A seawall or a revetment is a structure made of concrete, masonry or sheet piles, built parallel to the shore at the transition between the beach and the mainland or dune, to protect the inland area against wave action and prevent coastal erosion. Seawalls are usually massive structures designed to resist storm surges.
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.
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.
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.
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.
The construction of coastal works to mitigate erosion and hard sea defences ‘capable of altering the coast’, such as seawalls, fall into Annex II of the EIA Directive (codified as Directive 2011/92/EU): Member States decide whether projects in Annex II should undergo an EIA procedure, either on a case-by-case basis or in terms of thresholds and criteria. However, this requirement does not affect the maintenance and reconstruction of these works. Any infrastructure project likely to have a significant impact on a Natura 2000 site must be subjected to an ‘appropriate assessment of its implications for the site’ to determine whether the project will adversely affect the integrity of the site.
The Water Framework Directive calls for the good status of Europe’s water bodies, including coastal waters. Coastal defences could alter the hydromorphological characteristics of coastal waters – for example in terms of water flow, sediment composition and movement– and thus to a deterioration of ecological status. Any projects that do so would need to meet criteria set out in Art. 4(7) of the Directive. The EU Floods Directive (2007/60/EC) provides a legal framework for flood actions and defence. The construction and restoration of dikes could be part of measures under flood risk management plan. The 2014 Maritime Spatial Planning Directive requires consideration of the interactions between land and sea, along with maritime activities and adaptation to climate change. Seawalls could affect these land/sea interactions.
Depending on the complexity of the structure 5 to 20m per day can be realised during construction phase. Preparation before construction might double the implementation time.
Typically, 30-50 years life expectancy before major repair.
There are several ways how to implement a revetment. In their guide to managing coastal erosion in beach/dune systems, the Scottish Natural Heritage has provided information on different kind of revetments. The following measures are more described in detail on sub-pages of this guide.