蛇行流路におけるSorting現象および平衡河床形状に関する研究

Abstract

It is often found that a coarse surface layer exists in the outer-bank region, andmeanwhile a finer one in the inner-bank region of natural stream bends. Thisphenomenon generally results from the sorting of graded bed load. Some attemptshave been made to relate the sorting to bed topography as well as flowcharacteristics. The available models, however, practically deal with the case ofuniformly-curved bends in a steady-state. No comprehensive model is yet availablethat can predict the spatial and temporal variations of grain size distribution inmeander channels.In the present study, the sorting phenomenon and its effects on the bedtopography in a meander channel is considered. The mechanics of sorting is simplyillustrated by the force balance on a moving bed load grain. It shows that, thecoarser a grain is, the greater the ration of the lateral downslope force of gravity tothe upslope pull due to secondary currents is, so that coarser grains move towardthe outerbank, while finer ones toward the innerbank. Four experiments is alsoconducted to investigate the features of sorting in a flume meander channel. One ofthe results is that the coarse surface layer forming in the outer bank region reducesthe depth of deep scouring.Furthermore, based on a two-dimensional shallow water flow model, a two-dimensional numerical model is developed, which can predict the bed topographyand the surface bed material size distribution coupling with the consideration on thesorting of heterogeneous bed load. The validity of the model is verified by thecomparision of calculated results with experimental data.