposed stochastic model for bed material load could explain the simple non-equilibrium sediment transport that occurs along the reach from solid bed to fluvial bed. Besides, this model was applied to the more complicated process along the repetitions of reach that consists of solid bed and fluvial bed. These indicated the applicability of the previously-proposed model for bed material load.

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The purpose of this study is to understand the evolution process of river bed whose sediment size distribution is extremely wide. As a first stage of such study, movable bed experiments were conducted under the condition that the bed was composed of three different grain-sizes of sediment whose diameters take a different order of magnitude. One represents a group of larger size of sediment which can not move at all, and the others are in the group of medium or finer one which moves as a bed-load or a suspended load. In the process toward the stable state of bed, it was found that a typical vertical sorting occurred, and the characteristics of the bed structure were discussed on the basis of experimental results.

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A coupled modeling of surface-subsurface fluid flow, sediment transport and flow bed change has been proposed for water resources management in forest area. In the developed model, the surface water and subsurface water interactions, erosion of slope and production of solid particles by the surface flow, transport/sedimentation of solid, and resultant surface deformation are considered. Applicability of the model to the actual field has been checked through the comparison of the simulated and observed results such as hydrographs, suspended-load concentrations, and so on. Consistent and preferable matching was attained between them for all of the observed years.

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Erosion rate formula of cohesive sediment had been derived by the first author, but the there was very few information about it in the range that clay-content-ratio Rcc was less than 0.3. A series of erosion experiments were conducted in order to understand the effect of Rcc on the erosion rate. As a result, it was confirmed that the formula was valid in the range of R_cc between 0.1 and 1.0. The influence of watercontent-ratio on the test sample was discussed on the basis of experimental results. A deformation process and a stable profile of cohesive straight channel with a different value of R_cc were also investigated in order to understand the erosion process of cohesive sediment from a different point of view.

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The bed degradation is going rapidly in the upper part of the Ishikari River recently, owing to exposed bedrock erosion caused by bedload on the bedrock. In order to estimate the erosion rate of bedrock, we performed an experiment by a circular channel flume with artificial bedrock made of plaster. We applied the erosion rate estimated by the present experiment to a numerical simulation of bed deformation on the bedrock by using horizontal 2D flow model and the bedload layer model. The result of the simulation shows that gut pattern appeared in the simulated results which is very similar to the experimental result. It was found that the bed degradation was progressed by the erosion caused by sediment transport itself.

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Prediction methods of flow resistance and bed-load transport rate in alluvial streams with sand waves under lower-regime are proposed in this study. At first, the skin friction equation is derived by integrating the bed shear stress distribution equation on a sand wave surface. We propose the equation of hydraulic resistance based on the equations of skin friction and form drag next. The hydraulic resistance equation is verified with estimations of mean depth of flow over sand waves. The reach averaged bed-load transport rate can be calculated reasonably by the Meyer Peter & Müller’s equation using the proposed skin friction equation with the estimated mean flow depth. The proposed method in this study can predict flow depth, geometrical scale of sand waves, and bed-load transport rate by giving water discharge, bed gradient and grain diameter. The proposed method also clarifies that the ratio of a form drag to a skin friction for sand waves is different in a uniform sediment bed and a non-uniform sediment bed.

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Changes in bed condition, flow resistance and sediment transport are discussed in gravel-cobble covered bed where sand is supplied artificially like as river reaches downstream of dam reservoirs, on the basis of experimental results obtained in a large scale flume called "the experimental river" in the Aqua Restoration Research Center, PWRI. Variations of the bed condition were clarified minutely by the use of a sophisticated system that enables automatic measurement of 1cm mesh intervals even under flowing conditions. Flow resistance and non-equilibrium process of sediment transport are confirmed to correspond closely to bed states under subcritical condition.

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Distributions of river bed material are important to the bed variation as well as flood discharge and river plan form. The previous flume experimental studies have showed that the mechanism of bed variation in channels with sediment mixture is different from that of uniform sediment. But grain size distributions of the previous experiments are smaller than natural stony-bed rivers and gravel-bed rivers. In this study, we carried out field experiments by using compound straight-meandering channel in the Joganji river. And we compared the results of field experiments with the results of flume experiments. Furthermore, we performed the numerical analysis of the unsteady quasi-three-dimensional flow and bed variation in stony-bed channel and compared numerical computation results with field experimental results.

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These days, more information about the physical surface condition of riverbeds is needed for good river management to be able to take the habitat of river organisms into account. However, it is difficult to examine either sediment conditions at the surface or the substrate structure in detail because of problems in predicting sediment behavior in a real river. Furthermore, the attributes of mixed sediment transport vary depending on physical conditions, making it difficult to describe using numerical simulations. Therefore, a series of flume experiments were conducted in order to examine sediment behavior through alternate bars composed of sand and gravel. These experiments demonstrated the characteristics of fine and coarse sediment movement and indicated the plane distribution of sediment transport on the bars. Additionally, results indicated the impact of fine sediment supply on bed degradation, as well as on bar–morphology.

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For preserving a comfortable river ecosystem, it is important to restore the riffle and pools within engineered channels, where should play a role in the habitat diversity. It will be no easy task to restore the pool-riffle and bars in flat rock-bed river which is lightly covered with gravels, as contrasted with in the gravel river. Kinoshita and Miwa (1974) proposed the condition for preventing downstream movement of bars in meandering gravel rivers. This work is intended to apply this condition to meandering rock-bed rivers. By depositing sand and gravel in meandering section, it will be assumed to generate some bars in spite of rock-bed rivers. We investigate the validity of the Kinoshita and Miwa’s condition by experimental and numerical survey in mobile-bed and fixed-bed channel. For mobile-bed, we reconfirmed that the Kinoshita and Miwa’s condition is valid. For rock-bed, we derive the condition of deposition of sand in meandering section by relating with the ratio of meander length to channel width and the angle of meander.

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The dimensionless relationships of width(B*) and depth(h*) to discharge(Q*) for stable channels, such as the rivers with dike in Japan and natural rivers in other countries, having bed materials from gravel to silt-sand were presented linearly within one order zone on log-log graph. The linear relation represented by a set of exponent and coefficient for a wide variety of rivers can be regarded as quite new because those proposed conventionally based on regime concepts were specific for each river. The exponent of Q* for both B* and h* is determined empirically around to be 0.4, which also can be proven analytically and explained theoretically to relate with relative roughness. The coefficients of Q* for B* and h* can be functions of B/h and velocity coefficient. Thus such empirical relation supported by hydraulic parameters enables us to understand the variation of stable channel geometry.

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Estimation of sediment transport rate is important for flood control and river environment. In the previous studies, sediment transport formulae have been derived as functions of tractive stress at the river bed. However, the authors believed that sediment transport rate was regulated by the same mechanism as the determination of a stable cross-sectional form of rivers. In this study, we proposed a sediment transport formula considering the effect of river width and cross-sectional form. At first, we investigated relationships among dimensionless quantities of bed load movement by the dimensional analysis. Then, we derived the sediment transport formula using field observed data of bed load in the United States and Japanese rivers. Finally, we showed that the proposed formula was useful for river course design and river improvement work by the cooperation with the existing bed load formulae.

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Formation of bars indicates meandering of flow and deposition or erosion in river bed. It is necessary for river improvement and water utilization to clarify this phenomenon.Therefore many researches have been conducted on bar formation. In our previous study, it was found that the channel formation process was influenced by the form of hydrograph and duration of discharge recession period. The purpose of this study is to determine the influence given to channel formation by difference of discharge recession pattern in detail. Experiment conditions are case of constancy and variation in discharge recession speed.It is found that the channel formation process is strongly influenced by gradient of hydrograph at discharge recession period. That is, the relationship both discharge and gradient of hydrograph(discharge recession speed) is important for the channel formation process.channel width is able to estimated by the stable channel width theory proposed by Ikeda. In addition, it is found that the active.

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Recently, excavated or dredged sediment is often conveyed and placed to the downstream of dam site for the purpose of recovering river environment and solving sedimentation problem in reservoirs. To make appropriate sediment supply in consideration of the river environment, there is a need for a method to plan a volume, a shape of placed sediment. Additionally, it is also required to develop a prediction method to estimate the erosion process. In the study, laboratory experiments and three field observations were conducted to clarify the erosion process of placed sediment. As a result, relationships between hydraulic conditions and the erosion process of placed sediment were recognized.

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Riverbed aggradation in front of the power plant outlet causes the decrease of power generation efficiency. Although bed excavation and embankment construction are conducted to keep riverbed elevation, sediment inflow occurs by embankment collapse caused by snowmelt runoff. In this study, hydraulic model experiments and 2-D numerical simulation were conducted in order to investigate embankment and river bed level variation. It was clarified that erodible embankments composed of bed material could concentrate flow on inner bank of river bend and restrain riverbed aggradation.

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As a method of countermeasure to bed degradation and armoring phenomena of bed material in the downstream are of dam reservoirs, sediment augmentation has been carried out in many Japanese rivers.However, the effect of sediment augmentation on the bed elevation, sediment size and habitat in the downstream reach of the sediment augmentation has not been studied well because of the small amount of the replenished sediment. In this study, one dimensional bed deformation analysis is developed to simulate the sediment transport process of the replenished sediment in the Nakagawa, Tokushima. Here, the Japanese largest scale sediment augmentation has been performed in the Nakagawa.The results show that the developed model is able to simulate the sediment transport process from sediment augmentation. Furthermore, sediment deposition areas of each sediment size class are simulated. The coarse sediment is transported only within 3-4km. On the other hand, the fine material is transported to the downstream area from 10km to 20km. As a result, bed material from 4km becomes finer more than that near the replenished sediment area.

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When the flow and topography change around the structure are calculated by using Cartesian coordinate system, the conditioning at the corner is very important. In present study, the calculation method, namely coordinate system, is proposed. This method is not necessary to do the conditioning at the corner. Numerical analysis for flow and local scouring around a pier was carried out by the simulator which includes the proposed system. The method of the analysis is three-dimensional LES. The main object is to make certain of the effectiveness of the simulator for the prediction of flow and local scouring around the structure. The agreement between numerical and experimental results is good.

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Several studies have been made on local bed scour by movable bed experiments. Researches taking into account non-uniform sediment transport are still very few, in particular in case of the involvement of spur dykes. In this paper, we carried out movable bed experiments around an impermeable spur dyke under non-submerged condition. Both uniform and non-uniform sediment beds with similar mean diameter used and the changes of the bed configuration and bed surface composition have been considered. In addition, we used color sediment which is divided in three ranges of diameter with different colors in order to clarify the spatial differences of bed surface composition through pictures and visual observation. We focused on the relation between initial grain-size distribution and the final scour hole depth and the influence of vortices around a spur dyke on bed surface composition.

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This paper describes computations of dune formations in rivers by DEM-URANS coupling approach. In the present model, complicated turbulent flow structure on dunes was simulated by an Euler type vertical 2D flow model with a non-linear k-e turbulence model. The motions of sand particles were simulated by a Lagrange type approach utilizing the distinct element method (DEM). Dune formations have been successfully simulated by Sekine(2001). Sekine (2001) successfully simulated dune process for the first time using Lagrangian numerical approach called saltation model. On the other hand, the present model replicates each particle’s behavior including separation, collision and holding utilizing more physically based model. The computational result shows that the present method can capture reasonably the fundamental process of dune formation.

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The mechanism of three-dimensional levee breach by overflow (i.e., lateral overflow with consideration of river flow riverside land) has not been clarified in past studies. Elucidation of this mechanism is very important for disaster prevention as well as for the future progress of studies on levee breach by overflow. We conducted experiments of levee breach in three-dimensions by overflow using the Chiyoda Experimental Channel. The results of the experiment are as follows: Until the beginning of levee breach, phenomena near overflow area was similar to one by front overflow. It means that we can apply knowledge from anamnestic levee breach experiment involving front overflow to until the beginning of levee breach. After the beginning of levee breach, it related to unit width discharge and levee breach speed.

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River bank failure causes a heavy disaster. Many experiments had been conducted. Full-scale hydraulic experiment is very few, because of it has a limit to conduct in terms of space and budget. Therefore, many experiments were conducted in small scale and they need to be compared with the actual phenomenon. The large scale hydraulic experiment on river bank failure was conducted at Chiyoda experiment flume in 2008. We conducted small scale experiments on river bank failure and examined the reproducibility of the small scale experiments by comparison with the Chiyoda experiment. Fine-grained soil strongly influences the widening process of bank failure. In order to reproduce the results of experiment at Chiyoda experimental flume on dike failure, it is necessary to remove the fine material from the material of scaled dikes. And it is found that the time scale of widening process of bank failure follows the Froude similarity law.

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Flood in rivers is a common disaster all over the world. If a levee breach happens, it sometimes causes a fatal disaster. In addition, many buildings, urban facilities, lifelines, etc. are seriously damaged. Detailed mechanism of a levee breach has not been clarified yet. Therefore, it is important to predict the collapsing process of riverbank and behavior of overtop flow for reducing damage. We applied a two-dimensional shallow flow computational model to levee breach phenomena caused by overflow and the performance of the model was elucidated. A calibration of the numerical model is made through the comparison with field experimental data. Recently, a real-scale experiment on a levee breach was carried out at the Chiyoda Experimental Channel in Hokkaido, Japan. We performed the computation under the same conditions in the experiment. The computational results showed the excellent performance for simulating levee breach phenomena.

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This paper presents the laboratory tests on developing process of tidal creek network in the small flume and also validation of numerical model which can replicate the morphodynamic evolution of tidal channel and its geomorphic features. The experimental conditions fully satisfied the kinematic, dynamic and sediment transport similarities between the real and experimental scale. The channel network, which has the similar channel characteristics such as ratio between the channel width and depth observed in real field, was reproduced in the experimental flume. This study also conducted the numerical simulations with experimental conditions for validating the physically-based two-dimensional morphodynamic model. The results showed that the proposed morphodynamic model can simulate reasonably the geometry of channel network and its developing process observed in the experiment.

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In the existing linear stability analysis of fluvial bars, it is assumed that the river bed is in equilibrium, and the bed slope and flow are uniform in the streamwise direction. Though it is generally said that the bed becomes more unstable under bed aggradation, and becomes more stable under bed degradation, the existing analysis cannot shed further light on their effects. In this paper, linear stability analysis incorporating weakly non-equilibrium processes of bed aggradation/degradaion is performed by the use of the WKBJ method in order to clarify their effects. Bed aggradation or degradation is assumed to be sufficiently slow compared with the bed evolution due to instability, and its non-dimensional speed is used as a small parameter. The analysis explains that river beds become stable and sand bars tend not to be formed under degradation, and that river beds become unstable under aggradation. In addition, alternate bars are more strongly affected by bed aggradation/degradation than multiple bars.

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In the Asahi River, after the dam construction, flood disturbances on the river bed have reduced and stable vegetated areas have expanded on river bars. Because a development of riparian forest has reduced the number of gravel bars, peculiar river environments such as a diversity of river and a natural ecosystem have decreased. Furthermore, the developed riparian forest has degraded a riverscape and has reduced flood capacity. Artificial removal of vegetation is usually adopted to reproduce gravel bar, however self-sustaining gravel bar can not be well maintained in some cases. Therefore, to restore self-sustaining gravel bar is very important engineering problem. In this study, the effective management methods to maintain gravel bar at Tamagashi area in the Asahi River were proposed and the effects of them were investigated using two-dimensional flood flow simulation.

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A series of fisherie issues in the Ariake sea has been believed to be caused by a lack of sand supply from the Chikugo river on the basis of few investigations of characteristics and amount of sediment transport in the the Chikugo river estuary. The core sampling and super-sonic echo sounder indicates that the vertical structure of mid stream of the Chikugo river estuary is consists of complex alternate layer and the rate of water content is much different to each layer. To make the rate of sand transport in the Chikugo river estuary clear, the authors applied unsteady quasi-three dimensional flood flow and river bed variation analysis using observed temporal changes in water surface profiles of flood. In the analysis, the bed layer which the rate of water content upper than 70% is calculated 2009by the experimental equation of erosion speed of the cohesive material. The results of analysis indicates that the rate of sand supply is strongly affected by the tidal level change of the Ariake sea.

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At the mouth of Nanakita River, elongated sand spit can be frequently observed due to predominance of wave motion, and is occasionally flushed during big floods. For a river planning with the presence of elongated sand spit, the unsteady flushing process of sediment at the river entrance should be considered, instead of a simple assumption of constant water level at the downstream end as has been usually employed up to now. The present paper describes influence of flood hydrograph on the time variation of river stage and corresponding development of bed topography under the actions of both tide and high flood flows at the mouth of Nanakita River during a design flood. The water level thus obtained is utilized as downstream end water level for the river planning.

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The aim of this study is to clarify the influence of the riparian forest to the sediment transport process and bar formation. The Tokoro river is selected for this study because the large amount of fine material from cultivated fields flows into the river during flood. The invasion of trees on bars and the growth of bars with bank erosion are observed at the Tokoro River. The field surveys were conducted at 2009 and 2010 to understand an influence of vegetation to the sediment transport and a relationship between growth of vegetation and bar form. The results of surveys show that the vegetation strongly influences to the fine material transportation during flood and bar shape. In addition, reproducibility of field survey was confirmed by using a numeric calculation method.

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The urbanization in Sapporo was started about only 140 years ago. Before the urbanization, the topography of most part of Sapporo was wetland and not suitable for living and cultivation. Intensive river training works have changed this area dramatically for relatively short period, and made Sapporo as one of the biggest cities in Japan with a population of 1.9 million. In this study, the history of river training works and the transition of inundation characteristics in Sapporo are investigated through old literatures and numerical simulations. First, the historical process of river training constructions was clarified by inspecting old documents and found out some key works. Then numerical simulations of flood were performed in three cases, namely, original topography, after construction of Toyohira-River- Bank and after construction of new Toyohira River Channel. The comparison of simulation results elucidated the relation between river training works and change of inundation behavior.

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