Ebb- and flood tidal channels in scale-experiments of estuaries

The occurrence of mutually evasive ebb- and flood tidal channels is one of the most typical features in tidal basins and estuaries (Fig. 1). While Van Veen already observed and described these striking elements in the 1950s, their forming mechanism is still largely unknown because of the challenges in numerical modelling and physical models of tidal systems. Recently, it was showed that a periodically tilting flume can generate dynamic tidal morphology (Kleinhans et al., 2015). In this study we investigate the formation and evolution of mutually evasive ebb- and flood tidal channels with measurements of water levels and tidal currents. We define the confluences of ebb- and flood channels as nodes, for which we will show that they behave as asymmetric stable bifurcations.
We created estuaries in a tilting flume of 20 m long and 3 m wide. The maximum tilting slope was 0.004 m∙m-1 with a period of 40 s. The typical amplitude of tidal flow velocity was 0.3 m∙s-1 and water depths were 0.02-0.05 m. The experiment started with an initial converging channel at the centreline of the flume and was subsequently tilted for 15,000 tidal cycles. Detailed measurements of bathymetry, water levels and flow velocities were made during 12 stages of the experiment.
Bathymetry was created using Structure for Motion software (Agisoft) (Fig. 2a). Flow velocities were measured using Particle Image Velocimetry (PIV) and water levels were measured with echo sounders. Overhead cameras collected time-lapse imagery to monitor the evolution of the experiment. The water was dyed blue and the blueness was used as a proxy for water depth (Fig. 2b).
Dynamic channels and shoals evolved during the experiment. In the first 500 tidal cycles, an alternate bar pattern developed. This stage was followed by widening of the estuary, formation of flood tidal channels with a sill at their upstream end, and an increase in the number of channels and shoals (Braiding Index) in cross-section.
Water levels and tidal currents varied considerable in along-channel and cross-channel direction, creating cross-bar water level gradients. At these locations, cross-bar channels formed, because water levels and flow velocities in the ebb channel were generally larger than in the flood channel (Fig. 2). A growing cross-bar channel often reversed the role of the ebb- and flood channels.
Within 500 tidal cycles, mutually evasive ebb- and flood tidal channels formed in our experimental setup. Measurements of water level, bed level and flow velocities show that tidal bifurcations evolve into asymmetric stable bifurcations. After the formation of a flood tidal channel, gradients in water level generate cross-bar flow, which eventually cross-cuts the tidal bar that formed between the flood channel and the parallel ebb tidal channel.