breakwater

 

offshore structure to protect a harbor from wave energy or deflect currents. When it also serves as a pier, it is called a quay; when covered by a roadway it is called a mole. In the United States a breakwater commonly consists of a long mound of stone rubble and of cheaper materials like rubber tires and oil drums. The flow of waves up its slope, and the formation of swirls by its rough surface dissipate wave energy. A pneumatic breakwater consists of perforated pipes discharging air bubbles; another type has underwater pipes that direct streams of water against approaching waves to cause them to break. Breakwaters are also used to promote sedimentation, which, depending on the breakwater's alignment, will infill to produce a stable beach. The Chesapeake breakwater was the first built in the United States. See coast protection.

Breakwaters are structures constructed on coasts as part of coastal defense or to protect an anchorage from the effects of weather and longshore drift.

Purposes of breakwaters

Defense against coastal erosion

Offshore breakwaters, also called revetments or bulkheads, reduce the intensity of wave action in inshore waters and thereby reduce coastal erosion. They are constructed some distance away from the coast or built with one end linked to the coast. The breakwaters may be small structures, placed one to three hundred feet offshore in relatively shallow water, designed to protect a gently sloping beach. Breakwaters may be either fixed or floating: the choice depends on normal water depth and tidal range.

When oncoming waves hit these breakwaters, their erosive power is concentrated on these structures some distance away from the coast. In this way, there is an area of slack water behind the breakwaters. Deposition occurring in these waters and beaches can be built up or extended in these waters. However, nearby unprotected sections of the beaches do not receive fresh supplies of eroded sediments and may gradually shrink due to erosion.

Breakwaters are subject to damage, and overtopping by big storms can lead to problems of drainage of water that gets behind them. The wall also serves to encourage erosion of beach deposits from the foot of the wall and can increase longshore sediment transport.

Protection of anchorages

An anchorage is only safe when ships anchored there are protected from the force of high winds and powerful waves by some large underwater barrier which they can shelter behind. Natural harbours are formed by natural barriers such as headlands or reefs. Mobile harbours, such as the D-Day Mulberry harbours were floated into position and acted as breakwaters. Some natural harbours, such as those in Plymouth Sound, Portland Harbour and Cherbourg, have been enhanced or extended by breakwaters made of rock.

Types of breakwater structures

A breakwater is constructed some distance away from the coast or built with one end linked to the coast. Breakwaters may be either fixed or floating: the choice depends on normal water depth and tidal range. A breakwater structure is designed to absorb the energy of the waves that hit it. This is done either by using mass (e.g. with caissons) or by using a revetment slope (e.g. with rock or concrete armour units).

Caisson breakwaters typically have vertical sides and are usually used where it is desirable to berth one or more vessels on the inner face of the breakwater. They use the mass of the caisson and the fill within it to resist the overturning forces applied by waves hitting them. They are relatively expensive to construct in shallow water, but in deeper sites they can offer a significant saving over revetment breakwaters.

Revetment breakwaters use the voids in the structure to dissipate the wave energy. Rock or concrete armour units on the outside of the structure absorb most of the energy, while gravels or sands are used to prevent the wave energy continuing through the breakwater core. The slopes of the revetment are typically between 1:1 and 1:2, depending upon the materials used. In shallow water revetment breakwaters are usually relatively cheap, but as water depth increases, the material requirements, and hence costs, increase significantly.

The revetment breakwater at Douglas on the Isle of Man is made from unusual three-dimensional concrete structures called "stabits". The name is derived from the island's Latin motto "quocunque jeceris stabit" (wherever you throw it, it will stand) and refers both to the similarity between the shape of the stabit and the island's three-legged emblem.

See also

• Akmon
• Dolos invented by South African engineer in East London
• Groyne
• Harbour
• Jetty
• Mole (architecture)
• Pier
• Port
• Xbloc
• Tetrapod
• Accropode

External links

• Concrete Layer Innovations
• USGS Oblique Aerial Photography - Coastal Erosion from El-Niño Winter Storms October, 1997 & April, 1998
• Channel Coastal Observatory - Breakwaters

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