Rising sea levels are one of the results of global warming. As the atmospheric temperature of the Earth continues to increase, the polar ice continues to melt thus making the sea-level rise. Along with many others, a major impact of this is the increase in major flooding and erosion in low-lying, unprotected coastal land.
A solution to protecting land from flooding and erosion is the implementation of coast defences. This involves the use of hard and soft engineering to mitigate the effects of the energy transfer that occurs between ocean waves and the land.
Soft Engineering Solutions
Some examples of soft engineering in coastal defence systems are beach nourishment, dune regeneration and cliff stabilisation.
Beach Nourishment
Beach nourishment is the process of manually adding truckloads of sand and shingle to the shoreline to either restore or create a beach. This man-made beach can be used not just as a tourist attraction, but as a technique in absorbing wave energy which reduces the impact of the ocean on the land resulting in less erosion.
The benefits of this type of coastal defence are that it is commonly considered to improve the look and quality of the shoreline, therefore having a positive effect on the quality of life in the area. Beach nourishment can also be viewed as sustainable as it often involves the re-use of unwanted materials. The sand and shingle are often sourced from areas where it is unneeded and considered as waste.
As an example, during the construction of the British Airways i360 on Brighton beach in the UK, 7200 tonnes of shingle and gravel were excavated to make room for structural foundations in the form of 4150 tonnes of reinforced concrete. The excavated material was then transported to another beach and used as a form of beach nourishment.
The negative side of beach nourishment is that the procedure produces a high amount of CO2 emissions. Dependant on the size of the beach, many large vehicles are required to transport the shingle from other parts of the country. Due to long-shore drift, the new sand and shingle is moved along the coast and taken away from the proposed area. The life span of beach nourishment is between 1 and 10 years so the process must be performed regularly which makes this coastal defence strategy expensive and unsustainable.
Dune Regeneration
Dune regeneration is an alternative form of a soft engineered coastal defence. Dune regeneration involves creating or restoring sand dunes around the shoreline. The purpose of these dunes is to provide a barrier between the waves and society and to absorb wave energy.
Vegetation is planted in the dunes to help stabilise the dune, also further increasing its ability to absorb wave energy. In order to allow these plants to grow without being disturbed by human activity, the dunes are sometimes temporarily fenced off, or wooden walkways are built over them so humans can avoid stepping on the sand.
Similarly to beach nourishment, dune regeneration is accepted socially and environmentally as it often improves the sight of the coast while re-using unwanted materials. However, dune regeneration also provides extra vegetation which improves the sustainability of the coastal defence.
The negative impacts of dune regeneration are also similar to that of beach nourishment as the transportation of materials produces CO2 emissions while also having a low life span.
Hard Engineering Solutions
Examples of hard engineering in coastal defence systems include the likes of sea walls, rock armour, groynes and cliff fixing.
Sea Walls
Sea walls are large, normally concrete, walls that protect either flat land from flooding or lower parts of cliffs from erosion. The job of a sea wall is to absorb the impact of waves and redirect the wave energy back into the sea. Sometimes sea walls are curved, this is to improve the ability of deflecting waves.
A benefit of a sea wall is that they have a lifespan of around 30 to 50 years, meaning they can be considered as a long-term solution to minimise the erosion caused by rising sea levels. Sea walls can also have a secondary function; they can be used as public walkways. One example of this is Brighton’s long stretch of a sea wall that spans over 4 km from Brighton Marina to Saltdean. This sea wall is well used by the public for recreational activities such as walking, cycling and basketball whilst also being used for businesses such as small cafes which bring in tourism.
A negative to a sea wall is that they are very expensive to build; £200,000 to £500,000 for 100m in length. So even though the sea wall has a long lifespan, the initial cost of construction is very high. Another debatable negative to a sea wall is that some people will argue that it will ruin the look of the area. However, there are also people that think the sea wall improves the look of the area, so this is not a strong point to make.
Groynes
Groynes are also a form of hard engineering in coastal defences. Groynes are fence-like structures that are built at right-angles to the shoreline leading into the sea. Their purpose is to break up waves and reduce the waves energy before they reach land therefore reducing erosion. Groynes are usually made from wood but over time, stone groynes are increasing in popularity.
Stone groynes are more expensive however they are less prone to decay therefore having a longer lifespan. Groynes are also useful in trapping material being transported by long-shore drift. With this in mind, the combination of groynes and some form of soft engineering would be ideal as the groynes would preserve the beach materials therefore improving the beaches lifespan.
The main disadvantage of groynes is that while protecting the local area, they also increase the erosion in areas further along the coast. As the groynes trap material, beaches further along the coast receive less material through long-shore drift resulting in bare land without a beach to absorb the impact of the waves.
One example of this is Mappleton, Holderness Coast UK where the rate of retreat inland is between one and two metres every year. In order to protect an area of the coast, another part of the coast must be sacrificed which reduces the sustainability of the use of groynes.
Summary
Overall, the most sustainable form of coastal management against rising sea levels is a mixture of hard and soft engineering. Without taking expense into account, the combination of a seawall, stone groynes and beach nourishment seems the best solution. The seawall and the stone groynes already have a long life span due to their material and when combined with beach nourishment, the need to resupply the beach with new materials will not occur very often as the groynes help keep the shingle in place.
If the cost is an issue, then wooden groynes would be more suitable, and the beach nourishment is not necessary, but the construction of a sea wall seems to be the best solution even though it is expensive. In general, a long term solution would be the ideal choice as it sustains the coast for future generations.