COASTAL SAND DUNES PSAMMOSERES

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COASTAL SAND DUNES (PSAMMOSERES)

• Fragile Environments - All change
• and how to stop them looking like this

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Ecosystems
Ecosystems are natural habitats where the
interaction of living and non-living factors
creates a particular environment in which the
soil, vegetation and animals have adapted to the
particular physical factors. Where human factors
affect the balance, the ecosystem can be
seriously at risk. Where the imbalance is not
too severe, it can repair damage naturally. It
is when irreversible damage occurs that
ecosystems are destroyed for ever.
PHYSICAL FACTORS Climate, geology, drainage,
relief
HUMAN FACTORS Human activity, use of land
CHARACTERISTICS OF ECOSYSTEM BEING STUDIED
ANIMAL ACTIVITY (fauna)
SOIL (or regolith)
Figure 1 Interactions in an ecosystem
VEGETATION (flora)
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Introduction

• Dunes result from the transport of sand by wind
• They are unable to form if the wind velocity is
  too low, the sand too damp or there is too much
  vegetation
• They may be found in a zone up to 10km wide
  inland
• The height of the dunes ranges from 1 to 30m
• The upwind side (stoss) is steeper than the
  downwind (lee) slope
• The most important conditions for dune formation
  are strong offshore winds (at least 16 mph), an
  abundant supply of dry sand and a vegetation
  cover
• Ideal conditions can be found on low, near shore
  slopes with a large tidal range as this provides
  wide expanses of sand which dries out at low tide.

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Sand Transport

• On an exposed beach, wind speed increases with
  height
• There is a zone of no wind about 1mm thick but
  sand grains protrude through this layer
• The grains are moved by the wind due to saltation
  and creep (Figure 2) and sand collects on the
  leeward side of obstacles (driftwood, boulders,
  dead tramps?) where wind velocity decreases
• When vegetation begins to colonise the area it
  causes an increase in the height of the no wind
  zone which can be ten times greater

Windpeed 5m/s to initiate movement
Small grains less than 0,2mm carried in
suspension out of the dune zone
Smaller grains accelerate as they enter higher
wind velocity zones
Surface creep large grains jerked forward short
distance
Grain lands here and explodes other grains upwards
Figure 2 Movement of sand particles
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Embryo Dunes and Dune Migration

• As a result of saltation, wind shapes the surface
  into regularly spaced symmetrical 1-2cm high
  ripples
• Wind speeds are highest on the windward side of a
  dune ridge but decrease on the leeward side so
  erosion occurs on the windward side and
  deposition of sand particles on the leeward side
• First there is a long streamlined shadow dune
  parallel with the wind direction on the lee of
  the obstruction
• When the shadow dune reaches the height of the
  obstruction deposition ceases until vegetation
  develops
• Salt tolerant species such as sand couch and lyme
  grass affect sand transport rates and deposition
  may recommence
• Dunes formed like this lie on the upper slopes of
  the beach and are initially isolated low mounds 1
  or 2m high known as embryo dunes
• Because of erosion of the windward side and
  deposition on the leeward side, the dune migrates
  inland
• As it does so it accumulates more sand
• Dunes several hundred metres from the coast are,
  therefore, larger and older than those close to
  the sea
• Wind speeds are higher in the intervening valleys
  and this is what creates the slacks (low-lying
  marshy areas between the dunes)
• As the dune ridges move inland, new dune ridges
  are created on the seaward edge of the system

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Fore-dunes and Grey Dunes

• As embryo dunes increase in height, they also
  extend laterally and join together, forming
  fore-dune ridges parallel with the shoreline
• As they increase in height, xerophtic species
  colonise the fore-dunes (sand couch, marram
  grass, prickly saltwort)
• New embryo dunes form upwind from fore-dunes as
  dune ridges migrate landwards the time interval
  between development of successive ridges may be
  between 70 and 200 years
• Marram grass has a marked effect on saltation,
  reducing velocity below 10cm (zone of still air)
  and rapidly transporting sand above this height
  due to surface roughness
• Fore-dunes are also known as yellow dunes due to
  their appearance which is caused by a lack of
  humus
• They become increasingly grey as humus and
  bacteria from plants and animals are added and
  the gradually become more acidic and
  vegetation-covered
• Creeping fescue, sea spurge and some marram,
  cotton grass and heather can be found on the grey
  (mature) dunes
• Grey dunes may reach a height of 10-30m but then
  their supply of fresh sand is cut off by their
  increasing distance from the beach
• As the dune rises it also creates a stronger wind
  régime which increases erosion on the upwind
  surface (a self-limiting maximum height)

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Death of a Dune Older Ridges

• Slacks were though to be due to reverse eddies in
  the lee of dune crests
• They are now thought to be due to the wave-like
  pattern of wind speed which increases in the
  hollows, removing loose sand and causing erosion,
  sometimes down to the water table
• Vegetation in the dune slacks differs from that
  on the dry dune ridges with marshland vegetation
  of sedges and rushes
• Heath vegetation grows on the older dune ridges
  which are acidic and have a high humus and
  moisture content
• Reduction of height in older ridges is due to
  progressive lessening of saltation away from the
  source of sand so that older dunes are starved of
  sediment
• Paths cut by humans and animal burrows (eg.
  rabbit runs) expose areas of sand
• As erosive winds are channelled along these
  tracks, blow-outs form in the wasting dunes
• Dunes that were once straight become bulge-shaped
  and these may become separated to form parabolic
  dunes as the arms, which indicate the dominant
  wind direction, are anchored by vegetation and
  the central portion is moved downwind
• This erosion is being combated by methods such as
  fencing off wasting dunes and planting marram
  grass to restabilise the area

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A Transect Across Idealised Sand Dunes
Figure 3 Transects across idealised sand dunes
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Studland Problems

• Problems caused by
• Local residents walking their dogs
• Tourists sunbathing
• School parties carrying out beach surveys
• Problems are
• Dune erosion (leading to blow outs)
• Rubbish (12 tonnes per week)
• Heath fires
• Oil slicks

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Management Strategies
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The Management Dilemma

• To close off or not to close off that is the
  question!
• National Trust wants people to use the beach
  and facilities
• This will bring in money (0,5 million/year)
  which can be used to improve the beach, its
  facilities and for conservation
• Problem encourages more people to come
• English Nature wants to restrict access to
  specialist groups to protect and conserve this
  environment
• What might be the effects of the other options?
• Restricting access to 15 000 people
• Charge admission
• Remove all management strategies, leaving
  Studland to the public
• Close all access except by means of boardwalks
  and laid paths
• Establish Studland as an SSSI and restrict access
  to special study