Based on kindly provided information by the Scottish Natural Heritage
General description
Timber revetments can range from substantial, impermeable breastwork to temporary permeable upper beach wave barriers. The former is a final line of dune erosion protection, while the latter serves to partially dissipate wave energy before it reaches the dune face. Timber breastwork functioning successfully within an estuary.
Function
Timber revetments have been widely used in the UK for coast protection where the costs or impacts of a seawall may have been unacceptable. Construction flexibility allows timber revetments to serve various purposes. They can provide a partial barrier to wave energy when built as a permeable “fence” along the upper beach. Alternatively they can form a final wave protection wall when built as an impermeable vertical breastwork along the dune face.
Temporary structures can be built relatively cheaply of pressure treated softwood but more substantial structures are usually built of imported hardwood. Concerns over the sustainability of hardwood sources have increased material costs considerably, making it unlikely that large scale timber defences will be used in the future. Timber is now only likely to be viable for smaller schemes in relatively low energy areas. On an open beach exposed to large storm waves, hardwood structures will be abraded, giving a life expectancy of only 15-20 years. Within estuaries or on low energy beaches the timber may last 25-30 years before abrasion and wood boring invertebrates cause significant damage. Softwood structures are likely to have a life of only 5-10 years.
Methods
The flexibility of timber as a construction material allows designers to exercise their imaginations, provided that a few basic principles are be observed: two possible designs are illustrated above. Small schemes with short design life can be put in place without specialist skills, but larger schemes intended to last more than a few years require the services of competent coastal engineers and contractors. At the most basic level timber revetments may be little more than the dune fences discussed in Summary 4.
Structures built within the active beach zone must be designed to withstand wave impacts and changing beach levels. Support piles must be well buried to prevent rotational failure, particularly when beach levels drop during storms. All fixings must take account of the corrosive marine environment and must be secure against vandalism. Public access routes across the structures must be built in to the design.
Timber breastwork should be built above the limit of normal wave run up. Impermeable vertical structures built lower on the foreshore will suffer toe scour and may be undermined causing collapse. Recycling, fencing and transplanting in front of the breastwork will reduce the landscape impact and enhance the dune-beach interactions and habitats. Straight, shore parallel structures are likely to provide adequate service. More complex layouts (see photo) may offer some minor advantages in terms of encouraging dune recovery, but are unlikely to be as effective as a well designed and managed straight breast work with a programme of fencing and transplanting.
Costs for timber revetments are dependent on design, dimensions, quality of materials and the requirement for skilled contractors. Life expectancy will range from a few years up to 30 years, depending on rates of shoreline erosion, cross-shore location of structures and quality of timber.
Impacts
In common with all fixed dune face structures, solid revetments or breastwork disrupt the natural interchange of sand between dunes and beaches with the loss of the succession of habitats from foredunes back to the stable grey dunes or machairs. Sand can be blown from the beach and over the structures 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 piles leading to ultimate structural failure. Similarly, on-going erosion of adjacent unprotected dunes may result in outflanking necessitating extension of defences. Where the revetment is built high on the beach face the erosion pressures are much reduced.
Permeable wave barrier revetments are less disruptive of natural processes as they allow a continued interchange of sand from beach to dune. Used appropriately they can be beneficial to dune protection over a time scale of 5-20 years. They are also more intrusive on the coastal landscape as they are built on the active upper beach and will not be covered by dunes.
Both types of structures will impact on recreational use of the beach, and must include appropriate access routes.
Best practice and environmental opportunities
Timber structures offer great flexibility in design. They can be incorporated into recreation management schemes and are often readily accepted by the public. Timber structure are easily repaired or extended relative to concrete structures.