BREAKWATER, JETTIES AND

GROINS

Breakwater: 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 energy.

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 breakers.

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 situations

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

settlement.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 larger area.

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 water level. Mounds with super structure below low

water level.

Mound with super structure founded at low water level.

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 waters.

Jetties: 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: 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 heads.

Types of Groynes:Classification according to the

function it serves: Attracting Groynes. Deflecting Groynes. Repelling Groynes.Classification according to the

materials of construction: Permeable Groynes. Impermeable Groynes.

According to the condition of construction:

Fixed.Adjustable.According to their size as:Long Short.High.Low.

Attracting Groynes: 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.

Deflecting Groynes: This type of groynes are constructed at

right angle to the shore to deflect the flow. Repelling Groynes: 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 itself.

Permeable Groynes: 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 groyne.

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.