Response of a Channel With Alternate Bars to a Decrease in Supply of Mixed.Size Bed Load: A Flume Experiment

Gravel bed channels have a much greater capacity than sand bed channels to regulate the mobility of the bed surface through the formation of a surface layer of bed particles that is coarser than the particles supplied as load. The coarse surface layer affects bed mobility in two important ways. First, it reduces differences in mobility between coarser and finer particles on the bed surface by making the larger and less mobile particles more available for entrainment [Parker and Klingeman, 1982; Andrews, 1983]. Second, the coarseness of the surface of a streambed offers resistance to the entrainment of all bed particles [Dietrich et al., 1989]. If transport capacity exceeds the sediment load supplied from upstream, for example, the bed surface can coarsen, thereby increasing overall resistance to transport. These interpretations of the role of the surface layer do not conflict, because in the example above, a coarsening of the bed surface would not necessarily lead to a change in bed load particle size. Dietrich et al. [1989] fed mixed-size sediment at a high rate into a flume that contained sediment of the same size mixture as the feed and carried a flow with a low width: depth ratio. At a value of τ* = τ/[g(ρs ρ)(D50)load] equal to only 0.087 (τ is the mean boundary shear stress; g(ρs – ρ) is the submerged specific gravity of sediment; and (D50)load is the median particle size of the sediment load), which is exceeded at bank-full stage in many gravel bed rivers, a coarse surface layer was not evident. But as feed rate was reduced in two steps to one tenth the original while boundary shear stress was held approximately constant, the surface coarsened across most of the bed. Despite the bed's being nearly flat and uniform, most of the bed load transport was restricted to the remaining fine-grained zone of the bed surface. We extend their examination of bed surface response to changes in load into transverse as well as longitudinal