Grions and Gabions and Other Sea Defences

As Chaucer once so eloquently wrote, “Time and Tide Wait For No Man”. This seems to be especially true in this uncertain time of escalating climate change. As the IPCC predicts global sea level rise of anything between 20cm and 80cm by 2080 as a result of climate change (IPCC 2007), threats of flooding to low lying land, accelerated coastal erosion, and saltwater intrusion into freshwater habitats is becoming an ever increasing problem. Management techniques and sea defences are now becoming vital along the shorelines of the United Kingdom. These rising sea levels not only threaten wildlife and fragile habitats and put at risk some of our most valuable coastal habitats and natural features which may be progressively lost or drowned, they also threaten residential property, coastal industries and economies, tourism and our road and rail infrastructure.

Current Methods of sea defence, although not particualrly pleasing to the eye are obviously essential for the protection of our coasts. There are several methods of hard engineered protection used around the coastline at the moment.

Gabions are wire mesh baskets filled with cobbles or crushed rock. They are filled insitu, often with locally available material and therefore have a relatively low capital cost. Because they are flexible and porous they can absorb some wave and wind energy, thereby reducing the scour problems associated with impermeable sea defences such as concrete seawalls. Gabions can be placed as sloping “mattresses” or as near vertical cubic baskets. The latter are intended for bank or cliff stabilisation and are not normally suitable for use in shoreline situations.

The above image shows cobble filled gabions

Groins are man-made structures designed to trap sand as it is moved down the beach by the longshore drift. As the longshore drift current approaches the groin, it is forced to slow down and change direction. This chance in velocity causes sand suspended in the current to be deposited on the up-drift side of the groin. As the current then continues around the groin, it becomes turbulent and actually contributes to erosion on the down-drift side of the groin.

Groins and wooden revetments

Revetments both rock and wooden versions are widely used in areas with important backshore assets subject to severe and ongoing erosion where it is not cost effective or environmentally acceptable to provide full protection using seawalls The function of permeable revetments is to reduce the erosive power of the waves by means of wave energy dissipation in the interstices of the revetment. Rock revetments may be used to control erosion by armouring the dune face. They dissipate the energy of storm waves and prevent further recession of the backshore if well designed and maintained. Revetments may be carefully engineered structures protecting long lengths of shoreline, or roughly placed rip-rap protecting short sections of severely eroded dunes.

Rock Revetments.

Concrete Sea Wall Coastal Defences.