8.2 Characteristics and formation of coastal landforms Flashcards
Factors that affect cliff profiles
Bedding and jointing
Rock type
Dip of bedding planes
Composite rock
Relative position of weaker rock
Permeability of rock
Latitude affecting climate
Influence of bedding and jointing in influencing cliff profiles
Well-developed jointing and bedding of certain harder limestones creates geometric cliff profiles with steep, angular faces and a flat top bedding plane. Wave erosion at the base opens up these lines of weakness to create wave-cut notches and whole blocks of rock fall away to create angular overhangs and cave shapes. If cliffs are formed in tectonically active areas, faulting may have occurred which creates areas of weakness to be exploited by weathering and erosion
If at a headland, refraction will focus on the sides of the headland and areas of weakness to create geos, caves, arches, stacks and stumps
Cliff rock types erosion rates
Granitic = <0.001m/yr
Limestone = 0.01-0.1m/yr
Shale = 0.01-0.1m/yr
Chalk = 0.1-1m/yr
Glacial till = 0.1-10m/yr
Volcanc deposits = 10m/yr
Influence of dip of bedding planes in affecting cliff profiles
If bedding planes dip vertically then a sheer cliff is formed.
If bedding planes dip steeply seaward, steep, shelving cliffs with landslips result.
If bedding planes dip landward, sliding is unlikely.
Cliffs where the strata and planes dip seaward present the most challenges in terms of management as they are more unstable than those that dip landwards
Influence of composite rocks in affecting cliff profiles
Many cliffs are composed of more than one rock type
The exact shape and form of the cliff depends on factors such as strength (granite – relative slow retreat whilst glacial till cliffs will be rapid) and structure of rock, relative hardness and nature of waves.
Influence of relative position of the weaker rock in affecting cliff profiles
If the weaker rock is at the base of the cliff underlying more resistant rock: undercutting and cliff collapse will occur.
If the weaker rock is at the top – subaerial processes will operate from above whilst waves will also attack the base.
If rock type is uniform – more uniform retreat with sheer cliff faces if bedding planes are vertical.
Influence of permeability of rock in affecting cliff profiles
If impermeable rock overlies permeable rock – limited percolation and cliff is more stable
If permeable overlies impermeable – water passes through into the underlying rock and slope failure is more likely where the water builds up at the junction between the two rock types and sub-aerial processes maybe more important than wave erosion.
Influence of latitude affecting climate in affecting cliff profiles
Tropics: low wave energy levels and high rates of chemical weathering as climate warm and moist so cliffs are low gradient.
High Latitudes: past climate periglacial processes have produced large amounts of cliff-base materials which have deposited at the base of cliffs which creates relatively low gradient cliff profiles
Temperate climates of the Mid Latitudes: tend to have the steepest cliffs. Rapid removal of debris by high-energy waves prevents the build-up of material on the base whilst active cliff development occurs as a result of undercutting.
Bevelled cliffs
Formed at a number of stages (pre-glacial, glacial and post glacial)
1. Cliff formed due to marine processes during warm (inter-glacial) period when sea levels were higher than they were today
2. During glacial periods (colder), sea levels dropped as water locked on land as ice – periglacial processes removed the edge of cliff creating a bevelled edge whilst solifluction carries the material to the bottom of the slope and creates a sloping cliff profile
3. Post glaciation, sea levels rose again and marine action removed the soliflucted glacial deposits from the base of the cliff, steepening the cliffs at the base whilst leaving the bevelled cliff face above and a lower angle.
Bevelled cliff diagram
Wave cut platforms
These are remnants of the previous cliff line. They form as a ledge of bedrock left behind as the cliff retreats. The platform slopes at at 4-5 degree angle down to the sea. Traditionally it was thought that these form as waves erode the base of the cliff in the inter-tidal zone – hence the name. However, there is some controversy over the importance of other agents of weathering and erosion.
Some geomorphologists argue these are relict or ancient features originally cut long ago when sea level was more constant. They think that in post-glacial times, sea level has not remained sufficiently constant to erode such platforms and that they are today simply modifying these platforms slightly.
In addition to being modified by wave action, they are also being affected by weathering processes. In high latitudes, following glaciation, the land is rising (isostatic change) so arguably marine erosion is having less effect and the platforms are now equally weathered as a result of frost action as they are eroded by marine processes. In other areas, solution weathering through salt crystallisation and slaking may also support marine erosion especially in the tidal zone. Marine organisms (notably algae) may also weather the platform – they accelerate the weathering process at low tide. At night algae releases CO2 which combines with cool sea water to create an acidic environment causing ‘rotting’. Other organisms like limpets secret organic acids that slowly dissolve the rock and molluscs and sea urchins can actually ‘bore’ holes into rock surfaces especially chalk and limestone.
Wave cut platform anomaly/case study
In the case study of Kaikoura peninsula in NZ, the wavecut platform is now more modified by subaerial processes than wave action – waves are breaking early despite being high energy as they travel over the wavecut platform and orbits become more elliptical due to friction until they break (when wave reaches height to length ratio of 1:7 and usually happens in water that is 1.3 x the height of the wave)
Wave cut platforms diagram
Describe the marine erosion processes of hydraulic action, wave quarrying and corrasion/abrasion. Explain the factors that make these processes effective in the development of cliffs (10)
(mark scheme)
Hydraulic action is wave pounding, although often confused with quarrying; most effective when storm waves break against the base of a cliff releasing great energy (up to 30 tonnes/sq.m). Wave quarrying is the compression of air within cracks in rocks by wave impact followed by the sudden decompression as the water recedes. This creates an explosive effect which can in time open up cracks. Corrasion/abrasion is where breaking waves use available sediments such as pebbles and cobbles to grind, wear away, i.e. abrade or corrade, the cliff base. The factors are the resistance of rocks such as granite or less resistance of incoherent rocks such as weakly cemented sandstones or clays. Also important will be the degree of jointing or faulting making quarrying effective.
Explain how rock type and structure influence the development of different cliff profiles (cross sections)
(mark scheme)
Three different cliff profiles could be:
* vertical profiles are characterised by horizontal rock strata where marine erosion and removal of material keep pace with sub-aerial weathering/mass movements;
* seaward dipping profiles often reflect seaward dipping rock strata or cliff decline where marine erosion has ceased and sub-aerial processes have a greater influence;
* slope over wall profiles with less resistant strata overlying resistant strata or where sea level rise has allowed marine processes to erode a more gently sloping previous profile. However, other profiles could be a blocky profile, such as in well jointed granite; stepped profiles or profiles shaped by rotational slumping might also be considered.
Explain the importance of rock type, structure and erosional history on the evolution of cliff profiles.
(mark scheme)
Cliff profiles are influenced by a range of factors. Rock types, such as limestone and granite, and structure (jointing and bedding) control the cliff profile. Well-developed jointing and bedding can create steep, angular cliff faces with flat tops. Some cliffs are formed of a mixture of rock types – the exact shape of the profile being dependant on strength and structure of rock, relative hardness and the nature of the waves. Lines of weakness are opened up by erosion and complete blocks fall away leaving overhangs and caves. The dip of the bedding planes will create different cliff profiles – bedding dipping steeply seaward forms shelving cliffs with landslips. Beds dipping vertically create a sheer cliff face. Beds dipping landward tends to result in steep, possibly jagged, profiles with rockfalls. Erosional history may involve discussion of sub-aerial processes and changes in sea level. Cliff retreat will be slower in resistant rocks such as granite, and faster in glacial till.
Type of rock affecting cliff profile - resistant rock e.g. granite
Produces a vertical or very steep cliff face. Rock falls main type of mass movement. Cliff retreat is slow and cliffs are high
Type of rock affecting cliff profile - Less resistant or unconsolidated rock eg glacial till, sand and clay
Produces more gently sloping, stepped cliff profile. Rotational slumping is main type of mass movement with mudflows developing at base of cliff. Retreat is rapid and cliffs will be lower
How does Rock structure (including bedding/jointing and dip) affect susceptibility to erosion and weathering and mass movement
Rock types, such as limestone and granite, and rock structure (jointing and bedding) control the rates of erosion and hence shape the cliff profile. Well-jointed or faulted rocks are more susceptible to erosion – especially which types of erosion?
Dip of the bedding plane
Dip is the angle of inclination of the rock strata from the horizontal. It’s a tectonic feature from when the rocks were formed. Sedimentary rocks are deposited horizontally, but can be tilted by folding and faulting by tectonic forces. It can have dramatic effects on cliff profiles.
Effect of a low angel of seaward dip (<45*)
It produces a steep profile, that may even exceed 90⁰, creating areas of overhanging rock; very vulnerable to rock falls. Frequent small-scale mass movement of material weathered from cliff face. Major cliff collapse when undercutting by marine erosion makes overhang unsustainable
Effect of a landward dipping strata
Produces steep profiles on 70-80’ as downslope gravitational force pulls loosened blocks into place. Very stable profile with small rock falls. Produces very jagged and uneven profile but retreat by erosion is slow
Effect of bedding planes dipping steeply seaward
Cliff decline is occurring where marine erosion has ceased and sub-aerial processes have a greater influence as slides of large slabs occur relatively easily
Effect of beds dipping vertically or horizontally
The creation of a sheer vertical cliff face – here marine erosion keeps pace with subaerial processes of weathering and mass-movement. Geometric, angular and flat topped cliffs here
Bays and cliffs at different types of coastlines
Coastlines, that are discordant in geology help create typical headland and bay features that also erode over time to form distinctive erosional features like wave-cut platforms and arches and stacks.
Bays are sheltered, low energy zones that form in bands of weak geology, e.g. clays. Here the cliff erodes at a faster rate. Bays are flanked by headlands which are exposed rocky outcrops positioned at 90 perpendicular to the bay. They consist of more resistant rock, e.g. limestone. Due to the way waves refract around headlands, destructive waves concentrate their energy on their sides and over time develop unique coastal features, such as caves, arches and stacks.
At a discordant coastline headlands and bays may be formed because the cliff is subject to differential rates of erosion, due to bands of varying resistant geology. Wave refraction is the process by which waves become distorted by differentiated rates of friction caused by shallower water ahead of coastal features. In deep water waves are unaffected but in shallow water waves slow down. On approaching the shoreline, wave orthogonals will bend and concentrate around the sides of headlands; waves approaching headlands slow down and build height creating destructive waves. The waves become refracted around the headland and so wave energy becomes concentrated on the sides of the headland. In the bays, the waves run parallel with the coastline and spread out as they are slower and sheltered by headlands. This reduces drift and allows bays to build up beaches
Using a labelled diagram, describe the landforms shown in Photograph A and explain how they developed (10)
With the aid of a labelled diagram, describe the main physical features of the landscape shown in Fig. 4.1 (4)
Suggest how geology has influenced the landscape shown in Fig. 4.1 (6)
What processes are responsible for creating wave cut notches, cliffs and wave-cut platforms?
Cliffs overview + stat
Cliffs occur along approximately 80% of the world’s coastline.
There is a huge variety of cliff profiles and associated landforms due to the different factors and controls acting upon them, such as sea level, history, geology, climate, waves and tides. All rocky coasts are eroding coasts, and cliff erosion is the process by which they attempt to reach equilibrium with the dynamic driving conditions of waves, wind, tides and currents.
Caves, arches and stacks formation
Cracks can sometimes form in resistant rocks which steadily erode to become a cave.
When prominent bays and headlands are created waves begin to refract around the headlands, concentrating most of the energy on the sides.
This high concentration of energy further erodes cracks and caves. It is likely that a second cave may form on the opposite side of the headland.
The water erodes the caves simultaneously mainly through hydraulic action, until they eventually meet.
The resulting iconic land form is then considered an arch.
The roof of the arch has no support and is highly susceptible to weathering including salt crystallisation and biological weathering.
As the weathering continues, the arch will eventually collapse under its own weight. This leaves a stack, a tall, lone, column of rock sticking out in the sea.
This stack is now exposed to the full force of the sea, coming under heavy erosion. The stack will eventually collapse to form a stump.
Blow holes formation
When a cave is formed and erosion continues, the roof of the cave will become weakened. As the waves crash into the cave, they can be reflected upwards further eroding the top of the cave.
At the same time, weathering of the cliff, for example through carbonation, can weaken the rock above until eventually a hole appears.
As the waves continue to crash into the cave and are reflected upwards, water rushes through the hole and creates a blowhole.
Wave cut platform diagram
Wave cut notches and wave cut platforms formation
A wave-cut notch is formed at the base of the cliff simply where the majority of the waves force is concentrated.
As the waves erode the base of the cliff, an indentation at the base of the cliff is formed. This is a wave-cut notch.
As the notch enlarges, the cliff face becomes undermined until it can no longer hold its own weight and collapses.
Processes of attrition and transportation then clear the fallen debris washing it out to sea, leaving a small bedrock ledge.
This process is repeated many times leading to the formation of a wave-cut platform. This action of falling rock to create the platform is known as cliff-retreat.
Wave-cut platforms are characterised by gently sloping angles, hard bedrock and rock pools.
Over time as the platform grows the waves have further to travel to reach the cliff, by which time they will have lost the majority of their energy. The waves will no longer be able to undercut the cliff.
Caves, stacks, stumps etc diagram
Sediment input from rivers
Rivers transport a variety of materials - usually fine-grained silts and clays, but also larger particles of sand and gravel; this is known as alluvium.
Sediment deposited by rivers at the coastline may be intermittent, mostly occurring during floods when high rates of erosion have occurred along the river bed and banks.
Responsible for 70% of sediment at the coastline but in some locations may be as high as 90% This sediment may help shape the coastline since when deposited in low energy environments, it may result in formation of salt marshes and deltas.
Rivers erode the upland areas and bring sediment to the coast, either at estuaries or deltas where the sediment gets deposited or becomes entrained in suspension to be then moved by waves or currents.
Again, the nature of the sediment will reflect the local lithology and geography; in mountain environments in Scotland and Wales, coarse sediments are brought down by steep gradient rivers with more energy whereas in lowland Britain, rivers carry mostly clay and silt Sandy beaches of Caribbean are largely river sediment that has been re- worked by waves.
Example of sediment input from glacial deposits
GLACIAL DEPOSITS – shingle beaches of southern Britain are made of shingle derived from glacial and periglacial processes that happened 10000 years ago – showing importance of climate change and se level rise since this has caused these deposits to be rolled onshore
Sediment input from cliffs
Usually generate coarse sand and shingle (small rounded pebbles) which is produced as waves and longshore currents rework these through attrition and abrasion. These are common in temperate latitudes made up of quartz sand grains. This source from eroding cliffs is very important for maintaining beaches downdrift of the eroding cliff sections
Influence of subaerial processes on cliffs at higher latitudes
Physical weathering here may well produce coarser rock fragments and gravel eg frost action + salt weathering
Input of sediment from the sea
Huge volumes of sand and clay were deposited here in the ice age and may be brought onshore by waves and tides – in post-glacial times, offshore gravel deposits have been brought onshore creating significant build up at Chesil Beach in Dorset and Blakeney Point in Norfolk. This source is now considered exhausted but offshore sandbanks still provide a source of sediment that is brought by waves onshore
Accretion
The accumulation of sediment, deposited by fluid flow processes
Input of sediment from wind
This aeolian material is typically fine sand, as wind has less energy than water and so cannot transport very large particles
Aeolian
Relating to wind action
Inputs of sediment relating to humans
In recent decades, large-scale human intervention (including beach nourishment and large-scale coastal defences) has disrupted natural systems and affected sediment supply to the coast. This is why both spatially and temporally, it is important to calculate the sediment budget for any part of the coast as the amount and transfer of sediment is critical in coastal management decisions.
Flocculation importance specifics
In some cases, sediment will flocculate (eg clay which is cohesive in structure) and so become heavier and fall out of deposition – it is especially important in the formation of estuaries, salt marshes where only a small drop in velocity will lead to the sediment falling out of suspension and getting deposited
Describe the sources of coastal sediments and explain how waves transport and deposit sediments (10)
Describe the characteristics of breaking waves and explain the processes by which waves transport sediment on beaches (10)
Describe the nature of coastal sediment cells. How do processes of sediment movement contribute to the formation of coastal landforms? (10)
Where and when does deposition occur?
Deposition is when material is added to the coast
It occurs on beaches, in estuaries, salt marshes out to sea eg off-shore bars
When movement/current stops or slows and there is a loss in energy so capacity and competency reduced and load is deposited if accretion is greater than other shaping factors that would remove it eg waves, tides, currents or wind– e.g.:Updrift of groyne, sheltered locations e.g. estuary or bay, Where coastline changes direction abruptly, Offshore bars - A ridge of sand, gravel, or mud built on the seashore of gently sloping coastlines by waves and currents, generally parallel to the shore and submerged by high tides.
Describe the changes to the sediment cell shown in Fig 1B. (4)
Candidates should interpret Fig. 1B to identify the changes that have occurred. The main changes are:
* no input from cliff erosion
* no input of river sediment
* addition of groynes * construction of sea wall
* reduced input from offshore bar
* sand dunes starting to erode.
* 1 mark for each valid point.
Explain how the changes you have identified in (a) have affected the operation of the sediment cell shown in Fig. 1A (6)
- Candidates require an understanding of the nature of a sediment cell and how it operates as a coastal system.
- The following could be identified:
- reduction in input of river sediment will make the marine erosive processes more active and may lead to erosion of the coast in a downdrift direction
- the groynes will help to build up the beach and protect the town but may lead to erosion in the sediment starved areas downdrift especially the spit
- the sea wall will protect the town but will also starve downdrift areas of sediment
- there is reduction of input from offshore bars which will also make the marine erosional processes more active and beaches may be depleted.
- Level 3 5–6
- Response applies knowledge and understanding of the operation of sediment cells and convincingly explains both the major processes at work and the effect they have on the sediment cell. Response is well-founded in detailed knowledge and strong conceptual understanding of the topic. Any examples used are appropriate and integrated effectively into the response.
With the aid of diagrams, describe and explain the formation of beach profiles (10)
Explain how the interaction of winds, different types of breaking waves and longshore drift can lead to the development of spits and dunes along a stretch of coast (10)
Fig 2 shows the operation of some marine processes along a coastal area. Describe the processes shown in fig 2 and explain how these processes contribute to the formation of coastal landforms (10)
Explain how sedimentation and salt marshes develop in tidal estuaries along a depositional coast (10)
Describe the sources of coastal sediment and explain how sediment is transported along coasts (10)
Deposition overview
· Almost 20% of the world’s coastlines are depositional in nature.
· Deposition generally occurs in low energy environments where the effects of waves, storms and tides are much reduced.
· Deposition occurs when there is insufficient energy to move sediments further along with a supply of sediment (sand and shingle) which exceeds the rate at which it is depleted.
· This may take place in sheltered areas where there are low energy waves or where coastal erosion provides an abundant sediment supply which is in excess of the depletion rate.
· A range of coastal features result from these processes – these include landforms such as beaches and spits together with sand dune and saltmarsh ecosystems – these act as ‘sediment sinks’
· Large depositional landforms are only found where the tidal range is less than 3 metres
Swash-aligned beaches
These forming when waves break parallel to the coast. Bay beaches, such as those along Dorset coast, bay bars (Loe, Bar near Porthleven, in Cornwall) and barrier beaches, like Start Bay (Devon) may have similar origins. These create large beach profiles with berms, dunes and drainage features.
Drift-aligned beaches
These form when waves arrive obliquely at the shore caused by oblique onshore prevailing winds. This causes longshore drift currents which move material along the coast producing a range of partly detached depositional features. Spits like that at Orford Ness (Suffolk) are created in this way. Beach profiles of drift-aligned coastlines are narrow, especially at the near end where sediment is eroded from.
Sand dunes
These form when dry material from flat, open beaches is blown inland (aeolian transportation). Dunes migrate and a succession of plants colonise and adapt to this environment forming a dune or psammosere ecosystem
Mudflats and salt marshes (halosere ecosystem)
These are formed of finer material which flocculates (sticks together eg clay because of its cohesive nature) in the shallow water of estuaries. Here plants adapt to salt water and tidal conditions.
Beaches formation
· A beach is a depositional landform, produced by the build-up of sediment at the shoreline. They occur in the littoral zone between high and low tide
· Usually made of sand and/or shingle
· Deposition occurs where inputs exceed outputs i.e. rate of accumulation of sand and shingle exceeds rate of its removal
· Tidal range, amount of sediment and type of waves are significant influences on nature of beach
· Beaches are a dynamic landform – e.g. longshore drift; seasonal changes eg waves vary between winter & summer – destructive v constructive –storm v swell waves, tides – larger inter-tidal range exposes new beach environments even on a daily basis, impact of spring and neap tides, even humans etc – all affect gradient of the profile. They are steeper in summer than winter as constructive waves are more common which build up the beach with their strong swash. The strong swash of a constructive wave deposits the largest material at the top of the beach. As the upper beach builds up, the backwash becomes even weaker because a greater proportion of the water drains away by percolation, rather than running down the beach. The weak swash of a destructive wave deposits material at the base of the beach. It cannot advance further up the beach because it is destroyed by the backwash from the previous breaking wave. The strong backwash of these waves may erode material from the beach and take it back down the beach to be deposited as an offshore bar which reduces the angle at the bottom of the beach.
· Form best on lowland coasts (constructive waves) with a sheltered aspect in area of less resistant rocks (good supply of material) or in areas with significant material supplied by longshore drift
Beaches in summer vs winter diagram
Effect of constructive waves on beach profile
They alter beach morphology by causing net movement of sediment up the beach, steepening the beach profile. They produce berms at the point where the swash reaches the high tide line. (A berm is a ridge of material across the beach). The strong swash carries sediment of all sizes up the beach, but weaker backwash can only transport smaller particles down the beach.
This leads to a sorting of material in the foreshore zone, with larger, heavier shingle (pebble-sized sediment) at the back of the beach, and sand drawn back closer to the sea.
Since the backwash flows down the beach and loses energy through friction and depletion of water through percolation, sediment is further sorted as coarser sands are deposited in the middle of the beach and only fine sands are carried to the area of beach closest to the sea.
Effect of destructive waves on beach profile
Weak swash and powerful backwash produces a net transport of sediment down the beach, reducing beach gradient. Some sediment thrown forwards in detached spray of high impact breaking wave. Accumulates above high tide mark as storm ridge. Large, pebble-sized sediment dragged down beach by strong backwash to form wide ridge of material below low tide mark at start of offshore zone.
Explain how differences in wave enregy affect the cross section of beaches (6)
Decadal variation affecting beach profile
Climate change is expected to produce more extreme weather events in the UK and higher sea levels. Winter profiles may be present for longer time over course of year. More frequent and more powerful destructive waves may reduce beach size, allowing high tides to reach further inland and increasing rate of coastal erosion in what was backshore zone.
Seasonal variation affecting beach profile
Destructive, high-energy waves dominate in the winter, lowering angle of beach profile and spreading shingle over the whole beach. Offshore ridges/bars formed by destructive wave erosion and subsequent deposition of sand and shingle offshore.
In summer, constructive, low-energy waves dominate, steepening beach angle and sorting particles by size, with larger shingle particles towards back of beach. In summer, constructive waves build berm ridges, typically of gravel/shingle at high tide mark
Monthly variation affecting beach profile
Tide height varies over course of lunar month, with highest high tide occurring twice a month at spring tide and two very low high tides (neap tides)
As month progresses from spring down to neap tide, successively lower high tides may produce a series of berms at lower and lower points down the beach.
Once the lowest neap tide passes and there is a shift towards the next spring tide, the berms are successively destroyed as material pushed further up beach by as the swash reaches further up the beach with rising high tides.
Daily variations affecting beach profile
Storm events during summer will produce destructive waves that reshape beach profile in a few hours.
Calm anticyclonic conditions in winter can produce constructive waves that begin to rebuild beach, steepening profile for few days before storm.
Destructive waves change to constructive ones as the wind drops.
Storm beaches, high at the back of the beach, result from high energy deposition of very coarse sediment during the most severe storms
Ridges and runnels
form parallel to the shoreline in the foreshore zone. They form on gentle gradient beaches. They are the smallest of depositional features on the beach. Ridges : areas of the foreshore that are raised above the adjacent shore which dips into a runnel. They form as simple drainage routes for incoming and outgoing tides.
Berms
raised ridges that mark highest tidal point. Common that a beach profile supports several berms, that mark different tide levels. The highest berm is called the spring tide berm and is made up of the largest and most course sediment, which merges into the storm beach at the very back of the shore
Beach cusps
smaller scale feature of the beach profile. Beach cusps are found at the shoreline. They are made up of various grades of sediment that form a semi-circular, rhythmic scalloped shaped depressions on sand or gravel beaches at the water’s edge formed by swash action
Characterised by steep gradient seaward-pointing cusp horns and between them, gentle gradient cusp embayments
Storm beach
made up of large sediment (pebbles) which is pushed to the back of the beach by storm waves – it is the most landward feature on the beach. The breaking wave has enough energy to throw pebbles up the beach but the backwash is too weak to pull it back down. Forms only in the high-energy conditions of a surge or a spring tide.
Off-shore bars
Longshore bars (offshore bars) = ridges of sand, gravel, or mud running parallel to the coastline and submerged by high tides
Sometimes called breakpoint bars if they form at point waves break
Destructive waves contribute to sediment longshore bars
Breaking waves (slowed by friction, especially where gradient is shallow) also disturb sediment on seabed and push it up to form these bars
Tend to be formed of finer material worn down by attrition. They can form more substantial features ie barrier beaches and barrier islands in places with gently sloping beaches and a low tidal range and where sediment builds up above the sea surface
With the aid of diagrams, describe the characteristics and explain the formation of offshore bars, barrier beaches and barrier islands (10)
Explain how the profile and plan form of beaches are affected by the action of constructive and destructive waves (10)
Compare the characteristics of constructive and destructive waves. Explain the effects of waves on the development of beach profiles (10)
With the aid of diagrams, describe the characteristics and explain the formation of offshore bars, spits and tombolos (10)
Describe the characteristics of breaking waves and explain the processes by which waves transport sediment on beaches (10)
Spit
Depositional landform created where the accumulation of sand and shingle exceeds its depletion; due to sheltered low energy environment or where rapid erosion further along the coast provides an abundant supply of sediment. Spits are long narrow ridges of sand and shingle (depends on availability of sediment and wave energy) which project from the coastline out to sea (distal end) and is joined to the mainland at one end (the proximal end). May have a ‘curved hook’.
1. The formation of a spit begins when the prevailing wind is at an angle to the beach and when there is a change in the direction of the coastline, where a low energy zone is found. This can also be at the mouth of the estuary. The main source of material building up a spit is from longshore drift and currents, which bring material from further down the coast and extend the beach part of the way across the bay or inlet.
2. Where there is a break in the coastline and a slight drop in energy, long shore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up, projecting outwards into the sea - this continues to grow by the process of longshore drift and the deposition of material which will build up until it is above sea level. The spit may increase in height if sand dunes develop on them. Large size shingle and pebbles are deposited first in the lee of the headland, as it grows, storm waves throw larger material above the high-water mark which stabilises the spit whilst finer sand is carried towards the end of the spit. As wind picks up sand from the beach and dries it to blow it to form sand dunes which raise the height of the spit.
3. The process of deposition continues until equilibrium is reached at the distal end (seaward end) of the spit, between deposition and erosion by waves or the existing river current.
4. The length of the spit is determined by the existence of secondary currents causing erosion, either the flow of a river or wave action.
5. Spit cannot reach other side of estuary because of river current and depth of river channel. Curved hook on sand spits (at distal end) due to secondary wind and wave direction or wave refraction around end of spit.
6. On the spit itself, sand dunes often form and salt-loving vegetation colonises. Water becomes trapped behind the spit in a low energy zone, as the water begins to stagnate, mud and marshland often begins to colonise behind the spit
Simple vs compound spit
Compound spits exhibit a number of recurved ‘spurs’ along their length as each re-curvature represents a ‘break in coast orientation’ and the development of a new extension of the main spit under conditions of consistent longshore drift
Simple spit example
Spurn Point, Yorkshire
Sand & shingle, LSD along Holderness Coast (Prevailing NE wind) – long, narrow spit has grown across the Humber Estuary
Bars/barrier beaches
On drift aligned beaches, bars forms in a similar way to spits, as longshore drift transports sediment and shingle down the beach it deposits it in low energy zones, such as bays. If a spit continues to be fed by sediment and develops in a bay into which no major river flows, it may be able to build across that bay, linking two headlands, to form a bar. Bars straighten coastlines and trap water in lagoons on the landward side. ***Formation of some bars eg 9km Slapton Sands across Start Bay in Devon is possibly due to changes in sea level. The average position of the sea-level in relation to the land has remained relatively constant for nearly 6000 years. Before that, during times of maximum glaciation large volumes of water were stored on land as ice. This modified the hydrological cycle meaning that there was a world-wide or eustatic fall in se-level of about 120m. As ice accumulated, its weight began to depress the crust underneath it. This caused a local or isostatic change in land v sea level. The worldwide most significant rise in sea- level known as the Flandrian Transgression occurred as the ice melted and drowned may parts of the UK coastline. Old beaches became submerged and have since been rolled on shore as relic features to form bars.
Tombolo
A beach that extends outwards to join with an offshore island EG Chesil Beach in Dorset links the island of Portland to the mainland. It is 30km long and up to 14m high and presents a smooth face to the prevailing SW winds. These too may have rolled onshore like bars as a result of sea-level changes worldwide before 6000 years ago. Chesil Beach is an eg of this
Barrier islands
Series of sandy islands totally detached from, but running almost parallel to the mainland. Between the islands which may extend for several hundred km and the mainland is a tidal lagoon. Rare in UK but common worldwide – they account for 13% of the world’s coastlines. Found in USA , Gulf of Mexico, Fresian Islands off coast of Netherlands, west Africa and Western Australia. Longest stretches of barrier islands in the world: 2500km from New Jersey to southern tip of Florida and 2100km along Gulf coast states to Mexico. Develop on coasts with high energy waves and low tidal range. Formation on drift-aligned coastlines: Barrier islands form as waves repeatedly deposit sediment parallel to the shoreline. As wind and waves shift according to weather patterns and local geographic features, these islands constantly move, erode, and grow. They can even disappear entirely. They are an excellent defence against storm surges as energy is absorbed. They are stabilised by sand dunes and saltmarshes so whilst deposition is crucial in their development, vegetation stabilisation through succession becomes more important over time. **Formation on coastlines which have experienced sea level change – possibly formed below low-tide mark as off-shore bars of sand that have progressively moved landwards with eustatic sea level rise or that they are as a result of sea-level change where older beach ridges may have been submerged, leaving these exposed. Morphology – create smooth, straight ocean edges characterised by wide sandy beaches which slope gently up to sand dunes which may be anchored by high marram grasses. On the leeward side, shrubs and woodland, sheltered bays and lagoons and saltmarshes. In the tropics, mangroves would dominate here. Many are breached – probably due to storm waves and tidal action.
Describe the characteristics of the spit shown in Fig 4.1. (4)
Suggest how the spit shown in Fig. 4.1 has formed
Salt marsh formation
- They form in estuaries or other sheltered areas behind spits/barrier islands = partly enclosed body of brackish water with 1 or more rivers or streams flowing into which it provides enough sediment for them to form
- Subject to fluvial and coastal influences – river discharge, waves and tides influence them but note these are low energy coasts as sheltered
- Relatively large tidal range required to allow sediment to accumulate
- Incoming freshwater flow from the river(s)is opposed by mass of seawater and tides – they form at the inter-section of these two influences
- Short period between rising and falling tides when flow comes to rest which leads to decreased turbulence (movement of water & sediment) and subsequent deposition / sedimentation .
- To start with the area may only be uncovered by the sea for less than 1hr in every 12 hour tidal cycle.
This sediment accumulates in the estuary (lower layers) to form subtidal deposits which build up over time. This is aided by flocculation – negatively charged clay particles become neutralised in the salt water so they start to coalesce together (e.g. on entry into salt water) which encourages deposition due to increased mass enabling them to settle out of suspension
- As the tidal currents are slowed they begin to deposit material. This is fine-grained at first. It may be encouraged by the growth of algae and eelgrass which can tolerate being submerged for most of the 12 hr tidal cycles and the high levels of salinity. This is fundamental in the initial mudflats and helps further sedimentation
Halosere
A series of communities displaying a successional sequence where the plants are adapted to salt water (halophytes) (succession = process of change of an ecological community over time
Vegetation role in salt marshes
- Cordgrass or Spartina Anglica in submerged areas and are found in the mudflats as pioneer species. They are perfectly adapted to survive in the hostile conditions of tidal environments and thrive on mudflats. They have:
- glands to secrete salt and minimise dehydration
- deeply recessed pores to reduce water loss
- 2 root systems - a fine mat of surface roots to bind the mud and long, thick and deep roots with airways that can secure it in up to 2 metres of sloppy mud
- traps 4,000 tonnes of silt / hectare / year
- These pioneer species also help to trap more sediment creating a surface that remains exposed for increasingly longer periods between tides. This allows colonisation by further types of plants such as sea aster, marsh grass and sea lavender. These form a dense mat of vegetation up to 15 cm high.
- The vegetation creates friction to slow the tidal currents even further. This causes yet more sediment to be deposited. Additionally, the vegetation itself traps particles. These accumulate eventually on the mud. The plants also produce leaves and stalks that die and help build up the sediment level. This vegetation waste or detritus can be up to 15 tonnes of dry matter per year. These processes combine to increase the level of the mud flats by between 1mm and 30mm per year.
- As the mud levels rise, complex creek systems evolve that channel and drain the flowing and ebbing tides. These become deeper as the land rises. These creeks are eroded rapidly both laterally and vertically which gives saltmarshes their characteristic dissected appearance.
- Eventually the zone on the landward side of the inter-tidal mudflats rises above sea level as new species such as rushes and reeds become established. These species are perennials, as are the trees such as alder and ash that appear. This zone known as the Sward Zone may only be covered by sea for less than 1hr in each tidal cycle
- Salt marsh succession is complete. By this stage, the upper levels of marsh are rarely covered by the sea. Only the highest of spring tides and storms allow them to become inundated.
Factors affecting salt marsh development
Tidal regime - changes in tidal currents can increase erosion + alter species
Wave type - changes in direction, nature and size can affect marsh stability
Climate - affects species types, growth rates and sea levels
Weather - storms can erode the marsh
River regime - changes in currents an volume can affect erosion
Sediment supply - supply changes can enhance or diminish the available silt
Human action - commercial, industrial and recreational activity can damage marsh
Sea level - reses can upset equilibrium and destroy the marshes
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Coastal mangroves
Found in SALT water (brackish) i.e. salt tolerant inter-tidal zone– halophytic ie salt tolerant
Low Energy, sheltered coastal environments, in wetlands betweeen the land and sea the in tropics
- Found globally
- Between the tropics – warm tropical conditions. (>24 degrees in warmest month; 1250mm rainfall)
- Cover 60 – 75% of tropical shores.
- 40 species in E Africa, India SE Asia Australasia
- 8 species in West Africa South America Caribbean
- No. of species decreases with distance from equator
- Found along estuaries and marine shorelines. E.g.
- Shallow coastlines – Ganges-Brahmaputra (Bay of Bengal) + Mekong
- Often in protected areas. This is because seedlings cannot be established in high energy environments.
- Usually within 30 ◦N or S of the equator but not necessarily so. (some have adapted to temperate climates)
- Most prolific in SE Asia
Characteristics and adaptations
These trees are Anaerobic – survive in submerged conditions without oxygen at high tide
Found on FLAT land i.e. shallow slopes – NOT along cliffs.
Xerophytic adaptation (drought adapted – thick cuticle, sunken stomata, thick epidermis, leaf hairs)
Covered ONLY at high tide: found in the intertidal zones (between HWM and LWM)
Varied: 70 species from two dozen families—among them palm, hibiscus, holly, plumbago, acanthus, legumes, and myrtle. They range from small shrubs to 60 m timber trees.
- Adapted to deal with the salinity: some plants have a waxy covering to their roots which stops salt intake, salt glands secrete salt from their surface on leaves. Each mangrove has an ultrafiltration system with membranes that exclude salt to keep much of the salt out and a complex root system that allows it to survive in the intertidal zone.
- Adapted to deal with less oxygen (in marine environment): roots take in air when they can through little pores, some roots grow upward to allow them to break the surface and access oxygen. Some have snorkel-like roots called pneumatophores that stick out of the mud to help them take in air; others use prop roots or buttresses to keep their trunks upright in the soft sediments at tide’s edge.
- Adapted to deal with rising and falling tide with good stability –against strong wind and wave action using shallow cable roots which spread laterally and also have feeding roots that radiate from these. Prop roots also help stabilise from trunk of trees.
- Able to deal with poor nutrient availability
Mangroves in relation to the environment
The mangrove ecosystem is a sustainable resource that provides huge numbers of people with food, tannins, fuel wood, construction materials and even medicines. When a mangrove forest is protected, it will support an entire population of coastal residents. Mangroves offer protection of property and life from hurricanes and storms, as well as reduction in erosion and siltation. Plants in mangrove forests can absorb nitrates and phosphates, cleaning up and restoring water near the shore in a natural and completely cost-free manner. Unfortunately, as with many of our natural resources, mangrove forests are quickly being lost to pollution and development. The lenticels in mangrove roots are extremely sensitive to parasite attack, clogging by crude oil and unnatural prolonged flooding. The most severe problem is the clearing of thousands of hectares of forest to create man-made shrimp ponds for the shrimp aquaculture industry. Along with the impact from the charcoal and timber industries, the mangrove forest will eventually be lost to environmental stress if these trends continue. Another contributing factor to the devastation of mangrove forests is the governmental and industrial classification of these areas as useless swampland. Areas most severely affected by the devastation are Thailand (50% loss of mangrove forests since 1960), the Philippines (338,000 hectares lost between the 1920s and 1990), and Ecuador (20% loss of its mangrove coastline). Overall, up to 50% of the world’s mangrove destruction can be attributed to the shrimp farm activity.
Mangrove vulnerability
Salt marshes are dominant along colder coastlines where mangrove forests are damaged by freeze events. Winter climate change is expected to lead to poleward (northwood) expansion of mangrove forests at the expense of salt marsh.
EG Mangroves are a common sight in south Florida, but rare in the salt marshes of the north since they cannot survive extended periods of freezing temperatures. However, a warming climate has
made freezes less common, spurring a more than 100 % increase in mangrove cover in northern Florida since 1985. Areas that were once pure salt marsh are slowly shifting into mangrove forests.
Both coastal wetland communities are threatened by human impacts, especially reduced sediment supply and anthropogenic sea level rise. One key difference between these wetland communities is the latitudinal ranges they occupy, with mangroves dominating in the tropics and salt marshes dominating coastlines in temperate zones. Where these communities overlap around the world, marshes are often displaced by mangroves, which themselves are range-limited by physical factors
Rising sea level can convert land suitable for salt marsh vegetation into habitat for mangroves if the elevation of salt marsh does not increase at a rate proportional to sea level rise .
As a result, the primary direction of encroachment is landward rather than poleward
Coastal sand dunes
Sand dunes are an accumulations of sand grains, shaped into mounds or ridges by the wind under the influence of gravity
Found wherever loose sand is windblown – e.g. back of beaches
A depositional landform which helps to protect land behind them from erosion & flooding
A dynamic system: involves succession (slow evolution of plant & animal community over time) – known as a psammosere. Equilibrium depends on the inter-relationship between sediment content (sand) and vegetation.
Coastal dunes are depositional features reliant on a supply of sediment which form in conjunction with vegetation succession (which stabilises them).
Older, more mature dunes migrate inland, but new dunes then develop on the seaward side
Conditions needed for coastal dunes to form
Conditions needed:
- Deposits of Sand (usually from sea bed transported by swash or LSD which is deposited in the inter-tidal zone OR rivers flowing into sea) – RELIABLE SUPPLY OF SAND – MUST BE ENOUGH- LINK TO SEDIMENT CELL!!!
- Large tidal range – good as low tide exposes vast area of sand and allows it to dry out so it may be picked up by onshore winds and moved up the beach by saltation
- Wide beaches (provide sand that is then reworked by the wind). (Esp at low tide – wide sandy beaches at low tide). i.e. MACRO-TIDAL environment (LARGE TIDAL RANGE). Sand dries out and is exposed at low tide
- Wind (helped especially if ONSHORE prevailing wind) – carries sand up the beach + STRONG winds
- Obstacles – sand will become trapped by seaweed and drift wood on berms at the point of the highest spring tides. Wind velocity decreases on the leeward side of the obstacle so sand is deposited here.
- Plants will then start to colonise the area, stabilizing the sand and encourage further accumulation
Mangrove distribution stats
Mangroves cover 60 – 75% of tropical shores. There are 40 species in E Africa, India SE Asia Australasia and 8 species in West Africa South America Caribbean. The number of species decreases with distance from equator. It is estimated a total of 15.2 million hectares of mangroves exist worldwide and their main distribution is in the tropical areas (found within 30 degrees latitude of the equator). About one third of the world’s mangroves are found in Asia (39%), followed by Africa (21%) and North and Central America (15%)
Mangrove adaptations
Oxygen diffuses through the spongy tissue of the pneumatophore to the rest of the plant
Pneumatophores (breathing roots) arise from the cable roots
Cable roots radiate from the trunk. Fine feeding-roots grow off these radial roots and create a stable platform
Salt glands in the surface layers of leaves secrete salt
Salt may accumulate in older leaves before they fall
Prop roots descend from the trunk to provide additional support
Specialized root membranes in some mangroves prevent salt from entering their roots (salt excluders)
Threats facing mangroves
- Coastal development
- Extinction
- Aquaculture, agriculture and salt production
- Climate change
- Oil spills
- Deforestation
Coastal development threatening mangroves
Coastal development may be the primary threat to mangroves. Not only are the forests lost when a coast is developed, but a man-made structure almost always replaces them. That structure (e.g., a hotel, desalination plant, coal-fired power plant, nuclear plant, port facility, marina, cruise ship dock) inevitably brings with it associated issues of altered hydrology, erosion, and pollution. Rivers that once travelled through the mangroves before emptying into the sea are blocked or re-routed, causing changes in filtration, sedimentation, temperature, and salinity. These changes in turn can affect the aquatic species, including commercial or subsistence fish species for coastal communities. The developments are often associated with increased levels of pollution as well, including solid waste, pesticides, thermal, biological (invasive species), brine, and oil.
Extinction threatening mangroves
There are approximately 70 species of mangroves around the world. When activities such as logging, shrimp farming, coastal agriculture, hotel development, and other activities are valued over the ecosystem services the intact mangroves provide, genetic diversity is among the first—but least considered—casualty. The trees and associated species (e.g., birds, snakes, crabs) are visibly lost, but so too are the specific genotypes and phenotypes that have evolved in microhabitats around the world to withstand insects, tidal fluctuations, precipitation patterns and salinity regimes. Mangroves are not species-rich to begin with, especially in comparison with other tropical forests. And in the areas where replanting is attempted, it is often done with seeds from elsewhere, and often done with one species, rather than the mix of species that originally existed.
Aquaculture, Agriculture and Salt Production mangroves
The close proximity of mangroves to the ocean makes them ideal locations for shrimp farming
and other kinds of mariculture. Further, they are
areas rich in nutrients, and part of larger wetland systems, making them attractive as agricultural areas. Finally, these areas near the sea are prized for salt production. As a result, hundreds of thousands of hectares of mangrove forests have been cleared, and the hydrology has been altered, in order to intensify commercial production of shrimp and other species, cultivate agricultural crops, and create salt ponds. The delicate tidal regimes are interrupted and the balance between fresh and salt water is lost. The intensive mariculture operations are most often constructed for export. Shrimp farm activity alone has been responsible for the loss of 38 percent of the world’s healthy mangroves; the percent climbs to 52 if all agricultural activities are counted.
Climate change threatening mangroves
Climate change is causing two important impacts along the world’s coastlines. Sea levels are rising and the chemistry of the oceans is shifting. The rates at which these impacts are occurring is likely to exceed the ability of mangrove forests and the species that live within them to adapt. Mangroves around the world are adapted to specific tidal regimes. If they spend increasing amounts of time inundated, at some point they will not be able to rid themselves of the ocean salt quickly enough, and will whither and die. They will also not receive the nutrients and sediment from freshwater flowing seaward that they require to survive. Finally, mangroves are among the most important carbon sinks on the planet. Losing them will cause even greater carbon releases, creating a positive feedback loop that will further exacerbate sea level rise and increased ocean acidity.
Deforestation threatening mangroves
Most destructive uses of mangrove forests require their removal. The motivations behind deforestation include direct use of the mangrove wood and leaf products, use of the wetland habitat, or complete fill and conversion for coastal developments. Deforestation for fuel & timber accounts for the ongoing loss of approximately 26% of existing mangroves. Mangrove reforestation has had very low success, although new hydrology-based methods may be more promising.
Oil spills threatening mangroves
In 2014, the Sundarbans region of southern India and Bangladesh were heavily polluted by thick tar when an oil tanker collided with another vessel. Thick tar was reported to be clogging 350 sq km of delicate mangrove forest and river delta, home to endangered Bengal tigers and rare dolphins.
Mangrove importance
A crucial component of the coastal ecosystem and a powerful form of erosion control, mangrove trees provide shelter and nutrients to their ecosystems. Like salt marshes, these shallow, nutrient rich areas provide shelter to young fish, shrimps, crabs and molluscs where they can live safely and develop. Hundreds of bird species migrate and nest in mangrove forests such as those found in Belize that provide a home to over 500 species of birds. Other animals that inhabit mangrove forests include manatees, sea turtles, fishing cats, monitor lizards and mud-skipper fish. Not only do mangrove trees directly support countless food webs, they are also indirectly responsible for the survival of the most primary planktonic and epiphytic algal food chains, which in turn provide carbon for the mangrove tree. Mangroves protect coastlines from storm damage, wave effects, and erosion. Erosion is avoided when mangroves take on the force of the waves and help replace lost sediment by catching suspended particles in their root system while simultaneously keeping that same silt from covering (and damaging) coral reefs and sea grass beds
Sand dune evolution going from front to back
Yellow Dunes
More mobile - Mobile as sand is quite mobile at that point. (as sand uncompacted and not anchored by roots and is dry/arid)
Grey Dunes
These are semi-fixed and fixed – soil bound together by nutrients and roots of plants along with more moisture
Colour of dunes changes inland – yellower close to sea; greyer/ browner further inland due to humus as vegetation decomposes and more organic matter as more vegetation
Nearer sea – harsh environment – few nutrients and high PH (sea shells – alkaline), very dry (high rates of EVT). Windy, salty. (embryo dune) halophytes = plants that are salt tolerant as often submerged at high tide
Overtime, addition of HUMUS (organic material) as plant life dies AND therefore movement away from alkaline to acidic aided by the leaching of soils with increased moisture and removal of alkaline calcium carbonate. Trees fix sand in place. lots of organic content so evolution to mature dune. Also movement away from Salty to more fresh water due to mix with rainwater
Amount of vegetation increases – traps more sand! So dunes become higher – mature dunes may reach 30m in height. Vegetation also causes wind velocity to drop (esp down low cm close to ground) => reduces energy and increases deposition.
Diversity of vegetation increase
Mobile to fixed (soil more stable)
Evolution over time and space affects their development and how they change – to start with, sediment supply is arguably the biggest factor but over time, bigger role played by vegetation, climate, soil type etc as they get stabilized and move inland and succession more important as sediment supply becomes less important in the older dunes :
1) Once a fore dune is established, older dunes lose supply of sand. Marram dies out. Wind speeds lower; more moist (less EVT); nutrients added (e.g. decaying marram) => more acidic
2) Vegetation at back is variable – depends on nature of sand e.g. shells => calcium => grassland. But if less calcium => more acidic => heather and ling and pine. Oak prefers more neutral soils
Dune slack = where sand exposed to wind and blown away (blowout) – can fill with water
Describe the characteristics from A to B shown in Fig. 4.1. [4]
Explain how embryo dunes are formed and develop into fixed dunes. [6]
Compare the global pattern of coastal dunes and coastal saltmarshes shown in Fig. 4.1.
Dune evolution diagram + facts
Dune evolution
- Embryo dunes are the first to develop. They become stabilised by the growth of lyme and marram grasses. As these grasses trap more sand, the dunes build up and due to the high rate of percolation in sand, become increasingly arid.
Plants need succulent leaves to store water (sand couch) or thorn-like leaves (prickly saltwort) to reduce transpiration in these arid conditions with strong winds or long tap roots to reach water (marram grasses). These dunes are approx. 1m high with 80% of the sand exposed and have a high pH of 8 due to mineral content (calcium carbonate from sea-shells)
- As more sand accumulates, trapped by these species, the embryo dunes join to form foredunes which can reach a height of 5m. These form at the front of the beach but these also become hind dunes if another dune starts to form in front of it. These are ‘yellow dunes’ due to a lack of humus as there is little bacteria or decaying organics matter to be added at this stage. These are slightly less alkali than the embryo dunes and are also composed of marram grass and other drought-tolerant species like sand sedge (xerophytes). 20% of the sand is exposed here.
- Grey (mature) dunes – these become darker with increased humus and their pH lowers due to the humic acid from decaying vegetation. These can reach 10-30m high before their supply of sediment is cut off as they are too far away from the beach. Heath plants begin to dominate the area as acidity, moisture and humus all start to increase. Only 10% of sand is exposed
A sand dune system may take hundreds of years to develop but the process can be seen within a few hundred metres of the shoreline.
- Wasting dunes - Interference by humans or animals can cause dunes to be dissected and as wind funnels through these ‘blowouts’ the orientation of the dunes may change from being parallel with the coastline to start with shifting to a right-angle afterwards. These dunes decrease in height to 6-8m with 40% exposed sand
Eustatic sea level change
This is a global change in sea level relative to land level.
Negative eustatic change occurs when ocean volume decreases as temperatures fall and water is locked up as ice on land in ice sheets and glaciers. As the shoreline recedes, it is called a regressive coastline.
The world’s sea-level was at its minimum 18000 years ago when the ice was at its maximum. This led to a world-wide or EUSTATIC fall in sea-level of an estimated 120m.
Positive eustatic change refers to rising sea level causing land submergence producing transgressive coasts with distinctive features.
Localised Isostatic sea level change
As ice accumulated, its weight began to depress those parts of the Earth’s crust lying beneath it. This caused a local or ISOSTATIC change in sea-level/land level.
Isostatic change occurs when the land moves relative to the sea level and is generally more localised.
During glacial periods, the weight of the ice on the land can cause isostatic depression of the Earth’s crust. As temperatures rise, vast amounts of meltwater enter the oceans and the land, relieved of the weight and pressure of ice above it, rebounds and experiences isostatic uplift. The thickness of the ice sheets during the glacial period exceeded 1km so the amount of rebound is several hundred metres.
During the last ice age, ices sheets covered Scotland and the North of England and were sufficient enough to press down on the crust so much that they pushed mantle material away. The crust continues to sink in the SE of the country by 1mm/yr whilst land in the North and West is rebounding at a greater rate than eustatic rise producing an emergent coast and landforms.
What changes in sea-level have affected (2)
- The shape of coastlines and the formation of new features by increased erosion or deposition
- The balance between erosion and deposition by rivers resulting in eg the drowning of lower sections of valleys
Submergence summary relating to sea level rise and landforms
· Eustatic rises in sea-level following decay of ice-sheets led to the drowning of many low-lying coastal areas – this period of time is known as the Flandrian Transgression.
· It led to the formation of estuaries creating distinct, irregular coastlines which are now being infilled and therefore shaped by sediment deposition and may have led to the rolling on shore of offshore bars /tombolos which have straightened coastlines.
Contemporary transgressive coastlines are now widespread due to eustatic sea-level rise so this sea level rise has had an impact across the world.
