Hard engineered coastal defences generally involve the placement of artificial structures which are used to stop or slow the natural processes of erosion, flooding, and sea level rise. They are visible solutions which help to reassure coastal communities. However, they are expensive to construct and require expensive ongoing maintenance programmes.
The methods used to manage the effects of coastal erosion and flooding have typically involved building defences which deflect the power of waves and high tides. Hard engineered coastal protection can protect seaside towns, cities, and popular tourist destinations but they are constantly subjected to impacts from the sea and are costly to install and monitor. Rising sea levels, along with the high cost of maintaining sea defences, call for more cost effective and sustainable methods of coastal protection.
Here are some forms of hard engineering that are used to reduce coastal flooding and erosion risk:
Revetments
Revetments are sloping structures built on embankments or shorelines, along the base of cliffs, or in front of sea walls to absorb and dissipate the energy of waves to reduce coastal erosion. They can be made of concrete, stone, asphalt or wood, and the height of the revetments is designed to stop waves overtopping the defence. Revetments can be both permeable and impermeable. Permeable revetments are generally built from rock or concrete armour, gabions, and timber. They reduce the erosive power of waves by dissipating their energy as they reach the shore. Impermeable revetments are continuous sloping defences made of stone or concrete which act as a fixed line of defence and are designed to act as a barrier against hightides and storm surges.
Groynes
Groynes are low-lying wood or concrete structures situated out to sea from the shore. They are designed to dissipate wave energy, trap sediment, and restrict the transfer of sediment away from the beach through longshore drift. Longshore drift is caused when prevailing winds blow waves across the shore at an angle which carries sediment along the beach. Groynes prevent this process and therefore slow erosion at the shore. They can be permeable or impermeable, with permeable groynes allowing some sediment to pass through and some longshore drift to take place, and impermeable groynes being solid and preventing the transfer of any sediment.
Breakwaters
Breakwaters are offshore sloped or vertical concrete walls designed to dissipate the power of waves further out from the shore, so that once they reach the shore the wave energy and consequential erosion is reduced. This means that they can also create calmer waters for ships. Breakwaters can be made from rock, stone or concrete and some run parallel to the shoreline.
Sea Walls
Sea walls are solid barriers made from concrete, masonry, or gabions and are designed to prevent high tides and storm surges reaching inland and causing flooding. They can have a variety of profiles such as sloped, stepped, or vertical, and are designed to withstand the force of waves for around 30 to 50 years. Many sea walls have been constructed across the UK to reduce the risk of flooding, and they require frequent maintenance so that they don’t fail.
Image: The Flood Hub – Anchorsholme sea wall, Blackpool
Soft engineering techniques involve working with nature to manage the coastline. Natural flood management (NFM) is used to manage coastal flood risk and erosion using natural methods. This can be done by improving natural buffers which help to absorb tidal and wave energy through the replacement of eroded material, replanting beach plants which trap sand, and breaching or removing existing management schemes to allow the shoreline to roll back naturally. Soft engineering can be used on the coast, in intertidal areas or both. Some schemes, such as those where eroded material is replaced, will require ongoing maintenance programmes as they may have a relatively short lifespan.
Here are some forms of soft engineering that are used to reduce coastal flooding and erosion risk:
Managed Realignment
Managed realignment is the planned breach or relocation of sea defences further inland which creates sustainable, environmentally beneficial intertidal habitat in the form of mud flats and saltmarshes. These coastal marshes help to dissipate wave energy, protect against erosion, act as carbon sinks, and provide a rich environment for wildlife. Managed realignment is also a long-term alternative to building higher sea walls or larger defences as sea levels rise in the future.
Dune Regeneration
Sand dunes are natural barriers which protect coastal towns and villages from high tides and flooding. Dunes form above the level of high tide when dune plants trap wind-blown sand, which accumulates and increases the height and width of the dunes. They are constantly changing due to varying wind speed and direction, rising sea levels, storm surges, and recreational visitor activity. To slow the erosion of these natural coastal flood defences, there are techniques which can help to stabilise the areas where there has been dune loss:
Dune Fencing – Built seaward of the dunes, fencing prevents trampling from recreational beach users allowing sand to settle and increase dune size. This method can be used in combination with dune planting to encourage seaward dune growth.
Dune Planting – Plants such as Lyme or Marram Grass help to stabilise the dunes by trapping sand, as their root systems are extensive and mat together. Established plants also reduce wind speed over the dunes, slowing erosion. Plants may be self-sustaining after an initial period of establishment.
Dune Thatching – Covering the face of dunes with bundles of straw, branches, and old Christmas trees increases sand accretion and protects dune vegetation. A benefit of this technique is that there is no establishment time required.
Image: The Flood Hub – Dune regeneration
Beach Nourishment
Beach nourishment is the replacement of material which has been lost from beaches by sea or wind erosion. As opposed to building large hard engineered defences, this will keep the beach looking natural whilst providing protection to the local community from flooding. Long term management plans are required as the replenished material may erode in a relatively short space of time, meaning the process needs to be repeated periodically at considerable cost.
Dredging – Material is dredged from the seabed and pumped to shore along with large amounts of water. As the dredged material is pumped onto the beach the water drains away leaving the sand behind.
Reprofiling – The beach is reshaped by moving material by machine from areas of accretion to areas where erosion has taken place.
Transporting – Dredged material from other areas, or gravel/sand that has been quarried on shore, can be transported to site by truck to replace eroded material.
Cliff Stabilisation
Cliffs can fail due to several factors, such as composition or rock type, geological structure (faults, joints, etc.), climate, and wave energy. There are soft and loose cliffs made up of silt, clay, marl, or chalk which are more prone to erosion and landslides, and hard cliffs made up of limestone, granite, or sandstone. Erosion of cliffs in coastal areas results in the retreat of the coastline as the amount of material eroded exceeds that which is deposited. Stabilisation techniques are used to limit erosion and the potential for landslides, collapse and falling rocks.
One technique is to reduce the slope of the cliff and revegetate the cliff top. Planting vegetation helps to strengthen the cliff structure and increase cohesion, which will help to keep the cliff in place, prevent cliff material from slumping, and prevent landslides.
Another method can be to drain any excess water from the cliff. This prevents water from flowing along planes of weakness which can lead to rock and sediment sliding to the base of the cliff. It will also limit the likelihood of liquefaction which can cause mudflows.
For information on all of these techniques including case studies, download our Coastal Management Booklet here.
