Coastal landscapes

  • Created by: Georgie
  • Created on: 05-05-18 14:26

Formation of bays and headlands

Formed at discordant coastlines, where there are different bands of hard and soft rock

the weaker rock is eroded more rapidly to form bays and the harder rock outcrops into the ocean forming headlands

Rocks lying parallel form a concordant coastline, if the most resistant rock lies on the seaward side it protects weaker rock inland from erosion, the resultant coastline in straight and even

As waves near the coastline, it's slowed by friction in shallower water of the headland, at the same time part of the wave crest in deep water approaching the bay moves faster. The wave refracts around the headland and orthogonal coverage- erosion is concentrated at the headland 

As waves break on the side of the headland obliquely there's longshore drift of eroded material into the bay

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Cliffs and shore platforms

Destructive waves break repeatedly on relatively steep sloping coastlines, undercutting can occur between high and low tide levels where it forms a wave-cut notch

Continued undercutting weakens support for the rock strata above causing it to collapse, producing a steep profile and a cliff

The regular removal of debris at the foot of the cliff by wave action ensures that the cliff profile remains relatively steep and that the cliff retreat inland parallel to the coast

Profile varies depending on the geology 

Horizontally bedded and landward dipping rock strata support cliffs with steep, near vertical profile, if the rock strata incline seaward, the profile tends to follow the angle of the dipping strata

The sequence of undercutting, collapse, retreat causes the cliff to become higher. At its base, a gentle sloping shore-platform is cut into the solid rock

shore-platform is predominantly formed by erosion, weathering too can have an impact such as freeze-thaw and salt crystallisation

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Formation of Geos and Blowholes

  • Geos are narrow, steep-sided inlets 
  • Lines of weakness such as joints and faults are eroded more rapidly by wave action than more resistant rock around them. 
  • Hydraulic action forces air and water into the joints and weakens the rock strata 
  • Sometimes geos initially form a tunnel-like cave running at right angles to the cliff line, as they become enlarged they may suffer roof collapse, creating a geo
  • If part of the roof of a tunnel-like cave collapses along a master joint it may form  vertical shaft that reaches the cliff top, this is a blowhole
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Caves, arches, stacks and stumps

  • wave refraction means waves are concentrated on the sides of headlands, any points of weakness such as faults or joints are exploited by erosional processes and a small cave may develop on one or both sides
  • If a cave enlarges to such an extent that it extends through to the other side, possibly meeting with another cave, an arc is formed, Continued erosion widens the arch and weakens its support
  • aided by weathering the arc may collapse, leaving an isolated stack sperate from the headland
  • further erosion at the base of the stack will eventually cause further collapse leaving a small, flat portion of the original stack as a stump. This may only be visible at low tide e.g. Old Harry Rock
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Depositional landform formation- spits

Long, narrow beaches of sand and shingle that are attached to the land at one end and extend across a bay, estuary or indentation in a coastline

Formed by longshore drift occurring in one dominant direction which carries beach material to the end of the beach and beyond into the open water

As storms build up more and larger material they make the feature more permanent 

The end of the spit often becomes recurved as a result of wave refraction around the end of the spit and possibly the presence of a second wind/wave direction

Over time spits may continue to grow and a number of curves may develop

E.g. Spurn Head- Holderness coastline

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Onshore bar

  • Developed if a spit continues to grow across an indentation, such as a cove or bay, in the coastline until it joins onto the land at the other end
  • this forms a lagoon of brackish water on the landward side 
  • The lagoon may eventually be infilled as a salt marsh
  • e.g Slapton Sands
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Tombolos

Beaches connecting the mainland to an offshore island

formed by wave refraction and diffraction

As waves near an island, they are slowed by shallow water surrounding it 

These waves then bend around the island to the opposite side as they approach

Formed from spits that have continued to grow seawards until they reach and join and island

e.g. 30km shingle beach at Chesil Beach, Dorset

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Beaches

  • Accumulation of material deposited between lowest tides and highest storm waves
  • 3 main sources: Cliff (5%), Offshore (5%) and rivers (90%)
  • Sand produces beaches that are less than 5 degrees because its small particle size means it becomes compact when wet, allowing little percolation during backwash, therefore, the material is carried back down the beach rather than being left at the top, resulting in a gentle gradient and the development of ridges and runnels
  • Shingle (pebbles and small to medium size cobbles) produce steeper beaches because swash is stronger than backwash so there's a net movement of shingle onshore. Weak backwash means material accumulates at the top
  • Summer beaches (late spring, early summer)- smaller, calmer waves dominate and sand slowly returns to the beach, berms, and dunes recover as long as sediment is not lost offshore
  • Winter beaches- wave height increases eroding dunes and berms, lowering the beach profile. sand is pulled from the top of the beach and deposited in offshore sandbars. The result is a flat, concave beach shape
  • Berms- smaller ridges that develop at the position of mean high tide mark
  • Cusps- small semi-circular depressions  formed when swash and backwash have similar strengths, the sides of the cusp channel incoming smash into the center so backwash is stronger dragging material down the beach
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Salt marshes

  • Features of low energy environments
  • Vegetated areas of deposited silts and clays
  • Subject twice daily to  inundation and exposure as tides rise and fall
  • Salt-tolerant species such as eelgrass and spartina help trap sediment, gradually helping to increase the height of the marsh
  • Stems and leaves trap sediment swept in by tidal currents while roots stabilise the sediment 
  • Higher marshes have a shorter period of inundation and so are less saline
  • Low marsh on the seaward side = high salinity, turbid water (cloudy/muddy conditions from sediment held in suspension) and long periods of submergence, species diversity is poor
  • Further inland its the opposite
  • Extensive networks of small, steep-sided channels drain the marsh at low tide and provide routes for water to enter the marsh as the tide rises
  • Between the channels are shallow depressions, these trap water when the tide falls, these areas of salt water are called saltpans
  • Development depends on accumulation rate, with rates of 10cm per year common
  • Deposition of fine sediment as the river loses energy upon entering the sea
  • Flocculation- tiny clay particles carry an electrical charge and repel each other in the fresh water, in salt water they're attracted to each other and form flocs, which are larger and heavier and unable to be carried in the river flow so settle out of suspension
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Flows of energy and its impact on geomorphic proce

Mass movement

  • Rockfall- on cliffs of 40 degrees or more, especially is the cliff is bare, rocks may become detached by physical weathering. Wave processes remove material at the foot or it may form a scree slope
  • Slides- these may be linear, with movements along a straight line slip plane, such as a fault or bedding plane between layers of rock, or rotational (slumps) with movement taking place along a curved slip plane. In coastal landscape systems, slides often occur due to undercutting by wave erosion at the base of the cliff which removes support from the material above
  • Slumps are common in weak rocks, such as clay, which become heavier when wet, adding to downslope force. A layer of sand above a layer of clay may cause this as rainwater passes through the sand but cannot penetrate the impermeable clay below, thus increasing pore pressure in sand
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wave processes

    • Erosion
  • Abrasion- waves armed with rocks score the coastline
  • Attrition- rock particles, transported by waves collide with each other and the coast and become worn away. They become smoother, smaller and rounder eventually = sand
  • Hydraulic action- break against the cliff face, trapped air and water in cracks becomes compressed and as the wave recedes pressure releases, air and water expands widening the crack
  • Pounding- the mass of breaking wave exerts pressure on the rock causing it to weaken  30 tonnes per meter squared can be exerted by high energy waves
  • Solution- disolved minerals like Mg carbonate in coastal rocks 
    • Transportation
  • Solution- minerals dissolved into the mass of moving water 
  • Suspension- small particles of sand and silt are carried by currents (turbid water)
  • Saltation- irregular movement of material which is too heavy to be carried continuously in suspension
  • Traction- large particles are rolled along the seafloor by the force of the flow
    • Deposition 
  • The rate of accumulation exceeds removal
  • when slows immediately after breaking
  • top of swash- brief moment water not moving
  • during backwash when water percolates into beach material or low energy environments
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Fluvial processes

Erosion

  • Erosion in the upper catchment is the main source of a river's sediment load
  • Rivers use similar erosional processes to waves, with most channel erosion occurring during high-flow/energy events
  • Sediment is derived from mass movement and weathering 

Transportation

  • Traction
  • Suspension
  • Saltation
  • Solution

Deposition

  • As rivers enter the sea their velocity decreases
  • Tides and currents may also act in the opposite direction to river flow
  • Flocculation 
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Aeolian processes

Erosion

  • Deflation- sand and silt-sized particles are picked up by the wind
  • Abrasion- loose particles that are carried by the wind rub against coastal landscapes
  • Saltation- wind hits the ground causing turbulence, moving sand particles which hit other particles causing those to become air born
  • Creep- large particles rolled along when wind speed reaches 40km/h
  • Suspension- finer sand particles are moved by the wind high in the air 

Transportation

  • transport material using same mechanisms as water
  • once particles have been entrained they're moved at velocities as low as 20km/hr
  • saltating grains (0.15-0.25mm)
  • Creep (0.26-2mm)
  • Suspension (0.05-0.14mm)

Deposition

Wind speed falls as a result of surface friction

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Delta

  • Large areas of sediment found at the mouth of rivers
  • Deltaic sediments are deposited by rivers and tidal currents
  • Form when rivers and tidal currents deposit sediment at a faster rate than waves and tides can remove it
    • Form where:
  • Rivers entering sea are carrying large loads
  • A broad continental shelf margin exists at the river mouth to provide a platform for sediment accumulation
  • low-energy environments
  • low tidal range
    • Structure
  • Upper delta plain- furthest inland, composed of marine and river sediment 
  • Lower delta plain- inter-tidal zone, regularly submerged and composed of both river and marine deposits
  • Submerged delta plain- lies below mean low water mark, composed mainly of marine sediments and represents seaward growth of the delta
  • Deltas are crisscrossed by distributaries. Overloaded with sediment, deposition in the channel forms bars which cause the channel to split in two, each channel with reduced velocity so even more deposition occurs and thus more channels
  • Channels may be lined with levées on their banks, in times of flood these are breaches and deposition will take place in areas between levées called crevasse splays
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Types of Delta

Cuspate- pointed extension to the coastline occurs when sediment accumulates but this is shaped by regular, gentle currents from opposite directions

Arcuate- sufficient sediment supply is available from the delta to grow seawards, but wave action is strong enough to smooth and trim its leading edge

Bird's foot- distributaries build out from the coast in a branching pattern, with river sediment supply exceeding rates of removal by waves and currents

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Physical factors influencing coastal landscapes

Wind/Waves

  • Wave action is generated by the frictional drag of winds moving across the ocean surface 
  • The higher the wind speed and the longer the fetch (the distance of open water in one direction from a coastline, over which the wind can blow), the larger the wave and the more energy it possesses 
  • Onshore winds, blowing from the sea towards the land are effective at driving waves towards the coast
  • Oblique angle = wave approaching obliquely and generated longshore drift
  • The wind carries out erosion, transportation, and deposition (aeolian processes)

more on waves on next slide

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physical factors influencing coastal landscapes

waves

  • A wave possesses potential energy as a result of its position above the water trough, and kinetic energy caused by the motion of water within the wave
  • Waves impart a circular motion to individual water molecules
  • The relationship between wave height and wave energy is non-linear
  • Height = distance between crest and trough
  • Length = distance between two adjacent crests or troughs
  • wave behavior influenced by shape and gradient of seafloor and irregularity of coastline
  • Swell waves- long wavelength with a wave period of up to 20 seconds
  • Storm waves have a short wavelength, greater height, and shorter wave period
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Waves cont.

Breaking waves

  • Shallow water (depth of half the wavelength) causes water molecules to come in contact with the seafloor creating friction which changes the speed, direction, and shape of waves.
  • The wavelength decreases and successive waves start to bunch up.
  • The deepest part slows down more than the top. 
  • The wave begins to steepen as the crest advances ahead of the base.
  • When the water depth is less than 1.3 x wave height the wave breaks against the shore
    • Types
  • Spilling- steep waves break onto gentle sloping beaches; water spills gently forward
  • Plunging- moderately steep waves break onto the steep beach; water plunges vertically downwards as the crust curls over
  • Surging- low-angle wave break onto the steep beach; the wave slides forward and may not actually break
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Constructive and Destructive waves

Constructive

  • Low in height and frequency (6-8 per min)
  • Long wavelength
  • Spilling wave (steep waves on gently sloping beaches)
  • Strong swash travels a long way up the beach
  • Due to the long wavelength, backwash returns to the sea before the next wave breaks and therefore retains its energy and swash isn't interrupted (swash energy exceeds backwash

Destructive waves

  • Greater height and frequency (12-14 per min
  • Shorter wavelength
  • Plunging waves (moderate steep waves onto steep beaches)
  • Friction from the steep beach slows the swash and so it does not travel far before returning back down the beach, the swash of the next wave meets backwash or previous because of short wavelength (swash energy is less than backwash)

high-energy waves- remove material from the top of beach to offshore zones, reducing gradient whereas long-energy waves build up the beach, steepening profile

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Tides

  • The periodic rise and fall of sea surface and are produced by the gravitational pull of the moon and to the lesser extent the sun
  • Moon pulls the water towards it, creating a high tide, and there's a compensatory bulge on the opposite side of the earth
  • Between two bulges there's low tide
  • As the moon orbits the Earth, high tides follow it 

When the Moon, sun, and earth are aligned and gravitational pull is at its strongest (twice each lunar month) tides are at their heights = spring tides

When the Moon and sun are at right angles to each other and the gravitational pull is at its weakest, tides are low = neap tides 

Tidal range- in enclosed seas such as the Med, tidal ranges are low and so wave action is restricted to a narrow area of land, where the coast is funneled, such as Severn estuary, tidal range can be as high as 14m

Tidal range influences where wave action occurs, weathering processes that happen on the land exposed between tides and the potential scoring effect of waves along coasts with a high tidal range

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Geology

Lithology- physical and chemical composition of rocks

Some rocks have weak lithology e.g. clay with little resistance to erosion, weathering and mass movement, this is because they have weak bonds between the particles that make up the rock

Others such as Basalt are made of dense interlocking crystals and are highly resistant to erosion, they're more likely to form prominent features such as cliffs and headlands

Some such as chalk and carboniferous limestone are soluble in weak acids and thus vulnerable to chemical weathering

Structure- properties such as jointing, bedding, and faulting as well as permeability

In porous rock, such as chalk, pores sperate the mineral particles, these pores can absorb and store water (primary permeability), Limestone is also permeable but water seeps into it because of its many joints this is known as secondary permeability

Concordant and discordant coastlines

Horizontally bedded and landward-dipping strata support cliffs with steep, verticle profiles

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Currents

Nearshore and offshore

Rip currents- caused by tidal motion or by waves breaking at right angles to the shore

Cellular circulation is generated by differing wave heights parallel to the shore

Water from the top of breaking waves with a large height travels further up the shore then returns through the adjacent area where the lower height waves have broken. 

Once rip currents from, they modify the shore profile by creating cusps which help perpetuate the rip current, channeling flow through a narrow neck

Ocean currents are on a much larger scale, generated by the Earth's rotation and by convection, and are set in motion by the movement of winds across the water surface

Warm ocean currents transfer heat energy from low latitudes towards the poles- driven by onshore winds

Cold ocean currents do the opposite- driven by offshore winds

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Supply of sediment

Terrestrial

  • Rivers, particularly true of coasts with a steep gradient, where rivers directly deposit their sediments at the coast. Sediment delivery is intermittent, mostly occurring during floods. In some areas as much as 80% of sediment comes from rivers
  • Origin of sediment is the erosion of inland areas by wind, water and ice as well as weathering and mass movement
  • Wave erosion
  • Cliff erosion by rising sea level (70% in high energy areas)
  • Longshore drift

Offshore

  • Constructive waves bring sediment from offshore locations
  • Tides and currents
  • Wind blows sediment from other locations, including exposed sandbars, dunes + beach

Human

  • Beach nourishment- sediment brought in by lorry, dumped and spread by bulldozers or sand and water is pumped onshore from offshore sources. Low bunds hold the mixture in place while the water drains away and leaves the sediment behind
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How coastal landscapes can be viewed as systems

System- set of interrelated objects comprising stores and links that are connected together to form a working unit or unified whole

Energy available maybe kinetic, potential or thermal

Coastal landscapes are open systems (energy and matter can be transferred from neighboring systems as outputs or inputs e.g. input of fluvial sediment from a river

Inputs- wind, waves, thermal, potential, material from weathering and mass movement of cliffs

Outputs- marine and wind erosion from beaches and rock surfaces; evaporation

Throughputs-  stores including beach and nearshore sediment accumulation and flows, such as longshore drift

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System feedback

Equilibrium is achieved inputs = outputs 

Dynamic equilibrium occurs when the system undergoes self-regulation and changes its form in order to restore equilibrium (an example of negative feedback)

Positive feedback

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Sediment cell

Stretch of coastline and it's associated nearshore area within which movement of coarse sediment, sand and shingle is largely self-contained 

Closed system 

11 large sediment cells in England and Wales 

Boundaries of sediment cells are determined by the topography and shape of the coastline

Large natural features act as barriers to the movement of the majority of sediment

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Submergent landforms intro

Influenced by rising sea level

1-degree Celsius increase = 2m sea level increase

End of the Würm glacial period, 25,000 years ago, temp was 9 degrees Celsius lower than today and sea level was 90m lower 

since then sea level and temperatures have risen, this is called the Flandrian Transgression 

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Rias

Submerged river valleys

  • Lower part may be completely submerged 
  • Exposed river valley sides are gently sloping 
  • The plan view is winding, reflecting the original route of the river and its valley

Formation- sea level rises (eustatic or isostatic), low lying coastal environments become submerged and river valleys are drowned to form rias 

They have gently sloping sides, variable depth, and a winding plan form reflecting the original route of the river and its valley, formed by fluvial erosion (hydraulic action and abrasion) within the channel and sub-aerial processes (weathering and mass-movement) on the valley side

Rejuvenation in river valleys as sea level fell during earlier, colder period may have resulted in increased valley deepening before submergence occurred 

Interglacial periods- sea level rose, further deposition would have occurred as the river had less surplus of energy for erosion 

Increased water depth is likely to be associated with larger waves and greater energy, thereby increasing erosion rates and modification

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Delta formation

Large areas of sediment found at the mouth of a river.  Rivers entering the sea carry large sediment loads, a broad continental shelf margin exists at the river mouth to provide a platform for sediment accumulation. It's a Low energy environment with a low tidal range

    • Three distinctive components:
  • Upper delta plain- furthest inland, beyond the reach of tides, composed of river sediment
  • Lower delta plain- inter-tidal zone, regularly submerged and composed of both river and marine deposits
  • Submerged delta plain- lies below mean watermark, composed of marine sediment and represents seaward growth of the delta
  • Distributaries
  • Overloaded with sediment, deposition in the channel forms bars which cause the channel to split in two, so decrease in energy, more deposition and more splitting of channels
  • Breach of levées causes deposition in low lying areas between levées, called crevasse splays
    • Types
  • Cuspate: pointed extension to the coastline occurs when sediment accumulates, this is shaped by currents from opposite direction
  • Arcuate: sufficient sediment is available for sediment to grow seaward, wave action smooths and trims leading end
  • Bird's foot: distributaries build out from the coast, river sediment supply exceeds removal by waves and currents
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