The Environment Agency is investing £2.6 billion in flood defence construction projects to better protect 300,000 homes by 2021, in order to try to slow erosion of the coastline by the forces of the wind and sea.
The 4 main processes through which coastal erosion occurs are:-
Attrition – Stones and rocks smash against each other breaking them up.
Hydraulic action – Waves force pressurised water into voids or weaknesses in the rock eventually cracking and splitting it.
Abrasion – The continuous effect of sand and gravel suspended in waves wearing away at the land.
Corrosion – The acidity of seawater eats away at certain types of rock such as limestone.
Various techniques can be used to slow down the effects of coastal erosion and are generally separated into either soft or hard engineered options. Site specific risk will be taken into consideration when choosing which option will be used.
Beach nourishment is the replenishment of a beach where there has been significant shoreline erosion. The material used may be dredged from the area or transported from inland quarries. Beach nourishment may be an effective short term solution to erosion, but the process would need to be managed over time and would have ongoing costs.
In Norfolk, a 6km-long dune is being constructed which will protect Bacton Terminal, supplier of one third of the UK’s gas. The project is estimated to coast around £20million and will also provide protection from coastal erosion and flooding for two villages – Bacton and Walcott. The project is being led by Dutch Engineering company Royal HaskoningDHV. The sand will be redistributed along the coastline by the natural actions of the wind and sea. It will be the largest project of it’s kind to date in the UK and will provide around 15-20 years of protection.
The natural salt marshes on our coastline act as buffers against the sea and can dissipate the wave energy of storm surges. Salt marshes also have the added benefit of creating natural habitat for wildlife.
Managed Retreat or Realignment
In areas where land is considered to be of low value, coastal defences may be removed or relocated inland promoting the natural reinstatement of intertidal habitats. The introduction of these natural marshlands could save money in the long term due to the maintenance costs associated with hard engineered coastal flood defences.
Wood or concrete structures built out from the shore which are designed to trap sand and dissipate wave energy limiting the transfer of material along and away from the beach as a result of long shore drift. Long shore drift occurs when prevailing winds blow waves across the shore at an angle resulting in material being carried and deposited further along the beach.
Large rocks or pre cast concrete blocks interlock creating a mass which can deflect wave energy, also has gaps which help to absorb the forces of waves. Rock armour is placed around structures or parts of the coastline which may be susceptible to erosion, acting as a buffer.
Steel mesh baskets filled with stone/rock that can stabilise loose ground and also absorb the force of waves by allowing water to move through them. Gabion baskets have a relatively short lifespan in comparison to other protection measures such as rock armour as the mesh holding them together deteriorates or waves destroy them.
A solid barrier designed to stop high tides and storm surges reaching inland. As the tidal forces are not absorbed and dissipated sea walls can cause scour and further erosion of the beach/salt marshes that may be in front of them. The constant impact from the sea means that sea walls may require continual maintenance costs.
These are either concrete or wood constructed gradients which may be built in front of sea walls. Revetments help to dissipate the force of waves and may have surfaces which are textured to lessen wave energy further.
For many years there has been a “hold the line” hard engineered approach to coastal management which is costly to maintain, in more recent times, and where conditions are suitable there has been a shift toward letting the water in and creating a more natural habitats.
Sources used: BBC