Tidal dynamics and substrate response in a salt-marsh estuary

Abstract

The tidal wave in the Duplin River, a well-mixed salt-marsh estuary on the Georgia coast, displays distinctive forms of asymmetry in the horizontal and vertical tides. Ebb-current speeds reach a maximum of up to 30% higher than corresponding flood currents and maximum ebb and flood discharges are both displaced toward the time of high water. Duration of the flooding tide can be more than one hour longer than duration of the corresponding ebbing tide. Tidal asymmetry is clearly reflected in the tidally varying dynamic balance of the estuary estimated from a simplified equation of motion. The ebb-related pressure-gradient term and corresponding shear-stress term at the bed are an order of magnitude greater than flood-related pressure gradient and bed shear. Ebb-dominated asymmetry in the Duplin River is attributed to interaction of the principal lunar M 2 tide with its first harmonic overtide, M 4. Generation of the M 4 tide is thought to be from non-linearities that arise from continuity of the salt-marsh estuary, which is best described by a changing basin area model. Substrate response to ebb-dominated tidal dynamics is the development of largescale bedforms that maintain ebb-oriented geometry through both ebb and flood tide. Bedforms are in equilibrium with maximum ebb-related bed-shear values which result in sand transport rates that are an order of magnitude greater during ebb compared with flood tide.