coastal erosion

Typical features of coastal erosion: from the initial cracks in less resistant rock through to arches, stacks, and stumps that can occur as erosion progresses.

Coastlines are never truly static, although considerable human endeavour often goes into trying to make them so, with groynes and breakwaters and other forms of inshore barrier. This sea wall fortified with boulders to dissipate the force of the incoming tide is at Buzzards Bay, near Falmouth, Massachusetts, an inlet of Nantucket Sound.

Coastal arch at Durdle Door, on the Dorset coast, England. Beds of resistant limestone form the Durdle promontory. Weaknesses in the limestone, such as faults and joints, have been exploited by forces of erosion and mass wasting (downslope movements of loose materials such as sand). The sea has eroded the limestone to form first a cave, and finally an arch, as the cave reaches through to the far side of the Durdle promontory.

Chesil Beach in Dorset, England, is one of many shingle beaches found in southern England. The sediment originally comes from ‘periglacial’ deposits (material moved in the last cold glacial phase). Wave action stimulates the geological process of attrition, in which the rugged, sharp stones are transformed into smaller rounded pebbles.

The shingle beach at Lulworth Cove, Dorset, England. Wave action wears down rugged, sharp stones into smaller rounded pebbles. As the process of attrition continues, the pebbles become progressively smaller and smoother.

Cliffs in coastal areas are produced by erosion. Steep cliffs, such as these on the Cape Peninsula, in the Western Cape province of South Africa, occur when the sea removes all the material eroded by the waves. In other places, if erosion is rapid but transport is limited, material builds up, and a low-angle slope develops.

Stacks, such as this sea stack off the Dorset coast, are isolated upstanding rocks that are formed by erosion. Wave action pounds away at a rock barrier, such as a headland. First, it attacks weaknesses such as joints and faults, eroding a cave on the headland. The cave is deepened until it passes all the way through the headland to form an arch. If the top of the arch collapses then a stack is formed. Eventually the upstanding stack is eroded to form a stump.

Cliffs on Portland Island, Dorset, England. Jointed rocks, such as the limestone shown here, are vulnerable to erosion, especially along the joint. In this photograph, the dark shadow in the centre marks a geo, a steep inlet opened up by the force of the sea. On top of the limestone there are beach deposits. They were deposited about 125,000 years ago under different climatic conditions, when the sea level was higher.

Haystack Rock on the Oregon coastline. These outcrops or ‘sea stacks’ were once part of a headland; wave erosion and weathering have broken down the rock, producing a series of small craggy islands.

The erosion of the land by the constant battering of the sea, primarily by the processes of hydraulic action, corrasion, attrition, and corrosion. Hydraulic action occurs when the force of the waves compresses air pockets in coastal rocks and cliffs. The air expands explosively, breaking the rocks apart. It is also the force of the water on the cliff. During severe gales this can be as high as 6 tonnes/cm³ - the force of a bulldozer. Rocks and pebbles flung by waves against the cliff face wear it away by the process of corrasion, or abrasion as it is also known. Chalk and limestone coasts are often broken down by solution (also called corrosion). Attrition is the process by which the eroded rock particles themselves are worn down, becoming smaller and more rounded.

Frost shattering (or freeze-thaw), caused by the expansion of frozen water in cracks, and biological weathering, caused by the burrowing of rock-boring molluscs and plants, also lead to the breakdown of rock.

Where resistant rocks form headlands, the sea erodes the coast in successive stages. First it exploits weaknesses such as faults and cracks to form caves. Then it gradually wears away the interior of the caves and enlarges them. In some cases the roofs may be broken through to form blowholes. In other cases the caves at either side of a headland may unite to form a natural arch. When the roof of the arch collapses, a stack is formed. This may be worn down further to produce a stump. There are good examples of stacks at The Needles, Isle of Wight, England.

According to a government report released on 27 June 2000, coastal erosion could destroy a quarter of the buildings within 150 m/500 ft of the US coastline in the next 60 years. The Federal Emergency Management Agency (FEMA) said erosion could take about 1,500 houses and their land each year, with an annual cost of around $530 million. In 2000, FEMA was spending $2.6 billion a year in disaster response and recovery. The report recommended that preventative steps be taken and that FEMA should develop erosion maps detailing the risks to property buyers.

Beach erosion Beach erosion occurs when more sand is eroded and carried away from the beach than is deposited by longshore drift. Beach erosion can occur due to the construction of artificial barriers, such as groynes, or due to the natural high tides and high waves during winter storms, which tend to carry sand away from the beach and deposit it offshore in the form of bars. During the calmer summer season some of this sand is redeposited on the beach.