**2. Beach Sediment Budget**

Each stretch of beach, 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 easily 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]. It comprises multiple input and output items, each of them directly influenced by several factors, which, in their turn, respond to other forces, and none of these is stable over the time. In any beach the volume of input and output items can be many orders of magnitude different, and continuously changing, to make it statistically impossible for the statement to be equal.

**Figure 1.** Schematic beach sediment budget Input and Output. Arrows show the direction of the process; connecting lines thickness increases and decreases depending on the effect (positive vs. negative) that each item has on the following one (modified from the work in [29]).
