Gabions

Gabions are wire mesh baskets filled with cobbles or crushed rock. They are filled at site, often with locally available material and therefore have a relatively low cost. As they are flexible and porous they can absorb some wave and wind energy, thereby reducing the scour problems associated with solid 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 as they are more likely to suffer toe scour and structural collapse as they are less able to dissipate wave energy during storm wave attack.

Generally gabion revetments (foreground) are generally preferred to gabion walls (background) in coastal environments being less reflective of wave energy and more stable. Blown sand is also better able to accumulate on revetments, potentially softening their appearance.

A gabion will provide short term (5-10 years) protection from backshore erosion by absorbing wave energy. Their application is restricted to the upper part of sandy beaches, since they are not sufficiently durable to withstand regular direct wave action. They should not be installed on shingle beaches because wear and tear will rapidly cause damage to the baskets.

As they are porous structures they will tend to trap wind blown sand and allow the growth of vegetation under favourable conditions. This only applies to sloping structures: steep walls of cubic baskets will not attract sand or allow dune vegetation regrowth. They can also provide a short term alternative to rock armour structures in areas where large rocks are not available at an acceptable cost, or where long term protection is not appropriate.

Gabions used in lower energy or estuary situations can use PVC coated wire. Under more active conditions the coating is soon cracked, becoming relatively useless in preventing corrosion. In general galvanised wire of a larger diameter will provide better service than finer non-galvanised wire with a PVC coating.

An alternative type of gabion designed in Cornwall.

During placement, regrading of the shoreline may have to be carried out so as to adequately bed the baskets. A suitable geotextile is required to prevent the underlying sediment from being washed out through the gabions. Burial with recycled sand, combined with vegetation transplanting, thatching and/or fencing can enhance the aesthetic look of the area. Buried revetments provide a final line of storm protection, while allowing a natural dune/beach system to develop under less extreme conditions.

Regular basket maintenance is required to maximise the life of gabions. Gabions become unsightly and dangerous if they are damaged and not properly repaired. The use of angular stone is not recommended

as once the cages are broken the stones will be dispersed rapidly creating an unnatural beach. Mobile rocks on the beach may accelerate damage to adjacent gabion baskets.

The spit at Dawlish Warren in Devon developed many years ago. However, now it is in a serious state of erosion and much of it has already been lost. Hard sea defences and rock armour are present at the landward end of the spit. However, the more natural and further part with sand dunes has less artificial protection and has been subjected to more erosion.

The seaward line of dunes, the marram grass ridge, already has small cliffs. As a form of sea defence gabions have placed under the sand dunes at the front of part of the sand dune ridge. However these are now exposed by sea erosion in several areas on the seaward side of the sand spit.