*Review* **The Equilibrium Concept, or . . . (Mis)concept in Beaches**

**Enzo Pranzini 1,\* and Allan T. Williams <sup>2</sup>**


**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.

**Keywords:** beach sediment budget; coastal dynamics; beach evolution; coastal morphology; feedback processes
