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Erosion and Changing Coastlines Coasts are shaped by the sea and the action of waves. The processes that take place are erosion, transportation and deposition. The action of waves The power of waves is one of the most significant forces of coastal change. Waves are created by wind blowing over the surface of the sea. As the wind blows over the sea, friction is created - producing a swell in the water. The energy of the wind causes water particles to rotate inside the swell and this moves the wave forward. The size and energy of a wave is influenced by: how long the wind has been blowing the strength of the wind how far the wave has travelled (the fetch) Waves can be destructive or constructive. When a wave breaks, water is washed up the beach - this is called the swash. Then the water runs back down the beach - this is called the backwash. With a constructive wave, the swash is stronger than the backwash. With a destructive wave, the backwash is stronger than the swash. swash backwash Destructive Waves effects of a high wave Destructive waves are created in storm conditions. They are created from big, strong waves when the wind is powerful and has been blowing for a long time. They occur when wave energy is high and the wave has travelled over a long fetch. They tend to erode the coast. They have a stronger backwash than swash. They have a short wave length and are high and steep. Constructive Waves effects of a low wave They are created in calm weather and are less powerful than destructive waves. They break on the shore and deposit material, building up beaches. They have a swash that is stronger than the backwash. They have a long wavelength, and are low in height. Coastal Erosion The sea shapes the coastal landscape. Coastal erosion is the wearing away and breaking up of rock along the coast. Waves have the power to erode, or break up, and wear away rocks over time. Storm, or high-energy, waves breaking at the bottom of a cliff move blocks of stone - rocks. Waves fling pebbles at the bottom of a cliff and wear the cliff away. This process is called abrasion. Destructive waves erode (wear away) the coastline in a number of ways: Hydraulic action: Air may become trapped in joints and cracks on a cliff face. When a wave breaks, the trapped air is compressed (crushed) which weakens the cliff and causes erosion. Abrasion: Waves ‘throw’ pebbles, bits of rock and sand at the bottom of cliff surfaces which wears/grinds the cliff away like sandpaper. Attrition : Waves smash rocks and pebbles on the shore into each other, and they break and become smoother. Solution. Acids contained in sea water will dissolve some types of rock such as chalk or limestone. Cliffs and shorelines can get undercut or worn away at the bottom. Then the cliff above may collapse in a rockfall, or landslide. Video examples can be seen on our website. Along a coastline there are features created by erosion, these include cliffs, headlands and bays, caves, arches, stacks and stumps. Cliffs, wave-cut platforms and notches One of the most common features of a coastline is a cliff. Cliffs are shaped through a combination of erosion and weathering - the breakdown of rocks caused by weather conditions. Soft rock like sand and clay, erodes easily to create gently sloping cliffs but harder rock, like chalk, is more resistant and erodes slowly to create steep cliffs. Seven Sisters chalk cliffs on East Sussex coast The erosion of cliffs The process of cliff erosion Weather weakens the top of the cliff. The sea attacks the base of the cliff forming a wave-cut notch. The notch increases in size causing the cliff to collapse. The backwash carries the rubble towards the sea forming a wave-cut platform. The process repeats and the cliff continues to retreat (erode away). Headlands and Bays Headlands are formed when the sea attacks a section of coast with alternating bands of hard and soft rock. The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant (harder) Lulworth Cove in Dorset rock, such as chalk. This leaves a section of land jutting out into the sea called a headland. The areas where the soft rock has eroded away, next to the headland, are called bays. Lulworth Cove is a horseshoe shaped bay which formed because the inland rocks are less resistant than the headland. It is obvious to see from this bay, how different rock type are affected by the waves at different rates. These changes can happen quickly over a period of years or over thousands of years depending on the type of rock. Geology is the study of the types of rocks that make up the Earth's crust. Coastlines where the geology alternates between strata (bands) of hard rock and soft rock are called discordant coastlines. A concordant coastline has the same type of rock along its length. Concordant coastlines tend to have fewer bays and headlands. Caves, Arches, Stacks and Stumps Weathering and erosion can create caves, arches, stacks and stumps along a headland. See our video section. http://www.highviewschool.org.uk/coastal-erosion-wave-actions/ Caves Caves occur when waves force their way into softer rock with cracks in the cliff face. The waves contain sand and other materials which grind away at the rock until the cracks become a cave. This type of wave action is called hydraulic action. Cave at Hole Point, Dawlish, Devon Arch Durdle Door Arch in Dorset If the cave is formed in a headland, it may eventually break through to the other side forming an arch. Stacks and Stumps The arch will gradually become bigger until it can no longer support the top of the arch. When the arch collapses, it leaves the headland on one side and a stack (a tall column of rock) on the other. The stack will be attacked at the base in the same way that a wavecut notch is formed. This weakens the structure and it will eventually collapse to form a stump. Old Harry Rocks, Studland, Poole, Dorset Information sourced from: http://www.bbc.co.uk/schools/gcsebitesize/geography/coasts/ http://www.3dgeography.co.uk/coast-diagram