Abstract
Small, intermittently open or closed estuaries are characteristic of the coasts of South Africa, Australia, California, Mexico and many other areas of the world. However, modelling attention has tended to focus on big estuaries that drain large catchments and serve a wide diversity of interests e.g. agriculture, urban settlement, recreation, commercial fishing. In this study, the development of a simple, parametric, system dynamics model to simulate the opening and closure of the mouths of small, wave-dominated estuaries is reported. In the model, the estuary is conceived as a basin with a specific water volume to water level relationship, connected to the sea by a channel of fixed width, but variable sill height. Changes in the form of the basin are not treated in the model, while the dynamics of the mouth channel are central to the model. The magnitude and direction of the flow through the mouth determines whether erosion or deposition of sediment occurs in the mouth channel, influencing the sill height. The model is implemented on the Great Brak Estuary in South Africa and simulations reveal that the raised low water levels in the estuary during spring tide relative to neap tide, are occasioned by the constriction of the tidal flow through the shallow mouth. Freshwater inflows to the estuary are shown to be significant in determining the behaviour of the inlet mouth, a factor often ignored in studies on tidal inlets. Further it is the balance between freshwater inflows and wave events that determines the opening or closure of the mouth of a particular estuary.
Highlights
Freshwater inflows to the estuary are shown to be significant in determining the behaviour of the inlet mouth, a factor often ignored in studies on tidal inlets
The study confirms that it is the balance between freshwater inflows and wave events that determines the opening or closure of the mouth of a particular estuary
The parametric approach to modelling the closure and opening of the inlet of small, wave-dominated estuaries has established that the pumping up, or elevation, of low water levels over spring tides relative to neap tides is primarily determined by the constraining effect of the mouth on tidal fluxes
Summary
Open or closed estuaries characterize the coasts of South Africa, Australia, west Africa, California, Mexico and many other areas of the world (Cooper, 2001; Taljaard et al, 2009; Roy et al, 2001; Ranasinghe et al, 1999; Ranasinghe and Pattiaratchi, 2003; Goodman, 1996; Anthony et al, 2002; Jacobs et al, 2011; McLaughlin et al, 2013; Mendoza et al, 2009). The differences lie primarily in the manner in which the (maximal) volumetric flow through the channel is determined, and the sedimentation formulae that are applied Another parametric modelling approach focusing on sedimentation in a channel has been followed by Eysink and Vermaas (1983) and van Rijn (2013), who developed an empirically-based prediction tool Sedpit often used in conjunction with a harbour siltation model to determine dredging requirements for harbours. This model was applied to simulate sedimentation at Ijmuiden in the Netherlands with success (van Leeuwen, 2015).
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