Incipient Tidal Bar and Sill Formation

Abstract

Estuaries and rivers show dynamic patterns of bars and channels. Mutually evasive ebb-and flood-dominated tidal channels, that is, parallel tidal channels with net sediment transport in opposing directions, give tidal bars their characteristic shape. Here, we study incipient tidal bars with laboratory scale experiments in a periodically tilting flume. Bar patterns evolved from an initial straight channel in an erodible sand bed. Analysis of time-lapse imagery and bathymetry shows that alternate bars form within 300 tidal cycles. The bars become discrete, recognizable elements after an initial phase characterized by a rhomboid pattern (short and narrow ridges oblique to the channel). Tidal bars show similarities and contrasts with fluvial bars. In general, morphological shapes are similar, including the presence of alternate bars, midchannel bars, and point bars. Sills are superimposed on incipient tidal bars. Sills are narrow and straight and form by flow divergence and thus sediment convergence between alternate bars. This contrasts with rivers, where sills are remnants of alternate bars. Alternate tidal bars grow further by amalgamation of two opposing U-shaped lobate bars. The dimensions of incipient tidal bars emerge in the first 500 cycles, after which their length and position determine a large-scale quasiperiodic variation in the estuary width. The results imply that sills are initiated with the onset of tidal bar formation in straight channels and trigger formation of mutually evasive ebb-and flood-dominated channels, thus explaining their ubiquitous occurrence in tidal systems.

Nonlinear dynamics of tides and sandbars in tidal channels

“…However, to study the effect of variations in the phase difference between the currents and the free surface elevation, a global model is needed. The tidal bars simulated in the laboratory by Leuven and Kleinhans (2019) and those observed in the Exe estuary (Figure 1.1), extend over the middle axis of the channel. This corresponds to the results above.…”

“…Complementary to the models described above, tidal bars were also studied in a laboratory by Tambroni et al (2005) and Leuven and Kleinhans (2019). Tambroni et al (2005) report that the tidal bars that they simulated in their laboratory have characteristics that are consistent with those predicted by the theoretical model of Seminara and Tubino (2001), although the tidal bar wavelength was slightly smaller than that expected from the latter study.…”

“…The channel of Tambroni et al (2005) had one open end and one closed end. In contrast, the set-up of Leuven and Kleinhans (2019) was as close as possible to a local model, by using a periodically tilting flume with two open sides. Besides a correspondence with the theory of Schramkowski et al (2002) during the initial formation of the bars, they reproduced the formation of so-called sills.…”

“…The latter are narrow straight 'bridges' between the alternating tidal bars. Moreover, Leuven and Kleinhans (2019) attributed the differences between the initial formation and the long-term behavior to nonlinear processes and highlighted the need of a further development of the theory of nonlinear tidal bar dynamics to explain their observations.…”

“…The new local model has the option to integrate in time, such as the other (non local) numerical models. This allows one to study the transient behavior of the tidal bars and compare this with the evolution of tidal bars in a laboratory described by Leuven and Kleinhans (2019). Moreover, by using a root-finding algorithm, the model finds morphodynamic equilibria and their stability directly.…”