Equal mobility of gravel in streams: The remains of the day

[1] The concept of equal mobility of gravel transport is 2 decades old. The weak form of the hypothesis states that in order to transport the coarse half of the mean annual gravel load through a river reach at a rate equal to the fine half, the coarser material must be overrepresented on the bed surface, giving rise to a mobile-bed armor. The degree of mobile-bed armoring necessary to accomplish this diminishes with increasing bed shear stress. The strong form of the hypothesis states that the grain size distribution of the gravel portion of the bed load should be similar to that of the substrate and finer than that of the surface layer. Here the hypothesis is subjected to a rigorous test in the laboratory under controlled conditions which are nevertheless chosen to be as representative as possible of actual field conditions. The grain size distributions used in the experiment included both model gravel'' and model sand.'' At the end of the experiment the entire surface layer and a significant depth of substrate were removed from an entire reach of the channel and sieved. The results of the experiment confirmed the weak form of equal mobility: The surface layer was significantly coarsened compared with the bed load, even when the sand'' was excluded from the size distributions. The strong form of equal mobility was also supported by the experiment, in that the size distribution of the gravel'' substrate was nearly identical to the size distribution of the gravel'' portion of the load. The gravel substrate was, however, slightly but measurably coarser than the gravel load, a tendency that has been observed in the field in cases for which the strong form is not satisfied.