A breakwater is a coastal structure (usually a rock and rubble mound structure) projecting into the sea that shelters vessels from waves and currents, prevents siltation of a navigation channel, protects a shore area or prevents thermal mixing (e.g. cooling water intakes). A breakwater typically comprises various stone layers and is typically armoured with large armour stone or concrete armour units (an exception are e.g. vertical (caisson) breakwaters). A breakwater can be built at the shoreline or offshore (detached or reef breakwater). This measure is not directly addressed to protect the coast in flood events, but can indirectly stabilize the coast by preventing erosion.
Cliffs on coastal landscapes are formed by erosion processes of marine waves. This process, know as abrasion, can lead to precipitous formations. The shape of the cliff depends of a variety of factors including the jointing, bedding and hardness of the materials making up the cliff as well as the erosional processes itself. The speed, at which the erosion happens, depends on the strength of the surf, the height of the cliff, the frequency of storm surges and the hardness of the bedrock.
Rocky coasts are continuously cut back by the sea and are characterised by erosional features. They have a slow rate of morphological change, and experience the main erosional processes of: mechanical wave erosion, abrasion and hydraulic action; weathering - physical, salt, chemical and water-layer levelling; bio-erosion - biochecmial and biophysical; and mass movements by rock falls and toppling, slides and flows.
Both elements are characterized by erosional features, leading to a natural constant change of the systems. Eroding cliffs can be a risk for visitors at the beach and on the cliff and the erosion can lead to land losses in the backshore areas.