Breakwaters are used to protect a harbor, anchorage, basin, or area of shoreline from waves. Breakwaters reflect or dissipate wave energy and thus prevent or reduce wave action in the protected area. These structures must be designed to effectively serve competing requirements for wave blockage and safe vessel passage from fully exposed waters through a constricted entrance into tranquil harbor waters.
Breakwaters are often constructed as shore-connected structures, thereby allowing access from land for construction, operation, and maintenance. But this design may have an adverse impact on water quality or sediment transport, so detached offshore breakwaters are used in certain situations. Many systems utilize a combination of shore-connected and offshore breakwaters to protect anchorage or mooring areas.
It is preferable for breakwaters to prevent wave energy from entering a harbor rather than try to dissipate excessive wave energy inside the harbor. It is also important that breakwaters be constructed to limit wave reflection, which can cause hazardous navigation conditions.
Most breakwaters built on the open coasts of the United States consist of rubble-mound construction. Other structural types include concrete caisson, timber crib, sheet pile, composite, and floating. All breakwaters must be high enough to prevent excessive wave overtopping and sufficiently impermeable to deter wave transmission through the structure.