Based on kindly provided information by the Scottish Natural Heritage
General description
Impermeable revetments are continuous sloping defence structures of concrete or stone blockwork, asphalt or mass concrete. Revetments are built along the dune face, preferably above the run-up limit of waves under normal conditions. Where frequent wave attack is anticipated, the revetment may be topped by a vertical or recurved wall to reduce overtopping.
Seawalls are near vertical structures of concrete, masonry or sheet piles, designed to withstand severe wave attack. Their use was popular in the past but they are now normally considered to be costly, detrimental to the stability of beaches and unsuitable for erosion management along a dune shoreline.
Function
Rock faced concrete revetment with sheet piled toe and rock armour apron. The rock armour was placed after beach lowering exposed the toe of the revetment. The good medium term protection of such structures has to be balanced against considerable landscape impact and habitat damage.
Impermeable revetments provide a fixed line of defence for frontages with high value backshore assets. They are intended to withstand storm wave attack over a life expectancy of 30 to 50 years. Amenity facilities such as promenades, slipways and beach access steps can be built into the revetment.
Revetments will severely disrupt natural beach-dune interactions, and should not be used on frontages valued for natural heritage. Ongoing beach erosion may result in undermining of the revetment toe, leading eventually to structural failure or the need for repairs and extensions.
Typical impermeable revetment cross-section
Methods
The design and construction of impermeable revetments and seawalls requires the services of competent coastal engineers and contractors. Revetments should have a sloping face extending well below the normal beach level to allow for future foreshore erosion. Sheet piles at the toe should be driven down into the substrata to provide structural support and prevent leaching of sediment out from under the revetment. Rock armour placed in front of the sheet piles will provide additional support and reduce the potential for future toe scour in the event of ongoing foreshore erosion. The revetment crest should be set according to expected wave run-up levels, and can be topped by a vertical wave wall if further backshore protection is required. The crest elevation will be higher than required for a rock revetment as the structure will suffer greater wave run-up over the smooth and impermeable surface. A promenade can be added along the crest to reduce public pressure on the backshore dunes.
The face must be able to withstand impact and uplift forces during storms. Where blockwork is used the units must interlock or otherwise connect to form a unified surface, as the loss of a single unit will cause rapid deterioration of the whole structure. A sound foundation of geotextile and a crushed rock bedding layer will reduce the potential for damage due to hydraulic forces acting from below the revetment face.
Continued dune face erosion should be anticipated along adjacent frontages. The terminal ends of the revetment should be feathered smoothly back to the dune face, to prevent local scour from outflanking the structure. Feathering can be achieved using rock armour over the final 20m to 40m of the defence length line, with the rocks running back into the dune face to a depth of 5m to 10m depending on the anticipated rate of future erosion. Rocks can be placed at a flatter slope (1:3 or 1:4) to increase wave absorption.
Public beach access routes should be incorporated into the structure, linked to backshore walkways or a promenade along the crest. If the revetment face extends below normal spring tide level the surface may become coated with slippery algae, creating a public hazard. Appropriate hand rails or improved footings will be required for safe walking.
Costs for impermeable revetments and seawalls vary according to the type of construction, required dimensions, anticipated rates of future erosion, backshore facilities such as walkways and amenity add ons such as steps or slipways. Economic assessment should assume a life expectancy of up to 50 years before major repairs or reconstruction will be required.
Impacts
In common with all fixed dune face structures, revetments disrupt the natural interchange of sand between dunes and beaches with the loss of the succession from foredunes back to the stable grey dunes or machair. Sand can be blown from the beach and over the structure to reach the dunes, but cannot be returned to counter erosion of the beach during storms. The consequences can be increasing shoreline recession, with the exposure of the revetment toe leading to ultimate structural failure. Impacts may be felt along adjacent frontages that may become starved of sediment input, leading to further erosion and the demand for extended defences. Where the revetment is built high on the beach face the erosion pressures are much reduced.
Best practice and environmental opportunities
Revetments can be used to provide safe public access to the beach, and can support a promenade along the shoreline that may reduce the trampling of the remaining dunes. The backshore can be protected from marine erosion and a grey dune habitat can be maintained.