If the inputs and outputs of the systems are in balance, than the costal system is said to be in equilibrium. For example, the rate at which sediment is added to a beach is equal to the amount of sediment being taken out. However, should this equilibrium be interrupted, then the system will respond to the changes and attempt to restore the equilibrium. This is known as the dynamic equilibrium.$^2$
After a change, if the response causes further change that brings the system further away from equilibrium, then it is called a positive feedback. If the response brings the system closer to the equilibrium, then it is called a negative feedback.$^2$
Waves are generated by wind blowing over the sea. The characteristics of the waves are determined by the strength of the wind, its duration, and its fetch(length of water over which the given wind was blown).$^{12}$
Constructive waves have a low frequency and are low in height. They have a strong swash which carries materials up the beach. Their wave energy dissipates over a large area and thus creates a weak backwash.$^{12}$
Destructive waves have a large wave height and a short wavelength. They have tall breakers that have a high downwards force that creates a strong backwash. They also have a high frequency. They erode the beach and deposit it in an offshore sand bar under the sea.$^{12}$
If waves crashed against the beach at 90 degree angles, there would be no longshore drift. However that is not the case. Waves are created in the direction of the prevailing wind direction. As such, the swash also acts at an angle. However, backwash always acts at 90 degrees to the coast. This means that overtime, there is a net movement of materials along the side of the beach. This net movement is called longshore drift.$^{13}$