1.1.7 Aolian, fluvial and biotic processes

Aolian processes- the action of wind transporting, eroding and depositing material

• Suspension- finer sand particles are moved by the wind, high in the air. They are not affected by gravity and can travel far before landing

• Surface creep- larger particles are too heavy to be picked up and carries by the wind so instead, they move along the ground. When they become dislodged by the falling ones, they roll along the ground

• Saltation- when the wind hits the ground, it causes turbulence, disturbing the sand particles. If the wind has enough velocity, it will cause the particles to start moving. As the sand moves, it hits other grains which cause them to bounce up in the air. The wind then picks these airborne particles up and carries them. Gravity causes them to fall back down. If sand lands on a hard surface, it creates a chain reaction, causing other particles to bounce up

Coastal dunes are found where beach sands have been reworked by the wind. The predominant source of material for dune creation is the seabed and rivers which flow into the sea or through coastal areas. Macro-tidal environments (large tidal range) combined with onshore winds and wind-driven currents provide the ideal conditions for dune evolution. These are likely to form in zones of net sediment accumulation.

• In some areas, such as East Anglia, complex sediment transport systems can cause material which is eroded in one part of the coast to be transported, and eventually deposited in another location, sometimes fuelling the development of new dunes. Sand dunes are found on all British coasts, but they are more sparsely distributed in the south and south-east

• Aeolian processes in sand dunes:

  • Dunes develop above high tide level and can extend several kilometres inland. Some dune systems consist of a sequence of ridges and troughs parallel to the shore, whereas others are more complex dune fields with some ridges at right angles to the sea. The height of the dune ridges varies from 1-2m up to 30-40m above sea level.

  • Optimum conditions for a dune to form:

    • An abundant supply of sand

    • Low beach gradient

    • Macro-tidal range (range more than 4 metres)

    • Strong prevailing onshore winds

    • Inland space for dunes to develop

    • Vegetation e.g. marram grass to colonise the dunes

• If strong winds erode a section of the vegetated sand (commonly referred to as a blowout), a parabolic dune may form. Leeward motion occurs if sand from the blowout is deposited on the opposite slope of the parabolic dune.

  • How blowouts demonstrate positive feedback:

    • Loss of vegetation due to human or natural influence such as a storm→Sand removed during blowout→Increased wind speed as less friction from plants→more mobile sand→harder for vegetation to re establish

• Formation of a sand dune:

  • Constructive waves deposit sediment on the beach

  • Winds blowing onshore move sand inland by creep and saltation

  • Where an obstacle or vegetation reduces wind velocity, the sand is deposited and quickly accumulates

  • Plants such as sea couch grass and marram grass become established increasing accumulation and forming embryo dunes

  • Continued deposition leads to embryo dunes forming a ridge

  • New embryo dunes form in front of the ridge, repeating the process

  • Gradually older dunes become covered in vegetation and are stabilised (fixed dunes)

Non-marine influences in the formation of landforms

• Fluvial processes

  • Tidal flats

    • Tidal flats are level muddy areas bordering an estuary

    • Edges of estuaries are sheltered and so are low energy coastlines

    • Deposition of fine sediment is the main process creating a tidal mudflat

    • At low tide the mudflat is uncovered. Fresh water from tributaries flows across the mudflat to the sea in small channels and rills

    • Morecambe bay on the Lancashire coast is an example of a tidal flat

  • Salt marshes

    • Salt marches are gently sloping, vegetated areas of intertidal mudflats

    • Salt marshes are found in low-energy, sheltered locations such as estuaries , lagoons and behind spits

    • A lack of marine erosion allows salt-tolerant, pioneer plant communities to colonise the mudflats

    • Salt marshes are dissected by channels and rills

    • They are important wildlife habitats, some having been protected and managed for wildlife

    • Many areas in the past have been drained and used for agriculture, for example the Blyth estuary in Suffolk

    • Many examples of salt marshes are found along the 120 kilometers of Essex coast between the Blackwater and Crouch estuaries. It is known as the ‘Saltmarsh Coast’

• Biotic processes

  • Coral reefs

    • Coral reefs are calcium carbonate structures secreted by tiny animals called polyps. They build slowly over time and can be hundreds of meters thick

    • Biotic = Coral polyps are living creatures – form from the process of photosynthesis which is aided by the zooxanthellae which allows biological development of these huge structures

    • Importance of biotic? Very important -  essential – could not form without these processes

    • Other factors? Sheltered areas with weak currents, plenty of light available to aid photosynthesis, warm temperatures that allow biotic processes to occur

  • Mangroves

    • Mangroves form an ecosystem of small trees growing along tropical coastlines where fine sediment have been deposited

    • Biotic = Buoyant seeds settle in mud flats allows the beginning of mangroves to develop.  Deep roots develop in the flocculated sediment that can extract fresh water from deep under the sediment and can survive in the salty water

    • Importance of biotic? Very important – again essential – could not form without these processes

    • Other factors? Sheltered areas with weak currents, plenty of light available to aid photosynthesis, warm temperatures that allow biotic processes to occur

Summary:

• Non-marine influences in coastal  environments include wind action, river action and biotic processes

• Sand dune formation needs a supply of sand, a low beach gradient, a large area of exposed beach, strong prevailing onshore winds, an area for dunes to develop in and vegetation to colonise the dunes

• Sand dunes form, primarily by saltation, when wind speed is sufficient to move sediment of a given particle size. It is deposited once wind speed drops.

• Fluvial processes in estuarine environments form tidal flats, salt marshes and micro-features such as channel and rills

• Tidal flats are extensive, unvegetated intertidal areas. Salt marsh coastal systems form a near- horizontal platform sloping gently seaward, with micro-features such as channels, rills and salt pans around tidal creeks.