Gabions are wire baskets filled with cobblestones or gravel. They are filled in place, usually with locally available materials, and therefore have relatively low capital costs. Because they are flexible and porous, they can absorb some wave and wind energy, thus reducing the scour problems associated with impermeable sea defenses such as concrete seawalls. Gabions can be placed as inclined "mattresses" or near-vertical cubic baskets. The latter is used to stabilize river banks or cliffs and is not usually suitable for use in shoreline situations.
In coastal environments, gabion berms (front) are often preferred over gabion walls (background), reflecting less wave energy and being more stable. Blown sand also accumulates better on berms and may soften their appearance.
Smaller gabion structures can be designed and constructed by volunteer groups, but larger schemes should use qualified coastal consultants and contractors.
Gabions used in low-energy or estuarine conditions can use PVC-coated lines. Under more active conditions, the coating quickly breaks down and becomes relatively useless in preventing corrosion. In general, larger diameter galvanized wire will provide better service than finer non-galvanized wire with a PVC coating. Damaged gabions create a public safety hazard and release non-indigenous pebbles onto the beach.
The beach/dune may have to be re-graded during placement to allow for adequate basket placement. A suitable geotextile should be used to prevent the sand below from being washed away through the gabions. The manufacturer's recommendations should be followed regarding appropriate materials and installation methods. Careful attention should be given to edge details to prevent exposure to unsightly lengths of textile. Landward edges may be buried to hold the geotextile in place during gabion placement and filling. Seaward edges should be trimmed or securely fastened.
As with all engineered shoreline structures, gabion revetments may be subject to localized scour and may wrap around at the junction between the structure and adjacent unprotected dune faces. This problem can be minimized by turning the revetment face back into the dune and burying the end into the dune face. This plume end may extend along the shoreline for more than 20m-40m and may extend the landward of the main structure for 5m-10m. These dimensions will depend on the expected short- and long-term erosion rates. If erosion is likely to average more than 1m per year, then the gabions are likely to be unsuitable for defense.
Regular maintenance of the gabion baskets is required to maximize the life of the gabions. Severely damaged baskets should be refilled and closed with new mesh panels. If wear or corrosion is prevalent, then replacement mesh should be laid over the entire structure. In underexposed conditions, the maximum service life of 10 years is expected, after which the structure may need to be replaced.
Plans are best implemented in the spring and early summer when the work window is least restrictive and the shoreline is most likely to stabilize before winter storms begin to erode the beach.
The cost of a gabion program depends on the size required, labor, availability of fill material, method of transportation, and the amount of minor work required to enhance the dune system. Smaller schemes using volunteer labor and locally available fill materials can cost as little as £5,000/£100 million frontages. The initial cost of a large contractor-built gabion revetment could be £500,000/km, plus ongoing minor management works such as recycling to bury the gabions or other works to enhance the dune system. The economic analysis should anticipate a life expectancy of no more than 10 years.
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