Physical Management: Coastal Defences

The coast is an area where the consequences of depositional and erosional processes are clearly evident. The physical management of the coast means reacting to processes such as storm erosion, flooding, silting up of harbours and longshore drift. In the UK all coastal defence policies are set by the Government department known as DEFRA (Department for Environment, Food and Rural Affairs). Although DEFRA sets the policies, the Environment Agency, local authorities and drainage boards are responsible for making them work. These organisations are called coastal defence "operating authorities".

According to DEFRA there are an estimated 1.8 million residences and 140,000 commercial properties at risk of flooding and coastal erosion in the UK. They estimate that 4 - 5 million people could be affected. A few hundred years ago nobody was particularly worried about coastal erosion because very few people lived there. Since Victorian times, there has been a huge demand for homes and hotels by the sea and managing the land on which they are built has become very important.

Although in Victorian times the engineers didn't classify their techniques this way, all the techniques ever used fall into two management categories. These are called hard engineering and soft engineering.


Hard Engineering

Hard engineering options tend to be very expensive. They are often very obvious and have a high impact on the environment. Ironically, the fact that they are very obvious makes them seen reassuring to people living near them. They look strong and thus give people confidence in them.

Hard engineering aims to completely block waves and their effects, but this can have the unfortunate side effect of causing erosion of the beach in front of them. They are typically found in seaside resorts and where roads / buildings are sited right on the shoreline.

Examples of hard engineering include:


Wooden Groynes:

- Look like wooden “fences” that are built down the beach, at built at right angles to the coastline.

- They are designed to stop material being moved along the beach by long shore drift.

- They work by building up the amount of sand on the beach. The side of the groyne facing the waves suffers erosion, but the side protected from the waves allows deposition to occur and sand builds up there.

- Groynes have a life span of approximately 20 to 30 years.


Gabion Groynes:

- Large steel or stainless steel mesh cages that are filled with rocks.

- They run down the beach, at right angles to the coastline.

- They function in a similar way to wooden groynes.

- Expected life span of 20 – 25 years if made from steel because they will rust. Stainless steel ones last much longer.


Rock Armour / Rip Rap:

- Large boulders, of 10 tonnes or more, are piled up along the shoreline to form a type of sea wall.

- The rocks are dumped on top of eachother leaving gaps between them that allow water through. This disperses the energy of the waves and reduces their erosional power. They can be very effective.

- The boulders must be large, strong and resistant to erosion. Granite and basalt are often used. Small or weak rocks would not be able to withstand the impact from the waves and would quickly be eroded.


Soft Engineering

Soft engineering options are often less expensive than hard engineering options. They are usually also more long-term and sustainable, with less impact on the environment. There are two main types of soft engineering.

Beach Nourishment
This replaces beach or cliff material that has been removed by erosion or longshore drift. Sand is either brought in from elsewhere, or transported back along a beach, usually once a year.In tourist areas this is often done during the spring after the winter storms and before the tourists arrive to enjoy the beach. Beach nourishment is a relatively inexpensive option it requires constant maintenance. The annual costs are lower than installing hard engineering options, but to keep replacing the beach material as it is washed away requires annual expenditure.

Managed Retreat ( also doing nothing)
Engineers do nothing and the coast is allowed to suffer erosion, deposition and flooding naturally. This is an option considered when the land is of low value and there are no significant risks to the people. It is, of course, very inexpensive in the short term although if land erodes there may be a need to compensate people for the loss of businesses, land and homes.

Photograph of rip/rap and a coastal walld

Here we can see rip/rap adjacent to a concrete wall on the Cornish coast. The wall runs below a road and protects the soft cliff rocks from the sea where the waves hit it at an angle. As the coast bends, the wall is replaced by rip/rap that is capable of withstanding the direct impact from the waves. The rip/rap is also a less expensive alternative to building a longer wall.


Photograph of coastal flooding across a river floodplaind

This is Seaton beach, in Cornwall, UK. The photograph was taken in the winter and shows how the area is allowed to flood when rough seas force seawater up the river. Note that there is housing at the top of the beach and a road running down the side of the hill towards the beach. Both are above the flood level and the road is protected by hard engineering in the form of a concrete sea wall.

The use of soft engineering - letting nature take it's course - together with occasional dredging of the river is a low cost and safe policy. The flooded area is a natural flood plain for the river and will never be built on anyway, so the loss of economic land is minimal.

Photograph of wooden groyne with pebbles piled up high on one side

At St Margarets at Cliffe, near Dover in Kent, the beach is made from flint pebbles eroded from the chalk cliffs that extend for miles to either side. Longshore drift is quite pronounced and unless measures are taken to stabilise the beach there is a risk that it will be eroded and leave the sea wall vulnerable to undercutting.

Two forms of hard engineering are in use; a concrete sea wall to protect houses from landward erosion and wooden groynes to trap pebbles as they move across the beach.As you may have noticed, longshore drift is working from the left of the photograph to the right, with pebbles being trapped on the left side of the groyne.

Photograph of wooden groyne with pebbles piled up high on one side

This is another view of the beach at St Margarets at Cliffe, in Kent.

At the top left you can see the high chalk cliffs, made from a relatively soft and easily eroded rock. The cliff base is protected by a concrete wall with a curved lip to reflect the force of breaking waves, and the concrete wall is then protected by a series of wooden groynes.

Letting nature take its course at this site would almost certainly result in the beach being washed away, the sea wall being undercut and the cliffs becomming vulnerable to erosion. Several houses, a pub and other buildings lie between the sea and the cliffs, and more houses are located along the top of the cliffs so hard engineering has been employed to protect them all.

Photograph of wooden groyne with pebbles piled up high on one side

To the west of the beach at St Margarets at Cliffe, in Kent, is a headland made from chalk. Longshore drift is moving sediment from the west towards the east, so the pebbles on St Margarets beach will have come from the far side of this cliff.

The headland projects further out to sea than the beach at St Margarets and thus protects the beach to some extent. However, because the headland is subjected to the full force of the waves moving in from the west, it is vulnerable to erosion.

Stabilizing tall cliffs such as these can very difficult and expensive. Preventing these cliffs from eroding could also cut off the supply of sediment that is needed to maintain the beach.

This is an example of the soft engineering approach known as Managed Retreat. Nature is taking its course and a recent rock fall can be seen at the base of the cliff. This approach saves money and helps to ensure a supply of sediment for St Margarets beach, but at the cost of losing the buildings on the cliff top. Compensation may have to be paid for the loss of the buildings.



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