18.—Sedimentation in some Scottish Rivers of Low Sinuosity

Abstract

Synopsis Scottish streams of low sinuosity have four kinds of bars, three are bank-attached (lateral), and one is medial. Bar type is related to sinuosity, and as streams often increase in sinuosity away from source, there tends to be a variation in bar type successively downstream. The morphology and structure of the bars are related to grain size—gravel bars do not have the same bed forms and structures as those made of sand—and variations in flow stage. The bar head, composed mostly of gravel, forms during the high-flow stage, and as the flow falls so sediments of the lee face record the changing flow pattern. With further drop in water level the locus of sedimentation shifts to the bar tail where the dwindling flow may build diversely oriented bed forms and structures. Changes in the rate of fall, or differences in the maximum rise of water level, result in different proportions of sediment type being deposited. The mean orientation of the directional structures is not always alined with the mean stream direction, and the diversity of orientation is comparable with the diversity in orientation of directional structures in meandering streams. The variability in orientation is due to the effects of flood-produced bed forms in diverting the low-stage flow, stream sinuosity, and the changing orientation of the channel as it sweeps across the floodplain.