Coastal Landforms Notes (PDF)

8.2 CE Notes 8.2 - Coastal Landforms 8.2A - Erosional Landforms 8.2 CE Notes 1  Development of Cliffs and Wave-Cut Platforms During high tides, wave energy is sufficient to erode base of cliff, forming a wav...

8.2 CE Notes 8.2 - Coastal Landforms 8.2A - Erosional Landforms 8.2 CE Notes 1  Development of Cliffs and Wave-Cut Platforms During high tides, wave energy is sufficient to erode base of cliff, forming a wave-cut notch Weight of the cliff is unsupported this causes overhang to collapse and cliff to retreat Cliff increases in height Occurs on cliffs made of hard rock (eg. chalk) —> forms vertical cliff faces Wave cut platform is the flat part of land extending from cliff —> sea Cliff becomes ‘dead’ as wave-cut platform extends to the point where water no longer reaches it A singular cliff experiences the above process, different from entire discordant coastlines, where alternating resistant and less resistant rock produce headlands and bays 8.2 CE Notes 2  Variations in Cliff Profile Formation Cliff profile = shape and gradient of cliff, taken from a cross-section/side view 1. Rock Type and Structure More resistant vs less resistant Cliffs made out of more resistant rock have steep vertical cliffs (less erosion) Eg. White Cliffs of Dover are made of chalk (0.1-1m/yr) Cliffs made out of softer rock have gentle sloping cliffs (more erosion) Eg. Holderness coastline is gently sloping as made up from boulder clay (1.8m/yr) Composite cliffs Composite cliffs, which contain more than one constituent rock type, experience differential erosion Clear example of rock type being more important than marine erosion (as single rock cliffs are mostly eroded at a uniform rate) Eg. if softer rock underlies harder rock, then waterfall-like undercutting will occur. Leading to overhang collapsing (draw diagram) Faults Faults are major fractures in rock created by tectonic forces, with displacement of rock. More faults = more erosion, as more cracks that weakens rock for weathering and erosion Eg. Bantry Bay, Ireland contains a major fault that runs SW-NE down the centre of the bay, so it experiences erosion due to the faults formed from 8.2 CE Notes 3  this Dips Dips in the rock refer to the angle of inclination of bedding planes, which massively affects cliff profiles. Landward facing bedding planes produce stable cliffs that minimise marine erosion as the strata are ‘pushing into land’ - example of rock structure overriding marine erosion 2. Marine Processes Hydraulic action is the sheer force of waves entering cracks in rocks, exerting pressure and causing cliff material to be chipped away Cavitation may also be at play, as water trapped in cracks from hydraulic action are compressed by the pounding of waves Eg. Holderness coastline experiences 2m/yr of erosion, due to long fetch of high energy waves that travelled across North Sea 3. Sub-Aerial Processes During low tide, more of the cliff face becomes exposed, making it more prone to weathering processes Eg. freeze thaw in high latitudes weakens rock, making the cliff more susceptible to mass movement Carbonation weathering can occur from rainfall combining with CO2 in the atmosphere, and the resulting carbonic acid can attack rock = more likely 8.2 CE Notes 4  mass movement Cliffs that are on a tectonic plate boundary may also experience frequent mass movements In Kaikoura Peninsula, NZ, there are high energy waves (greater than 1.5m at times), but they break before they hit the coastal platform. Instead, wetting and drying weathering is the main process affecting the coastal platform. Landforms Created at Cliffs Crack —> cave —> arch —> stack —> stump process along a headland on a cliff. When drawing own diagram in exam 8.2 CE Notes 5  STEPS: 1. Cracks (areas of weakness) at the base of the headland within the intertidal zone are attacked by hydraulic action and abrasion, widening the crack. Cracks may be due to faults, jointing, bedding planes. 2. Cracks are further widened by sub-aerial weathering processes (eg. salt crystallisation, wetting and drying) 3. Over time cracks widen to form wave-cut notches. Further erosion and weathering will deepen notches to form caves 4. Cave gets attacked by wave energy from 3 sides due to wave refraction which results in wave energy being concentrated on headlands 5. This deepens the cave, and once 2 caves are aligned, an arch is formed 6. Over time, roof of arch becomes unstable and collapses, leaving behind a stack 7. Stack eroded at its base, creating further wave cut notches and sub-aerial processes weaken stack from above, leading to collapse and forming a stump GEO: A geo is a steep sided inlet, formed where a joint is eroded If a geo extends into a cliff, a cave will form 8.2 CE Notes 6  An example is Duncansby Head less resistant siltstone along fault = eroded quicker, forming geo Case Study: Jurassic Coast Discordant coastline N-S direction (clay and sands, chalk, limestone alternating) Concordant coastline E-W (all Portland Limestone) Examples of landforms: Cliff - West Bay Cliffs (sandstone = resistant to erosion, forms vertical cliffs) Wave cut platform - Dancing Ledge (Portland limestone) Caves - Stair Hole (Portland limestone) Arches - Durdle Door (Portland limestone) Stacks/stumps - Old Harry Rocks (chalk stacks) 8.2 CE Notes 7  Cove - Lulworth cove (Portland limestone) Headland - Durlston Head (headland - portland limestone)) 8.2B - Depositional Landforms Beach Profiles A beach is the zone of loose sediment that extends from the mean low water line to the place where there is marked change in material or physiographic form (form in the intertidal zone) A beach profile is a cross section of the beach, from the edge of the sea to the base of the cliff. It shows distance on the x-axis and height above the seashore on the y-axis The beach is divided into 5 zones: Onshore, backshore, foreshore, nearshore, offshore (from cliff —> sea direction) ONSHORE ZONE All land, never covered by water unless extreme storm 8.2 CE Notes 8  Consists of cliff BACKSHORE ZONE Rarely covered by waves, but waves frequently throw pebbles up the beach leading to storm beaches Consists of storm beach, berms and cusps FORESHORE ZONE Zone between normal high tide mark and normal tide mark, so width of foreshore depends on tidal range Contains ridges and runnels, ripples, and drainage channels NEARSHORE ZONE Zone between low tide mark and the level of lowest spring tides Longshore bars formed OFFSHORE ZONE Zone is always underwater Bars can eventually form barrier beaches and barrier islands Variations in Beach Profiles ROLE OF TIDES Width and nature of foreshore and nearshore zones determined by tidal range and amount and type of sediment Spring tides (when full or new moon) = forms berms in backshore ROLE OF WAVES Constructive waves move material up the beach, forming berms and increasing gradient of beach Destructive waves comb material down beach, forming longshore bars and reducing gradient of beach Type of wave that break on a beach will change from day to day, depending on changing weather - so type of waves = short term factor affecting beach 8.2 CE Notes 9  profiles ROLE OF SEDIMENT Larger particles = steeper beach (shingle = steeper, sand = less steep) Shingle is more permeable and porous that sand, so backwash percolates into beach, rather than back out to sea (weaker backwash). In contrast, sand is less permeable and porous, so has a relatively stronger backwash. Sand beaches are wider as particles are easily distributed due to less friction from smooth sand, compared to shingle SEASONAL CHANGES Beach profile can change depending on season Winter = steeper beach gradient, less steep offshore gradient Summer = less steep beach gradient, steeper offshore gradient 8.2 CE Notes 10  WINTER In winter, rougher conditions with stronger winds may cause more severe storms, causing destructive waves which have a weak swash and strong backwash Sediment is removed + eroded from the top of beach, steepening the top of beach Material eroded is then deposited in the form of offshore bars So offshore gradient is less steep 8.2 CE Notes 11  In summer, the reverse is true - calmer conditions lead to constructive waves that build material back up the beach, creating a wider and less steep beach Berms and dune crests formed The offshore gradient is now more steep Small Scale Beach Features As mentioned earlier, each zone of the beach contains its own small-scale features BACKSHORE FEATURES 1. Storm Beaches During spring tides, large destructive waves push large sediment (pebbles) far up the beach 2. Berms Marker of highest tide 8.2 CE Notes 12  Formed by constructive waves during calm weather, where material is added to a beach Storms and spring tides can move existing berms up the beach, meaning that a new berm can develop, which changes the beach profile 3. Cusps Naturally formed semi circular patterns on a beach, that are on the seaward edge of berms Formed due to wave refraction along irregular coastline beaches, causing energy to be dissapated in certain areas So swashes are deflected as they approach irregular coastline, so 2 swashes come together to create a stronger backwash This backwash erodes the beach to create semi-circular shape The point on beach where swash is forced to divide is known as a horn The area of beach being eroded is known as embayment Horn/embayment essentially ‘mini headlands/bays’ within beaches FORESHORE ZONE 1. Ridges and Runnels Beaches are rarely flat 8.2 CE Notes 13  As tide goes out (low tide), it encounters elevation, so sand is slowed and deposited So sand builds up in height, forming ridges Any water left from outgoing tide creates a runnel - natural ditches of water between ridge and beach 2. Ripples Formed when tides go out (low tide) As waves retreat, they create patterns in the sand known as ripples Only visible in low tide Parallel to shoreline 3. Drainage Channels Perpendicular to shoreline Exposed at low tide Swash vs Drift Aligned Beaches Swash-aligned coasts are orientated parallel to wavefronts, and perpendicular to wave energy Swash-aligned are closed systems Swash-aligned feature larger beach profiles (more cliff), cusps, riffles, ridges & runnels, sand dunes Drift-aligned coasts are orientated obliquely to wave fronts, so is controlled by longshore drift Drift-aligned are open systems Drift-aligned feature smaller beach profiles (less cliff), spits, bars, tombolos 8.2 CE Notes 14  Depositional Landforms Simple Spits Drift-aligned feature created by deposition An extra stretch of beach material that moves out to sea Joined at one end to the mainland FORMATION 1. Prevailing wind (most common wind direction) dictates direction of longshore drift, causing swash (at oblique angles) and backwash (at perpendicular angles to shoreline) to transport sediment in a zig-zag motion 2. This is repeated along the coastline, transporting sediment further along coastline towards headland, and eventually extends out into sea 3. The distal end of the spit is exposed to a change in wind direction, from a secondary prevailing wind, forming a recurved end. Sediment is deposited at a different angle from previously 4. Sheltered area behind a spit is protected by waves of sea, so material accumulates —> forming a salt marsh (where plants can grow) 5. Process continues until equilibrium is reached at the distal end, between deposition and erosion 8.2 CE Notes 15  Example: Spurn Point, Holderness Coastline, UK Compound Spits A spit with multiple recurved ends that curve back into bay/inlet Recurved ends form because wave refraction around the distal end transports and deposits sediment for a short distance in the landward direction So, end of spit curves back and becomes abandoned, and a new spit forms on the old spits’ recurved ends Narrow proximal = curve to the left, recurved distal = final curve enclosing the bay on right 8.2 CE Notes 16  Compound spits contain a narrow proximal end and a recurved distal end - see above Example: Presque Isle, Lake Erie Example: Orford Ness Tombolos Ridge of material connecting offshore island to mainland Joined by land on both sides On drift aligned coastlines, when longshore drift builds a spit out from land until it contacts with an offshore island There is then wave refraction around both sides of the island, causing waves to lose energy and deposit sediment in areas of calm water sheltered by the island On swash aligned coasts, there may also be a collision of wave fronts on the landward side, cancelling each other out and producing a zone of still, calm water where deposition occurs, between the island and the coast Tombolos can also form due to onshore migration of sediment as sea levels rise Example: Chesil Beach - connects Isle of Portland to mainland Dorset Coast Offshore Bars Ridges of sand or shingle running parallel to the coast in an offshore zone. 8.2 CE Notes 17  Formed when waves approach a gently sloping coast, and sediment is deposited at boundary of offshore/nearshore zone, when wave energy is insufficient so these large particles are deposited Then exposed at low tides as below sea level Behind bars, lagoons can form Example: Slapton Sands, Devon Cuspate Forelands Cuspate forelands are low lying triangular shaped headlands, formed from deposited sediment Formed when longshore drift currents from opposing directions (two LSD currents) converge at the boundary of two sediment cells The sediment is deposited into sea, creating a triangular shaped area of deposited material Example: Dungeness, Kent - 11km triangular extension of coastline, due to interaction of the main W-E LSD with the N-S LSD produced by waves from North Sea 8.2 CE Notes 18  Barrier Beaches/Barrier Islands/Bars Barrier Beaches are linear ridges of sand/shingle extending across a bay and are connected to land on both sides It traps a body of seawater behind it, forming a lagoon Can form on drift-aligned coastlines, when longshore drift extends a spit across the entire width of the bay. Or when rising sea levels cause constructive waves to drive a ridge of sediment onshore to coastlines with a gently sloping shallow sea bed (barrier islands) (Beach ridge formation) BEACH RIDGE FORMATION THEORY (see diagram below) 1. 8000 years ago sea level was 100m lower than now. Dunes were formed along the continental shelf 2. Sea level has risen, breaking through the dunes and flooding the low area behind it. Forming a lagoon, and former dune becomes an island. 3. Continuing rise in sea level caused the island to migrate landwards, as sand was removed from the beach and deposited inland 8.2 CE Notes 19  Example: Fire and Jones Island, along South Shore of Long Island 8.2 CE Notes 20  Sand Dunes Sand dunes are small ridges of sand found at the top of a beach, above the maximum reach of waves Example: Harlech Beach, North Wales - features large tidal range which aids development of the dune Form in temperate areas, not tropical. Tropical areas have stormy conditions + high rainfall —> wet sand is difficult for wind to blow Form when deposition > erosion Factors Required For Development 1. SAND Large supply of sand Dry sand (so easily carried by wind, wet sand clumps together) Large tidal range (so time for sand to dry) 2. ENVIRONMENT Large flat beach (so large area where sand can be deposited) Obstacle for dune to form against (eg. pebble, driftwood, plastic bottle) 3. WIND Onshore wind (sea—>land) so sand can be moved to top of beach Types of Aeolian (Wind) Transport These processes happen when onshore winds blow sand to top of beach 1. CREEP (4% of sediment moves this way) Largest sediment ‘rolled’ along beach 2. SALTATION (95% of sediment moves this way) 8.2 CE Notes 21  Medium sized sediment ‘bounced’ along beach Sand moves this way 3. SUSPENSION (1% of sediment moves this way) Smallest sediment ‘carried’ by wind Clay and silt moves this way FORMATION PROCESS Sand dunes develop over time, slowly building the land out into sea. A well defined sequence of dune ridges develops - each with its own distinctive size and plant community Plant succession (change of plant species) stabilises sediment, which is vital for dune formation Plant succession = The gradual evolution of a series of plants within a given area. This occurs in a roughly predictable order, while the habitat changes Plant succession in sand dunes is an example of primary succession - Area of newly created land (sand blown on land = naked in the case of sand dune) will be colonised by pioneer of plants which are able to live in very harsh abiotic (climate) environmental conditions. A psammosere is the plant succession of a sand dune, referring to the sequence of changes in vegetation across the sand dune with increasing 8.2 CE Notes 22  distance from the sea. Overall Trends (from shore —> inland) More stability by vegetation away from beach Darker colours of soil away from beach and Lower pH away from beach because of.. 1. Lower % calcium carbonate (alkali) away from beach because of leaching 2. More % organic matter and humus away from beach - caused by death of plants (eg. pioneer species) which adds organic material and improves soil for more diverse species. Humus is acidic. More stable environments (fixed dunes) away from beach 8.2 CE Notes 23  1. Strand Line (pH 8.5) As tide goes out and retreats, sand is dried and blown up the beach Sand blown by aeolian transport - saltation, suspension, creep Forms a line of seaweed and litter at top of beach called a strand line 2. Embryo Dune (pH 8.0 from shell fragments CaCo3-) Young, very small dunes Sand carried by aeolian transport processes from strand line is deposited above the high water mark Deposition when sand comes in contact with obstacle (rock, driftwood, litter) Colonisation by pioneer species (prickly saltwort, lime grass) Sand accumulates around newly colonised plant Fragile dunes that can be easily washed away in high tides 3. Foredune (pH 7.5) Behind embryo dune, foredunes form as embryo dunes build up Marram grass begins to grow, stabilising the dune further More diverse species, ground is covered 4. Yellow Dune (semi-fixed) Forms at 10-20m high Made of sand (not soil) which is yellow Once over 10m high, less sand builds up and marram grass dies - forming a thin humus layer Humus layer + more organic matter = more soil - allows other plants to grow Known as a semi-fixed dune 5. Grey Dune (fixed dunes) (pH 7.0) No seawater reaches here, so not salty meaning lots of species can grow 8.2 CE Notes 24  Gorse and heather grow here as soils improve (lots of humus and organic matter) Soil becomes damper and richer —> flowering plants can grow 6. Mature dune (woodland) (pH 6.0) Climax community (the dominant plant species at the end of plant succession that reaches a state of ecological equilibrium appropriate to climate) is reached Moist and nutrient rich soils Woodlands form (pines & birch tree)s Additional feature #1: Dune slacks Dune slacks are dips between sand dunes, and form when the water table is higher than the land height, so freshwater pools form Can form because of a dune blowout (a gap in a dune caused by strong winds) exposing a dip of bare sand (see image below for blowout causing a dune slack) Slacks provide habitat for dune wildlife (eg. Natterjack toads), which use the freshwater pools found in dune slacks for breeding Water-loving plants are found (willow, moss, reeds) 8.2 CE Notes 25  SAND DUNE MANAGEMENT Case Study: Sefton Coast, north of liverpool = one of UK largest dune systems Threats 1. Over 5 million people live within an hour drive away = lots of human pressure from tourism and recreation (eg. dogwalking) 2. Spread of scrub vegetation has led to drying out of dune slacks 3. Removal of invasive species can cause damage to dunes themselves (unintentionally) 4. Grazing of rabbits was impacted following myxomatosis outbreak during 1950s, which killed many rabbits = dunes became overgrown = resulting loss of plant species Solutions 8.2 CE Notes 26  1. Fewer visitors are allowed on the Ainsdale National Nature reserve (need a permit) than on the beach where anyone is allowed 2. Paths are covered by boardwalks to reduce impact to dune and beach 3. Restricted parking during spring tides HOWEVER OVERALL, management has not been great: Sefton Council has cut back on the amount of money it spends on area = fewer wardens, fewer repairs to fences and boardwalks —> more vehicles driving into dunes = more damage overall Salt Marshes and Mudflats SALT MARSHES Wetlands formed in the intertidal zone of sheltered coasts, in temperate, high- latitude areas (mangroves would be destroyed by freezing) Contain grasses and herbs Found on low-energy coastlines - behind spits, barrier islands, estuaries FORMATION 1. Begins when mud and silt are deposited along a sheltered part of the coast 2. Deposition over time = mud breaks the surface to form mudflats 3. Pioneer plants (eg. Cordgrass) begin to grow - they are tolerant to seawater and help bind sediment together 4. There is now more sediment, so covered by less tide, so less salty as you move inland 5. More diverse plants can grow with these more fertile soils. Mud continues to get deeper. 8.2 CE Notes 27  Mudflats are only exposed during low tide Upper marshes are always exposed, not even covered by spring high tides MUDFLATS Found in intertidal zone, and is regularly flooded by tides and is usually barren (no vegetation) Mudflats experience submersion and exposure twice daily ALL FORMED DUE TO: SEDIMENTATION (basically deposition but in a fluid medium) Process of deposition of solid material that is suspended or in solution, when velocity of water no longer supports the transport of the particles Sediment is brought down by rivers + sediment left by receding tides SEDIMENTATION OCCURS ALONG: Along low energy shorelines In estuaries - which are tidal river mouths - at high tide = seawater floods river, you get combination of fluvial and marine processes The presence of a spit across an estuary mouth 8.2 CE Notes 28  Mangroves Mangroves are SALT TOLERANT forests of trees and shrubs that grow in the tidal estuaries and coastal zones of tropical, low-latitude areas Formed due to deposition of sediment in protected shorelines, plants grow and are adapted to low oxygen Contain trees Example: Everglades national park CONDITIONS REQUIRED Temperature greater than 24C in warmest month Annual rainfall exceeds 1250 mm Brackish water - mix of salty and fresh Low shoreline gradient - for accumulation of sediment Gentle wave action - slow-moving water to accumulate sediment and build up muddy soil ADAPTATIONS Aerial roots allows transport of oxygen + carbon dioxide to underground roots (soil has little oxygen) Buttress roots provides stability by extending plants vertically Pneumatophores are tiny side branches around the primary root, and allow for oxygen intake 8.2 CE Notes 29  Stilt (prop) roots provide extra stability and provide aerial ventilation 8.2C - Role of Sea Level Change Eustatic vs Isostatic 2 main types of sea level change: Eustatic and isostatic EUSTATIC Global Changes in actual sea water level Related to temperature (more temp = more melted ice + thermal expansion, and vice viersa) 8.2 CE Notes 30  ISOSTATIC Local Changes in the land level (up = uplift, down = subsidence) Usually because of glaciation Valentin’s Classification of Advancing and Retreating coasts Advancing coastline = land emerging or deposition being the prominent process. Retreating coastline = land submerging or erosion becoming the prominent process. Landforms of Submergence Submergence = RISIN˝G SEA LEVEL Cause by isostatic subsidence, or eustatic rise https://www.youtube.com/watch?v=bq684YOUjDo 1. Rias Flooded river valleys 8.2 CE Notes 31  As sea levels rise, they flood to river valleys, leaving only the higher areas visible V-shaped cross sections Are a type of estuary Example: Milford Haven, Pembrokeshire, UK 2. Fjords Flooded glacial valleys U-shaped cross section Have shallow sections at seaward end, called a threshold Example: Oslofjord, Norway 8.2 CE Notes 32  3. Dalmatian Coastline Coastal mountain range turned into a chain of long, narrow, chain of islands parallel to the coastline because of sea levels rising Separated from coasts by narrow sea channels called sounds Example: Dalmatian region of Croatia, 1,240 islands Landforms of Emergence Emergence = FALLING SEA LEVEL Caused by isostatic rebound, or eustatic fall 8.2 CE Notes 33  1. Raised beaches with relict cliffs Relict features, such as wave-cut platforms, cliffs, caves, arches, and stacks and all found above present day level due to falling sea levels (eg. isostatic uplift) This results in the beach being higher than it once was Example: Isle of Arran, Scotland. Because of isostatic rebound/uplift due to post glaciation. 8.2 CE Notes 34