Topographic forcing of estuarine channel networks by fixed banks

Sandy estuaries are characterized by braided channel networks with continuously shifting channels and bars induced by tidal currents. Many estuaries have planforms confined by bedrock or human structures, which can topographically force local morphology by suppressing channel and sandbar migration. To what degree topographic forcing determines channel pattern and mobility is poorly understood, because it is challenging to obtain sufficient temporal and spatial data to completely grasp the timescales at which the morphology changes. In this study, we therefore acquired large temporal datasets through conducting scale experiments of sandy estuaries in the periodically tilting tidal flume, the Metronome. We conducted several experiments with initial flat beds and fixed banks of different configurations using rough sandpaper, with at least one repeat experiment for each configuration, as well as a control run without fixed banks. From our data animations, we observe the formation of topographically forced sandbars as well as channel scours at fixed banks that are consistent between repeat experiments. Moreover, we discovered quasi-periodic formation and disappearance of non-forced sandbars and channels. For one fixed embankment configuration, the channel network rapidly shifted between three stable states, suggesting that aspects of the system morphodynamics may be described by coupled oscillators. Repeat experiments exhibit notable differences in periodicity of this cyclic behaviour, implying sensitivity of the complex system dynamics to initial conditions. These results confirm suspicions based on observations and linear stability analyses that shifting between alternative quasi-stable states can happen in estuaries, which provides a major challenge for field observations and numerical modelling of the dynamics of natural estuaries.