Seawall or Revetment

The height of a seawall will at least cover the difference between the beach level and the mainland, though commonly seawalls are built higher to protect the land against wave overtopping. Seawalls are also used to stabilize eroding cliffs and protect coastal roads and settlements. The crest of the wall often extends into a stone covered part that may be used for a road, promenade or parking area. A seawall creates a distinct separation between the beach and the mainland. Seawalls are often found in the case of narrow or steep beaches, where a typical breakwater is either too large or not economical.

Stakeholder participation

If an EIA is undertaken, the EU Directive provides for the right to access information and to participate in the environmental decision-making procedures to the public concerned by the project. If a project could create a significant impact on a Natura 2000 sites, the ‘appropriate assessment’ of the infrastructure project may include a public participation process, though this is not mandatory. Similarly, the Floods Directive, the Water Framework Directives and the Maritime Spatial Planning Directive establish public participation processes that may be relevant for these projects.

Several categories of stakeholders can be affected by strengthening seawalls. A seawall can negatively affect the landscape and the use of the beach, or can be used to artificially widen the beach to create recreational space. Tourists and tourism industry as well as other recreational users will therefore be affected. However, the protection against flooding offered by the seawall can benefit settlements and economic activities.

Success factors

A seawall provides a high degree of protection against coastal flooding and erosion.  It fixes the boundary between the sea and land which can be beneficial if important infrastructure or buildings are located on the shoreline.

Seawalls have a lower space requirement than other coastal defences such as dikes. Seawalls can be heightened to face sea level rise, which requires simultaneously a widening of the foundation.

The high level of security provided by a seawall can favour the development of the hinterland. The crest of the seawall often extends into a stone covered part providing other functions, e.g. road, promenade or parking places.

Limiting factors

Artificial structures such as seawalls can have adverse effects on the coastal environment. Seawalls often interfere with natural processes such as habitat migration, causing the reduction of intertidal habitats. However, these effects depend very much on the main wave and sediment transport direction and the design of the seawall. The choice of coastal defences will be made according to site-specific conditions and primary and secondary goals (such as wave protection, road stabilization, space conservation and mooring capabilities). Where sufficient space is available and no conflict with other primary or secondary goals exists, green measures (such as beach nourishments and dune restoration) are often preferred.

Seawalls often do not stop erosion in front of the structure but prevent erosion of the dunes and hinterland. Vertical seawalls often reflect wave energy instead of dissipating it, which makes the shoreline more subject to erosion. Many seawalls have therefore been more recently conceived to integrate slopes.

When seawalls are regularly overtopped, or when this occurs in major storms, the water can remove soil or sand behind the wall and weaken it. Overtopping water saturates the soil and increases pressures from the landward side, which can cause structural collapse. Sea rise level and potential overtopping must be taken into account in the construction of the seawall. In general, continued erosion can undermine the foot of the structure and threatened its stability.

Seawalls can have negative impact on the landscape and can reduce the attractiveness of the landscape. However, seawalls have also been used, more or less successfully to artificially widen a beach at steep shores to create recreational space.

Costs and Benefits

Construction costs are high but these structures usually require low maintenance. Construction costs vary according to the shape of seawall structures: the volume of the seawall depends on the required crest level, the foundation level, the wave loading, and acceptable overtopping rates. In the Netherlands, it has been estimated that a seawall would cost 300-500€ per m3 of concrete (Deltares, 2014).

Estimates made by Scottish Natural Heritage in 2000 show that in the UK, costs at the time varied from £200,000 to £500,000 (250,000 – 625,000€)/100m length for seawalls and impermeable revetments. The English Environment Agency (2007), gives an average construction cost for seawalls of US$ 2.65 million (at 2009 price levels; about 1.85€ million) including direct construction costs, direct overheads, costs of associated construction works, minor associated work, temporary works, compensation events and delay costs. No indication on the length of the seawall was provided. These and other sources note that costs for seawalls vary according to the type of construction, dimensions, availability and proximity of construction materials, anticipated rates of future erosion and wave loadings, facilities such as walkways and steps or slipways.