On Morphological Change and Tidal Energy Dissipation in a Modelled Estuary

As the morphology of a tidal estuary evolves, the overall rate of energy dissipation changes as adjustments occur to channels, meanders and shoals. This occurs in accordance with the principle of increasing entropy production as evolving seabed features add to the disorder of the system. Model simulations of bathymetric evolution over 250 years in a converging tidal estuary show how energy dissipation is reduced in time, becoming localised in evolving channels while also being redistributed down-estuary from its initial predominance in the intertidal reach. Less morphological activity occurs as the bathymetry seeks an equilibrium state, characterised by a tendency towards an approximately linear decrease up-estuary in both the mean channel width and the mean energy dissipation rate. Simulations carried out both with and without morphological acceleration show the same general behaviour regarding the reduction in energy dissipation, but with the dissipation decreasing less rapidly if a morphological factor (MF) is implemented. This seems due in part to the finer texture of shoals and channels predicted in the inner estuary without an MF applied, which leads to reduced dissipation rates compared with the predictions made with an MF. The study confirms that entropy production decreases as an estuary evolves towards equilibrium.