Offshore breakwaters

Offshore breakwaters, act as a direct barrier to waves, reducing the wave energy before a wave reaches the shoreline, therefore consequently reducing erosion potential at the shoreline. The gaps between segmented structures allow some wave energy to reach the shoreline, but this is dissipated by refraction and diffraction. Erosion may continue in the lee of the gaps leading to formation of an embayed shoreline as sand moves into the shelter of the structures.

The breakwaters are either constructed some distance away from the coast or built with one end linked to the coastline.

The breakwater design and size, together with placing will be dependant on the beach type, morphology and wave patterns in the area.

When oncoming waves hit these breakwaters, their erosive power is absorbed and disapated around the breakwater.

This can cause an area of slack water behind the breakwaters where sand can build up (salients). these may grow seawards sufficiently to connect with the structure, forming an artificial tombolo.

If the salient is stable, the beach may grow as sediment is 'trapped'. However, as a result nearby unprotected sections of the beaches will not receive these fresh supplies of sediments and may gradually shrink due to erosion, namely longshore drift.

As breakwaters can have a such a strong influence on longshore drift they are generally not normally used on long expanses of open coast or within estuaries if strong wave or tidally induced currents are present.

In 1994 a breakwater was constructed as part of a coast defence scheme at Sidmouth in Devon. Sidmouth is a popular holiday destination which depends on it shingle beach for both protection from storms and as an am entity for tourists.

The severe storms in 1989 and 1990 caused substantial depletion of the beach and exposure of the foundations of the seawall. A coastal protection scheme comprising of an offshore rock breakwater, rock groynes and beach replenishment scheme. The construction works were carried out in the winter months to minimise disruption.

The breakwaters at Sidmouth offer protection from the prevailing south-westerly seas