Morphology and Dynamics of Channels in the Coastal Zone: A Review

Channels in the coastal zone respond to water fluxes with two very different flow patterns and periodicities: unidirectional fluvial discharge which varies stochastically on time periods ranging from annual to decadal (or even longer); and bidirectional tidal flux which varies in a very regular manner on periods ranging from semi-diurnal to annual and even longer, but with most of the energy concentrated at short periods of hours to days. Maximum shear stresses for river currents typically occur when water depths are greatest, whereas those for tidal currents are greatest at depths less than bankfull. The relative importance of these two processes varies seasonally and over longer time periods. When averaged over periods of many years, this ratio appears to exert a first-order control on the morphology and dynamics of coastal-zone channels. This ratio varies along the length of individual channels through the fluvial-marine transition, between laterally adjacent channels because of differences in fluvial input, and between systems because of variations in runoff and tidal prism. The parts of channels with a tidal dominance tend to have a net landward movement of bed material, whereas the parts of channels that are river-dominated have a net seaward transport of bed material. Within 'estuaries' (i.e., transgressive coastal systems), which have unfilled accommodation because of relative sea-level rise, the bedload convergence that lies between the areas of seaward and landward bed-material transport lies landward of the coast. By contrast, channels such as active delta distributaries are river-dominated along their entire length and have expelled the bedload convergence during the transition from an estuarine phase. Within a single coastal system, channels that are more strongly fluvially dominated show less seaward flaring (i.e., a less pronounced funnel-shaped geometry) than adjacent channels with less fluvial influence. Because of the landward narrowing, most channels become more sinuous inland of the coat. Systems that are 'estuarine' as defined here, including tidal channels in the 'abandoned' portions of delta plains, reach very high sinuosities (> 2.5) in the area of the bedload convergence, and then become straighter in the fluvially dominated zone, producing a 'straight'-meandering-'straight' channel pattern. In the transition from a sediment-importing to a sediment-exporting system, these tight meanders are lost by neck cut-off. In at least some estuarine systems, successive meanders appear to have alternately higher and lower radii of curvature. The direction of net sediment movement in the channel appears to exert some influence on the shape of the meanders (e.g., symmetrical vs. skewed), but more work is needed to confirm possible trends. The limited amount of long-term morphological data for tide-dominated channels suggests that their meander bends migrate more slowly than do those of fluvial channels: scroll-bar morphology is never (?) present in channels with significant tidal flux. These observations have significant implications for the nature and distribution of channel deposits in coast-plain successions.