PMT notes notes - Coasts

Classifying coasts

• The coast is an open system as it receives inputs from outside the system, transfers within the system and gives outputs away from the coast and into other systems

• The other systems that interact with the coastal system can be terrestrial, atmospheric or oceanic

• However, sediment cells are closed systems, as sediment does not get transferred from one cell to another

Sediment cells and Feedback Loops

• There are 11 sediment cells in England and Wales. They have:

  • Sources - where the sediment originates from (cliffs or eroding sand dunes)

  • Through flows - Movement of sediment along the shore through longshore drift

  • Sinks - Locations where depositional landforms are created (ie. spits, salt marshes, dunes)

• Under normal conditions, the coastal system operates in dynamic equilibrium - where inputs and outputs are in a constant state of change but remain in balance

• The coastal system has mechanisms which enhance changes, taking it away from dynamic equilibrium, but it also has mechanisms that balance changes, taking the system back towards equilibrium

• Negative feedback loops lessen change which has occurred within the system

  • For example, a storm could erode a large amount of a beach, taking it out of dynamic equilibrium, as there is larger input of sediment into the system than output. This works thus:

    • When the destructive waves lose their energy, excess sediment is deposited as an offshore bar

    • The bar dissipates the wave energy and protects the beach from further erosion

    • Over time the bar gets eroded instead of the beach

    • Once the bar has gone normal conditions ensue and the system returns to dynamic equilibrium

• Positive feedback loops exacerbate change in a system, making it more unstable and taking it away from dynamic equilibrium:

  • People walking over sand dunes destroys vegetation that is growing there and causes erosion

  • As the roots from the vegetation have been holding the dunes together, the dunes now erode quicker.

  • Eventually the sand dunes will be completely eroded, leaving more of the beach open to erosion taking the beach further away from dynamic equilibrium

The Littoral Zone

• The littoral zone is the area of the coast where land is subject to wave action. It is constantly changing and varies due to:

  • Short-term factors such as tides and storm surges

  • Long-term factors such as changes in sea level and climate change

• There are several subzones within the littoral zone:

  • Backshore - area above high tide level only affected by very high tides

  • Foreshore - area where most wave processes occur

  • Offshore - the open sea

Valentine’s Classification

• A Valentine’s Classification describes the range of coastlines that can occur

• An advancing coastline is a coastline that is growing out into the foreshore - land emerging or deposition is usually the prominent process here

• Alternatively, a coastline may be retreating due to the land submerging or erosion being the prominent process

Post-glacial adjustment / post-glacial rebound - When there is isostatic sea level change as a result of the land rebounding from melting glaciers

• Emergent or submergent coastlines may be due to post-glacial adjustment

• One axis on the classification shows emergence to submergence, and the other shows erosion to deposition. One diagonal axis shows a coast in equilibrium, and the other diagonal axis goes from fully advancing to fully retreating

Erosional processes

Erosion is a process which involves the removal of sediment from a coastline by different types of erosion, not by one type acting by itself

Processes of erosion:

• Corrasion - Sand and pebbles are picked up by the sea from an offshore sediment sink are hurled against the cliffs at high tide, causing the cliffs to be eroded

• Abrasion - The process where sediment is moved along the shoreline, causing it to be worn down over time

• Attrition - Wave action causes rocks and pebbles to hit against each other, wearing each other down to become smaller and rounder - is an erosional process on the coastline, but has little impact on erosion of the coastline itself

• Hydraulic action - As a wave crashes onto a rock or cliff face, air is forced into cracks, joints and faults within the rock. The high pressure causes the cracks to force apart and widen when the wave retreats and the air expands. Over time this causes the rock to fracture.

• Cavitation - Bubbles within the water may implode under high pressure creating tiny jets that erode the rock

• Corrosion (Solution) - The mildly acidic seawater can cause alkaline rock such as limestone to be eroded in a process similar to carbonation weathering

• Wave Quarrying - Where breaking waves that hit the cliff face exert a pressure up to 30 tonnes per square metre. These waves can enlarge joints and remove large chunks of rock in one vibration

Erosion rates are highest when:

• Waves are high and have a long fetch

• Waves approach the coast perpendicular to the cliff (so all the wave energy is in the direction of the cliff)

• It is high tide - waves travel higher up the cliff so a larger area of the cliff face can be eroded

• Heavy rainfall has occurred - water percolates (filters gradually through porous rock) through permeable rock, weakening the cliff

• It is winter - destructive waves are the largest and most destructive during winter

Vulnerability to Erosion

Clastic rocks - Made of sediment particles cemented together

Crystalline rocks - Made of interlocking mineral crystals

The resistance of a rock will determine its vulnerability to erosion, and is influenced by various factors:

• Whether rocks are clastic or crystalline - sedimentary rocks are clastic and are therefore more vulnerable to erosion, whereas igneous and metamorphic rocks are crystalline, and are therefore less vulnerable to erosion

• The amount of cracks, fractures and fissures - the more weaknesses there are in a rock, the more open it is to erosional processes, especially hydraulic action

• The lithology of the rock - the type of rock directly affects its vulnerability to erosion, with igneous having the most resistant lithology

Erosional Landforms

The sequence from a fault to a stump occurs on a headland.

• Erosion widens faults in the base of the headland, widening over time to create a cave

• The cave will widen due to both marine erosion and sub-aerial processes

• This will then erode to the other side of the headland, creating an arch

• The arch continues to widen until it is unable to support itself, falling under its own weight through mass movement, leaving a stack as one side of the arch becomes detached from the mainland

• With marine erosion attacking the base of the stack, eventually the stack will collapse into a stump

Wave cut platforms:

• Marine erosion attacks the base of a cliff, creating a notch of eroded material between the high and low tide mark

• As the notch becomes deeper, the cliff face is undercut and becomes unstable, falling under its own weight

• This leaves behind a platform of the unaffected cliff base beneath the wave-cut notch, as the cliff retreats

• Through the process of repeated wave-cut notches and platforms, new cliff faces are created as the land retreats

Blowhole:

• A blowhole is a combination of two features: a pot hole on top of a cliff created by chemical weathering, and a cave formed through marine erosion

• As the cave erodes deeper into the cliff face and the pothole deepens, they may meet

• In this case, a channel is created where the incoming waves can travel through and up the cliff face, causing water to splash out the top of the blowhole when energetic waves hit the cliff face

Transport and deposition

Longshore drift (littoral drift):

• Sediment is predominantly transported along the coast through the process of longshore drift

• It transports sediment along the beach and sometimes even between sediment cells

Process:

• Waves hit the beach at an angle determined by the direction of the prevailing wind

• The waves push sediment in this direction and up the beach as the swash

• Due to gravity, the wave then carries sediment back down the beach in the backwash

• This moves sediment along the beach over time

• It is one of the reasons that you move across while swimming in the sea

Traction - Large, heavy sediment rolls along the sea bed, being pushed by currents

Saltation - Smaller sediment bounces along the sea bed, being pushed by currents

Suspension - Small sediment is carried within the water column

Solution - Dissolved material is carried within the water

Effectiveness of transportation:

• The impact of transportation depends on the severity of the angle that waves travel onto the land

• Swash-aligned - Wave crests approach parallel to the coast so there is limited longshore drift. Sediment does not travel far or fast

• Drift-aligned - Waves approach at a significant angle, so longshore drift causes the sediment to travel far up the beach

Likelihood of deposition:

• Deposition occurs when a wave loses energy, meaning that the sediment becomes too heavy to carry

• Deposition is a gradual and continuous process - a wave will not just spontaneously drop all of its sediment

Gravity-settling - Wave energy becomes low so heavy rocks and boulders are first deposited, and then the next heaviest sediment and so on

Flocculation - Rock particles that float (such as clay) will in theory never be deposited, however they will clump and stick together due to chemical attraction and then sink due to the resulting weight

Depositional Landforms

Spits:

• A spit is a long, narrow strip of land which is formed due to deposition

• Longshore drift occurs along the coast line but as the waves lose energy (every time the wave breaks it loses energy), they deposit their sediment

• Over time this creates a spit, which forms when the coastline curves around or reaches an estuary, but longshore drift and deposition carries on in the original direction of the coast

• Periodically, the prevailing wind will change direction, causing a hook shape to appear

• Over time, the sheltered area behind a spit can turn into a salt marsh

• The length of a spit is influenced by surrounding currents or rivers

  • For example, if a spit spans part of an estuary, then the river current will prevent it from going all the way across the river, and therefore a recurved spit end is formed

Bars:

• A spit is formed

• Over time, this spit crosses across a bay and links up two sections of coast

• The water within the bay is then called a lagoon, as it is sheltered from the open sea

Tombolo:

• A tombolo is a bar or beach that connects the mainland to an offshore island

• It is formed due to wave refraction off the coastal island, leading to deposition of sediments

• Sometimes they are covered at high tide, which can mean people get trapped sometimes

Cuspate Foreland:

• Only occurs with triangular shaped headlands

• Longshore drift along each side of the headland will create beaches

• These beaches meet and form a cuspate foreland, which sticks out perpendicular to the direction of the coastline

Offshore bars:

• A region offshore where sand is deposited, as waves don’t have enough energy to carry the sediment to the shore

• Formed if the wave breaks early, depositing its sediment as an offshore bar

• They can also be formed from the strong backwash in storms, as sediment is carried off the beach and offshore

Sand dunes

Tidal range - The distance between high tide and low tide

Berm - Raised section in which shingle or sand is deposited, it is above the high tide mark

Sand dunes occur when prevailing winds blow sediment to the back of the beach, and therefore the formation of dunes requires large quantities of sand and a large tidal range. This allows the sand to dry out, so that it is light enough to be picked up and carried by the wind to the back of the beach. Frequent, strong winds are also necessary

The dunes develop thus:

• Embryo dunes - Upper beach area (above the berm)where sand starts to accumulate around a small obstacle, such as driftwood, a wooden peg or a ridge of shingle

• Yellow dunes - As more sand accumulates and the dune grows, vegetation may develop on the upper and back dune surfaces, which stabilise the dune - yellow dunes are the tallest of all the dune types

• Grey dunes - Sand develops into soil with lots of moisture and nutrients as vegetation dies and fertilises the soil, enabling more varied plant growth

• Dune slack - The water table rises closer to the surface, so plants that need lots of moisture can be present here

• Heath and woodland - Sandy soils develop as there is a greater nutrients content, allowing for less saline plants to grow. Trees can also grow, with the coastal woodland also acting as a windbreak

Stability of depositional landforms

• Depositional landforms are mostly formed of unconsolidated sediment (sand with very little vegetation and with small sediments)

• Therefore, during major storms, large amounts of sediment can be eroded or transported elsewhere, which can remove a landform, sometimes entirely, from one region of the sediment cell

• Depositional landforms depend on a continuous supply of sediment to stay in dynamic equilibrium, which may see some landforms changed as their dynamic equilibrium shifts

Sub-aerial Processes - Weathering and Mass Movement