The coast The coast is a narrow contact zone between the land and sea. It is constantly changing due to the effects of land, air and marine processes.
The Coastal System INPUTS Marine: Waves Tides salt sprau Atmosphere: Sun Precipitation Air pressure Wind speed & direction Humans: Pollution, recreation, settlement, defences Terrestrial: weathering, erosion, deposition, rock type /structure PROCESSES Erosion Transport Deposition OUTPUTS Beaches Sand Dunes Spits Bars and Tombolos Headlands & bays Cliffs Wavecut notches Wavecut platforms Caves Arches Stacks Stumps.
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 which drags the water in the same direction that the wind is blowing. If the wind blows for long enough the strength of the wave continues to increase. Eventually the wave may gain enough energy to move under its own momentum, and therefore it no longer needs the wind to blow it.
The size and energy of a wave is influenced by: the length of time that the wind has been blowing. the strength of the wind. how far the wave has travelled (called the fetch). (A long fetch will create big waves and a short fetch small waves)
Constructive Waves Small break, wave spills onto the shore. Strong swash and weak backwash Swash takes load the wave is carrying up onto the beach and deposits it creating new landforms. Hence the name CONSTRUCTIVE!!!
Destructive Waves Large break, making wave plunge onto shore. Wave has large backwash and small swash. The backwash removes the load and thus erodes the coastline. Hence the name DESTRUCTIVE!!!
Wave Terminology The Crest and trough are respectively the highest and lowest points of a wave. The wave height (H)is the distance between the crest and the trough. Wave period (T) is the time taken for a wave to travel through one wave length. This can be timed either by counting the number of crests per minute or by timing 11 waves and dividing by 10, i.e. the number of intervals. Wave length (L) is the distance between two successive crests.
Wave Velocity (C )is the speed of movement of a crest in a given period of time. Wave Steepness (H ÷ L) is the ratio of wave height to wave length. Most waves have a steepness of between 0.005 and 0.05, the ratio cannot exceed 1:7 (0.14) because at that point the wave will break. Waves formed by distant storms and travelling large distances are known as swell. These waves are characterised as having a low height (in relation to wave length), gentle steepness, long wave length and long period. Waves that result from local winds and travel only short distances are known as sea. They are high (in relation to wave length) and steep and have a short wave length and short period.
Diagramto show characteristics of swell and sea Peruazul.com Wetsand.com
Why a wave breaks. As waves move into shallower water at a depth of approximately one half of their wavelength they begin to interact with the sea floor. The wave speed slows down and the wave length decreases, so the wave becomes steeper and grows in height. This process is called shoaling. As the wave moves further inshore and continues to ‘shoal’ it grows to a size where it can no longer maintain its height and the wave breaks. The point at which the wave breaks is known as the plunge line. The water that rushes up the beach is called the swash while any water returning down to the sea is the backwash.
Wave lengthdecreases. Wave getshigher Wavesbeginstointeractwithseafloor. Frictionslows wave down. Wave steepensuntilit can no longermaintainheight (exceeds 1:7 ratio) and breaks
Other Inputs Marine – Tides Atmosphere – Air Pressure, Wind Speed and Direction, Sun, Precipitation Humans Terrestrial – Rock Type
CoastalProcesses: Erosion, Transport and Deposition Erosion is the wearing away of rocks, there are four main types: Hydraulic: as waves break against the cliff face, the pressure of the breaking wave can compress air in cracks. This compressed air gradually forces open the crack in the rock - as this process continues, the rock becomes increasingly weakened. Abrasion: (this is also known as corrasion) - this is where rock fragments are hurled at cliffs by breaking waves, gradually scraping away at the cliff face; the load acts as a grinding tool. Attrition: this is where rock fragments carried by the waves hit against each other and gradually wear down to form sand and silt
4. Chemical: (this is also known as solution) - this occurs where the salt water is able to dissolve some of the chemicals in rocks - for example, limestone cliffs are gradually weakened as the salt water dissolves the calcium carbonate in the limestone. Originally known as KoTapu (Nail Island), this island gained fame when it was featured in a 1974 James Bond film “The Man with the Golden Gun”.
Transport Transportoncoastsis a result of twomain inputs: waves and wind. LongshoreDrift: LSD
Longshoredriftcausedbywaves LSD: The zigzag movement of material along a beach. Swash carried material up the beach following the angle of the waves. Backwash takes material straight down the beach under gravity. Material is moved!
Saltationcausedbythewind Thishappenswhenwindlifts fine and coarselygrainedsand. As windcontinuestoblow, thesandparticlesbouncealong.
Deposition When the sea loses energy, it drops its load of sand, rock particles and pebbles, which it has been carrying. This is called deposition. Deposition happens when the swash is stronger than the backwash and is associated with constructive waves. Deposition is likely to occur when: waves enter an area of shallow water. waves enter a sheltered area, eg a cove or bay. there is little wind. there is a good supply of material. Sources of material include: erosion from cliffs. transported by longshore drift along the coastline. brought inland from offshore by constructive waves. rivers carrying sediment to the coastline.
On the beach different features arise as a result of the action and strength of waves on sand and shingle. Storm beaches: Noticeable, semi-permanent ridge, found a the level of the highest tide. Berms: Small-scale beach ridges built up by successive levels of tides or storms. Cusps: Semi-circular scalloped embayments found in the shingle or at the shingle-sand junction. Ripples: Formed by wave action or tidal currents Berms are more common on shingle beaches, whilst ridges and runnels are found more on sandy beaches. Both form 'crests'.
The material found along a beach tends to vary in size and type as you move further away from the shoreline (where the waves break on the beach). The smallest material tends to be deposited near the water, while larger material is found nearer to the cliffs at the back of the beach. Large material is deposited here in times of high energy, for example during a storm. Waves break most frequently near the shoreline, so sediment nearer the water is broken down more effectively by attrition.
Sanddunes Sand dune characteristics: Embryo dune The first part of the dune to develop. Stabilisation occurs via marram and lyme grass, which act as traps for sand. Conditions are dry (due to percolation) and plants adapt to this via long roots, or thorny leaves to reduce evapotranspiration. Yellow dune Colour is due to a lack of humus, but with distance inland they become increasingly grey due to greater amounts of humus. Heights can reach 5m and plants include sand sedge, sea holly, and red fescue. Fixed grey dunes Limited growth due to distance from beach. Far more stable as shown by existence of thistle, evening primrose, bracken, bramble and heather. Dune slacks Depressions between dune ridges, which will be damp in summer and water-filled in winter. Species include water mint, rushes, and weeping-willow. Blow outs Often evidence of over use by humans. Large 'holes' that appear in the dunes.
Spits Spits are formedbecausetheprocess of LSD transportsbeachsedimentalong a coastline, and thenthissedimentisdepositedoutacrossanarea of lowerenergy, usuallycreatedwhere a change in theshape of thecoaslineoccurs.
Wave Refraction Where waves approach at an irregular coastline they are refracted, i.e. they become increasingly parallel to the coastline.
Cliffs, WavecutNotches and WavecutPlatforms 1. Weather weakens the top of the cliff. 2. The sea attacks the base of the cliff forming a wave-cut notch. 3. The notch increases in size causing the cliff to collapse. 4. The backwash carries the rubble towards the sea forming a wave-cut platform. 5. The process repeats itself and the cliff continues to retreat.
Caves, Arches, Stacks and Stmps Caves occur when the waves force their way into cracks in the cliff face. The water contains sand and other materials that grind away at the rock until the cracks become a cave. Hydraulic action is a predominant process. If the cave is formed in a headland, it may eventually break through to the other side forming an arch. 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 wave cut notch is formed. This will weaken the structure and it will eventually collapse to form a stump.