Factors Affecting Coastal Recession & Stability (Edexcel A Level Geography)

Coastal Recession

• Coastal recession is the retreat of the coastline inland, this may be due to:

  • Erosion

  • Sea level rise

  • Submergence

• Where coastal recession is the result of erosion, this is dependent on the lithology of the coast

  • Mineral composition 

  • Rock classification 

  • Structure 

Mineral composition

• Some minerals are more reactive than others

  • This affects the rate of chemical weathering for example:

    • Calcite is reactive and so easily chemically weathered 

    • Quartz is not reactive (inert) so chemical weathering is much slower

Rock classification

• Sedimentary rocks form as a result of compaction and cementation of sediment called lithification

  • Examples of sedimentary rocks include shales, sandstone, and limestone

  • Sedimentary rocks: 

    • Erode and weather more rapidly than other types of rock

    • Form in layers

    • Have weak bedding planes

    • They are clastic which means they are made of clasts (sediment particles)

    • Are heavily jointed

    • Often have many bedding planes and fractures

• Metamorphic rocks form when sedimentary and igneous rocks are altered through heat and pressure, but do not melt in the process

  • Examples of metamorphic rocks include slate and marble

  • Metamorphic rocks:

    • Have a crystalline structure

    • Are often folded and faulted

    • Are more resistant than sedimentary rock and less resistant than igneous rocks

    • The crystals have a parallel arrangement (foliation) which means they are weaker than igneous rocks 

• Igneous rocks form when molten rock from the Earth's mantle cools and hardens

  • Examples of igneous rocks include granite and basalt

  • Igneous rocks:

    • Erode and weather very slowly

    • Can be categorised into two types:

      • Intrusive igneous rock - forms within the ground, cools slowly, and has large course crystals 

      • Extrusive igneous rock - forms on the Earth's surface, and cools quickly, forming smaller crystals

    • Has interlocking crystals 

    • Fewer joints and weaknesses than in other rocks

Differential Erosion

• The changing rates of erosion of different rock types is known as differential erosion

• More resistant rocks erode more slowly than less resistant rocks

• Differential erosion:

  • Leads to complex cliff profiles

  • Influences rates of recession

• The differences in geology contribute to landforms generated by processes on the coast

• On a discordant coastline differential erosion leads to headlands and bays

• Where there are layers of alternating rock types (strata) this creates a cliff profile where the more resistant rock layers jut out due to the faster erosion of less resistant rock

• Permeability can also impact on the rate of erosion:

  • Where permeable rock overlies impermeable rock, the permeable rock is vulnerable to mass movement because the additional weight and lubrication the water creates leads to instability

Role of Vegetation

• Vegetation helps to stabilise coastlines because:

  • The plant roots help to bind the soil/sand together reducing the impact of erosion

  • Wind speeds are reduced by vegetation which decreases erosion and increases deposition

  • Dead plant material adds organic matter to the sand and eventually leads to the formation of soil

• Vegetation is often sparse in coastal environments because:

  • Exposure to salty/saline water

  • Evaporation of water leads to increased salinity

  • High wind speeds

  • Lack of shade 

  • Lack of nutrients

  • Free-draining sediment means water is not retained

• Pioneer species in the coastal environment are the first plants to grow in the harsh coastal environment, paving the way for other plants that are less able to survive the conditions

• Pioneer species:

  • Help to stabilise the sediment

  • Add organic matter

  • Increase shade

  • Trap more sediment

Sand dune succession

• Sand dune succession is an example of this process 

• The succession in a sand environment is called a psammosere

  • Sand dunes are a dynamic environment

• Sandy beaches usually have sand dunes at their rear, because of strong onshore winds transporting dried, exposed sand

• Sand grains are trapped and deposited against any obstacle (rubbish, rocks, driftwood etc) and begin to form embryo dunes

• Dune ridges move inland due to onshore winds pushing the seaward side to the leeward side

• It is the interaction of winds and vegetation that helps form sand dunes

Formation of a sand dune

• Windblown sand is deposited against an obstruction: Pebbles or driftwood

• As more sand particles are caught, the dunes grow in size, forming rows at right angles to the prevailing wind

• Over time, the ridges of the dunes will be colonized and fixed by vegetation in a process called succession

• The first plants (pioneer species) have to deal with:

  • Salinity

  • Lack of moisture as sand drains quickly (highly permeable)

  • Wind

  • Temporary submergence by wind-blown sand

  • Rising sea levels

• Embryo dunes

  • Wind-blown dried sand is trapped by debris and deposition begins

  • Pioneer species such as lyme grass and sea couch grass begin to colonise, these are halophytic plants which can tolerate high salt levels

  • There is little soil content and high pH levels (alkaline)

  • Embryo dunes are very fragile and reach a maximum height of 1 metre

• Fore dunes

  • The embryo dunes bring some protection against the prevailing wind

  • This allows other species of plant to grow such as marram grass

  • Marram grass begins to stabilise the dune with its root system which can grow to 3m to reach the water table

  • These plants add organic matter to the dunes making the dunes more hospitable for plants that later grow

  • A microclimate forms in the dune slack

  • Maximum height is 5 metres

• Yellow dunes

  • These are initially yellow but darken as organic material adds humus to the soil

  • Marram grass still dominates the vegetation, but more delicate flowering plants and insects are found in the dune slacks

  • 20% of the dune is exposed, down from 80% 

  • Height does not exceed 8 metres

• Grey dunes 

  • Grey dunes are more stable, with less than 10% of exposed sand and have a good range of biodiversity

  • Soil acidity and water content increase as more humus is added

  • Shrubs and bushes such as gorse begin to appear

  • Height is between 8 - 10 metres

• Mature dunes

  • As the name suggests, these are the oldest and most stable of the dunes

  • They are found several hundred metres or more from the shoreline

  • The soil can support a variety of flora and fauna such as oak trees and alders (climax vegetation)

  • This is the final stage in succession which is known as the climax community stage

Salt marsh succession

• A succession in salty water is called a halosere

• The plants which are adapted to these conditions are called halophytic plants

• Salt marshes often develop in estuarine areas because of:

  • A steady supply of sediment from the river

  • Shelter from strong waves

• In the estuarine environment fresh water and seawater mix
  

  • This causes flocculation where clay particles stick together

  • The clay particles sink causing sediment to build up

• The deposited sediment is colonised by algae which:

  • Binds the sediment together

  • Increases organic matter 

  • Traps more sediment

• The sediment builds up and is covered by the tide for less and less time

• This allows other plants such as cord grass to colonise

• The height of the sediment increases and is only covered by the highest tides

• More plants colonise such as sea lavender and sea thrift

• Saline levels decrease and other plants colonise

• This continues until a climax community is achieved