The domain of bedload sheets

Bedload sheets are low-amplitude bedforms the are formed by sorting patterns in mixed-size sediment (gravel or gravel-sand mixtures). The features have been functionally linked to dune forms developed in sandy sediments and gravel dunes formed in river channels. There have been a number of attempts to place bedload sheets on extended versions of the bedform phase diagrams used to predict sand bedform occurrence. There is no reason to suspect that this is problematic from the perspective of fluid and sediment particle interactions. However, it is well-known that bed surface texture and sediment flux in gravel bedded rivers are controlled by the magnitude and grain-size distribution of sediment supply to the channel. Here, we demonstrate that the occurrence of bedload sheets is not controlled hydraulically and that their occurrence, dimensions, and dynamics are primarily controlled by sediment supply. The results suggest that placing bedload sheets, and perhaps all bedforms developed in mixed size sediments, on hydraulically-based bedform phase diagrams is problematic. We suggest that a more appropriate approach may to be construct bedform occurrence diagrams for mixed-size sediments in the phase space formed by a sediment supply parameter and a bed mobility parameter.