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Breakwater (structure)

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Breakwaters create safe harbors, but can also trap sediment moving along the coast. Long Beach Peninsula in Long Beach, California, April, 1998.
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

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Schematic of shifting coastline with breakwaters


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.

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3 of the 4 breakwaters forming Portland Harbour
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The eight offshore breakwaters at Elmer, UK

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

External links

Coastal defense
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weather is the set of all extant phenomena in a given atmosphere at a given time. The term usually refers to the activity of these phenomena over short periods (hours or days), as opposed to the term climate, which refers to the average atmospheric conditions over longer periods of
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Longshore drift (sometimes known as shore drift, LSD or littoral drift) is a geological process by which sediments such as sand or other materials, move along a beach shore.
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coast is defined as the part of the land adjoining or near the ocean. A coastline is properly a line on a map indicating the disposition of a coast, but the word is often used to refer to the coast itself.
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coast is defined as the part of the land adjoining or near the ocean. A coastline is properly a line on a map indicating the disposition of a coast, but the word is often used to refer to the coast itself.
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beach, or strand, is a geological landform consisting of loose rock particles - such as sand, gravel, shingle, pebbles, cobble - or even shell fragments, along the shoreline of a body of water.
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Ocean surface waves are surface waves that occur in the upper layer of the ocean. They usually result from wind or geologic effects and may travel thousands of miles before striking land. They range in size from small ripples to huge tsunamis.
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coast is defined as the part of the land adjoining or near the ocean. A coastline is properly a line on a map indicating the disposition of a coast, but the word is often used to refer to the coast itself.
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Slack water, or slack tide, is the time during which no appreciable tidal current is flowing in a body of water.

Slack water usually occurs near high water and low water, and are occasions in between when the direction of the flow reverses.
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Deposition is the geological process whereby material is added to a landform. This is the process by which wind and water create a deposit, through the laying down of granular material that has been eroded and transported from another geographical location.
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Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. Sedimentation is the deposition by settling of a suspended material.
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Erosion is displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).
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ship is a large watercraft capable of offshore navigation. Ships may be operated by:
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Underwater is a term describing the realm below the surface of water where the water exists in a natural feature (called a body of water) such as an ocean, sea, lake, pond, or river. Three quarters of the planet Earth is covered by water.
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harbor or harbour (see spelling differences), or haven, is a place where ships may shelter from the weather or are stored. Harbors can be man-made or natural. A man-made harbor will have sea walls or breakwaters and may require dredging.
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headland is an area of land adjacent to water on three sides. A bay is the reverse, rather an area of water bordered by land on three sides. A large headland may also be called a peninsula. Long, narrow and high headlands may be called promontories.
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reef is a rock, sandbar, or other feature lying beneath the surface of the water yet shallow enough to be a hazard to ships. Many reefs result from abiotic processes—deposition of sand, wave erosion planning down rock outcrops, and other natural processes—but the
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D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and
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A Mulberry harbour was a type of temporary harbour developed in World War II to offload cargo on the beaches during the Allied invasion of Normandy.

The Mulberry harbours were two prefabricated or artificial
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Plymouth Sound, or just The Sound, is a bay at Plymouth in England.

Its south west and south east corners are Penlee Point in Cornwall and Wembury Point on Devon, a distance of about 3 nautical miles (6 km).
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Portland Harbour is located beside the Isle of Portland, off Dorset, on the south coast of England. It is one of the largest man-made harbours in the world. Grid reference: SY 685 765 .
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Commune of
Cherbourg-Octeville

Administration
Country  France
Arrondissement Cherbourg-Octeville
Canton Chief town of 3 cantons
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In geotechnical engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships.
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berth is used to describe a bed on a boat or a location in a port or harbour used specifically for mooring vessels while not at sea (or as a verb to describe bringing a vessel alongside - to berth
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Revetments are structures placed on banks or cliffs in such a way as to absorb the energy of incoming water or explosives. They are usually built to preserve the existing uses of the shoreline and to protect the slope, as defense against bombs or artillery, or to secure an area
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Douglas
Manx - Doolish


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Ellan Vannin
Isle of Man


Flag Coat of arms
Motto
Quocunque Jeceris Stabit
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akmon is a multi-tonne concrete block used for breakwater and seawall armouring. It was originally designed in the Netherlands in the 1960s, as an improvement on the tetrapod.
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dolos (plural dolosse) is an unusually-shaped concrete block weighing up to 20 tons which is used in great numbers to protect harbour walls from the force of the sea.
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