Dynamics of intertidal drainage channels on a multi-barred macrotidal beach

摘要

The intertidal drainage channels on a macrotidal bar-trough (ridge-and-runnel) beach were monitored during a 17-day survey. Type 1 channels were persistent, dominantly longshore systems essentially limited to the wide intertidal zone between mean high and low water neap tidal levels. The cumulative length of this channel type fluctuated as a function of topographically controlled through-flow or flow impedance in troughs, and showed no correlation with the semi-lunar tidal cycle. Smaller, ephemeral type 2 channels appeared as dominantly cross-shore systems incising bars on the narrower upper and lower beach zones during spring tides. They disappeared during neap tides through infill by waves and aeolian activity. The only significant phase of type 1 channel mobility occurred during a brief moderate-energy storm at the start of the survey. The effect of this mobility on beach morphology was inextricably linked to that of waves and currents. Meander bend migration, forced by wave- and longshore-current-induced migration of a bar during the storm, resulted in important but highly localized morphological change that was only a minor part of an irregular saw-tooth pattern of change that affected the entire beach profile, and that was largely controlled by wave processes and longshore currents. The flow velocities in channels on this beach are too weak to generate the formation and longshore migration of high-energy bedforms. Channel mobility and impact on beach morphology are expected to increase under storm conditions. Copyright (c) 2007 John Wiley & Sons, Ltd.