Types of Coastal Defenses
As the strong wave action starts causing issues like shore erosion or threatening the structures, people adopt various coastal structures made of concrete, stone, or timber to fight against it. These coastal structures are built where there is enough fund to deal with the issue.
For instance, the longest breakwater in the world is the South Breakwater in Texas, which is about 10.85 kilometers long. South Korea has the longest sea wall, spans about 34 kilometers, called the Saemangeum sea wall.
Every coastline is constantly shaped by waves, winds, and tides. This natural process erodes rocks and carries away sand and pebbles. While this process is inevitable, but we could slow it down to protect things like ships, houses, farms, or recreational areas.
We have seen how concrete arms are used at the entrance of harbors or ports to break the forces from the wind and waves, protecting the boats and ships. If positioned correctly, they can also assist to prevent silt from building up at the entrance channel.
Hence, these structures serve two important purposes — protection and controlling the flow of water — and they are used in different ways along coastlines to manage these challenges.
Groynes structure
Longshore drift is a process happens along coastlines where waves move sand and small pebbles along the shore in the direction of the wind.
In places where this causes erosion issues, like resort areas, people adopt structures called groynes to manage it. Groynes are structures made of rock, concrete, or heavy wooden boards fixed to timber piles. They are constructed perpendicular to the shore and extend into the sea. They absorb the wave energy and reduce the sand movement along the shore. Each groyne helps to retain sediment on its ‘up drift’ side and can contribute to sand build up at the beach.
However, there is a phenomenon called ‘terminal groyne syndrome,’ a phenomenon where the shore on the side without a groyne (the last groyne in a series) can experience severe erosion. This occurs because the groyne acts as a barrier, preventing the natural movement of sediment along the shore. As a result, the area down-drift or downstream of the last groyne becomes deprived of new material, leading to increased erosion and degradation of the shoreline in that area.
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Concrete groynes can also act like ‘entrance training walls’ in some places, helping keep a river channel deep and free from silt. However, they might disrupt the natural movement of sand along the shore, potentially causing more erosion on either side.
Sea wall
In low-lying coastal regions, sea walls play a crucial role in not just stopping erosion but also in keeping the sea from flooding inland. In the past, sea walls had straight vertical faces that faced the waves directly. However, this design led to damage and actually increased longshore drift. Nowadays, sea walls are often sloped with a curved top edge that redirects the waves and absorbs their energy, which is more effective.
Recurved sea wall
Although they are costly and could cause limitation on public beach access, seawalls offer a durable solution and are highly effective in protecting cliffs and coastal areas from erosion. Their concave design redirects waves, reducing the force of impact.
You can watch an extremely useful video demonstration below showing how different types of coastal defenses impact flood risk. The video clearly showcases the effectiveness of a recurved seawall shape in reducing wave impact.
Tetrapod
The above concrete structures are known as Tetrapods, were invented in France by Pierre Danel and Paul Anglès d’Auriac in the late 1950s as a response to the need for more effective coastal protection structures.
Despite being sometimes criticized for their appearance, Tetrapods have proven to be extremely effective when it comes to coastal protection. These durable structures have the capacity to withstand the impact from hurricanes and typhoons.
Revetment
A revetment is a gently sloping wall designed to absorb wave energy gradually without disrupting sediment and currents along the shore. They are usually construccted using specially shaped flat-faced stone or riprap, which are irregular boulders, with some weighing several tons. In certain cases, revetments can also be made of timber piles and sloping wooden planks, resembling groynes but running parallel to the coastline.
Gabion
Gabions are wire mesh cages filled with rocks, forming either cubes or rectangular sections for constructing walls. While it may not be as durable as concrete structures, they have the advantage of allowing some water flow through them. Additionally, they trap sand and stones between the rocks, creating a robust defense.
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