- 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 storm waves build up more sediment the landform becomes more substantial and permanent eventually the end of the spit will become recurved as a result of wave refraction around the end of the spit and possibly due to a secondary wind/wave direction. As time goes by the spit may grow further hooks along its length.
- If a spit grows across an estuary the length of the spit may be limited by the actions of the river current. In the sheltered area behind a spit deposition, is likely to occur due to it being a low energy zone, silt and mud will slowly build up and eventually salt tolerant plants may colonize and may become a salt marsh.
Coastal Land forms - Depositional - Onshore bars
- Bars form just the same way as spits, the only difference being that the landform grows across some kind of coastal indentation such as a bay or cove and then connects to the other end of the indentation. This forms a lagoon of brackish water behind the bar which may fill in or form a salt marsh. The bar may also be breached if there is a reduction in the LSD or if the sea levels rise or in a storm.
Coastal Land forms - Depositional - Tombolos
- Tombolos again form very similarly to spits the only difference being that a tombolo is a spit that connects to another piece of independent land such as an island
Coastal Land forms - Depositional - Salt Marshes
- Salt marshes usually occur in low energy zones like estuaries and on a landward side of spits.
- Salt marshes are vegetated areas of deposited silts and clays, They are subject to flooding twice a day as tides rise and fall. The marshes are populated with salt-tolerant plants such as Eelgrass and Spartina which helps trap sediment and gradually builds up the height of the height of the marsh by trapping with their leaves and stabilising the landform with its roots.
- The higher the marsh the shorter the periods of inundation and the less saline the environment becomes.
- The low marsh nearer the sea is characterised by high salinity, turbid water and long periods of submergence. Due to these environments, plant diversity is quite poor.
- The further inland the marsh goes the conditions improve meaning more diverse plant life can survive such as reeds and rushes.
- Salt marshes have a shallow gradient which slopes seaward, many have a low cliff which separates the salt marsh from the unvegetated mudflats.
- Even though the higher parts of the marsh are inundated less but the rate of deposition is still quite high as at the high water mark, low energy, slack water may be present for several hours. At the higher point of the marsh, more permanent larger plants populate this area with extensive root networks. As well as this lots of creeks and steep-sided channels help drain the higher areas much quicker than the lower areas.
- The development of these marshes also depends on the rate of accumulation with 10cm a year being quite common. Deposition of small material tends to occur when the river mouth meets the sea and velocity drops. Flocculation also occurs when the electrical charge of clay particles changes and they start to stick together to form larger particles
Coastal Land forms - Depositional - Deltas
- Deltas are large amounts of sediment deposited at the mouth of many rivers.
- The sediment comes from rivers and tidal currents and forms when waves cannot remove sediment as quickly as it is being deposited.
- Deltas form typically:
- where rivers entering the sea are carrying large sediment loads
- A broad continental shelf exists at the river mouth to provide a platform for sediment accumulation.
- a low-energy zone exists in a coastal area
- tidal ranges are low
- There are three distinct components in the structure of a delta:
- Upper plain delta - furthest inland, composed only of river sediment
- Lower plain delta - in the inter-tidal zone composed of both kinds of sediment and regularly submerged
- Submerged plain delta - below the water line is mainly coastal deposits
- Deltas are covered with distributes ciss-crossing the whole delta, these occur due to bars forming and splitting the channels, this reduces the velocity and energy of the water and causes more deposition to occur.
- Although these channels may have levees on their banks and in times of flood the natural embankments are breached and deposition of lobes of sediment will then take place in the low-lying areas between the levees called crevasse splays.
- They are three common kinds of delta:
- Cuspate: a pointed extension to the coastline caused by regular gentle currents from opposite directions.
- Arcuate: the delta has enough sediment to grow but the currents are strong enough to trim the leading edge.
- Birdsfoot: branching distributes with riiver rates ighyl excedding the rates of removall
How do landforms evolve as the climate changes
- Global changes in the volumes of water are known as eustatic changes. These changes are influenced by variations in mean global temperatures affecting both the amount of water in the ocean store and its density.
- There are a number of physical factors that affect changes in global temps and the volume of water in the oceans. These include:
- Variations around the earth solar orbit, usually every 400,000 or so years.
- Variations in the amount of solar energy produced with a maximum about every 11 years
- Changes in the atmosphere composition due to things like major volcanic eruptions which reduces incident solar radiation.
- Variations in the tilt of the earth's axis occurring every 41,000 years
Climate Change and Sea Level Fall
- Decreases in global temperatures result in more precipitation being in the form of snow and eventually turning into ice and so more water is stored on the land in solid form rather than becoming a liquid and returning to the ocean store leading to a worldwide drop in sea levels.
- As global
- As the temperature falls, water molecules contract leading to increased density and a reduced volume.
- As far as numbers go it is estimated that a 1-degree fall in global temperatures causes sea levels to fall around 2m.
- Approximately 130,000 years during the Tyrrhenian inter-glacial period global temps were 3 degrees higher and the sea levels where 20m higher whereas during the Riss glacial period 108,000 reaching 7 degrees lower than they are today and sea levels dropped by 100m making them 83m lower than today.
Emergent Landforms - Raised beaches, marine terrac
- Raised beaches are areas of former shore platforms that are left at a higher point than present sea levels, they are usually found a distance inland from the present coastline.
- Behind the beach along the coastline abandoned cliffs, wave cut notches, as well as caves arches and stacks are usually found.
- Marine terraces are much larger landforms compare to raised beaches which tend to be localized to the base of cliffs, terraces, however, do not necessarily have cliffs above them but their formation is essentially the same as raised beaches - marine erosion during a previous period of higher sea levels.
- In Portland on the southern tip in Weymouth Dorset, there is a distinct raised beach about 15m above present day sea level. This is thought to have formed around 125,000 years ago in the Tyrrhenian inter-glacial period when sea levels were much higher than they are presently.
- The Portland limestone here was eroded by hydraulic action partly through the exploitation of the bedding plane weakness. Erosion rates at that time are estimated to have been as much as 1m/year. Other raised beaches at Portland are thought to date to about 210,000 years ago.
Modification of Landforms
- After emergence, these landforms tend to not be affected by wave processes but are by weathering and mass movement.
- For example on top of the abandoned cliffs in Portland is a 1-1.5m layer of frost-shattered limestone debris deposited when the area experienced periglacial conditions during the last glacial period. During the same time, the cliff face itself was gradually being weathered away leading to rockfall.
- Evidence of other processes like cryoturbation is also evident from contortions in fragmented limestone. These occur through freezing and thawing of the permafrost in the subsoil during the late Pleistocene period the final glacial phase.
- In the post-glacial period more warmer and wetter conditions have led to the development of more vegetation cover on many such exposures, often making them more difficult to recognize.
- With more warming predicted for the future climate, more degradation is likely to occur with chemical weathering perhaps becoming more influential especially by carbonation of limestone cliffs and platforms.
- Biological weathering on the raised beach may also become more significant with the appearance and colonization of mollusks such as limpets and whelks which exuded corrosive material slowly making depressions in the rocks.
- If temperatures increase sufficiently, the associated sea level rise could lead to these emergent landforms being found again much closer to or even ar, the coastline. They would then be subjected to wave processes once more.
Climate Change and Sea Level Rise
- Increase in global temperatures usually results in the thawing of ice stored on lands like ice caps, ice sheets, and valley glaciers, therefore, leading to a global increase in the volume of water in the ocean store and therefore sea levels rise.
- As well as this as temperatures rise water molecules also expand also leading to higher sea levels, a 1-degree increase in global temperatures will result in 2m sea level rise.
- At the end of the Wurm glacial period about 25,000 years ago, global temperatures were about 9-degrees lower than today and the sea level was about 90m below the present as well. Since then temperatures and sea levels have risen to the rate they are at now this rise in sea levels is known as the Flandrian Transgression.
Submergent Landforms - Rias
- Rias are submerged river valleys formed due to sea level rise that floods the river's course and the floodplains. Usually, the higher land that forms the tops of the valley sides, as well as the middle and upper part of the river's course, remain exposed.
- In cross sections, rias have relatively shallow sides becoming increasingly deep towards the centre this is because exposed valley sides are quite gently sloping while in the long section they exhibit a smooth profile and water of uniform depth while in plan view they tend to be winding, reflecting the original course of the river and its valley, formed by fluvial erosion within the channels and sub ariel processes on the valley sides.
- A number of the rias present in the UK are situated on the south coasts of Devon and Cornwall, including those at Salcombe, Kingsbridge, and Fowey. These particular rias were formed in during the post-glacial sea level rise known as the Flanderian Transgression.
Submergent Landforms - Fjords
- Fjords are similar to rias but instead of river valleys, fjords are submerged glacial valleys. They have deep nearly cliff-like valley sides and the water tends to be uniform deep often reaching 100m deep.
- The Sogne Fjord in Norway is nearly 200 km long although those in Scotland are less well developed as the glacial ice was not as thick during the glacial period.
- The U shaped cross section is due to the original shape of the glacial valley itself, the valley consists of a glacial rock basin with a shallower section at the end known as the threshold. This results from lower rates of erosion at the seaward end of the valley where the ice thinned in warmer conditions. The valleys also tend to have much straighter platforms than rias sd the glacier would have truncated any interlocking spurs present.
- Due to the depth of water that was present in the fjords during the Flandrian Transgression, marine erosion rates are still high and in some cases, the fjords were further deepened.
Submergent Landforms - Shingle Beaches
- When sea levels fall due to a higher volume of land ice forming large areas of new land emerges out of the sea.
- These new areas accumulate sediment through deposition from rivers, meltwater streams, and low-energy waves. As sea levels rose at the end of the last glacial period, wave action pushed this sediment onshore.
- In some places, they beached at the base of the former cliff lines; whereas elsewhere they may form tombolos and bars.
- For example, the tombolo at Chesil Beach is thought to have formed in this way during the last Flandrian Transgression. Sediment was carried into the English Channel by meltwater during the Wurm glacial and accumulated in locations such as Lyme Bay. As sea levels rose, the sediment was carried northeast by prevailing winds and the resultant waves. It moved around 50 km until it became attached to the Isle of Portland at one end and at the other end at the mainland Abbotsbury. The beach now contains around 100 million tonnes of shingle varying in size from 1-2cm pea sized material to 5-7cm pebbles. Originally it was thought this tombolo was formed by the extension of a spit to the Isle of Purbeck but the lack of recurves and the complex grading of pebbles suggest this was not formed by LSD alone.
- After rias and fjords have formed they may still be modified by the wave processes acting on their sides at the present-day sea level.
- The valley sides may also be affected by the operation of sub-aerial processes in the present climatic conditions or in future climatic conditions. This may lead to a reduction in the steepness of the valley sides of fjords.
- As sea levels are predicted to rise by another 0.5m in the next 100 years, this will lead to increased water depth in rias and fjords. Marine erosion will also increase due to stormier conditions, larger waves, and increased water depth.
- Shingle beaches are especially vulnerable to modification due to the fact they are made from unconsolidated material. For example, the tombolo at Chesil beach has been significantly affected by present LSD and is likely to continue and even increase in the future.
- With the sea level rising it is likely shingle will move even more northeast, currently, the rate of movement is around 17 cm/year, and a breach of the tombolo is highly likly in future storm conditions, more recent storms have seen waves over-topping the beach, in 2009 1 metre sized pieces of clay were washed up on to the beach during a winter storm, this will be more likely with more storm events occurring.
Human Impact on Coastal Landscape Systems
- Human Impact in any coastal landscape system will either purposefully or accidentally cause a change in the coastal system.
- Such impact causes changes in the transfers of energy and sediment in the system which will, in turn, affect the overall coastal landscape.
- Humans may deliberately change the coastal system through hard and soft engineering methods to protect certain areas.
- Coastal environments provide a large potential area for economic and human activity development.
- Coats provide an attractive area for tourism as well as industries like fishing, trade, and transport. Issues can arise, however, when taking advantage of these opportunities can unintentionally cause change within the coastal landscape system.
- Things like: changes in the sediment budget may occur and therefore rates of natural processes may be altered. These unintentional changes may need to be managed through certain hard or soft techniques.
Case Study: Coast Management: Location
- The Sandbanks peninsula separates Poole Harbour from Poole Bay and is heavily managed mainly by the Poole Harbour Commissioners, Poole Council and the Environmental Agency, and the strategies employed form part of the Two Bays Shoreline Management Plan, based on the sediment cell covering Poole Bay and Christchurch Bay.
Case Study: Coast Management: Need for Management
- The Sandbanks peninsula has a large number of high-value commercial properties like the Sandbanks and Haven Hotel which provide significant employment and tourism opportunities which are an important part of the area's economy.
- Residential properties in this area are in high demand meaning house prices are incredibly high (4th most expensive per square meter). Large detached houses can get prices in excess of £10m.
- The beach is a major tourist attraction with a blue flag for water quality and gently sloping beaches good for swimming.
- It also provides protection for Poole Harbour, therefore it is a popular and safe place for water sports like windsurfing, sailing, and water skiing. It is also home to numerous yacht clubs and marinas.
- Near the end of the peninsula is the entrance to the harbour used by cross channel ferries, catamarans and commercial ships carrying goods like timber. Issues like LSD could clog up the entrance and make it shallow.
- Climate change means it is predicted to rise by about 0.6m in the next 100 years which is likely to flood some of the peninsula as well as cut it off from the mainland. It is predicted that if no management strategies are applied £18m of damage will occur to residential properties in the next 20 years.
Case Study: Coast Management: Strategies and their
- Rock groynes have been constructed to maintain a wide and deep beach as well as to minimize the movement of sediment through LSD along the beach at Sandbanks. This restricts the movement of sediment into the harbour entrance, therefore, keeping access free for shipping as well as absorbing wave energy and reducing rates of erosion. It is estimated that without this strategy erosion rates would be about 1.6m per year.
- Beach recharge is also being used to conserve the beaches using sand dredged from offshore stores through rainbowing. This costs about £20 per meter cubed, but a recent trial of dumping sediment dredged from the habour just of offshore is much cheaper only costing £3 per meter cubed. Natural currents will eventually transport this sand onshore where it will build up beaches. In total over 3.5 million meters of sediment has been added to Poole Bay beaches.
Case Study: Sand mining in New Zealand: Economic D
- Sand is a vital mineral resource in a modern economy used in construction, concrete making, glass manufacture, and beach replenishment.
- A high-quality sand resource is present in the nearshore zone at Mangawhai-Pakiri on the east coast of New Zealand Northland Penisula.
- The sand present here is high quality and suitable for the construction industry. The store is located 50km north of Auckland and it is convenient for NZ largest and economically most dynamic metropolitan region.
- With a population of over 1.5 million, the Auckland region accounts for the third of NZ total population and 35% of the country's GDP. The region is growing rapidly. Apart from business, finance, and tech industries, tourism centered on Auckland's outstanding coastal amenities is booming with 2.3 million foreign visitors in 2015.
Case Study: Sand mining in New Zealand: Sand Minin
- Nearshore sand dredging on the 20km coastline between Mangawhai and Pakari has operated for 70 years between 1994 and 2004, 165,000 meters cubed per year were extracted.
- Mining ended at Mangawhai in 2005 but continued at Pakirir Beach. Current rates of extraction are 75,000 meters cubed per year until 2020. A large amount of this sand is used for replenishing Auckland's tourist beaches.
- The sand currently present was deposited during the Holocene (past 9000 years). Sand is a non-renewable resource along this coastline as there are few rivers depositing material it is thought to have come from offshore and therefore the sediment budget is essentially a closed system.
- This means any sediment taken out from nearshore mining are not replaced by any inputs and extraction rates at Pakari Beach exceed inputs by a factor of five. The effect of mining is, therefore, depleting the overall sand supply stored in dunes, beaches, and sea-bed as a result of this, movement of sand between the major stores have diminished.
Case Study: Sand mining in New Zealand: Impact on
- Due to the coastal system being a closed one, current rates of extraction at Pakiri Beach are clearly unsustainable. The depletion of sand is having a massive impact on landforms as well as landscapes. Beaches starved of sediment have become wider and flatter and less effective in absorbing the energy of waves increasing rates of erosion making dunes and spits vulnerable. Foredune ridges are undercut by wave action, developing steep, seaward-facing scarps.
- Loss of vegetation covering the dune makes them more susceptible to aeolian erosion. In 1978, storms caused a 28-meter beach at the base of the Mangawhai spit. This added to a second breach massively altered the tidal currents which has led to the sedimentation of Mangawhai harbor this causing shallower in the harbor has threatened the waterfront communities of the area. They decided to dredge the harbour and construct groynes on the spit to help restore some equilibrium.
- Studies along the coast by the Auckland Regional Council suggest rates of coastal erosion are likely to increase in the future with declining natural protection from storm events.Coastal retreat is already evident pegged mainly down to sand extraction however, there are issues with the data due to climate change and rising sea levels. Long term retreat is estimated at 35m by the end of the century and the width of the zone susceptible to erosion varies from 48 to 111m a significantly higher value than any of the other regions beaches.