Abstract
Beaches, as deposits of unconsolidated material at the land/water interface, are open systems where input and output items constitute the sediment budget. Beach evolution depends on the difference between the input/output to the system; if positive the beach advances, if negative the beach retreats. Is it possible that this difference is zero and the beach is stable? The various processes responsible for sediment input and output in any beach system are here considered by taking examples from the literature. Results show that this can involve movement of a volume of sediments ranging from few, to over a million cubic meters per year, with figures continuously changing so that the statistical possibility for the budget being equal can be considered zero. This can be attributed to the fact that very few processes are feedback-regulated, which is the only possibility for a natural system to be in equilibrium. Usage of the term “beach equilibrium” must be reconsidered and used with great caution.
Highlights
Equilibrium is a term having many different meanings
Even if delimited by coastal structures, such as harbors, jetties, groins, and detached breakwaters, is not a closed system where the amount of initial free energy is less available as the system moves towards a state with maximum entropy [20], as it might appear from previous descriptions, but it is crossed by a significant flow of entering and exiting material, some entering and some exiting, all contributing to a beach sediment budget [28]
Contrary to what one is led to believe, erosion of high coasts generally provides a very low sedimentary input, and those models of coastal evolution, which see the retreat of promontories and the filling in of the gulfs to the complete rectification of the coast, appear to be linked to a Davisian concept of coastal geomorphology largely outdated
Summary
Equilibrium is a term having many different meanings. In physics, it describes the average condition of a system, as measured through one of its elements or attributes over a specific period of time. For a short time scale, the beach is subject to feedback processes that make it assume the form that best dissipates wave energy, and as this changes moment by moment, even the beach adapts and changes In these short periods some sediment can enter/exit, but does this infer that a beach is stable? What is under discussion here is the existence of that hypothetical equilibrium around which the beach should fluctuate The approach to this problem in this paper starts with processes that form the beach sediment budget looks at how they can modify sediment input and output from the beach, considers whether related processes are feedback regulated or not, and, to try to find a functional definition of beach stability (if any).
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