Groynes

The best form of natural defence is a beach which efficiently absorbs the energy of the waves. However, along many coasts longshore drift causes the beach to thin out in places and erosion of the land behind becomes a problem.

Groynes are regularly used as part of sediment control systems, designed to slow down longshore drift and build up the beach.

As a result beach material builds up on the updrift side, where littoral drift is predominantly in one direction, creating a wider and a more plentiful beach, therefore enhancing the protection for the coast because the sand material filters and absorbs the wave energy. However, there is a corresponding loss of beach material on the downdrift side, requiring that another groyne to be built there.

However, groynes do not protect the beach against storm-driven waves and if placed too close together will create currents, which will carry sand material offshore.

Traditionally they were constructed from tropical hardwoods which are more resistant to marine borers and erosion. However rock groynes have been introduced at a number of sites since the 1980s. Considerable investment has been made at sites incorporating groynes and beach recharge simultaneously.

They are built at right angles to the shore and spaced about 50-100 metres apart. Without regular maintenance of groyne systems they degrade and can become dysfunctional.Many experts consider groynes to be a "soft" solution to coastal erosion because of the enhancement of the existing beach.

Groynes are increasingly viewed as detrimental to the aesthetics of the coastline, and face strong opposition in many coastal communities.

In some areas a problem called Terminal Groyne Syndrome can occur. This is where the last groyne that has been built or the terminal groyne, prevents longshore drift from bringing material to other areas along the coast, effectively moving the problem further along the shoreline.

An example of this can be found between Holcombe and Dawlish Warren in South Devon.

The net direction of littoral drift between the Parson and Clerk headland and Langstone Rock is southwest to northeast. This can be seen from the sand accumulation against the headland, and the breakwater, which was deliberately built to encourage beach accretion against the wall protecting the coastal railway line.

From the evidence of beach setback immediately north of Langstone Rock, it is effective in trapping sediment drifting north-eastwards, thus functioning as a large terminal groyne.

However, as some bypassing would appear to take place during periods of high wave energy, it is classified therefore as a fixed, but partial, transport barrier.