Breakwaters are offshore structures that are built (both parallel and perpendicular to the coast) to protect the coast from longshore drift. While there are a wide variety of breakwaters, their most common purposes are to ensure safe harborage and reduce coastal erosion. They are constructed to absorb the energy of incoming waves, and are usually built with rock, concrete, and/or caissons. Unlike the other forms of hard stabilization, breakwaters can be implemented through a range of strategies, including: rubble, caisson, and wave attenuator breakwaters.
Rubble breakwaters use structural voids, or empty pockets, to diminish the energy of incoming waves. They are constructed with piles of stones, using smaller stones for the core, surrounded by heavier stones that serve as a form of armor for the structure. Rubble breakwaters are a form of revetment breakwater, which are land-backed rather than sea-backed. Rubble structures are used in shallow water, because they are extremely costly as the depth of the water increases.
Caisson breakwaters, on the other hand, are not a form of revetment breakwater. Rather, they are built in deeper waters for the primary purpose of berthing ships on their inner face. That said, they are also sometimes constructed to absorb waves and prevent erosion.
Wave attenuator breakwaters, seen below, are partially submerged, floating structures that are generally more temporary in design. Because currents can continue to travel beneath them, they do not disrupt the flow of sediment as much as other breakwaters. Instead, they simply reduce the force of coastal waves, which often serves the purpose of protecting nearby marinas.
With the exception of wave attenuators, breakwaters have similar consequences to groins and jetties; they disrupt longshore currents and cause one side of the breakwater to trap sediment, and the other side to become starved. As is the case with the other hard stabilization strategies, this results in accelerated erosion over time. Breakwaters are usually relatively cheap to install, but the costs of the coastal erosion on the downdrift side quickly outweigh the economic benefits.
Works Referenced:
https://www.britannica.com/technology/breakwater
https://www.sciencedirect.com/topics/engineering/breakwaters