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.