breakwater

Breakwaters create safer harbours, but can also trap sediment moving along the coast. Balboa Peninsula in Newport Beach, California, April, 1998.

Breakwaters are structures constructed on coasts as part of coastal defence 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. 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. Breakwater construction is usually parallel or perpendicular to the coast to maintain tranquility condition in the port. Most of Breakwater construction depends upon wave approach and considering some other environmental parameters

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 sediments and may gradually shrink due to erosion, namely longshore drift.

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.

3 of the 4 breakwaters forming Portland Harbour

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.

Rubble mound 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.

Advanced Numerical Study

3D Numerical Simulation - MEDUS 2009

The Maritime Engineering Division University Salerno (MEDUS) developed a new procedure to study, with a more detailed and innovative approach, the interactions between maritime breakwaters (submerged or emerged) and the waves, by an 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, not using a classical equations for porous media.

The breakwaters were modelled, as it happens in the full size construction or in physical laboratory test, by overlapping three-dimensional elements and the numerical grid was thickened in such a way to have some computational nodes along the flow paths among the breakwater’s blocks (AccropodeTM, Core-locTM, Xbloc®).

References

• Ciria-CUR (2007) - Rock Manual - The use of rock in hydraulic engineering.
• N.W.H. Allsop (2002) - Breakwaters, coastal structures and coastlines.

See also

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

External links

Wikimedia Commons has media related to: Breakwater

• USGS Oblique Aerial Photography — Coastal Erosion from El-Niño Winter Storms October, 1997 & April, 1998
• Channel Coastal Observatory — Breakwaters
• Shapes of breakwater armour units and year of their introduction
• SeaBull Marine, Inc. — Shoreline Erosion Reversal Systems