HOW DO SAND-BEDDED RIVERS SCOUR?

Alluvial rivers adjust their geometries to balance bed stresses induced by channelized flow and the resistance of the sediment in the bed and banks to transport. In sand-bedded rivers sediment transport occurs in bedforms and through suspension, and much of this sediment is generally assumed to come from an upstream source. Riverbed scour (temporary lowering of the riverbed) is also a potential large source of sediment, particularly in channels which have substantial alluvial thickness. Scour processes have been widely studied in the engineering discipline because of the obvious importance to civil infrastructure. But riverbed scour is also an important geologic process because it directly relates to the potential preservation thickness of alluvial deposits, and may be an important component of fluvial sediment budgets. In this paper we use bed-adjustment data from several sand-bedded rivers to examine riverbed scour as a morphodynamic problem, speculate on the spatial variability and control of scour, and address the role it may play in fluvial sediment budgeting.