Stability of riverbanks under three unsteady river flow conditions is simulated. First is the case of constant water level in the river, second is the case of filling the river to the riverbank top and finally the case of drawdown. Two modes of drawdown are analyzed; the rapid drawdown and the slow drawdown. A finite element model of saturated unsaturated seepage flow was de-coupled with a plain strain elastic-plastic finite element model using strength reduction technique to calculate the stability factor of safety (FOS) of bank material. The influences of location of phreatic surface, pore-water pressure, drawdown rate and ratio on the riverbank stability are discussed in details for each case. Results showed that safety factor for saturated riverbank is nearly 83% of its dry value (for the studied case). The case of filling is most likely to stabilize the riverbank. For the case of slow drawdown, the minimum FOS occurs when the water depth is about 0.25~0.3 the bank height. The case of rapid drawdown is the most critical case.
Longitudinal and transverse sediment sorting and the formative mechanism of bars on non-uniform sediment bed in a straight open channel are studied in terms of flume tests, numerical analysis and theoretical analysis. The results support that the sediment sorting is formed on bars; finer sediment deposits around crests of bars and coarser sediment is left in troughs. Furthermore, it is found that the sediment sorting produces the difference of geometric and migration characteristics of bars between non-uniform and uniform sediment beds. Wave height, wavelength and migration velocity of sand bars on non-uniform sediment bed become smaller, shorter and faster, respectively, than those of bars formed on uniform sediment.
The aim of this study is to establish a methodology that estimates flood flow resistance of riverine woody communities from measurements of wind flows. Similarity between water and airflows affected by woody plants was investigated basing on results of flume and wind tunnel experiments carried out with the same tree models. It was clarified that there are significant differences between the turbulent flow structure of water flows and airflows mainly due to existence or absence of the water surface. The drag coefficients of canopies in the airflows were found to be about 40% greater than in the water flows. In addition, numerical models to simulate the water flows and airflows passing through woody communities were developed based on a κ-ε turbulent flow model.
A new approach to simulate the mass failure process in riverbanks is developed and verified by experimental work. Both planer and circular failure modes can be expressed by this approach. The proposed approach is capable of distributing the collapsed bank material. Failure mechanism of riverbanks is described, failure plane is determined, shape of collapsed material is proposed, and travel slide distance is estimated. Eight experimental cases were conducted to investigate the processes of mass failure that follows the hydraulic erosion of an artificial riverbank with different heights and slopes. Deformations, developed cracks, plane of failure, and shape of deposited material were recorded. It is watched from experimental results that there exist many cracks are developed and thus many planes of failure are found, while in numerical simulation only one failure plane exists. The difference between simulated and experimental results may return to the large deformation of collapsed bank material which can't be simulated by simplified assumptions. Further improvements to the proposed approach are needed.
Hot-stamped galvannealed steel sheets (GA) are widely applied to produce automobile components that require high strength. The hot-stamped components tempered for the control of strength improve the collision property. This study investigated the structural changes in the hot-stamped GA coating and the influence on corrosion resistance by tempering. It has been revealed that by tempering between 500-600 degrees Celsius, the Fe-Zn solid-solution phase in the hot-stamped GA coating changed into two phases: the Γ phase and the Fe-Zn solid-solution phase with less Zn, and these changes in the coating structure improved the corrosion resistance. The phase decomposition is considered to result from discontinuous precipitation of the Fe-Zn supersaturated-solid-solution.
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.
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.
