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
wave 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.
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
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
See also
External links
.jpg/220px-Breakwater_break1_new(USGS).jpg)
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