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Osaka, Japan

Kabir M.A.,Monash University | Dutta D.,Monash University | Dutta D.,CSIRO | Hironaka S.,NEWJEC Inc. | Pang A.,Monash University
Water Resources Management | Year: 2012

Bed load transport is a key process in maintaining the dynamically stable channel geometry for restoring the form and function of river ecosystems. Bed load consists of relatively large sediment particles that are moved along the streambed by rolling, sliding or saltation. Currently, various empirical correlations are used to estimate bed load transport rates since no single procedure, whether theoretical or empirical, has yet to be universally accepted as completely satisfactory in this aspect. Bed load particles are primarily sourced from river bed materials or banks. The amount of bed load and its spatial distribution contributes significantly to river bed level changes. Hillslope sediment contribution, mostly available to the river in the form of suspended load, also plays an important role in river bed level changes. This study aims to analyse different bed load equations and the resultant computations of river bed level variations using a process-based sediment dynamic model. Analyses have revealed that different bed load equations were mainly deduced from the concept of relating bed shear stresses to their critical values which are highly factored by the slope gradient, water discharge and particle sizes. In this study, river bed level variations are calculated by estimating total surplus or deficit sediment loads (suspended loads and bed loads) in a channel section. This paper describes the application of different widely used bed load equations, and evaluation of their various parameters and relative performances for a case study area (Abukuma River Basin, Japan) using a basin-scale process-based modelling approach. Relative performances of river bed level simulations obtained by using different bed load equations are also presented. This paper elaborates on the modelling approaches for river bed load and bed level simulations. Although verifications were not done due to unavailability of field data for bed load, qualitative evaluations were conducted vis-à-vis field data on flow and suspended sediment loads as well as the bed loads presented in different past studies. © 2011 Springer Science+Business Media B.V.

Kabir M.A.,The Bureau of Meteorology | Kabir M.A.,Monash University | Dutta D.,CSIRO | Dutta D.,Monash University | Hironaka S.,NEWJEC Inc.
Water Resources Management | Year: 2014

The paper presents a process-based distributed modelling approach for estimating sediment budget at a river basin scale with partitions of suspended and bed loads by simulating sediment loads and their interactions. In this approach, a river basin is represented by hillslopes and a network of channels. Hillslopes are divided into an array of homogeneous grid cells for modelling surface runoff and suspended sediments. Channels are defined by incorporating flow hydraulic properties into the respective hillslope grids as sub-grid attributes for modelling both suspended and bed loads. Suspended sediment transport is modelled using one dimensional kinematic wave approximation of Saint-Venant's principles of conservation of mass and momentum. Transport capacity of runoff or streamflow is used to set the limit of suspended sediment transport rate. Bed load in channels is estimated based on the instantaneous water and hydraulic parameters. Fractional interchange between suspended load and bed load is then back calculated. The performance of the model was evaluated through a case study application in a large river basin in Japan. The model satisfactorily calculated the sediment transport and total sediment budget in the basin. The simulated bed load was found to be reasonable and consistent with the water flow and suspended sediment flux. The results showed the bed load can be expressed as a linear function of the suspended load. The fractions of different sediment loads also exhibit linear relationships with water discharge for the rising and recession limbs of the flood hydrographs. The case study has demonstrated that the process-based distributed modelling approach can efficiently describe the basin-scale sediment budgets with due consideration of the suspended and bed loads and their interactions in the hillslopes and channels. © 2014 Springer Science+Business Media Dordrecht.

Fukuda T.,NEWJEC Inc.
Journal of Geosciences | Year: 2011

In Japan, slope failures frequently occur during heavy rain, and there have been large-scale disasters.Techniques to reduce the damage due to slope failures can be characterized as hard or soft measures. For instance, a hard measure would be setting up a retaining wall,and a soft measure would be making a hazard map. Although hard measures proved to be effective, their construction is often difficult to justify in terms of economic efficiency; the cost of management is also expensive. Therefore, the making of hazard maps to reduce the effects of the disaster is very important. However, present hazard maps do not provide sufficient information on hazard risks. In this study the author investigated making hazard maps of the Shirasu slope failure, oriented to the prediction of the time of occurrence and probability of the debris flow area, called "real-time hazard map." The proposed real-time hazard map is a map to represent the product of the probability of slope failure occurring and the probability of debris flow area. In order to organize the timeline of the probability of slope failure, the setting of Fragility Curve to relate the rainfall pattern to the probability of slope failure occurrence is needed. Fragility Curve on the Shirasu slope failure becomes a relational expression of the effective rainfall and the probability of slope failure occurrence if a supposed instability by the prime factor of the Shirasu slope is uniformity because early rainfall greatly influences the moisture state in the Shirasu slope from the continuous measurement result of the resistivity etc. The probability method mainly utilizes the angle of elevation formed by slope height and distance between end margin of debris and slope head crown, 6. Using the above 6 distributions, they developed a stochastic simulation system for predicting the hazard areas of shallow slope failure debris, and this simulation called SLSS (Shallow Landside Simulation System). SLSS simulation is applied for the calculation of probability of debris flow area. To use the real-time hazard map drawn up in the results of this study, the methodology of this study is available for making hazard maps of slope disaster risks.

Mizutani H.,Kyoto University | Nakagawa H.,NEWJEC Inc. | Yoden T.,Kyoto University | Kawaike K.,Kyoto University | Zhang H.,Kyoto University
Journal of Hydraulic Research | Year: 2013

This paper reports laboratory experiments and numerical simulations of river embankment failure due to overtopping flow for different sediment sizes and different saturation conditions of embankment body. The effects of saturation and sediment size of embankment materials on the erosion process are discussed based on the results of the laboratory experiments. A numerical model is proposed to simulate the erosion process of embankments by overtopping flows. The proposed model considered the effects of infiltration process and resisting shear stress due to suction of unsaturated sediment. To simulate the embankment erosion phenomenon, the numerical model consists of four modules: two-dimensional (2D) shallow-water flow, seepage flow, sediment transport using a non-equilibrium model framework, and 2D slope stability. The validity of the developed model is tested using experimental data on embankment erosion. The numerical results on progressive embankment erosion agree well with the results of the sandy river embankment experiments. © 2013 © 2013 International Association for Hydro-Environment Engineering and Research.

Goto H.,Kyoto University | Sawada S.,Kyoto University | Hirai T.,NEWJEC Inc.
Wave Motion | Year: 2011

We introduce a new conserved quantity, Normalized Energy Density (NED), alternative to the conventional definition of energy for a layered structure in a 2D SH problem. NED is defined by the average of power of a half transfer function multiplied by the impedance, and the conservation across the material interface is analytically proved for a two-layered case. For three, four, and ten-layered cases, the conservation is examined by applying the Monte Carlo simulation method, and then NED is supposed to be conserved through the layers. © 2011 Elsevier B.V.

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