Physical Management: Coastal Defenses

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 defense 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 defense "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:

They look like wooden “fences” that are built down the beach 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:

They are large steel or stainless steel mesh cages that are filled with rocks running down the beach, at right angles to the coastline. They function in a similar way to wooden groynes. Their expected life span is 20 – 25 years if they are made from steel because they will rust. Stainless steel ones last much longer.

Rock Armour / Rip Rap:

These are large boulders, of 10 tonnes or more, that are piled up along the shoreline to form a type of sea wall. The rocks are dumped on top of each other 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.

Cliffs and Sea Stacks

Cliffs are common coastal features. They are formed by a combination of erosion and weathering, weathering working on the upper parts of the cliff and erosion wearing away the base of the cliff.

Steep cliffs are formed where the land consists of hard, more resistant, rocks, their height obviously being determined by the difference between the sea level and the level of the land. Hard rocks erode and weather slowly, and the less fractured the rock is, the better it will resist breaking down.

Igneous rocks such as granite and basalt form rugged vertical cliffs such as those along the Cornish Atlantic coast at Lands End. Granite is a very strong rock and such cliffs can withstand constant pounding by Atlantic storm waves.

Softer rocks, such as clay, chalk and some sandstones erode more easily and create more gently sloping cliffs.

Sedimentary rocks that have been laid down in distinct strata may dip towards the sea or away from it. This has the effect of either presenting a smooth surface towards the sea or a rough broken surface. A smooth angled surface will better resist waves than a rough broken surface, so the same rock can form different types of cliff according to how the rock surfaces face the sea.

Generally though, remember that hard rocks form steep cliffs and soft rocks and badly broken/fractured rocks form more gently sloping cliffs.

Stacks are easy to recognise because they stand alone, no longer attached to the cliffs. At high tide they will usually be tall rocks rising from the sea just off the shore, though at low tide you may be able to walk out to them across the beach.

Stacks form when there is an area of weakness in the cliffs that can be attacked by the sea, weakened further, and eroded away. The most typical way in which this happens is when a weakness in an exposed headland becomes a cave, the cave is enlarged from both ends to form an arch, and finally the top of the arch collapses leaving the seaward pillar (the upright bit nearest the sea) as a stack. Thus, stacks are erosional features, because they are formed by erosional processes.

The presence of isolated stacks may indicate a change in the type of rock forming the cliff line. For example, a volcanic intrusion or a lava flow may be much more resistant to erosion than the surrounding rocks. Over time the softer rocks erode away leaving a strip of much more resistant rock exposed to the sea. Harder rocks will stay around longer, increasing the chances of the rock remaining long enough for a stack to be formed. You can deduce from this that very soft and easily eroded rocks are unlikely to form a stack because they will not withstand the action of the sea for a long enough period of time.

Sometimes a line of stacks will be formed, such as The Needles off the Isle of Wight, UK. When this happens the stacks furthest from the coastline will often be more worn down than those closer to the shore because they are the older ones.

Over time, all stacks are worn down. A stack that has been reduced to a short pillar is known as a stump. Stumps may be hidden at high tide and only become visible as the tide goes out.

Two additional features about stacks are worth noting. Firstly, there is a darker line around the base of the stack, separating a lower, brown area from the upper dark grey area. This line represents the high tide mark and indicates the general boundary between areas subject to sea water and atmosphere based weathering processes. Secondly, there are light grey / white streaks running down the upper slopes of the stack. These are droppings left by sea gulls, cormorants and other sea birds. Stacks are often popular as nesting sites for birds because of their seclusion, the lack of predators and easy access to food. Many stacks have become important bird breeding sites and are protected by law.

Mud Flats and Salt Marshes

Mud flats are typically found in areas where the tidal waters flow slowly, such as sheltered bays, estuaries, rias and along gently shelving coasts.

A mixture of very fine silts from tidal waters and alluvium from rivers dropping their load as they reach the sea is deposited, causing a build up of mud layers, called mud flats. Mud flats are covered at high tide and exposed as the tide drops. In rias, mud flats are often found in the remains of the valleys that were tributaries to the main submerged river. In such areas, remains of the original watercourses can sometimes be seen as channels carved out of the mud, down which a little fresh waters may be seen flowing at low tide. All mud flats are usually crisscrossed by winding channels that are kept open by tidal action. Unless these channels are fed by active water sources, such as streams and rivers, they will usually dry out at low tide and contain no water.

Where the muds are sufficiently stable to support vegetation, salt marshes may form, or in tropical areas, mangrove swamps.

Mud flats are well named. They are muddy, and often very flat! The reflective surface of the mud flat is created by very high water content in very fine slit, producing a mirror like surface. This mud is very unstable and anyone trying to walk on it would sink up to the knees within seconds. Further out into these mud flats you would sink without trace!

Despite not being a good place for heavy humans, the mud is teeming with life and is a popular feeding ground for birds.

Salt marshes form in coastal areas that already have mud flats. They usually form in areas that are well sheltered, such as creeks, inlets and estuaries where fine sediments can be deposited. They also form behind spits and artificial sea defences where tidal waters can flow gently and deposit fine sediments. Salt marshes are exposed at low tide and at least partially flooded at high tide, so only salt resistant forms of vegetation can grow there. They are typically very flat, with numerous muddy channels and creeks cutting through them, making them very dangerous places to be as the tide comes in.

In some areas salt marshes are left as natural coastal defenses because they can be safely flooded by high tides and thus protect the more valuable commercial, housing or agricultural areas lying further inland. Salt marshes can be reclaimed as agricultural land, as happened in Norfolk, but they are also very important in their natural state. They form a specialised ecosystem supporting numerous species of insects, birds and plants, as well as shellfish, and invertebrates such as lug worms.

Human use of salt marsh areas varies but can include tourism in areas designated as nature reserves, bait collecting areas for fishermen, and they also provide ideal conditions for the farming of some species of shellfish. Hundreds of young shellfish are placed in large sacks that allow the water to flow through whilst stopping the creatures from wandering off. The sacks are pegged out on the mud and left for a year or two until the creatures have grown to a marketable size.

Dunes

Sand dunes are small ridges or hills of sand found at the top of a beach, above the usual maximum reach of the waves. They form from wind blown sand that is initially deposited against an obstruction such as a bush, driftwood or rock. As more sand particles are deposited the dunes grow in size, forming rows at right angles to the prevailing wind direction. If vegetation, such as Marram Grass and Sand Couch, begins to grow on the dune its roots will help to bind the sand together and stabilise the dunes.

Dunes are subject to different forms and sizes based on their interaction with the wind. Most kinds of dune are longer on the windward side where the sand is pushed up the dune, and a shorter "slip face" in the lee of the wind. The "valley" or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as ergs.

Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds.

Sand dunes can be important ecosystems supporting unique plant life and a healthy population of small animals and insects. An example of a sand dune ecosystem is found at West Wittering on the South East coast of the UK. Dune habitats provide niches for highly specialized plants and animals, including numerous rare and endangered species.

Dunes are very vulnerable to erosion by natural processes and by human activity. It is common to see vulnerable sections of dunes fenced off to prevent public access, or for paths to be laid to prevent people from eroding the dunes. Due to human population expansion dunes face destruction through recreation and land development, as well as alteration to prevent encroachment on inhabited areas. Some countries, notably the U.S., New Zealand, Great Britain, Australia, Canada and the Netherlands have developed extensive programs of dune protection. In the UK, a Biodiversity Action Plan has been developed to assess dunes loss and prevent future dunes destruction.

One of the biggest problems posed by sand dunes is their encroachment on human habitats. Sand dunes move through a few different means, all of them helped along by wind. One way that dunes can move is through saltation, where sand particles skip along the ground like a rock thrown across a pond might skip across the water's surface. When these skipping particles land, they may knock into other particles and cause them to skip as well. With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of meters through such sheet flows. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.

Sand threatens buildings and crops in Africa, the Middle East and China. Drenching sand dunes with oil stops their migration, but this approach is highly destructive to the dunes habitat and uses a finite resource. Sand fences might also work, but researchers are still analyzing optimum fence designs. Preventing sand dunes from overwhelming cities and agricultural areas has become a priority for the United Nations Environment Programme.