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Recently, several extreme flood hazards with active sediment and driftwood transport have occurred in mountainous areas of Japan, as typically observed in the Akatani river flood disaster in 2017. Therefore, in order to mitigate such hazards, it is required to develop methods to evaluate flood, sediment, and driftwood runoff from a basin during heavy rainfall. This research proposes integrated methods to evaluate flood, sediment and driftwood runoff from a basin during heavy rainfall using numerical methods. The proposed methods are applied to the Terauchi Dam basin, where large amounts of sediment and driftwood were discharged during the 2017 Northern Kyushu heavy rainfall event, to discuss the applicability of the methods.The methods are composed of a distributed rainfall-runoff model, slope stability analysis, sediment and driftwood transport in the slope based on mass system equation, and sediment and driftwood transport in the river channel by unit channel model. These models are integrated as Rainfall-Sediment Runoff (RSR) model to evaluate sediment and driftwood runoff from a basin. As a result of its application to the Terauchi Dam basin, we found that whether or not the debris flow enters the river channel, i.e., the definition of the upstream end of the river channel, has a significant impact on the results. Therefore, we investigated whether the sediment supplied by debris flow enter the river channel during the actual event. The results show that the upstream end of the river channel in the computation should be defined as approximately 4% to 10% slope, and in case the mesh size is sufficiently fine, the debris flow inflow into the channel is sufficiently evaluated. In conclusion, this research proposes an integrate methods to evaluate flood, sediment and driftwood runoff from a basin, and discusses its applicability to the disasters such as the 2017 Northern Kyushu heavy rainfall event.

River flow, sediment transport and channel width influence one another, and channel width generally changes toward the equilibrium state in sediment transport. When the tidal range is large with the water level fluctuating periodically, the sediment transport capacity of the river channel increases in the river mouth due to the drawdown effect during low tide, whereas the backwater effect becomes dominant during high tide. Therefore, the channel width is determined by the flow discharge during low tide and tends to be wider toward the mouth. On the other hand, when the channel in the river mouth is submerged during the rainy season in tropical regions, the channel width is defined by the flow discharge during the transition period from the rainy season to the dry season. This presentation explains a mechanism that determines the channel width in the river mouth by taking rivers in Japan and Southeast Asia as examples.

This study reveals the characteristics of flood flow with active sediment transport that caused the Sozu River flood disaster in July 2018. The results of field surveys revealed that the basin contains steep mountains and flat areas, facilitating the rapid transport of sediment produced in the mountains to downstream areas during the disaster. Two-dimensional numerical simulations of flood flow with sediment transport are conducted to realize the flood flow characteristics. As a result of the simulation, a huge amount of sediment deposition in the channel near the top of the alluvial fan increases the inundation damage at downstream areas. The results indicate that within two flood peaks during the disaster, the inundation in the second flood peak is more severe than that in the first peak, whereas the discharge of the second peak is much less than that in the first peak, caused by the reduction in the capacity of the original channel owing to the sand deposition during the first and second flood peaks.

Present study discusses the formation process of present geomorphology and sediment size distributions of bed material of Seri River, Japan to identify the physical meanings of the data measured in a low water stage after six years of a large flood event. Numerical simulations are conducted to reproduce present channel morphology using a specified flood series based on 2013 event as well as on floods from 2014 to present. Planform of the channel shape, cross-sectional geometry, and lateral and longitudinal sediment size distributions of bed material are reproduced well. Numerical results suggest that the present channel morphology is mainly determined by the flood event of 2013. The flood events from 2014 to present only contribute to small scale adjustments in sediment size distributions of riverbed material, lateral and longitudinal channel geometry as well as channel planform.

We examined channel bars at two sites in the lower reach of the Stung Sen River, which flows into Lake Tonle Sap, Cambodia, to identify sediment transport and accumulation processes during monsoon-related flood events and to investigate how sediment transport capacity changes as a result of enhanced backwater effect of the lake. Channel bars in the lower reach of the Stung Sen River that emerge in the dry season were classified into type A (lateral bars), type B (point bars), type C (concave-bank benches) and type D (diagonal and island bars, or fluvial dunes) based on Nagumo et al.'s previous study. Type B, type C, and type D channel bars were at our study sites of SEN-01 and SEN-02, about 83 and 77 km from the river mouth respectively. Computation of water surface profiles showed that sediment transport capacity of the Stung Sen River decreased abruptly after the peak flow stage because of the backwater effect from Lake Tonle Sap during the decreasing flow stage. Our observations suggest that alternating layers of sand and silt to clay layers accumulate to form type C channel bars, corresponding to changes in sediment transport capacity controlled by backwater effects from Lake Tonle Sap and by changes in flow depths and associated slackwater systems. The accumulation of alternating silt and sand layers of type B channel bars results from lateral sediment transportation that is accelerated with an increase of secondary flow, whereas development of type D channel bars is related to bedload mobility.

The present study demonstrates a method to specify critical rainfall conditions for the occurrence of a sediment disaster and identify areas prone to landslides using a simulator proposed by the current authors for sediment hazards. The simulator predicts spatial and temporal distributions for surface and subsurface flows, landslides, and debris flow resulting from rainfall events. The method to develop a critical curve for the occurrence of a disaster is proposed using simulated landslide data derived from artificially specified rainfall conditions, past rainfall data, and disaster records. Usually, a rainfall event also constitutes a period without rain, and this method can be used to evaluate the influence of the no-rain period. In addition, we propose a method to classify slopes according to the probability of landslide occurrences on a domain defined by slope gradient versus catchment area, using data on landslides resulting from a specified rainfall amount and intensity. Areas identified as having a high probability of landslide occurrences correspond to the runout mark of landslides and debris flow in August 2014.

A method is proposed to predict bank erosion and sand bar migration in river reaches where suspended sediment transport is dominant. The method focuses on the influence of the lateral bed slope on the erosion and deposition rate of suspended sediment, as well as on the profile of lateral bedload transport, assuming that geometric similarity holds in the bank region. In the proposed model, the erosion and deposition rate can be evaluated using either the bed shear stress at a reference location or the average bed shear stress in the bank region. In order to simulate bank erosion and associated bank shifting with a depth-integrated-base treatment, stretchable grids were added to the conventional coarse grid system near the bank. The proposed method, including the bank erosion model, is applied to the lower reach of the Brahmaputra River, which is ∼90 km long and ∼12.50 km wide. The computed results on bank shifting, sand bar migration, and sediment transport rates are compared with data obtained from field investigations and remote sensing. These results suggest that the proposed method is applicable for predicting sediment issues in river reaches dominated by suspended sediment.

In recent years, flood damage caused by flash floods in mountainous rivers has been frequently reported in Japan. In order to ensure a sufficient lead time for safe evacuation, it is necessary to predict river water levels in real time utilizing a hydrological model. In this study, we conducted flood prediction using the RRI model and rainfall forecasted for the next 6 hours in the Kagetsu River basin (136.1 km2) in July 2017, evaluated the uncertainty regarding the prediction, and illustrated the results using a box-plot. The evaluation found that the mean error of the forecasted water level was approximately - 0.3 m in the prediction for the initial 3 hours and -0.97 m at the 6th hour. Also, the study investigated the possibility of correcting water levels forecasted by clarifying an uncertainty distribution. As a result, the water level forecasted was found to be underestimated because it was predicted to rise as high as Warning Level 2, while the water level forecasted with bias correction was predicted to reach Warning Level 4. Moreover, the lead time was estimated to prolong by 2 hours. Overall, the study suggested that flood forecasting can be improved by considering the uncertainty involved in prediction.

In this study, the characteristic of multiple glacial lake outburst floods (GLOFs) in the Pho Chu River basin in Bhutanese Himalayas is evaluated to help assess the potential impact. Thorthormi Cho (TC) and Lugge Cho (LC) in the east branch and two unnamed lakes labelled A and B in the west branch of Pho Chu are chosen for the study. Numerical models were employed to simulate different involved processes. The results show that the peak sediment discharge in the east branch of the Pho Chu River by the TC dam breach reached about 5000 m3/s (during the first GLOF) at 4 km whereas by the LC dam breach is about 600 m3/s (second GLOF) at 6 km. However, the highest peak hydrographs (sediment and water mixture) calculated during the first and second GLOF are about 10 000 m3/s at the 18-km section and about 23 000 m3/s at the 10-km section, respectively. In the west branch of Pho Chu, erosion and depositions are the frequent intermittent local processes during the first GLOF event from Lake A. Because the first event stabilized the irregular river bed profile, there is not much sediment discharge developed during the second GLOF from Lake B. At the 17-km section of the west branch, the peak hydrograph reached about 9000 m3/s during the first event against the peak of about 800 m3/s during the second event. The results suggest that even if multiple dam breaches occur simultaneously, GLOF surges pass through the main river channel at different times with very different flood characteristics. The differences in travel time and flood characteristics mostly depend on the distributions of bed slope and potential erosion depth along the reach. Further, the amount of sediment accumulated in and transported by each surge is reliant on the temporal geomorphologic setting of the river and therefore on the impact of the previous GLOF on riverbed profile and potential erosion depth. The robustness in peak GLOF hydrographs is associated with sediment flow dynamics. As a consequence, serious inundation of Punakha, Lobeysa and major portion of Wangdue Phodrang is anticipated. Copyright © 2011 John Wiley & Sons, Ltd.

The author thinks keys to debris flow studies lie in 1) sediment sorting in debris flow body, 2) phase shifting to or from fluid to solid, 3) difference between sediment concentration and flux sediment concentration, 4) constitutive relations and 5) governing equations employed in numerical simulation. In discussing 3)-5), the author stresses that 1) Eq. (1) predicts the spatial average sediment concentration of the flow body well from debris flow to bed load, and thus it should be prized, 2) researchers must be careful for the difference between sediment concentration and flux sediment concentration and for different flow structures over erodible and rigid beds, and realizes that 3) many problems associated with governing equations such as bed shear stress, erosion and deposition rates and correction parameters for sediment transport still remain to be solved.

The Tam Pokhari glacial lake outburst flood (GLOF) in the Mount Everest region of Nepal is described primarily based on field observations by a survey team studying GLOF transmission areas and flood impact along the Inkhu River. An estimated accumulation of 18 billion cubic meters of water was suddenly released down the Inkhu River when a Tam Pokhari glacial lake moraine dam was breached. Field data suggests that the downstream flow consisted of massive sediment erosion and deposition resulting from bed slope changes and flow width and that river channel morphology was dramatically changed by this GLOF. As a consequence, the characteristics of GLOF changed drastically in downstream areas, resulting in infrastructures damage. In addition, the GLOF created numerous debris fans now occupied by settlements, especially tourism-based industries.

Many sediment related disasters have occurred in many areas of the world. The table of sediment related disasters from 1997 to 2006 is shown. It shows strong earthquakes and super hurricanes/typhoons cause large landslides and debris flows. Climate change may trigger larger disasters more frequently in the future. Stratovolcanoes are geologically weak and cause huge landslides and debris avalanches. Active volcanoes release lava flows and pyroclastic flows, which cause serious damages. As an example of a typical sediment disaster, a disaster which occurred in Venezuela, in 1999 is briefly reported. The disaster was caused by unusual heavy rainfall. Many people were killed by many debris flows and shallow landslides. The disaster shows information on hazards such as hazard maps and rainfall is necessary and control structures may reduce damages if they had existed. Proper land-use and hazard education are needed.

The Tam Pokhari glacial lake outburst flood (GLOF), which occurred in 1998 in the Mt. Everest region of Nepal, was evaluated using hydrodynamic models to gain a better understanding of the flow behaviour. The flood wave was analysed separately under rigid and erodible boundary conditions. In both cases, the calculated dam-breach hydrograph, which had a peak discharge of about 10 000 m3/s, was routed through the Inkhu River, which originates from the lake. The morphologic changes along the river were also analysed and the results were compared with satellite images, field observations and recorded data. In the case of rigid boundaries, the routing procedure gradually attenuated the peaks of the hydrographs to account for hydraulic pooling in narrow gorges and storage in the channel. In the case of erodible boundaries, such effects were minimized due to the increment in channel capacity associated with erosion by debris flow. The study revealed that the GLOF event produced a large-scale debris flow. Additionally, the results revealed that erosion and deposition took place intermittently, but that approximately 440 000 m3 of sediment was deposited about 14 km downstream from the lake mouth. The calculated peak of the water and sediment mixture at 14·4 km was found to be 30 000 m3/s, which is almost 6 times as large as that observed when the rigid boundary conditions were used. Further, the increase in the peaks of the hydrographs due to sediment transport was the primary reason for the destruction associated with the GLOF. These findings suggest that the local sedimentology and topography, as well as other geo-hazard conditions in the area, should be carefully evaluated before recommending any control measures against GLOFs in the Himalayan region. Copyright © 2009 John Wiley & Sons, Ltd.

The advance of technology has led to more competent countermeasures, but lives and properties still continue to suffer from water-induced disasters, such as floods, landslides, and debris flows. To increase the effectiveness of counter systems, improved methods of planning and designing such systems are prerequisite. This paper describes briefly a methodological approach for predicting debris flow characteristics, and proposes techniques for evaluating and improving the mitigative effectiveness of check dams against debris flows in steep mountain torrents. Additionally, a non-dimensional parameter, namely potential storage volume, is introduced to generalize the evaluation processes. As an example, the 1999 debris-flow event in the San Julian River, Venezuela, is chosen for discussion. The paper also proposes a method of evaluating the control function of a series of check dams as well as the criteria for the selection of their sizes, numbers and locations. It is hoped that this work will help to determine which combinations of check dams will fit best together for the optimal control of debris flows and available resources in any river basin. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of shape of sediment size distribution on alternate bar geometry is discussed by use of linear bed stability analysis and numerical analysis. Normal distribution, Talbot distribution, and bimodal distribution are chosen for sediment size distributions in the analysis. The results show that bar geometry on bed with bimodal type sediment has large difference from the others. Normal distribution with wide sediment size range is also tested and regularity of bar geometry is improved. Furthermore, bed load layer model is applied for reproduction of alternate bars and compared the results to those by exchange layer model. Spatiotemporal change of layers thickness is considered in bed load layer model and makes difference of reproduced bed geometry between two models.

The annual number of deaths resulting from landslides and debris flow in Japan over the past 40 years has shown a decreasing tendency as corresponding countermeasures are implemented. The countermeasures constituted of structural and non-structural methods are introduced, focusing on their phenomenological function to clear up roles of sediment research on them. In addition, governing equations which are employed currently in numerical predictions are shown, including formulas for bed shear stress and sediment erosion and deposition in order to illustrate differences between the flow of water and sediment mixture in steep areas and the flow of usually observed floods in rivers. It is recommended that equations which are able to distinguish rapid changes of flow characteristics due to internal solid type friction, sediment erosion and deposition and the corresponding changes of bed elevation should be applied in the numerical computations.

Results obtained from both of flume data and theories suggest that equilibrium bed slope in flow over an erodible bed are determined uniquely by sediment discharge rate when the movements of sediment particles are laminar and thus no suspended transportation take place. This means that the static friction force is dominant in debris flow and that sediment concentration is determined by shear stress balance on the bed surface as seen in our previous studies. On the other hand, if part of sediment particles in debris flow body is transported in suspension, sediment concentration will be larger and the equilibrium bed slope will decrease. These facts are supported by Egashira et al. 's and author's flume data and others' experimental data. The present study discusses experimentally phase transition from solid phase to liquid phase based on experimental data and proposes thickness of laminar sediment moving layer and the methodology for the thickness if part of debris flow body is turbulent.

Alternate bars on bed with non-uniform sediment are difficult to be approached to equilibrium state under some hydraulic conditions. It seems likely that sediment sorting is responsible for the instability of alternate bar bed geometry, because sediment sorting affects local sediment discharge resulting in difference bed geometry. In the present study, the mechanism of the spatiotemporal change of geometric and migration characteristics of alternate bar formed on bed with non-uniform sediment is discussed in terms of numerical analysis and linear bed stability analysis. The results support that alternate bars are difficult to be approached to equilibrium state because of coexist of the same-scaled complex migration velocities.

Braided streams and alternate bars under unsteady water and sediment supply conditions were reproduced via flume tests and numerical analysis, and the effects of the unsteady characteristics of water and sediment supply on the geometric characteristics of streams and bars are discussed. The results show that sediment deposits in smaller channels at higher locations during floods and bed at riffles is eroded during low flows. These phenomena force water to flow into larger streams. As a result, the bed elevation in streams decreases. Furthermore, the presence of vegetation enhances sediment deposition at higher locations and contributes to a decrease in the number of streams. These results also indicate that the unsteady condition of water and sediment supply and vegetation growth in rivers are absolutely essential for formation of single thread meandering channels.

A preliminary study is conducted to propose countermeasures for residential areas with cultural heritage as well as for residential area against sediment hazards such as landslide and debris flow, focusing on part of Kyoto City. Firstly, the spreading of residential area in Kyoto City is discussed using both a latest topographic map and an old map published before about 90 years. Secondary, in order to obtain the simple and physically reasonable methods for evaluating the sediment hazards, the validities of predictions obtained using the governing equations of mass point system for soil block's dynamics is confirmed in terms of the calculated results using 1 D. governing equations of debris flow and the data obtained by field investigations.A critical line of sediment hazards zone predicted using momentum conservation equations of mass point system suggests that occurrences of sediment disaster are inevitable in present residential regions and, additionally, several cultural architectures are located in sediment hazards zone.

So far huge sediment production due to the eruption of Mt.Kelud has given serious damage to the Brantas river basin. On the other hand the eruption has provided soil and sand for agricultural land and construction material. So, as a first step to establishing an advanced control system both of the sediment runoff and the sediment utilization, we have investigated the change of sediment transport properties after the eruption in 1990. The field survey result shows that the sever sediment deposition occurred immediately after the eruption, but 1 year after the eruption the bed aggradation changed into the bed degradation because of active sand mining. Also we obtained the sediment supply condition at the confluences with the tributaries after the eruption and presented a way to estimate the condition. Furthermore the field observation result on wash load indicates the wash load transport rate decreased rapidly after the eruption.

Spatial distribution of transported sediment in channels with vegetation was investigated with attention focused on sediment sorting via field observation at the Kizu River. Results are summarized as follows.(1) Cross-sectional averaged bed elevation has not been changed in Kizu River. On the other hand, cross-sectional bed geometry has been changed dramatically because of the change of spatial distribution of transported sediment.(2) Sediment deposition near the boundary between vegetated area and non-vegetated area is much and the deposited sediment is overpass load.(3) Sediment deposition at the center of vegetated area is few and the deposited sediment is wash load.(4) Bed deformation in non-vegetated are caused principally by overpass load.(5) Downstream-fining of sediment size was observed in vegetated area located near main flow.

Results obtained from both of flume data and theories suggest that equilibrium bed slope in flow over an erodible bed are determined uniquely by sediment discharge rate when the movements of sediment particles are laminar and thus no suspended transportation take place. This means that the static friction force is dominant in debris flow and that sediment concentration is determined by shear stress balance on the bed surface as seen in our previous studies. On the other hand, if part of sediment particles in debris flow body is transported in suspension, sediment concentration will be larger and the equilibrium bed slope will decrease. These facts are supported by Egashira et al.'s flume data and others' experimental data.The present study discusses experimentally an influence of flow scales on an equilibrium bed slope and flow structure, based on experimental data and emphasizes that the equilibrium bed slope decreases with increasing of flow scale if part of debris flow body is turbulent.

International audience; The present study describes entrainment characteristics of bed material into debris flow, based on flume tests, numerical and dimensional analyses. Flume tests are conducted to investigate influences of bed sediment size on erosion rate by supplying debris flows having unsaturated sediment concentration over erodible beds. Experimental results show that the erosion rate decreases monotonically with increase of sediment size, although erosion rate changes with sediment concentration of debris flow body. In order to evaluate critical condition of bed sediment entrainment, a length scale which measures an effective bed shear stress is introduced. The effective bed shear stress is defined as total shear stress minus yield stress on the bed surface. The results show that critical entrainment conditions can be evaluated well in terms of Shields curve using the effective bed shear stress instead of a usual bed shear stress.

In this paper, a 1-D governing equations proposed by Egashira et al. is put forward first to describe the mechanics of debris-flow. Debris-flow simulations were conducted in order to understand what happens when rainfall occurs to cause numerous landslides on steep river basin, supposing several realistic occurrences in several tributaries. Multiple debris-flow surges were observed in 1999 San Julian debris-flow event, which was simulated in different contexts. The simulated results suggest that the different possible sequences can produce different amounts of sediment volume. Maximum sediment-volume difference between independent and dependent predictions was 0.12×106m3, which is very large and cannot be neglected. This shows that the order of debris flow occurrences should be taken into account for simulating debris-flow behavior at the fan-head.

This paper presents field data for channel change, bed material, water surface elevation and hydrograph in Tan Chau reach in Vietnam to understand mechanism of the river change. These show that bank shifting tends to occur toward anti-clock wise and sand bars migrate downstream, resulting in a much larger bar, which is influenced by co-presence of non-cohesive and cohesive sediment and by artificial protection works. In addition, bed material loads are originated by bank erosion. A 2-D numerical model is proposed to treat such river changes by introducing erosion rate formula for cohesive material and formula for estimating the thickness of sediment transport layer into a usual method. The new method is applied to the study reach which is 12km long and is supposed to be applicable for predicting channel change of natural river with fine sediment.

Debris flow characteristics such as flow discharge, mean flow velocity, mean sediment concentration, sediment runoff volume and so on are usually predicted by solving the governing equations numerically. Formulas for erosion/deposition rate and flow resistance play important roles in mass and momentum equations. In addition, potential erosion depth and sediment correction factor also affect largely the debris flow characteristics.The present study emphasizes the importance of the correction factor for sediment concentration and velocity profiles in predicting sediment transport rate and sediment runoff volume. According to Egashira et al.'s theory, sediment is concentrated near the bed, and this non-homogeneity increases with decreasing bed slope. This means the flux sediment concentration of a debris flow differs from the volumetric cross-sectional mean concentration. Correspondingly, sediment runoff volume simulated without a correction factor is very different from actual runoff volume. In this paper, we present a method for predicting the correction factor for sediment discharge, and discuss simulated results of runoff process of debris flow in a flume.

Dynamic characteristics of braided streams formed on beds with non -uniform sediment are discussed by means of numerical analysis. Results are summarized as follows.(1) Sedime nt particles are exchanged to fine ones in dry bed regions and coarse in streams, respectively, under steady flow conditions, whereas, under unsteady flow conditions, mean diameter in trough increases in the rising stage of water discharge and decreases in the falling stage.(2) If the frequency with which water covers bars decreases by reducing peak flood discharge, vegetation growth will be promoted. Furthermore, vegetation suppresses the development of small streams and grows large streams.(3) If huge amount of sediment is excavated artificially, number of streams will decrease with eroding bed in main streams.