SUPPLY LIMITED BEDFORM PATTERNS AND SCALING THROUGH A GRAVEL-SAND TRANSITION

A distinct suite of bedforms has been observed as sand supply to the bed progressively increases. Initially, sand ribbons form, followed by barchans, and eventually channel spanning dunes. While the characteristics of supply-limited bedforms have been documented in laboratory settings, there are relatively few observations from natural river channels. Moreover, there are even fewer observations of transitions from supply-limited to transport limited conditions in the field. Here we examine bedforms developed downstream of the abrupt gravel-sand transition (GST) of the Fraser River in British Columbia using multi-beam swath-bathymetry obtained at high flow. This is an ideal location to study supply-limited bedforms because an abrupt change in slope associated with the GST leads to an increasing sand supply to the bed in the downstream direction. We observe barchanoid and isolated dunes in the reach immediately downstream of the slope beak and most of the bedform field has gaps in the troughs consistent with sand moving over an immobile gravel bed. Linear, along-stream bedform fields exhibit characteristics of sand ribbons with superimposed dunes. Once the bed is entirely sand covered and transport limited, channel spanning dunes develop. There is a 4-fold increase in sediment transported via bedform migration between the supply- and transport limited reaches downstream of the GST. Bedform dimensions (height and length) increase in the downstream direction. The conventional bedform scaling with flow depth (height is equal to 1/6 flow depth; length is equal to 5x flow depth) does not emerge in our data set. Instead, dunes heights are 1/20 flow depth and lengths are on the order of the flow depth throughout most of the bedforms field with a large amount of scatter about these mean values. Only where the channel is transport-limited in terms of sand does the conventional scaling emerge for dune dimensions. The results question the conventional scaling of bedforms with flow depth in river channels where sand is supply-limited. We provide some evidence from the literature that the conventional scaling with flow depth is, in fact, a myth.