8.2 Coastal landforms of cliffed and constructive coasts

• Concordant Coastline


  Has rock strata, which when viewed from the air runs parallel to the coastline.

  • Fault is widened by marine processes, once waves got through 'Portland and Purbeck', to 'Wealden Beds', marine action widens the bay.
  • Chalk at back of bay prevents/slows further retreat.

  
• Discordant Coastline


  Has rock strata, which when viewed from the air runs perpendicular to the coastline.

  -Landforms, e.g headlands, bays, stacks, stumps.
• Influences of Concordant/Discordant Coastlines

  • Geology
  • Rock structure
  • Differential rates of erosion and denudation
• Factors Influencing Coastal Landforms

  Marine Factors-waves, winds, tides, salt spray, currentsTectonics-coastal uplift, volcanic activityGeology-structure, lithologyBiotic Factors-impact of vegetation, coral reefs etcClimatic Factors-winds, weather, climate change, glaciationGeomorphic Factors-rivers, glaciers, mass movementHuman Factors-pollution, conservation management, buildings, recreationSubaerial Factors-temperatures, weather
• Erosional Landforms-Cliffs

  • Described as a steep, high rock face usually determined by jointing bedding and rock hardness, along with marine processes such as wave action.
  • Produced by coastal erosion and wave action which undermines the land leading to slumping or rock falls.
  • Wave cut notch is created which will get larger and eventually collapse.
  • Process is repeated and results in cliff retreating
• What Determines the Nature of a Cliff

  i)Hard vs soft rocks (lithology)

  ii)Relationship between marine and sub-aerial processes

  iii)Geological structure
• Slope-Over Walls Cliffs (Bevelled Cliffs)

  1)Marine processes dominate, which causes the cliff to retreat.

  2)During and ice-age etc, sub-aerial processes dominate which causes the profile of the cliff to lessen.

  3)Marine processes return as sea levels rise at the end of an ice-age causing the formation of a slope over wall cliff.

  -E.g North Devon Coast and Lundy Island
• Cliff Morphology and Geological Structure

  i)Uniform horizontal strata produce steep cliffs

  ii)Rocks dip gently seawards with near-vertical joints (rock toppling)

  iii)Steep seaward dip (rock slabs slide)

  iv)Rocks dip inland producing a stable, steep cliff profile

  v)Rocks dip inland but with well developed joints at right angles to bedding planes

  vi)Slope over wall cliffs
• Relationship Between Subaerial and Marine Processes

  1)Marine erosion processes are more effective than subaerial processes-slow subaerial weathering rates and marine removal.2)Subaerial processes and marine processes are in balance3)Subaerial processes are more effective than marine erosion processes-slope angle declines to produce a low angled cliff
• Factors Affecting Cliff Profiles

  Hard Rock-produces vertical or very steep cliff, retreat is slow and rock falls main type of mass movement.Soft Rock-gently sloping cliff profile, retreat quick and rotational slumping main type of mass movement.Rock Dipping Towards the Sea-cliff face slopes towards the sea.Rock Dipping Away from the Sea-cliff face almost vertical but rugged and uneven, retreat is slow.Alternating Band of Soft and Porous Rock-porous upper layer absorbs water becoming heavy and unstable, lower layers form a slide plane.Raised Beaches-sea level was much higher in the past, cliff lines formed above current sea level.
• Erosional Landforms-Caves

  -Wave refraction concentrates wave energy onto a headland and waves attack it from both sides.

  -Cracks and weaknesses within the rock are exploited by corrasion and hydraulic action, which widens the crack to produce a cave.

  -Erosion enlarges the caves, following the line of weakness back into the headland.
• Erosional Landforms-Arches


  -The caves are eroded until they cut completely through the headland and meet to form an arch.

  -If the line of weakness runs across the headland from side to side, two caves can meet to form a tunnel through the headland, which is a natural arch.
• Erosional Landforms-Stacks


  -Base of the arch continues to be eroded by the waves and the roof of the arch is weakened by sub-aerial weathering.

  -Eventually the roof becomes too heavy and collapses forming a stack.

  -Stack continues to be eroded to form a stump.
• Erosional Landforms-Blowholes

  -If the line of weakness continues upwards into the body of the cliff, the processes of cavitation and wave quarrying can continue to erode the crack, forming a tube that grows upwards from the roof of the cave.

  -Waves rushing into the cave act as a piston, compressing the air, which when expands bursts out of the cracks with explosive force.

  -The tube eventually breaks through the top of the cliff, forming a blowhole during storm conditions.
• Depositional Landforms-Beaches


  -Usually made of sand/shingle, with a shingle/pebble ridge at the top of the beach, with a wider sandy section nearer the sea.

  -From between HWM and LWM (inter-tidal zone).

  -Sediment is moved along the beach by LSD, so therefore a dynamic landform.
• Beach Zones


  Backshore-consists of a storm beach made up of large sediment, coarse material pushed up to top of beach by storm waves, berms formed by spring tide.Foreshore-lies between HWM and LWM, features formed mainly by sand, from the action of swash and backwash, ridges and runnels common, dynamic area, ripple marks also found.Nearshore-also known as breaker zone, longshore bars formed here, ridges of sand run parallel to coastlines.Offshore-always under water, longshore bars found here when circular movement of water in the waves starts to touch the sea bed.
• Role of Tides in the Formation of the Beach Profile


  -Ebb and flow of tides that makes the beach a transitional zone between land and sea.

  -Width and nature of foreshore and nearshore are determined by tidal range and by amount/type of sediment.

  -Spring tides shape berms at the top of the beach.

  -High tidal range means each falling tide reveals new beach environment.
• Role of Waves in the Formation of the Beach Profile

  Constructive waves-move material up the beach, contributing to the formation of berms and increasing the gradient of the beach.

  Destructive Waves-comb material down the beach, contributing to longshore bars and reducing the gradient of the beach.

  -Local conditions, e.g presence of headlands, can also influence the type of waves and nature and position of beaches.

  -Angle at which waves approach the beach is determined by wind direction.
• Role of Sediment in the Formation of the Beach Profile

  -Larger size of particle=steeper beach.

  -Shingle more permeable than sand so backwash drains into it.

  -Shingle makes up upper part of beach-thrown up by strong waves that can't be pulled back by backwash.

  -Backwash on a sandy beach drags material down the beach producing sand ripples and leading to build up of longshore bar.

  -Sand is smoother than shingle and causes less friction with the waves but the beach is gentle so wider, therefore wave energy is dissipated over a wide area.
• Beach Cusps


  -Relatively rare features to find on a beach

  -Horseshoe shaped depressions cut into the lower part of a storm beach

  -Origin is uncertain

  -Once formed they are self-sustaining

  -Form where waves approach the beach at 90 degrees, so that swash and backwash both run straight up and down the beach

  
• Swash-Aligned Beach


  -Waves break parallel to the shore.

  -Beach is parallel to swash alignment of incoming waves.

  -Because swash and backwash run straight up and down the beach, longshore drift does not occur and the net movement of sediment is zero.

  -Beach cusps often form under these conditions.

  E.g, Lulworth Cove, Dorset

  
• Drift-Aligned Beach


  -Coast is aligned at an angle to the crests of the prevailing waves.

  -Swash moves up the beach at an angle and backwash runs straight back down the beach.

  -Longshore drift controls the movement of sediment.

  -Beach progrades to become parallel to the line of maximum drift.

  -Optimum angle for LSD is 45 degrees.

  E.g, East Norfolk Coast

  
• Zeta-Form Beach

  -Headland forms a partial block to sediment movement.

  -Beach plan is swash aligned at near (proximal) end of the bay.

  -Beach plan becomes drift aligned at the far (distal) end of the bay as sediment builds up.

  -Beach profile is reflective type.

  -Final beach form depends on prevailing wave approach and the distance between headlands.

  -Beach gradient, wave energy and grain size increase as you get closer to distal end.

  E.g, Dawlish Beach.
• Depositional Landforms-Spits

  -Occur when LSD extends the beach part of the way across an estuary/bay/inlet.

  -Sand spits produced by constructive waves.

  -Shingle spits produced by destructive waves.

  -Sand dunes often form on the top of the spit, raising its level above HWM.

  -Saltmarshes form in the slack water behind the spit, mainly because of alluvial sedimentation in the estuary.

  -Spit can't reach other side of the estuary because of river current and depth of river channel.
• Simple Spit


  -Can be straight but usually have curved ends because of change in wind direction of wave refraction around end of spit.

  E.g, Spurn Point (Yorkshire), Dawlish Warren.
• Compound Spit


  -Has a narrow base attaching it to the mainland but it widens into a broad re-curved end, consisting of a series of ridges which have grown successively over time.

  -Ridges often have sand dunes on them and illustrate the way that the coastline can build out into sea.

  -Spit may or may not shelter a lagoon between itself and original coastline.

  E.g, Orford Ness, Suffolk.
• Depositional Landforms-Tombolo


  -Occur when a beach extends outwards to join an offshore island, E.g Burgh Island.

  -Tombolo at Burgh Island formed by waves approaching from SW and refracting around Burgh Island.

  -This brings waves on the leeward side of the island into opposition with each other.

  -When the waves collide, they lose energy and deposit material to form a beach. This may be covered at high tide or exposed permanently.
• Depositional Landforms-Bar


  -Formed when a spit created by LSD, extends across a water inlet, joining to the far side.

  -Can create a lagoon containing brackish water behind.

  -Bars can be breaches in big storms.

  E.g, Looe Bar, Cornwall.
• Depositional Landforms-Offshore Bars


  -Elongated depositional landforms, comprised of coarse sand or shingle.

  -Develop on gently sloping coastlines where incoming waves start to interact with the sea bed. The forward movement is impeded by friction with the seabed, which causes the waves to slow and deposit their load.

  -Usually underwater, even at low tide, but can occasionally be exposed.

  -Most prominent bars will create a lagoon or a "sound" between the bar and the land.
• Depositional Landforms-Barrier Beaches/Islands

  -Elongated islands that are parallel to the coast

  3-160km long

  1-3km wide

  -Found on shallow coastlines (separated by inlets)

  -Separated from mainland by bays, lagoons and marshes
• Barrier Beach Formation (Submerged Beach Ridge Theory)

  -During pleistocene (glacial period) sea levels dropped (up to 85m).

  -Beach ridges (storm ridges) formed along coastlines.

  -At the end of the ice age (11,000-12,000 years ago), sea levels began to rise (known as flandrian transgression).

  -As sea levels rose they formed a lagoon separating the beach ridge from the mainland to form an offshore barrier island .

  -Over time the barrier island can migrate towards the coastline.
• Processes that Shape Barrier Islands

  Tidal Inlet Migration

  -inlets formed because tides have to go in and out of the lagoon or after a storm

  -inlets migrate because of LSD, however inlet is eroded on down drift side.
• Overwash fans


  -Occur during storms

  -Sand is deposited to the back of the island (in a tongue or fan)

  -They move islands landward

  -Transgressive barrier island is one that migrates landward.
• Depositional Landforms-Sand Dunes


  Ridges of sand that form at the back of a beach but also on spits and barrier islands.
• Importance of Sand Dunes

  -Very important coastal landform and also distinctive coastal ecosystem.

  -Help protect the land behind them from coastal erosion and flooding.

  -Easily damaged so are protected.

  -Home to many rare plants and animals.
• Conditions for Sand Dune Formation

  -Wide sandy beach that dries out at low tide.

  -Strong, prevailing, onshore wind to carry the sand up the beach.

  -Obstacle at top of beach, e.g driftwood, to cause the wind to deposit the sand.

  -Come from macro-tidal environments; bays, indented coastlines, prograding coastlines, offshore islands, barrier beaches.
• Sand Dunes-Succession

  Succession=a series of changes that take place to a plant community, from the original pioneer species through to climax vegetation (woodland).

  -A dune system is called a 'psammoseres
• Types of Sand Dunes

  Embryo Dunes-first to form, very dry, high pH, few nutrients, only hardy plants.Fore Dunes-hardy plants help more sand to build up.Mobile Dunes-as hardy plants grow and die they add organic material to the soil allowing more water to be retained, less harsh environment, marram grass develops.Grey Dunes-soil becomes richer and moister, range of herbs/shrubs start to grow.Dunes Slack-sand is exposed to the wind and it blows away lowering surface, water loving plants.Fixed Dunes-trees grow and fixed dunes dominate.
• Coastal Saltmarshes


  In sheltered river estuaries, often behind a growing spit, silt and mud is frequently deposited to form inter-tidal mudflats/saltmarshes.

  E.g, Otter River, Exe Estuary, Dawlish Warren, Lees Foot (Thurlestone)
• Conditions for Saltmarsh Development

  -Low wave energy

  -Shelter (estuaries, barriers, spits)

  -Fine sediment (silt/clay)

  -Tidal rhythms

  -Flocculation (fresh water meets seawater, causing silts/clay to coagulate and fall to sea bed).