Estuaries, tidal inlets, Escoffier, O,Brien and morphological attractors

Jon Hinwood, Errol McLean

Friday 3 july 2015

13:45 - 14:00h at Antarctica (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) Morphodynamics of estuaries and coastal areas

Parallel session: 16A. Sediment - Coast

In a previous paper the authors showed that over very long time scales a barrier estuary or tidal inlet will tend towards one of two states, called attractors. In this paper it is shown how that analysis represents an extension and generalisation of three earlier analyses. The class of estuaries we consider is the barrier estuary in which a tidal basin is connected to the sea by a constricted entrance channel through an erodible sand barrier. In such estuaries, tidal exchange is the dominant process controlling water and sediment transports. The entrance channel through the barrier provides a significant hydrodynamic resistance and has a regulating effect on the tidal prism. The three widely recognised quantitative “laws” describing the long term equilibrium dimensions of the entrance channel are: the tidal prism-entrance area relations, often referred to as the O'Brien equation, secondly the Escoffier or closure diagram, based on a simple hydrodynamic relationship between the entrance area and the entrance velocity plus an empirical "equilibrium velocity", and thirdly the Bruun rule relating the entrance channel stability to the longshore sediment supply. Each of these is based on major simplifications that restrict its utility and range of validity more than is usually recognised. The attractor analysis, while still based on a lumped model, adds sediment transport and deposition/scour equations and enables the use of more realistic entrance hydrodynamics. It predicts the rates of change and presents the results on a practical “attractor map”. The predictions of the three laws are shown on the attractor map and their practical application is critically compared.