Turning the tide: Growth and dynamics of a tidal basin and inlet in experiments

Abstract

Dynamic equilibrium of short tidal systems with ebb deltas, inlets, and basins is poorly understood. Observations suggest the possibility of equilibrium with sediment import balancing export, while individual channels and shoals at the local scale remain dynamic. Our objectives are to ascertain (1) whether tidal systems under entirely steady forcing can attain this state and (2) under what conditions cyclic channel-shoal migration occurs. We present experiments of tidal systems developing from an initial breach in the coast. We periodically tilted the entire flume to obtain reversing tidal currents and sediment transports. The surface area of the back-barrier basin with an inlet channel with erodible boundaries continued to enlarge while sediment mobility decreased. Experiments with fixed inlet boundaries remained smaller and much more dynamic and had cyclically migrating ebb and flood channels. The same cyclicity with a period of about 80 tides is observed in shifting dominance of the two channels of the inlet and in the shifting channels in the basin. Experiments with stepwise sea level rises resulted in more rapid channel and bar shifting, increased channel dimensions and basin size. We conclude that cyclic migration of channels is coupled between inlet and basin but the ebb delta did not show such cyclicity. Furthermore, tidal basins with erodible boundaries slowly enlarge by margin erosion toward a system where sediment mobility is at the threshold for motion, as in braided gravel-bed rivers. Consequently, in nature dimensions of tidal systems are partly determined by naturally formed cohesive and vegetated margins and geological context.