Based on kindly provided information by the Scottish Natural Heritage
General description
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.
Gabion revetments (foregound) 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.
Function
The purpose of a gabion revetment is to provide short term (5-10 years) protection from backshore erosion by absorbing wave energy along the dune face. 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.
Gabions 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.
Methods
Small gabion structures can be designed and built by volunteer groups, but larger schemes should make use of a competent coastal consultant and contractors.
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. Damaged gabions forming a public safety hazard and releasing non-indigenous cobbles onto the beach.
During placement, regrading of the beach/dunes may have to be carried out so as to adequately bed the baskets. A suitable geotextile should be used to prevent the underlying sand from being washed out through the gabions. Manufacturer recommendations should be followed as regards appropriate materials and installation methods. Edge details should be carefully addressed to prevent exposure of unsightly lengths of textile. Landward edges can be buried to fix the geotextile during gabion placement and filling. Seaward edges should be trimmed or firmly secured.
As with all engineered shoreline structures, gabion revetments are likely to suffer from local scour and possible outflanking at the junction between structure and adjacent unprotected dune face. This problem can be minimised by turning the revetment face back into the dunes and burying the end into the dune face. This feathered end may extend alongshore over 20m-40m and may end 5m-10m landward of the main structure face. These dimensions will depend on the expected rate of short and long term erosion. If erosion is likely to average more than about 1m per year then gabions may well be inappropriate for defence.
Regular basket maintenance is required to maximise the life of gabions. Severely damaged baskets should be refilled and closed with new mesh panels. Replacement mesh should be laid over the entire structure if abrasion or corrosion is widespread. Under exposed conditions a maximum life of 10 years should be anticipated, after which time a replacement structure may be required.
Schemes are best implemented in the spring and early summer when work windows are least restricted and the shoreline has the greatest chance to stabilise before winter storms start to erode the upper beach.
Costs for gabion schemes depend on required dimensions, labour, availability of fill material, transport methods and the amount of minor works required to enhance the dune system. Small schemes using volunteer labour and locally available fill material may cost as little as £5,000/100m frontage. Large gabion revetments built by contractors may have an initial cost of £500,000/km, plus minor ongoing management works such as recycling to bury the gabions or other works to enhance the dune system. Economic analysis should anticipate no more than a 10 year life expectancy.
Impacts
Gabions become unsightly and dangerous if they are damaged and not properly repaired. Released cobbles are not a problem to coastal processes, but can detract from the general dune/beach environment and may accelerate damage to adjacent baskets.
Exposed gabions tend to trap strand line debris. As with all fixed dune defences the gabions will interfere with the natural dynamic interchange of material between beach and dune. They will also influence the longshore transfer of sand, modify dune habitats, disrupt the natural landform and potentially result in localised dune face scour at their terminal ends.
Vertical gabion walls are more prone to structural failure and outflanking, more intrusive on the landscape and are much less likely to become buried by new foredunes relative to sloping gabion revetments.
Best practice and environmental opportunities
Sloping gabions can provide good erosion protection for periods up to 10 years (longer if normally buried). They are often more acceptable and less costly than rock armour. When carefully built and placed they can blend into the dune landscape, and are only exposed during eroding storms. They are best used in areas where episodic erosion takes place and where natural burial by new foredunes is possible under favourable conditions. Their applicability in areas subject to persistent erosion is limited to temporary protection. Removal of temporary structures is more difficult with gabions than with sand bag, rock or timber structures. Recycling, fencing and transplanting will encourage growth of new foredunes over buried structures.