The effects of grain sorting on the stability of gravel and cobble channel beds

Abstract:

The frictional stability of grains in a channel bed exerts a key control on sediment transport rates. The metric of grain stability, the particle friction angle, is observed in laboratory measurements to decrease with increasing grain size for a given substrate. This phenomenon, along with greater protrusion into the flow, tends to push channel beds towards the condition in which all grain sizes move at similar shear stresses. Qualitative observations of grain stability in mountain channels, however, suggest that a significant portion of the channel bed remains immobile during bankfull flood events. Steps composed of coarse grains sometimes span the channel width and are more resistant to erosion. In addition, size-selective transport is more common in these channels, and coarse grains tend to remain in place as finer grains are transported. This anomalous stability may be explained by the effects of grain sorting. To test this hypothesis we measured in-situ particle friction angles at eight steep sites in California with slopes ranging from 0.9% to 42%. The results indicate that in moderately steep beds (0.9% to 6.5%) particle friction angle is not a function of relative grain size, contrary to previous studies of particle friction angles. On steeper slopes (15% to 42%), however, the typical relationship is partially recovered, and coarser grains have lower particle friction angles than finer ones. Subtle textural differences between moderately steep and very steep channel beds suggest that incipient fluvial sorting into sub-meter-scale patches may be responsible for the grain-size-independent friction angles. In the very steep channel beds, where debris flows tend to dominate transport, grains appear randomly organized with respect to size. Downstream, in moderately steep channel beds, sorting into small clusters places particles next to those of similar size. Sorting into channel-spanning steps occurred at all slopes, however, and the grains within these steps were, on average, three-times harder to move than grains outside of steps when normalized by grain weight. These results suggest that sorting, and the resultant particle friction angles, is at least partially responsible for the stability of steps and coarse grains in moderately steep channels.