Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2006
Publication Date: 9/30/2006
Citation: Simon, A., Rinaldi, M. 2006. Disturbance, stream incision, and channel evolution: The roles of excess transport capacity and boundary materials in controlling channel response. Geomorphology 79: 361-383.
Interpretive Summary: The causes of river channel incision are numerous, but the morphological effects and hazards associated with incised channels are often similar, across a spectrum of. Incision is a common response of alluvial channels that have been disturbed such that they contain excess amounts of flow energy or stream power relative to their sediment load. However, channel incision represents only one of a range of possible adjustment scenarios for disturbed streams that are free to adjust their boundaries. The purpose of this paper, therefore, is to not only present findings on the nature of channel incision and related processes in disturbed alluvial systems, but to also place this important process in the broader general context of the driving and resisting forces that govern channel adjustment. This is accomplished by analyzing channel adjustment in terms of the imbalance between flow energy or stream power and sediment load by presenting examples of unstable channels using field and numerical-modeling techniques.
Technical Abstract: Channel incision is part of denudation, drainage-network development, and landscape evolution. Large, anthropogenic disturbances, similar to large or catastrophic “natural” events, greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to anthropogenic and “natural” disturbances are presented for fluvial systems in the mid continent and Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically-controlled sediment (sand) provided by bank erosion. The sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape, with erosion from the channel banks generally the dominant source. Disturbances that effect available force, stream power or flow energy, or change erosional resistance such that an excess of flow energy occurs can result in incision.