Jump to content

Breakwater (structure)

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by 80.63.55.170 (talk) at 13:36, 22 May 2012. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Breakwaters create safer harbours, but can also trap sediment moving along the coast. Alamitos Bay, CA entrance channel.

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

Offshore breakwaters, also called bulkheads, reduce the intensity of wave action in inshore waters and thereby reduce coastal erosion or provide safe harborage. Breakwaters may also be small structures designed to protect a gently sloping beach and placed one to three hundred feet offshore in relatively shallow water.

An anchorage is only safe if 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 such barriers as headlands or reefs. Artificial harbors can be created with the help of breakwaters. 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.

Unintended consequences

When oncoming waves hit breakwaters, their erosive power is concentrated on these structures, which are some distance away from the coast. This creates an area of slack water between the breakwaters and the coast. Sediment deposition can thus occur in these waters and beaches can be built up or extended there. Breakwaters also prevent nearby unprotected sections of beaches from receiving fresh supplies of sediments and they may gradually shrink due to erosion in a process known as longshore drift. On the other hand, breakwaters can also encourage erosion of beach deposits from their base and thus increase longshore sediment transport.

Breakwaters are subject to damage, and overtopping by big storms can lead to big problems with draining any water that gets behind them.

Three of the four breakwaters forming Portland Harbour
The eight offshore breakwaters at Elmer, UK

Construction

Breakwaters are either constructed some distance from the coast or built with one end linked to it (in which case they are usually called sea walls. They may be either fixed or floating, the choice depending on normal water depth and tidal range. They usually consist of large pieces of concrete spaced about 50 m apart. Their design is influenced by the angle of wave approach and other environmental parameters. Breakwater construction can be either parallel or perpendicular to the coast, depending on what will maintain tranquil conditions.

Types of breakwater structures

A breakwater structure is designed to absorb the energy of the waves that hit it, 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 erected 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.

Rubble mound breakwaters use structural voids 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 prevent the wave energy's 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 inexpensive. As water depth increases, the material requirements, and hence costs, increase significantly.[citation needed]

Advanced numerical study

3D Numerical Simulation - MEDUS 2009

The Maritime Engineering Division of the University of Salerno (MEDUS) developed a new procedure for studying in greater detail the interactions between maritime breakwaters (submerged or emerged) and the waves that hit them by making integrated use of CAD and CFD software.

In the numerical simulations, the filtration motion of the fluid within the interstices, which normally exist in a breakwater, is estimated by integrating the RANS equations, coupled with a RNG turbulence model inside the voids, instead of using classical equations for porous media.

The breakwaters were modelled, in analogy to full size construction or physical laboratory tests, by overlapping three-dimensional elements and having the numerical grid thickened in order to have some computational nodes along the flow paths among the breakwater’s blocks.

See also

References

External links

Retrieved from "https://en.wikipedia.org/w/index.php?title=Breakwater_(structure)&oldid=493822371"