Coasts 1 Waves How waves form? Waves are usually formed by the wind blowing across the sea. This causes ripples on the top of the surface and these develop into waves. The stretch of open water over which the wind blows is called the fetch. The longer the fetch the more powerful the wave. Video showing wave formation - http://www.bbc.co.uk/learningzone/clips/understanding-waveformation/4018.html The above diagram shows you what happens when a wave moves closer towards the coast and crashes onto a beach. Crest - this is the top part of the wave and eventually topples onto the beach. Swash - this is the water that rushes up the beach Backwash - this is the water that flows back towards the sea Types of waves There are two types of waves; constructive and destructive waves. Constructive waves: 2 Constructive waves are waves that surge up the beach and have a powerful swash They carry large amounts of sediment and 'construct' the beach making it more extensive Main characteristics: They have a strong swash and a weak backwash and are smaller in height compared to destructive waves. They are waves which contain a small amount of energy. Destructive waves Destructive waves are named because they 'destroy' the beach. When the waves hit the beach they rear up and smash down onto the beach. There is very little swash when the wave breaks but has a powerful backwash. The backwash removes the sediment which leads to the 'destruction' of the beach Main characteristics: They have a strong backwash, lots of energy and are waves which are high in height 3 Processes influencing our coastlines Coastal processes can be split into two different categories: Land processes influencing our coastlines Marine coastline influencing our coastlines Video showing the processes of erosion - http://www.bbc.co.uk/learningzone/clips/weatheringerosion-and-coastal-features/4022.html Excellent animation showing coastal processes and defences http://www.as.uky.edu/academics/departments_programs/EarthEnvironmentalSciences/EarthEnviron mentalSciences/Educational%20Materials/Documents/elearning/module14swf.swf Weathering The processes of weathering affects rocks exposed at the coast. Freeze-thaw weathering in particularly effective if the rock is porous (contains holes) and permeable (allows water to pass through it). This can lead to dramatic rockfalls as shown below. Full explanation as to how freeze-thaw weathering works can be found at the following link: http://onlinegeography.wikispaces.com/Ice+as+a+shaping+agent Mass movement Mass movement is the downhill movement of material under the influence of gravity. In 1993 60m of cliff slid onto the beach near Scarborough in North Yorkshire, taking with it part of Holbeck Hall Hotel. 4 5 Marine Processes Coastal erosion Erosion – use the acronym CASH Corrasion – (abrasion) is caused by large waves hurling beach material against the cliff Attrition – is when waves cause rocks and boulders on the beach to bump into each other and to break up into small particles Solution – (corrosion) is when salts and other acids in seawater slowly dissolve a cliff Hydraulic pressure (power/action) – is the force of waves compressing air in cracks in a cliff One final process of erosion is: Abrasion: this is the 'sandpapering' effect of pebbles grinding over a rocky platform, often causing it to become smooth. Coastal transportation Processes of Transportation Traction – rolling stones along the sea floor (needs the most energy) Saltation – sand-sized particles bounce along the sea floor in a ‘leap frog’ movement Suspension – silt and clay-sized particles are carried within the water flow· Solution – some minerals dissolve in the water (this needs the least energy) Longshore Drift Video showing how Longshore Drift works - http://www.bbc.co.uk/learningzone/clips/landformscreated-by-longshore-drift-and-coastal-deposition/9965.html Waves approach the beach in the same direction as the wind When the wave breaks, swash carries material up the beach at the same angle as the wind The backwash carries the material straight back down the beach under gravity· This process slowly moves material along the coastline 6 Remember when drawing: To draw in a beach An arrow showing the prevailing wind An arrow showing the direction of longshore drift To draw in the movement of the pebbles To use the terms ‘swash’, ‘backwash’ and ‘gravity’ when describing the movement of the pebbles Coastal deposition Deposition occurs in areas where the flow of water slows down. The sediment can no longer be carried or rolled along an has to be deposited. Coastal deposition most commonly occurs in bays, where the energy if the waves is reduced on entering the bay. This explains the presence of beaches in bays and accounts for the lack of beaches at headlands, where the energy of the waves is uch greater. 7 Landforms of erosion Very useful BBC videos: Formation of landforms - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-andlandforms/9966.html Wave-cut platforms and headlands/bays - http://www.bbc.co.uk/learningzone/clips/wave-cutplatforms-and-headland-erosion/4021.html Weathering and erosion - http://www.bbc.co.uk/learningzone/clips/weathering-erosion-and-coastalfeatures/4022.html An example of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocksdorset/3244.html An example of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-coastalprocesses-and-landforms/3245.html More landforms - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-west-wales/3084.html Cliff slumping - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-blowholes-and-cliffslumping/3085.html Headlands and bays Cliffs rarely erode at an even pace. Sections of cliff that are particularly resistant to erosion stick out to form headlands. Weaker sections of coastline that are more easily eroded form bays. Form where there are alternating outcrops of resistant outcrops of resistant (harder) and less resistant (softer). · Destructive waves erode the soft rock move quickly to form bays· 8 The harder rock is more resistant and are left protruding into the sea· The headlands protect the adjacent (next by) bays from destructive waves As the headlands protect the bays sand is deposited to form a beach. At the headlands there often wave-cut platforms and notches. Cliff and wave-cut platforms Wave erosion is greatest when large waves break against the foot of the cliff · The waves undercut the foot of the cliff to form a wave-cut notch · Over time the notch enlarges the until the cliff above it is unsupported and collapses · The gentle sloping expanse of rock marking the foot of the retreating cliff is called a wave-cut platform · Wave-cut platforms are exposed at low tide but hidden at high tide 9 Caves, arches and stacks When drawing a diagram label: What and where the different processes of erosion are occurring · Where wave-cut notches may be formed · Check you have the correct sequence· Explain what hydraulic power, corrosion and abrasion 10 Landforms of deposition Video showing all of the coastal landforms of deposition http://www.bbc.co.uk/learningzone/clips/depositional-coastlines/4023.html Beaches They are usually found in sheltered bays between two headlands The headlands protect the area from erosion Low constructive waves deposit material on the shore Gradually a beach is built up Material on a beach is well sorted – the biggest pebbles are nearest the land with the smallest nearest the sea The larger the material the steeper the beach – pebble is steeper than sand Spits http://www.geographyalltheway.com/ks3_geography/coasts/imagesetc/deposition.swf - an amazing animation which explains the formation of a spit!! Video showing the formation of a spit - http://www.bbc.co.uk/learningzone/clips/the-formation-ofspits/3246.html A spit is an area of sand or shingle that has been transported by longshore drift and then deposited as the coastline has changed direction. It is attached to the land at one end. It is a depositional landform. Hurst Castle Spit in Hampshire is a very famous example. Where the coastline changes direction, sediment is deposited in the same direction as the original coastline (i.e. in line with the prevailing wind direction). Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up. The material is deposited in the deeper water offshore until the ridge is built above the level of the sea. Drift continues along the seaward side of the spit extending it further 11 down the coast while salt marsh develops in the slow-moving water on the landward side. Spits can become a permanent feature. This happens when the prevailing wind picks up sand from the beach and blows it inland across the spit to form sand dunes. These dunes will then be colonised by vegetation, which stabilises them. It is common for a salt marsh to develop in the sheltered area of water behind the spit. Water is trapped behind the spit, creating a low energy zone. As the water begins to stagnate, mud and marsh begin to develop behind the spit. A spit may grow out across a river estuary. Where the spit is crossing a river mouth, the river will be diverted so that it follows the coastline for some miles before reaching the sea. Bars Bars can form in several ways: (a) a spit grows the whole way across a bay (b) a sandbank develops offshore, parallel to the shore, and is moved towards the coastline by the waves and wind until it joins the mainland Slapton Sands is an example of a bar. The lagoon of water than has formed on the landward side of the bar is called Slapton Ley. A tombolo is formed where a spit joins an island to the mainland. An example is the Isle of Portland which is joined to the mainland by a shingle ridge known as Chesil Beach. Useful weblinks: BBC Class Clips video - Blakeney Point BBC Class Clips video - Kaitorete Spit, New Zealand BBC Scotland video about spits, bars and tombolos BBC Bitesize - Spits BBC Bitesize - Tombolos 12 Sea-level rise Videos on Youtube channels The causes of sea-level rise: One of the effects of global warming is sea-level rise. Over the last 15 years, global average sea levels have risen by 3mm a year. The latest estimate from the Intergovernmental Panel on Climate Change (IPCC) suggests a rise in global sea levels of between 28 and 43 cm by the end of the century. The main cause of sea-level rise is thermal expansion of seawater as it absorbs more heat from the atmosphere. Also the melting of ice on land e.g. from Greenland and will increase the level of the oceans. The actual amount of sea-level rise will vary from place to place due to variations in the level of the land and the amount of deposition of sediment occurring at the coast. The effects of sea-level rises: In the UK, East Anglia is likely to be hardest hit and this threatens coastal defences and natural ecosystems. Elsewhere in the world, vast areas of low-lying coastal plains such as Bangladesh and whole chains of islands such as the Maldives and Tuvalu could disappear. More than 70% of the world's population live on coastal plains so the effects of sea-level rises are going to be devastating. 13 Cliff Collapse BBC videos on cliff collapse: Buildings threatened by erosion - http://www.bbc.co.uk/learningzone/clips/a-dairy-farm-is-threatenedby-coastal-erosion-at-holderness-east-yorkshire/3190.html Buildings threatened by erosion - http://www.bbc.co.uk/learningzone/clips/a-farm-is-threatened-bycoastal-erosion-at-holderness-east-yorkshire/3189.html Buildings threatened by erosion - http://www.bbc.co.uk/learningzone/clips/a-house-threatened-bycoastal-erosion/3188.html Erosion at Hallsands - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-at-hallsands-causesand-effects/9967.html Erosion at Holderness - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-at-holderness-eastyorkshire/3187.html Erosion at Hallsands - http://www.bbc.co.uk/learningzone/clips/hallsands-destruction-ofcoastland/3242.html Case study on Happisburgh - http://www.bbc.co.uk/learningzone/clips/living-with-coastal-erosion-inhappisburgh-east-anglia-pt-1-2/7361.html Case study on Happisburgh - http://www.bbc.co.uk/learningzone/clips/living-with-coastal-erosion-inhappisburgh-east-anglia-pt-2-2/7364.html Various factors can contribute to cliff collapse. These include: weathering processes - such as heavy rainfall that can saturate the land and make it unstable mass movement such as sliding and slumping - which is more likely if the land is made of soft weak rock types the power of the waves - crashing against the cliffs and undercutting them from below Barton-on-Sea, Hampshire Barton-on-Sea is a small village in Christchurch Bay and has long been affected by coastal erosion and cliff collapse. Over the years a number of buildings and most recently a cafe have been lost to the sea. Sea defences have been built to prevent coastal erosion but in 2008 there was a fresh landslip. The older houses in the settlement are now only 20m from the sea. It is expected that the houses will be lost to the sea in the next 10 to 20 years. Why are the cliffs at Barton-on-Sea so prone to collapse? the rocks are weak sands and clays. They are easily eroded by the sea and have little strength to resist collapse. the arrangement of the rocks causes water to 'pond-up' within the cliffs. This increases the weight of the cliffs. The increase in water pressure within the cliffs which encourages collapse. the coastline is exposed to the direct force of the prevailing south-westerly winds. This means a very long fetch and as a result the waves are very powerful leading to rates of erosion as much as 2m a year in places. 14 small streams increase the amount of water entering the cliffs which increases the weight of the cliff and then leading to collapse buildings on the cliff top have increased the weight on the cliffs, making them more vulnerable to collapse 15 Sea Defences – See Website for excellent videos and animations http://onlinegeography.wikispaces.com/C+-+Sea+defences Salt Marshes Salt marshes are areas of periodically flooded low-lying coastal wetlands. They are often rich in plants, birds and animals. A salt marsh begins life as an accumulation of mud and silt in a sheltered part of the coastline for example in lee of a spit or bar. As more deposition takes place, the mud begins to break the surface to form mudflats. Salt-tolerant plants such as cordgrass soon start to colonise the mudflats. These early colonisers are called pioneer plants. Cordgrass is tolerant of the saltwater and its long roots prevent it from being swept away by the waves and the tides. Its tangle of roots also helps to trap sediment and stabilise the mud. As the level of the mud rises, it is less frequently covered by water. The conditions become less harsh as rainwater begins to wash out some of the salt and decomposing plant matter improves the fertility of the newly forming soil. New plant species such as sea asters start to colonise the area and gradually, over hundreds f years, a succession of plants develops. This is known as a vegetation succession. Salt Marsh Vegetation Succession 16 Case study: Keyhaven Marshes Make sure you can describe the threats to the ecosystem and management options there. 17 Glossary Fetch: the distance of open water over which the wind can blow Beach: a deposit of sand or shingle at the coast, often found at the head of a bay Crest: the top of a wave Swash: the forward movement of a wave up a beach Backwash: the backward movement of water down a beach when a wave has broken Constructive wave: a powerful wave with a strong swash that surges up a beach Destructive wave: a wave formed by a local storm that crashes down onto a beach and has a powerful backwash Rockfall: the collapse of a cliff face or the fall of individual rocks from a cliff Hydraulic power: the sheer power of the waves Corrasion: the effect of rocks being flung at the cliff by powerful waves Solution: the dissolving of rocks, such as limestone and chalk Attrition: the knocking together of pebbles, making them gradually smaller and smoother Traction: heavy particles rolled along the seabed Solution: the transport of dissolved chemicals Saltation: a hopping movement of pebbles along the seabed Suspension: lighter particles carried (suspended) within the water Longshore drift: the transport of sediment along a stretch of coastline caused by waves approaching the beach at an angle Headland: a promontory of land jutting out into the sea Bay: a board coastal inlet often with a beach Wave-cut platform: a wide, gently sloping rocky surface at the foot of a cliff Cave: a hollowed-out feature at the base of an eroding cliff Arch: a headland that has been partly broken through by the sea to form a thin-roofed arch Stack: an isolated pinnacle of rock sticking out of the sea Spit: a finger of new land made of sand or shingle jutting out into the sea from the coast Salt marsh: low-lying coastal wetland mostly extending between high and low tide Bar: a spit that has grown across a bay Shoreline Management Plan: an integrated coastal management plan for a stretch of coastline in England and Wales 18 Hard engineering: building artificial structures such as sea walls aimed at controlling natural processes Soft engineering: a sustainable approach to managing the coast without using artificial structures Managed retreat: allowing controlled flooding of low-lying coastal areas or cliff collapse in areas where the value of the land is low Pioneer plant: the first plant species to colonise an area that is well adapted to living in a harsh environment Vegetation succession: a sequence of vegetation species colonising an environment 19