Summary 8: GABION REVETMENTS

Appropriate locations Sandy beach sites suffering periodic moderate to severe erosion where backshore assets are at risk. Useful for estuary bank protection.
Costs Moderate, but require maintenance (£5000-£50,000/100m of frontage, plus minor works and maintenance).
Effectiveness Well placed gabions provide reasonable fixed defences, but have a limited life of 5-10 years due to deterioration of the baskets.
Benefits Useful solution where armour rock is considered inappropriate or too costly. Various forms available. Can be buried by sand and vegetation. Permeable face absorbs wave energy and encourages upper beach stability.
Problems Limited life, leading to unsightly and hazardous wire baskets along beach and the release of non-indigenous cobbles to the beach system. Wire affected by saltwater, vandalism and abrasion by trampling or gravel beach impacts.

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.

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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.

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Cross section of gabion revetment

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.

The manufacturers recommendations should be followed as regards basket materials, methods of filling, lacing up of individual panels and the placement of spacer wires needed to maintain panel shape. The need for careful hand packing cannot be over emphasised. Poorly packed gabions will be rapidly damaged due to abrasion caused by movement of the stone.

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.

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Damaged gabions forming a public safety hazard and releasing non-indigenous cobbles onto the beach.

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.

Gabions should be placed as a sloping revetment as shown in the figures. Near vertical gabion walls are more likely to suffer toe scour and structural collapse as they are less able to dissipate wave energy during storm wave attack. They are also much more obtrusive to the dune landscape and will not become buried by new foredunes.

The figure provides some guidance for structure design, but this should be reviewed for each site. Structure face slopes are a compromise between flatter faces that absorb more wave energy, and therefore suffer less toe scour, and steeper faces that give the structure a smaller footprint. A slope of 1:2 is a reasonable compromise, and is in keeping with natural dune slopes.

The approximate limits of wave run-up can be established by observing and recording the location of the strand line over Spring tide periods during both winter storms and more normal wave conditions. The toe of a freshly eroded dune face is normally just below the run-up limit of the most recent severe sea.

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.

Small schemes can be undertaken by hand, apart form the transport and delivery of rock. Larger schemes will need heavy equipment to regrade the dune face, excavate trenches for the foundation layer of baskets and transport rock from stock piles to the baskets. Even on large schemes baskets must be hand packed to ensure minimum post-fill movement and settlement.

Burial with recycled sand, combined with vegetation transplanting, thatching and/or fencing (Summaries 2 to 5) can enhance the recovery of the dunes over the gabions. Buried revetments provide a final line of storm protection, while allowing a natural dune/beach system to develop under less extreme conditions.

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.

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Gabion revetments may permit sand accumulation and dune grass growth.

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Embryo dune formation upon a sloping gabion revetment (hidden). The dune slope above the gabion wall has been reprofiled and replanted with marram grass.

All dune management schemes should observe the following guidelines to maximise the probability of success and minimise impacts on the natural and human environment:

In addition to these general guidelines, the following are of specific importance to gabion revetments: