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.

Effects of mud supply on large-scale estuary morphology and development over centuries to millennia

“…4d) with dynamic channels and bars, but in the absence of mud they expand continuously by bank erosion due to channel migration. This agrees with the continuously exporting estuaries in the numerical models of Van der Wegen et al (2008) and with the physical experiments of Kleinhans et al (2015) with perpetually expanding tidal basins in cohesionless sand. After a rapid adjustment of basin size and bar and channel pattern the experiments developed to near equilibrium but never attained equality of sediment import and export.…”

Growing Forced Bars Determine Nonideal Estuary Planform

“…The latter, for example, occurs around elongated tidal bars, where the opposite sides of the bar crest have opposing directions of residual sand transport and residual water flow, forming a circulation pattern. The difference between the latter and former group of authors is that the macrocells describe only the largest scale of bars, whereas the mutually evasive transport paths occur at a range of scales, including that of the smallest shoals as also observed in experiments (Kleinhans, Scheltinga, et al, ; Kleinhans et al, ).…”

“…This has previously been reported for natural tidal systems as well as experiments. For example, experiments of short tidal basins show periodic migration of channels and shoals, which is coupled to reorganization of the channels in the tidal basin (Kleinhans et al, ). Most of the studies so far focussed on cyclicity on the ebb tidal delta (e.g., Elias & van der Spek, ; Israel & Dunsbergen, ; Oost, ), on which channels migrate from one side to the other, after which they disappear and reappear at their initial position.…”

Topographic forcing of tidal sandbar patterns for irregular estuary planforms

“…, ), for which boundaries are well known but bars are usually not preserved, as well as estuaries in numerical models and laboratory experiments (Kleinhans et al. ). This will allow us to systematically study the effects of boundary conditions and vegetation on the dynamics of tidal bars over time and on the evolution of the entire estuary from initiation on inherited landscapes.…”