HR: 13:55h
AN: H53D-02    [Abstracts]
TI: Coupling sediment transport and channel morphology: must we?
AU: * Wilcock, P
EM: wilcock@jhu.edu
AF: Johns Hopkins University, Geography and Envionmental Engineering, Baltimore, MD 21218 United States
AB: There is broad agreement that stream channels adjust to the water and sediment supplied to them, indicating that sediment transport and channel morphology are coupled. There is contrary evidence. The hydraulic geometry provides broad empirical description of channel morphology using water discharge, but not sediment transport. Most stream channel designs make little, if any, reference to sediment transport and, once constructed, many appear to maintain their size and shape. The disruption of water and sediment supply by reservoirs produces downstream channel change, although the effect is often attributed to flow control and vegetation establishment, rather than a change in sediment supply. Is sediment transport relevant to channel morphology? There is some evidence. Stream channels subjected to high rates of sediment supply may not fit the broader trends of the hydraulic geometry. Some stream channel designs fail through excessive deposition. Some rivers below dams undergo considerable downcutting and armoring, indicating that they had once been adjusted to the sediment supply rate now interrupted by the reservoir. Evidently there is a threshold in the rate of sediment supply that determines whether sediment transport matters or not. Below the threshold, streams are shaped hydraulically, with a form determined by water discharge and channel materials, but not sediment transport rates. Above the threshold, channel morphology expresses an adjustment between water discharge and sediment transport rates. Given the range of streams whose morphology is described by the hydraulic geometry, and the limited adjustment observed in many built channels with a sediment-free design, the magnitude of the sediment supply threshold may be larger than typically thought. As models of channel morphology and sediment routing are refined and applied at increasing scales, and as countless miles of stream channel are rebuilt, a clearer definition of the sediment supply threshold will be useful.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting