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Breakwater, jetties and groins.Presentation Transcript
JETTIES AND GROINS
Breakwater are the structures constructed
to enclose the harbours to protect them from
the effect of wind generated waves by
reflecting and dissipating their force or
Such a construction makes it possible
to use the area thus enclosed as a safe
anchorage for ships and to facilitate loading
and unloading of water by means of wave
Classification of Breakwaters:
Vertical wall breakwater.
Heap or mound breakwater.
Mound with super structure.
Vertical wall breakwater:
The construction of vertical breakwater is
found advantageous under the following
The depth of water is sufficiently large to
prevent the breaking of waves.
Sea bed is resistant to erosion.
Foundations are not subject to uneven
Heap or mound breakwater:
It is simplest type of breakwater and
constructed by dumping of rubbles, stones
into the sea till the heap or mound emerges
out of water. The rubble mound breakwater
comparatively is safe. The looseness of the
elements permits them to settle with out
damage. In this way they provide a broad
base. This helps in distributing the load on a
Usually the concrete cap should be cased
after two years of dumping the rubble. The
quantity of rubble depends upon the depth,
rise of tides and exposure.
Mound with super structure:
There are two cases:
Mound with super structure founded at low
Mounds with super structure below low water
Mound with super structure founded at low water
A solid super structure consisting of a quay
protected by a parapet on the sea face is constructed
on the top of rubble mound. Such a construction is
founded about the low water level.
Mounds with super structure below low water level.
In this type of construction, the superstructure can
be founded well below the level of disturbance. The
maximum disturbance is created between the low
and high water levels. Thus at such low levels below
the low water level, waves practically have no
disturbing effect. This type of construction is very
economical in terms of mound material in deep
A jetty is a narrow structure projecting from
the shore into the water with berths one or both
sides and sometimes at the end also.
These are structures in the form of piles
projections. They are built from the shore to the
deep water. They may be constructed in the sea
or in a navigable river.
In the sea, jetties are provided at places
where harbour entrance is affected by littoral
drift or the sea is shallow for a long distance. The
jetties extend from the shore to the deep sea to
receive the ship.
Jetties are exposed to severe wave action
and their structural design is similar to that of
breakwaters. The impact caused by the
berthing ships depends upon the local
conditions of the currents, winds, waves and
the skill of the navigation officer coupled with
the efficiency of the tug assistance. Some
authorities advocate that the berthing effect
should be taken into account while designing
the jetties, while others do not agree to this.
Note that the wharf is a berth parallel to the
shore, where as a jetty is perpendicular to the
shore or breakwater.
Groynes are structures constructed
traverse to the shore and extend from the
shore into the water far enough up to a limit
to accomplish their purpose. Groynes are
constructed for the following purposes:
To build up a protective beach.
To retard erosion of an existing or restored
beach by trapping of littoral drift.
To protect the toes of sea water or bulk
Types of Groynes:
Classification according to the function it
Classification according to the materials of
According to the condition of construction:
According to their size as:
These groynes are constructed at the shore in
such a way that it points towards the
downstream of the direction of normal flow. Its
inclination varies from 45 degrees to 60 degrees.
This type of groynes are constructed at right
angle to the shore to deflect the flow.
This type of groyne is constructed in such a way
that it points up stream at an angle of 10 degrees
to 30 degrees to the line of normal to the shore.
The head deflects the current at right angles to
They permit the flow of water through
them. They dampen the velocity and reduce
the erosive action of the stream. The
permeability of the groyne helps in avoiding
abrupt offset in a shore. A part of materials
and littoral drift pass through the groyne. This
results in sand deposition on both sides of
High and low groynes:
High groynes compeletly block sand
moving in the zone of its influence and called
terminal groynes.i-e they act as impermeable
groynes. Where as low groynes serve the
same purpose as that of a permeable groynes
i-e thay allow sand supply down stream.