Hard Coastal Protection Structures

Hard coastal protection structures are fixed engineering works such as seawalls, revetments, groynes and offshore breakwaters, built to resist waves or retain beach sediment. They may protect one coastal reach, but can intensify erosion elsewhere by altering wave energy, currents and sediment transport within an interconnected shoreline system.

Interruption of sediment movement

Waves approaching obliquely generate longshore drift, which transfers sediment along the coast. A groyne or breakwater interrupts this movement, causing deposition on the updrift side while reducing the sediment supplied to the downdrift beach.

• When downdrift sediment losses exceed fresh supply, the beach narrows and its shoreline retreats.
• A structure therefore often redistributes a limited sediment budget rather than creating new sediment.

Wave reflection, scour and flanking

Vertical or poorly dissipative seawalls can reflect substantial wave energy instead of allowing it to dissipate across a beach. Reflected waves and turbulence may remove sediment from the structure's base, producing toe scour and beach lowering.

• Wave refraction and diffraction around breakwaters can concentrate erosion in particular neighbouring reaches.
• If the protected shoreline remains fixed while an adjoining unprotected shore retreats, erosion may develop around the structure's end through flanking.

Why impacts differ between coasts

The severity of adjacent erosion depends on the direction of littoral transport, wave climate, tides, sediment availability, coastal geology, and the structure's orientation and permeability.

• Effective management therefore considers the wider sediment cell, not merely the threatened site.
• Beach nourishment, sediment bypassing and nature-based measures can complement structures where suitable, but require monitoring and periodic maintenance.