Typhoon No. 26 in 2013 attacked the Izu-Oshima Island with record heavy rainfall and caused a disaster resulting from landslides and mud flows. This extreme event is the motivation of our study on how we evaluate hazardous zones at risk of mud flows and how we design structural and non-structural measures accordingly. The present study describes sediment runout processes, mud flow control by means of a guide wall, and a method to evaluate topological conditions in which landslides and mud flows avalanche into unexpected areas. The landslides took place in the western slope of Izu-Oshima, which is only about 2500m wide. Analyses on phase shifting from solid to liquid as well as on mobility of the soil masses suggest that the soil masses released by the landslides transformed directly into mud flows, and that the mud flows developed in size through sediment erosion in their run-out processes. The predicted results by means of a numerical model based on depth-integrated governing equations of sediment-water mixture flow suggest that mud flows could be controlled well using a guide wall, which shows a high possibility of mud flow control using a storage structure with a guide wall. In addition, we propose a simple method to evaluate topological conditions to judge whether mud flows will enter unexpected areas, which will provide a key to identify hazardous zones including even those that have been missed out conventionally.

Erosion process of clay riverbed was investigated. The clay riverbed is eroded by the action of water flow. It is also eroded by moving sand and gravel. In this study, the influence of moving sand and gravel on erosion of the clay is investigated. This process is affected by the amount of sand and gravel transport in the river. Movable bed experiments were conducted under the several conditions of both the flow discharge and the rate of sediment supply. As a result, we found that the relationship between the erosion rate of the clay riverbed and the rate of supplied sand has extreme value. It was also found characteristic vertical structure under the bed surface.

Experimental investigation was conducted on the dynamic equilibrium state of bed which was composed of extremely wide range of sediment. A void of larger stones was filled with medium gravel which moves as bed load and finer sand which moves as suspended load. Experiment was conducted with supplying gravel and sand. The flow discharge and the sediment supply discharge of gravel were set as fixed value, and the sediment discharge of sand was changed. The layer of only gravel was exposed on the bed surface under the condition that the discharge of sand was small. On the other hand, if the supplying rate of sand was large, the layer which was composed of both gravel and sand was formed below the bed surface. It was verified that this results were not changed even if the flow rate and the discharge of gravel changed.

The Kanda river basin is a highly-urbanized area located next to the Tokyo metropolitan area. In this area, most ground was paved, and high density drain network has been constructed under the ground. Because of such condition, rain water concentrates rapidly to the drainage pipe, and it flows into urban river system. In this study, numerical simulation was conducted to investigate a process of urban inundation in this basin by focusing on the interaction between river and drainage system. Two rain events, Suginami torrential rain in 2005 and heavy rain event on June, 29 in 2014, were paid attention in this study. In the latter case, the rain data by X-MP radar which was provided by MLIT was applied. As a result of this analysis, it was revealed where the points with higher inundation risk lay hidden in this basin.

Recently extremely strong rain came to attack the urban areas of Tokyo. As a result, urban inundation came to be frequent. Because there are several underground spaces including subway stations in this highly urbanized area in Tokyo, appropriate measures must be taken beforehand. Numerical simulation is effective to reduce damage of the inundation. In this study, we applied numerical prediction technique developed by the first author to the Tokyo downtown area. Based on the result, the inundation risk of the actual large-scale underground was made clear.

Recently, the warming on the earth advances gradually. Under such circumstances, occurrence frequency of the torrential rain increases in Tokyo. The technique of numerical prediction of the inundation is an effective tool in order to reduce damage due to such a torrential rain in urban area. In this study, the numerical prediction was conducted by using the rain data of the X-band MP radar as the first step to realize a real-time inundation forecast at the time of the localized torrential rain occurrence. Inundation in Shinjuku area that was highly-urbanized was investigated in this study by using the actual rain data that was measured locally on August 26, 2011. It was suggested that an inundation forecast could be realized soon in future.

In this study, inundation process in lowland areas of Tokyo which is bordered by the Ara River and the Sumida River was investigated numerically. Numerical technique developed by the first author was applied in order to estimate how hazardous the urban lowland area under two different conditions. The state of inundation which is caused by a localized torrential rain was predicted. Inundation rocess in this area was also investigated numerically under the assumption that the overflow from the Ara River into this area occurred. As a result of these simulations, the time series of information about the flow path of inundated water were obtained, and the aracteristics of inundation in this area were made clear.

The objective of this study is to clarify the deformation process of river bed which is composed of extremely wide range of sediment. The frame of bed is constituted by the larger rock or stone which cannot move at all. In this study, "the dynamic equilibrium state of river bed" was investigated experimentally. The flow discharge and the sediment discharge of supply were the parameters in experiment. The sand and gravel existing in a void of the stones of the large particle size is affected by the stones. It is called a hiding effect. It was found from this study that the riverbed reaches different dynamic equilibrium state according to a supplying rate of the sand or gravel, and the surface elevation of sand and gravel exposed on the riverbed surface changes.

Experimental investigations were conducted on the process of surface erosion and channel network formation on the bare slope. This is the process of sediment yields on the river basin. Some experiments were conducted to clarify the process of the erosion by varying both the gradient of slope and the width of mouth at the downstream end. It was found that characteristics of channels were different among the experiments, such as length, width, branches and interruptions. As a result, it became clear that the steepness of slope affects the length of the channel network in longitudinal direction. The width of the mouth, on the other hand, affected the width and the pattern of channel network in lateral direction.

Fundamental process of surface erosion on bare slope which is caused by artificial rainfall was studied in this study. Numerical simulation was attempted under the same condition as the experiment that we conducted at the same time. Investigations were carried out for two type of the bare slope. Erosion process of the bare slope of sand which contains clay by 12% was one of the targets of this study. Numerical predictions technique was newly developed in consideration of being a non-equilibrium sediment transport process. Results of the numerical computation agreed well with experimental results, and the validity of the technique was confirmed.

Recently, we often experience a localized torrential rain whose intensity is much larger than the design value for drainage network system. Numerical simulation ofinundation was conducted in order to estimate how hazardous the highly urbanized area of center of Tokyo is if such a heavy rain attacks there. There are several underground spaces including subway stations there, and a number of gates for stairs toward the space exist in the spots where the ground elevation is lower than the surrounding area. As a result of this study, it was concluded that serious inundation may occur in large scale underground space in this area. We also found out the gates leading to the space through which water flowing on road can enter.

The purpose of this experiment is to understand the effect of sand transported from upstream reach of river on both deformation of bed with cohesive sediment and vertical bed structure. In this experiment, the amount of sediment feeding is equal to the amount of equilibrium bed-load transport, and experiment was continued until the deformation of the bed reaches the dynamic equilibrium state. As a result, characteristic vertical structure consisting of "sand layer", "sand-clay mixed layer" and "clay layer" were confirmed on the bed. In addition, it was seen that the existence of waves formed over the surface of cohesive sediment leads such vertical bed structure.

In this study, series of experiments were conducted in order to investigate the effect of permeability of sediment on slope erosion by artificial rainfall. In this study, the slope was mainly composed of sand whose grain size was different in each case. In addition to such experiments, another investigation was carried out to find out the effect of adhesiveness on the erosion of slope which contains some amount of clay. As a result, it was found that overland flow affects surface erosion, and therefore the slope permeability governs the slope erosion considerably. And it was also confirmed that adhesiveness affects channel network formation due to the surface erosion considerably.

Numerical simulation of inundation in actual underground space in Tokyo was conducted. Urban inundation around the space was also computed simultaneously. Characteristic of the inundation process in the space was made clear as a result of this investigation. Evacuation movement of persons from the space was also studied by using the newly invented numerical model. An evacuation route from this space was successfully found and recommended in this study. It was confirmed that the guide for evacuation was effective to make the persons evacuate orderly and safely. The announcement for the persons there to be urged toward the evacuation is also effective so that the evacuation can complete more rapidly. But the special consideration must be needed about the timing not to make the persons be in panic.

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.

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.

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.

Evacuation movement of persons from an inundated underground space was investigated in this study. Numerical simulation was conducted under the two conditions; (1) the persons were guided to evacuate in a recommended manner or (2) they escaped through the nearest stairs without guide. Simulation was also conducted in order to make clear the effect of evacuation announcement. It was confirmed that the guide was effective to make an orderly and safe evacuation movement. And the announcement was also effective to make persons escape more rapidly to a ground level in case that it was made at just the right time. This kind of computation must be needed before we make a plan of the announcement.

Recently, we often experience a localized torrential rain whose intensity exceeds much beyond the designed magnitude for both a drainage system and an urban river in Tokyo. On September 4-5, 2005, such an extremely heavy rainfall caused the inundation damage in the Myosyoji River basin. And also on August 5, 2008,five workers who worked in a trunk drainage pipe drowned because of the sudden concentration of rainwater in it in Zoshigaya area, Toshima Ward. In this study, numerical simulation model was developed in order to investigate how the rainwater can be removed by both a drainage system and an urban river in these areas during such a rainfall event.The validity of the model was verified by comparing the numerical results and the observed data. As a result of this study, the expanding process of inundation in each region explained above was made clear for some extent hydraulically.

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 Rcc 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 Rcc were also investigated in order to understand the erosion process of cohesive sediment from a different point of view.

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.

Channel deformation process of a gravel-bed river with vegetation zone was investigated in the present study. The channel is assumed to be constituted by gravel-silt mixture. When we simulate this deformation process, the evaluation of erosion rate of silt from gravel-bed is important. The formula by Ashida and Fujita [1986] is only the applicable one. In this study, a "gravel-silt-packed bed model" was newly invented and introduced in order to make the computation effectively with the above mentioned formula. The model enables us to predict a temporal variation of sediment composition in bed as well as the bed elevation reasonably. In this paper, the model was applied to investigate the effect of both a zone of vegetation and a silt erosion and deposition on the channel deformation. As a result, it was confirmed that a depositional terrace like landform formed just inside the vegetation zone around a water margin, which was observed and reported by Fujita et al. [1996].

The formation process of depositional landforms on a 2-D permeable plain was studied numerically. In this paper, such landforms are called a micro-scale alluvial fan. The purpose of this study is to develop a numerical model which enables us to simulate this formation process. To understand the formation mechanism more precisely is also the purpose of it. The Slope collapse model was applied in order that the local slope angle does not to exceed the angle of repose. And the modified version of bed load transport function was introduced so as to consider the gravity effect associated with the slope angle in longitudinal and lateral directions. Water flow was computed on the basis of both the 2-D shallow water equations for surface flow and the 3-D Richards' equation for subsurface flow. Comparison was made between a computed result and that of the experiment by Sekine et al. (1998). As a result, it was verified that the numerical model works fairly well, and the formation process of micro-scale alluvial fan can be simulated reasonably. The process is closely related to the channel deformation process, that is, channel migration, disappearance of the channel, and a new channel formation.

Main purpose of this study is to evaluate the erosion rate of cohesive sediment. More than 100 experiments were conducted in a closed conduit, and the test sample was the mixture of kaolin, silica sand, the composition of which was a variable. It was found from this study that a frictional velocity of flow, a water temperature and a water content ratio are the most dominant factors, and the amount of sand or gravel and its grain size contained in the sample, on the other hand, are independent of the erosion rate approximately. Erosion rate formula was derived in this study on the basis of the experimental results. Erosion process was also investigated qualitatively through an image processing.

The formation process of depositional landform over a one-dimensional permeable flat plain was investigated numerically in this paper. In such a process, a water exchange between a surface flow and a subsurface flow is important to be taken into account. The governing equations of the flows are one-dimensional shallow water equations for surface flow, and two-dimensional Richards' equation for subsurface flow. As a result of this study, the following two processes were confirmed: (a) a front of surface flow migrates a little in the downstream direction due to the loss of surface-flow discharge, and the thickness of the depositional landform increases; (b) the front migrates with relatively high speed due to the recovery of surface-flow discharge, and the depositional landform spread in the downstream direction.

In this study, numerical simulation of suspended sediment transport was conducted on the basis of an equation of motion in mass point system. LES turbulence model was introduced to simulate a coherent structure of turbulent shear flow which is composed of a series of ejection and sweep. The motion of multiple sediment particles were computed in this flow field, and their trajectories were analyzed statistically. And then a diffusion coefficient of sediment was evaluated numerically on the basis of diffusion theory by Taylor (1953). It was found that the diffusion coefficient of sediments approaches to that of turbulence as the sediments continue to move. This result coincides with the assumption of traditional analysis.

In this study, numerical simulation of suspended sediment transport was conducted on the basis of the equation of motion in mass point system. LES turbulence model was introduced to simulate a coherent structure of turbulent shear fow which is composed of a series of ejection and sweep. Several hundred of sediments were traced in such a flow field, and their trajectories were analyzed statistically. Diffusion coefficient of sediment was evaluated numerically on the basis of diffusion theory by G.I. Taylor, and was compared with the value of turbulent diffusion coefficient. As a result, these two coefficients were found to take approximately same value, which coincides with the assumption of traditional analysis.

Numerical analysis of the flow, which is established under the effect of interaction between surface flow and subsurface flow, was conducted, together with the analysis of the bed evolution due to a sediment transport in this study. Basic equations of water flow are one-dimensional shallow water equation for surface flow, and two-dimensional Richards equation for subsurface flow. Numerical model was applied to simulate the formation process of depositional landform on one-dimensional permeable bed, which is just like a fl uvial fan.

he knowledge about an actual condition of sediment transport at any local point in river is now needed as basic information before we discuss the preferable state of river system. In this study, semi-2D numerical model was developed to make the long-term prediction of river bed deformation as well as the local sediment sorting be possible. Gravel-silt packed bed model(2005) was included in the developed numerical model, which was applied to Sakawa River, which was a steep gravel bed river in Kanagawa Prefecture. Based on the result of sample calculation, the numerical model was found to work fairy well in terms of qualitative consideration.

Erosion rate of soft cohesive sediment was investigated experimentally in the present paper. In this study, we developed a movable apparatus of erosion test, which was designed to conduct a field measurement of erosion rate. In order to check the validity and utility of this apparatus, the several series of measurements were carried out outside in large-scale experimental flume. It was verified that the results by this new measuring system has almost same order of accuracy as that by our standard test in laboratory. Effect of water temperature on the erosion rate were also evaluated quantitatively.

Numerical model of evacuation behavior of people from the inundated underground space was investigated in this study. Information about a human behavior was taken into account as much as possible, a walking velocity, for example, was evaluated on the basis of relation between it and the congestion density of crowd. It was considered that the movement of each people was affected by both the movement by others and the approching flow of water on the floor. Numerical simulation was conducted in the relatively large scale model of underground space. It can be concluded that the present model works well and the reasonable simulation of evacuation behavior is possible to be realized.

The channel deformation in Kinu River during a flood period was investigated in the present study. Field measurement and film analysis were conducted to quantify the temporal and spatial changes in channel profile in the reach of 54.5-56.5 km from the confluence point to Tone River in the term between 2001 and 2003. Considerable amount of bank erosion and channel shift were observed, and the alternative channel was created just after the final flood during this term. As a result of this analysis, it was found that the magnitude of channel deformation depends on an individual flood discharge and also on the history of the former flood events. Numerical analysis was also conducted to understand more deeply the mechanism of this channel deformation. It was confirmed that the numerical model worked fairly well.

The channel deformation of gravel-bed river with vegetation zone was investigated in the present study. The channel is assumed to be constituted by a gravel-silt mixture. In order to simulate such a phenomenon, the erosion rate of silt from gravel bed surface is the important factor to be considered. The formulation by Ashida and Fujita (1986) was applied here in the numerical model. Numerical computation was conducted by considering the effect of silt erosion and deposition on the channel deformation. As a result of this, it was confirmed that the depositional configuration like a terrace formed just inside the vegetation zone, which was mentioned by Fujita et al.(1996).

Numerical simulation of sediment wave formation was conducted in the present study. Saltation model has been extended in order to analyze the entire motion of particle which starts by being picked-up from a bed and ceases its motion by depositing on it. The procedure in which a mutual collision between moving particles can be explained physically was newly invented here to consider the effect of interaction between them. Based on such models, the multi-particle motion can be analyzed simultaneously. In this paper, the gravel bed deformation which is caused by each particle motions was predicted under the same condition of flume experiment.

The effect of dry-wet history on the erosion process of cohesive sediment was investigated experimentally in the present study. Detailed discussion on the mechanism of this process was conducted here on the basis of enormous number of experimental data. In this experiment, the test sample keeps under dry or wet conditions every 6 hours, and erosion test was conducted just after each period. It wasfound that this process can be classified into two parts in their mechanisms according to the total period of this history. The erosion rate of test sample which experienced this history more than one day keeps almost constant value which agrees well with the predicted value of the erosion rate formula.

Inundation process in underground space which connecting to a subway station in urban area was studied numerically. Actual data sets of (1) road network, (2) drainage network and (3) underground space in the area around Shibuya Station were considered here, and a simplified numerical model was constructed. Based on the results of numerical computation, the characteristics and the weak points of the drainage system in this area were quantitatively understood. It was revealed that (1) the local area whose ground elevation is relatively lower and (2) the undergournd space are the possible area where extensive damage will result if there happenes to be the rainfall whose intensity exceeds the designed one.