- Created by: Natwallace
- Created on: 17-09-18 18:13
The Coastal System - Overview
- Marine – waves, tides, and currents
- Energy – kinetic energy from waves and wind, thermal energy from the sun and potential energy from material on cliffs/slopes and material from processes of weathering, mass movement, erosion, and deposition
- Geological – rock type, structure, and tectonics. material from marine deposition, weathering, and mass movement
- Atmospheric – climate, weather and climate change
- People – urban planning, housing, industry, coastal management/ defenses, leisure
- Stores such as sediment on a beach, and flows such as longshore drift moving sediment along the coast.
- Erosion - attrition, corrosion/abrasion, hydraulic action, and pounding
- Weathering - freeze-thaw, solution, salt crystallization, biological weathering, onion skin etc.
- Mass Movement - slumps, rockfall, slides, soil creep
- Landforms from erosion - cave, stack, stump, wave-cut platforms etc.
- Landforms from deposition - spits, tombolos, on-shore bars, dunes, beaches, salt marshes
Coastal Processes - Sediment Cells
- Sediment cells are areas along the coast and their nearshore zones where sediment moves mainly in a closed system not outputting any sediment to other locations.
- There are 11 sediment systems around England and Wales
- The location of sediment cells are related to the topography of the coastline
- Although most cells are quite closed off there is some loss of sediment
- Within the larger cells there a smaller sub-cells
Coastal Processes - Wind
- Wind is a source of energy for coastal erosion and transport of sediment by waves.
- Wave energy is generated by the friction of the wind blowing across the top of the ocean, this is known as fetch. The larger fetch the more energy in the waves
- Onshore winds, blowing from the sea are the most effective at driving waves towards the coast
- If winds blow in at an oblique angle then the waves will also do the same generating LSD
- The wind can also perform erosion, transportation, and deposition, when done via winds these are known as aeolian processes.
Coastal Processes - Waves
- A wave posses potential energy as a result of its position above the wave trough and kinetic energy caused by the motion of the water within the wave.
- The amount of energy in a wave in deep water can be approximated by the formula: P=H2T
- Waves are mainly influenced by wind as the wind blows across the surface the friction causes kinetic energy to be transferred into the wave increasing the wave energy. Fetch is the distance a wave blows over the wave until it breaks.
- Wave refraction: Due to the coastline not being completely uniform, things like headlines and bays cause the depth of water to be irregular around the coast, this alters the wave and its influence. As waves approach the coast waves concentrated around headlands and are reduced around bays.
Coastal Processes - Wave Anatomy
- Crest: the highest point of a wave
- Trough: the lowest point of a wave
- Height: the distance between trough and crest
- Wavelength: the distance between one crest/trough and the next
- Swash: water movement up a beach
- Backwash: water movement down a beach
- When a wave breaks water moves up the beach as swash, driven by the transfer of energy that occurs when the wave breaks. The speed of this water movement will decrease as it travels further up the beach due to friction caused by the gradient of the beach. When it has no more energy to travel up the beach it is drawn back down the beach as backwash. This energy comes from gravity and always occurs perpendicular to the coastline down the steepest coastline.
Coastal Processes - Breaking Waves
- When waves move into shallow water their behavior changes as the bottom of the column of moving energy in the water hits the sea floor creating friction changes the speed, direction and shape of the waves.
- Firstly, waves slow down as they drag across the bottom, this decreases the wavelength and successive waves bunch up the deepest part of the wave slows down more than the top of the wave.
- The wave begins to steepen as the crest advances ahead of the base and when the water depth is less than 1.3* wave height the wave topples and breaks against the shore.
- Breaking waves is the only significant forward movement of water as well as energy
- There are three kinds of breaking waves:
- Spilling - steep waves breaking onto gently sloping beaches; water spills gently forward as the wave breaks.
- Plunging - moderately steep waves breaking onto steep beaches: water plunges vertically downwards as the crest curls over.
- Surging - low-angle waves breaking onto steep beaches; the wave slides forward and may not actually break.
Coastal Processes - Constructive and Destructive W
- Constructive waves - These tend to be low in height, long wavelength and occur usually where there is a large amount of fetch. They tend to have a low gradient, a larger swash weaker backwash, low energy, and an elliptical orbit. They have a low frequency of about 6-8 waves per minute. Constructive waves normally break as spilling wave this means backwash is weaker and returns to the sea before the next wave breaks and so the next swash is uninterrupted and so retains its energy.
- Deconstructive waves - These tend to be high in height, shorter wavelengths and are the main eroding waves, they have backwash is greater than swash, and circular orbit a steep gradient and they tend to break as plunging waves meaning there is little forward-moving energy to move the waves up the beach, this and the friction results in swash being slowed down and traveling only a small way before returning as backwash. Due to the shorter wavelength, the swash of the next is slowed by the returning back was of the previous wave. They have a high wave frequency of about 12-14 per minute.
Coastal Processes - Tides
- Tides are the periodic rise and fall of sea surface and are caused by the gravitational pull of the moon and to a lesser extent the sun.
- As the moon travels around the earth it pulls the water towards it creating a high tide and compensatory bulge on the other side of the planet, between the bulges, is a low tide zone.
- Twice each lunar month the earth moon and sun are aligned pulling the sea at it's strongest, this happens twice a lunar month (29.5 days) and results in spring tides with high tidal ranges. Also occurring twice a month the moon and sun are at right angles to each other and therefore the gravitational pull is at the weakest producing neap tides with low tidal ranges.
- Tidal ranges can have significant effects on coastal process and landscapes. In places like the Mediterranean sea, there is a low tidal range and so wave action is restricted to a narrow area of land. in the Severn estuary has a high tidal range of around 14m due to the fact the coast is funneled there.
Coastal Systems - Geology
Lithology - describes the physical and chemical composition of rocks. Rocks like clay have a weak lithology, with little erosion weathering and mass movement due to the facts the bonds between the particles are weak. Others like basalt, made of interlocking crystals, are highly resistant to erosion and are more likely to form cliffs and headlands. Others, like chalk, are soluble in weak acid and are thus vulnerable to the chemical weathering via carbonation.
Structure - structure concerns the properties of individual rock types such as jointing, bedding, and faulting. It also involves the permeability of rocks. Porous rocks like chalk have tiny air spaces that separate the mineral particles. These pores can absorb and store water, known as primary permeability. Secondary permeability is when water seeps into the joints between limestone, these joints are easily enlarged by solution. Structure of the rocks is an important factor in the shaping of the coastline, for example, a coastline with uniform rocks parallel to the coast known as concordat usually results in straight coastlines. Whereas discordant coastlines where different rocks outcrops exist is more likely to result in irregular coastlines with headlands and bays. Structure of rocks also has an impact on cliff profiles.
Coastal Systems - Currents
- Offshore and nearshore currents have an influence on coastal landscapes systems.
- Riptides play an important role in the transport of coastal sediment. They are caused by tidal motion or by waves breaking at right angles to the shore. A cellular circulation is created by differing wave heights parallel to the shore. Water from the top of breaking waves with a large height travels further up the shore and then returns through the adjacent area where the lower height waves have broken. Once this has occurred they modify the shore profile and create cusps which help perpetuate the rip current channeling flow through a narrow neck.
- Ocean currents also have much larger effects on the coastal system, generated by the Earths rotation and by convection and are set in motion by the movement of winds across the surface. Warm ocean currents transfer heat energy from low latitudes towards the poles. They particularly affect western facing coasts where they are driven by onshore winds. Cold water currents do the opposite by moving cold water from the poles to the equator and are usually driven by offshore winds and so tend to have less effect on coastal landscapes. Although the strength of the current has a limited effect on geomorphic processes it is the transfer of thermal energy that has the biggest effect as it changes air temperature and therefore sub-aerial processes.
Coastal System - Terrestrial
- Rivers are major sources of sediment for the coastal sediment budget. This is particularly true of coasts with a steep gradient where rivers directly deposit their sediment at the coast. Sediment delivery to the shoreline can be intermittent only really occurring during storms. In some cases, 80% of coastal sediment comes from rivers.
- The sediment comes from erosion inland from river valleys.
- Wave erosion is also the source of large amounts of sediment in the sediment budget, cliff erosion can be increased by rising sea levels and may be amplified by storm surge events. The sediment supplied by cliffs can make up 70% of the budget in high-energy wave zones. Usually, though it only provides a little sediment.
Coastal Systems - Offshore
- Constructive waves bring sediment to the shore from offshore locations and deposit it adding to the budget, tides, and currents do the same. Winds also do the same including exposed sandbars, duns and beaches to elsewhere along the coast known as Aeolian processes and usually involves fine material as the wind has less energy to carry heavier material.
Coastal Systems - Human
- When a budget is deficit nourishment may be the only way to achieve equilibrium and protect the coastline. Sediment may be brought in by truck and dumped or through rainbowing where sand is taken from sediment stores offshore by boats and pumped onshore by a pipe.
Geomorphic processes - Weathering - Physical
- Weathering is the use of energy to weaken, break and chemically alter the surface and near-surface rocks through physical and chemical methods.
- There are three types of weathering:
- Physical - when a rock is broken into smaller pieces without any chemical alteration taking place.
- Freeze-thaw - water gets trapped in cracks of rocks when the temperature drops the water freezes and the water particles expand widening the crack and weakening the rock.
- Exfoliation/Onionskin weathering - Usually this occurs in warm areas, as the sun shines on the rock it expands, during the night it then cools and contracts. This process occurs, again and again, causing small pieces of rock to flake off.
- Salt Crystallisation - evaporation of salt water of the surface of rocks can form crystals in the cracks of rocks, as these crystals get bigger the cracks can widen and weaken the rocks and fracture them.
Geomorphic processes - Weathering - Chemical
- Solution/Carbonation - This is when acidic rain causes the calcium carbonate to react and wear away.
- Hydrolysis - The breakdown of rock by acidic water to produce clay and soluble salts
- Oxidation - The breakdown of rock by oxygen and water usually giving iron-rich rocks a rusty-colored weathered surface.
Geomorphic processes - Weathering - Biological
- Roots - Roots from plants can work their way into cracks in rocks and expand, weakening the rocks and even breaking them apart.
- Animals - Large animals can burrow through cliffs and around rocks weakening them. Smaller organisms like mollusks and algae may also weaken rocks through boring their way into the rocks or secreting acids to protect them.
Geomorphic Processes - Wave Processes - Erosion
- Abrasion - as waves break against the land, rocks, pebbles and sand grind against the land wearing it down slowly like.
- Attrition - again as waves break rocks and sand get flung together grind against one another and become smaller and smoother.
- Hydraulic action - air and water get trapped in gaps within the cliff, waves break compressing the fluid as the pressure is released the fluid suddenly expands and the cracks widen and weaken the cliff.
- Pounding - this happens when the mass of a breaking wave exerts massive amounts of pressure as much of 30 tonnes per meter squared on the rock weakening it.
- Solution/Corrosion - the ocean contains weak acids like Carbonic acid which is capable of dissolving limestone. The evaporation of salt in the ocean produces crystals and their formation can lead to the disintegration of rocks.
Geomorphic processes - Mass Movement
- Mass movement happens when the forces keeping material on a slope, mainly friction, are exceeded by the forces acting on the slope mainly gravity.
- Mass movement usually occurs on cliffs in the coastal landscape system which adds regolith and rocks into the sediment budget.
- The main processes involved are:
- Rock Fall: On cliffs of 40 degrees or more, especially if the cliff face is bare, rocks may become detached via physical weathering processes, the rock then falls to the bottom of the cliff and is removed by wave processes or accumulates as a relatively straight, lower angled, scree slope.
- Slides: Usually linear with movement occurring along a straight line slip plane like a fault, bedding plane between layers of rock, or rotational, with movement taking place along a curved slip plane. Rotational slides are known as slumps, these usually occur when waves undercut the base of a cliff and support for the material above is weakened. Slumps are more common in weak rocks like clay which become heavier when the water is added this contributes to the downslope force. A layer of sand above the clay may particularly add to this as water passes through this layer but can not through the clay and so increases pore pressure in the sands and exerting more pressure on the cliff to give way.
Coastal Processes - Wave Processes - Transport
- Waves, tides, and currents can move material
- Listed below are ways wave processes participate in transportation:
- Solution - some minerals from rocks etc. are dissolved in the water and carried around in small usually invisible particles.
- Suspension - small more visible particles like silt and clays are carried around in the water creating cloudy water.
- Saltation - sediment loads are bounced along the seabed, things like small rocks, shingles, and large grains of sand.
- Traction - larger sediment and rocks that cannot be suspended by currents are rolled along the seafloor.
- Once deposited onshore sediment may be then moved along the coastline by longshore drift, this occurs when waves approach the coast at an oblique angle due to the dominant wind direction. When the waves break the swash carries sediment diagonally up the beach and then under the force of gravity it is moved perpendicularly down the beach. If this process is carried out repeatedly then a net movement of sediment occurs along the coast evidence of this can be seen on beaches where sediment becomes more rounded and smaller with increasing distance along the beach.
Coastal Processes - Wave Processes - Deposition
- Material and sediments deposited when there is a loss of energy caused by a loss of velocity and/or volume of water.
- Deposition takes place in coastal systems when: the rate of sediment accumulation exceeds the rate of removal through erosion and transportation
- When waves slow down immediately after breaking
- At the top of the swash where there the water is not moving for a brief movement
- During the backwash when percolates into the beach material
- In low energy zones like a sheltered area or estuary
- The velocity the sediment will deposit at is known as the settling velocity, the larger the particles the more energy needed to transport them. As velocity decreases the largest particles are dropped first and so on until the smallest sediment is dropped.
Coastal Processes - Fluvial Processes
- Erosion in the upper catchment are usually the main source of a river's sediment load, rivers use similar erosional processes to waves with most erosion taking place during high-flow and high-energy.
- Sediment also comes from similar weathering and mass movement processes that the ocean also suffers from
- Transportation - Rivers also use the same methods of transportation of sediment.
- Deposition - When rivers enter the sea there is a significant drop in velocity due to faster water meeting a largely static body of water and/or there may be currents and tides actively going against the river currents. This drop in velocity results in most of the sediment being dropped with bigger particles first and then smaller and smaller sequentially. As well as this the meeting of fresh and saltwater flocculation of clay particles occurs due to electrical charge changing, as they get bigger the eventually drop onto the seafloor.
Coastal Processes - Aeolian Processes
- Wind can pick up sand particles and move them via deflation. With speeds of 40 m/hour, sand grains are moved by rolling over the surface (surface creep) as well as salvation. Due to the size of the sand grains compared to clay and silt, they are rarely carried by suspension, this restricts the erosion ability using abrasion to about 1 m and has limited effect in the erosion of rocky coastlines and cliffs.
- As the velocity of the wind increases the erosion capacity also does, for example with a velocity increase from 2 to 4 meters per second causes an eight-fold increase in erosive capacity.
Coastal Land forms - Erosional - Shore Platforms
- Wave-cut notch - forms when destructive waves break repeatedly, this causes a wave-cut notch between the high and leave-cut zone. As this notch gets bigger the support for the rock strata above is weakened and eventually the strata collapses, this produces a steep profile and a cliff. Due to the resultant wave action removing the debris at the base of the cliff the profile remains pretty steep and the cliffs retreat inland parallel to the coast.
- Wave-cut platforms - These depend a lot on the geology. Rock strata with more horizontally bedded and landward dipping strata tend to support cliffs with a steep near verticle profile. If the strata incline more seaward the profile tends to follow the angle of the dipping strata.
- As the process continues the cliff becomes higher and higher while at its base a gently sloping platform is cut into the rock, Although originally being flat the surface will eventually have deep cuts caused by abrasion though larger rocks will accumulate at the base.
- In the end, the platform will get so wide it will produce shallow and small waves even at higher tides, this lowers the friction power of the waves and slow downs the undercutting process and eventually halting the cliff being eroded anymore. This is about 500m before stopping.
- Although waves play the biggest role in this landform, depending on climate, temp and, geology. Weathering can also have a big effect on things like carbonation, solution, freeze-thaw, and salt crystallization. Organisms can also have an effect due to being slightly acidic and releasing carbon dioxide.
- Platforms usually slope seawards at angles of about 0 and 3 degrees, erosion occurs between high and low tide zones but is particularly constant in these areas, this explains the formation of a ramp and small cliff at the low tide level. These features occur more if the tidal range is less than 4m.
Coastal Land forms - Erosional - Headlands and Bay
- These tend to form adjacent to one another normally due to a discordant coastline with bands of differeing rock next to one another.
- As the waves come in perpemdicular to the coastline the weaker rocks are eroded quicker creating bays whereas the harder rock is eroded slower forming headlands. The width of these landforms will be determine by the width of the bands of rock. Bay depth will be determined by the differential rates of erosion between the more resistant and weaker rocks.
- Rocks lying parellel to teh coastline will produce a concordant coastline, usually harder rock on the outside will protect weaker rock further inland from erosion although small bays and headlands may form due to faults in the rock.
- Waves will be concetrated around headlands and the orthongals will converge and diffused around bays where the orthongals will be diffused.
Coastal Land forms - Erosional - Geos and Blowhole
- Geos are narrow steep-sided inlets, these are usually present in resistant rock coastlines where there may be weaknesses in the rock such as joints and faults. These points get eroded more by wave action and especially hydraulic action which forces air and water into these cracks and weakens the surrounding rock.
- Huntsmans Leap in Pembrokeshire which is 35 m deep and eroded along a large joint in the Carboniferous limestone.
- Geos can form initially as tunnel-like caves running at right angles to the cliff line and as the roof is eroded away a geo is formed and if the roof collapses along a master joint it may create a vertical shaft known as a blowhole and in storm conditions, water may b forced out and spray out of the hole as white aerated water.
Coastal Land forms - Erosional - Caves, arches, st
- Although all of these can be seen independently along the coast they are all part of a sequence of landforms that develop around headlands.
- As stated before wave refraction means energy is concentrated around headlands meaning any weak points are exploited by erosional processes and a small crack or cave may form on either side of the headland, these will usually form around the tidal zone if these caves grow larger and meet on each side an arch will be formed.
- Further erosion will weaken the support for the arch and the roof will collapse forming a stack, and through more erosion, a stump will be formed which may be only visable at low tide.
- Examples include Old Harry in Dorset and Green Stacks Pinnacle - Flambourigh Head, Yorkshire.
Coastal Land forms - Depositional - Beaches
- The most common depositional landforms beaches occur due to the deposition of sediment taken from sediment stores such as cliffs (5%), offshore beds (5%) and rivers (90%).
- Sandy beaches tend to have quite gently sloping beaches of about 5 degrees, this kind of slope means little energy is lost due to friction and little volume is lost due to percolation this results in ridges and runnels being created parallel to the water, there may be occasionally breached by channels draining water of the beach.
- Shingle and/or pebble beaches tend to have a higher gradient and be steeper mainly because the swash is stronger than the backwash and so there is a net movement of material onto the beach as well as this due to rapid percolation from larger air spaces being present little backwash occurs and material is left at the top of the beach. Another feature that can form on beaches like these ridges, storm beaches, and berms, the first two commonly caused by storm waves throwing material up to the back of the beach. Berms are smaller ridges that develop at the position of the mean high tide mark.
- Cusps can also form on beaches, they are temporary, small, semi-circular depressions formed by the collection of waves reaching the same point and when the swash and backwash have a similar strength. The sides of the cusp channel swash into the center of the depression and produces a strong backwash which drags material from the center back into the water deepening the depression further. Closer to the water line ripples may develop due to the orbital movement of water in waves.
- Due to many environmental factors, beach profiles change a lot but eventually developing an equilibrium with a balance between erosion and deposition. High energy, destructive waves remove sediment offshore and create flatter beach profile and therefore shallower water and more friction and a reduction in wave energy whereas low-energy, constructive waves do the opposite.
Coastal Land forms - Depositional - Spits
- Spits are long narrow beaches of sand or shingle that are usually attached to one end of the land and extend across a bay, estuary or indention in the coastline. They are usually formed by longshore drift occurring in one dominant direction which carriers beach material to the end of the beach and then further into the open water.
- As storms build up more sedimen