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.
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