These defensive structures might be subject to Environmental Impact Assessment, depending on national laws and regulation. Stakeholder consultations should include local authorities, tourism operators, coastal communities, NGOs and coastal research institutions. The decision to construct seawalls and jetties must therefore consider and balance different stakeholders’ perspectives and needs considering both the implementation and maintenance phase. At the same time, by exploiting sand accumulation in the updrift side of the structure, these structures can be used to artificially widen the beach and therefore create additional space for human activities. However, they can negatively affect the coastal landscape and environment as well as the natural value of a beach and its use. Seawalls and jetties are primary implemented to ensure protection against flooding and storms of valuable human settlements and activities, particularly at risk. However, downdrift erosion can lead to major setback of the coastline. Sand accumulation in the updrift side can create space for land-based activities, for example for tourism and ports. In this way, they can cause sediment accumulation at the updrift side and erosion at the downdrift side, due to lack of transported sediments. Jetties (like groynes ) are designed to interrupt long-shore sediment transport. A wide variety of materials including rock armour, concrete, tetrapod, and steel piling can be employed for construction of jetties. In this way, the navigability of the channel is improved or, for tidal channels of lagoon areas, a sufficient opening is ensured for an adequate water exchange. Jetties are also employed to avoid large volumes of sand filling the inlet. This integration needs coordination among different governance levels to ensure long-term sustainability of the adaptation interventions.Ī jetty is a structure built to protect a navigation channel or a tidal inlet, stabilising one or both sides. To counteract disadvantages of seawalls and other grey protection measures, an increasing number of projects started to integrate seawall construction and strengthening with nature-based solutions in the framework of large coastal interventions (see for example Saltmarsh recreation by managed realignment in Hesketh Out Marsh ). While seawalls protect infrastructures and settlements in the hinterland, erosion in front of the seawall and in the adjacent coastal stretches will continue also at increasing rate, without solving the causes of erosion. Possible increase in wave heights and storminess due to climate change should also be considered to ensure durability and efficacy of the structure under more severe climatic conditions. To be effective for flooding in the medium and long period, regional and local projections of sea level rise must be incorporated in the design specifications. They are frequently used in locations where further shore erosion will result in excessive damage to coastal roads and settlements, providing a high degree of protection also from flooding. Seawalls are often found in the case of narrow or steep beaches, where a typical breakwater is either too large or not economical. Slopes and irregular surface of the structure design of seawalls can improve their performance, allowing wave breaking, energy dissipation and scattering of the direction of wave reflection.
Vertical seawalls can also cause a scour hole at the foot of the structure leading to its instability. They possibly create turbulence and suspending sediments, thus further increasing the risk of erosion. Smooth, vertical sea walls mainly reflect wave energy seawards. The shape of the seawall face determines its capacity to reflect or to dissipate wave energy. The crest of the wall often extends into a stone covered part that may be used for a road, promenade or parking area (see for example the “ wave-damping” square in Ostend ).
Seawalls are also used to stabilize eroding cliffs and protect coastal roads and settlements. 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 usually massive structures designed to resist storm surges. Seawalls can be combined with jetties in port areas, to stabilise the sides of navigation channels and tidal inlets and to avoid siltation. It is 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. A seawall is a structure made of concrete, masonry or sheet piles.
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