ESPL lecture 4
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- Created on: 30-05-22 14:12
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- ESPL AEOLIAN PROCESSES lecture 4
- DUNE TYPE
- free’
dune = primarily controlled by wind &
sediment supply
- Coastal
dunes
- Cluster of dune types
In small Ares in coast a variety of dunes
are seen in small areas compared to in deserts
- •Can
include free dune types e.g. transverse ridges
- •Ephemeral
barchans are common on beaches
- •Typical
coastal dune
assemblage may
include shadow or embryo dune, foredune, transverse ridges, parabolic dunes,
blowouts
- •Suitable sand transporting winds Coastal dunes have: •Plus complicating factors => moisture + vegetation Interaction with beac
- Dunes need a surface to deposit sand upon
- Require a sediment supply – coastal dunes need a process tough which sand can be transported and a mechanism by which they can be deposited
- Beaches are often the source and areas upon witch sand is deposited
- Require a sediment supply – coastal dunes need a process tough which sand can be transported and a mechanism by which they can be deposited
- All dunes need: •Surface on which to deposit sand •Sediment supply
- •Typical
coastal dune
assemblage may
include shadow or embryo dune, foredune, transverse ridges, parabolic dunes,
blowouts
- •Ephemeral
barchans are common on beaches
- The foredune – distinctive to coats
Dependant form – owes existence to the
interaction of the nearshore sand and wave action/wind action
- Bauer & Sherman 1999 p. 73
- •Can
include free dune types e.g. transverse ridges
- Distribution
of coastal dunes
- •Windward
coasts with wide sandy beaches
e.g.
European & southern African Atlantic coasts, Pacific coasts of Americas
- •Near well-defined sand source areas e.g. Aquitaine coast, France (Gironde R.) Dutch dunes (River Rhine).
- Coastal dunes require an engine to provide the sand for the dunes
- Incipient
foredune
- The dune formed from vegetation
- •New or developing foredunes
forming in pioneer plant communities
•
•Maybe discontinuous, discrete
deposition due to vegetation/driftwood/litter
•e.g. a few embryo dunes
- Development = f(plant density,
distribution, height, wind velocity, rates of sand transport)
- Secondary factors = frequency of swash inundation, storm wave erosion, overwash, wind direction
- Development = f(plant density,
distribution, height, wind velocity, rates of sand transport)
- •New or developing foredunes
forming in pioneer plant communities
•
•Maybe discontinuous, discrete
deposition due to vegetation/driftwood/litter
•e.g. a few embryo dunes
- DEVELOPMENT OF ESTABLISHED FORE DUNES
- Magnitude & frequency of wave
& wind forces
- sand supply and human activity
- long term beach state
- Occurrence and magnitude of storm
erosion, dune scarping and overwash
processes
- rate of accretion and erosion
- interactions with water and water level
- vegetation and plant species
- interactions with water and water level
- rate of accretion and erosion
- Occurrence and magnitude of storm
erosion, dune scarping and overwash
processes
- long term beach state
- sand supply and human activity
- flow dynamics
- Embryo dunes
1-1.5m high windward side (experiencing
wind)
Compression of streamline à acceleration (over the top)
Expansion of flowlines à deceleration (in
the lee)
- Dune grwoth à putrebation to wind field is greater Flow is no longer attached to surface = recirculation of the air flow in the lee side Pressure difference in this area High pressure where flow attaches Low pressure in the cavity where the flow is not attaches No linear profile bc of pressure difference and lack of attachment Will no reattach if there is a second dune
- Impact of vegetation Vegetation absorbs shear stress Protects surface Flow deflection Along the crest line or along the crest line – air flow is 3D- the wind will not only come straight on to the dune
- •increases surface roughness (z0) • •promotes deposition both upwind and downwind • •has to tolerate saline/burial conditions • •has to withstand sandblasting
- Plants adapted to being hit by sand Marram grass – resistant to the sand battering
- •increases surface roughness (z0) • •promotes deposition both upwind and downwind • •has to tolerate saline/burial conditions • •has to withstand sandblasting
- Embryo dunes
1-1.5m high windward side (experiencing
wind)
Compression of streamline à acceleration (over the top)
Expansion of flowlines à deceleration (in
the lee)
- Magnitude & frequency of wave
& wind forces
- The dune formed from vegetation
- •Windward
coasts with wide sandy beaches
e.g.
European & southern African Atlantic coasts, Pacific coasts of Americas
- Cluster of dune types
In small Ares in coast a variety of dunes
are seen in small areas compared to in deserts
- Coastal
dunes
- ‘anchored/impeded’ dune = significantly influenced or dependent upon vegetation, topography etc.
- free’
dune = primarily controlled by wind &
sediment supply
- Hesp et al. 2005
- To
understand foredune dynamics, need to understand airflow over them. Much from a
host of studies carried out by Canadian-Australian group (lot of coastline!)
- Reduction in speed from mast 1 à mast 2
Air flow approaching dune declines
Deposition in the zone between mast 1 and
à increase in elevation
- Dune crest Mast 3 and mast 7 at crest The air speed increases from 3 to 7 at 1 m above surface Where lines intersect there is a reverse of this At 10cm 3 if faster than 7 à this is due to the vegetation percent
- Reduction in speed from mast 1 à mast 2
Air flow approaching dune declines
Deposition in the zone between mast 1 and
à increase in elevation
- To
understand foredune dynamics, need to understand airflow over them. Much from a
host of studies carried out by Canadian-Australian group (lot of coastline!)
- dune sequence
- 1. parabolic
- 2. blowouts
- 3. foredune
- 4. beach
- 4.Source of sand
- 4. beach
- 2.Erosional hollows in the fore dune structure
- •saucer-,cup-
or trough-shaped depression or hollow formed by wind erosion on a pre-existing
sand deposit
- Shapes can change
Not true dunes – features formed from
pre-deposited sands
- causes of blow outs...
- Require
disruption of veg…
•
- Wave erosion along seaward face of dune •Climate change
- •Vegetation variation in space or change through time •Grazing Human activities
- •Topographic acceleration of airflow over dune crest
- •Vegetation variation in space or change through time •Grazing Human activities
- Wave erosion along seaward face of dune •Climate change
- Require
disruption of veg…
•
- causes of blow outs...
- Steep erosional walls
Have wet sands and vegetation
interactions forming soils
Narrow entrance
Distinctive shape
Depositional lobe – shear stress
decreased
Compression though blowout = faster
speeds
- saucer Subject to deflation = erosion
- Fractional speed-up within a saucer
blowout
- Hugenholtz & Wolfe, 2009
- Fractional speed-up within a saucer
blowout
- troughNarrower width More acceleration though gap
- positive relationship between
- Length of deflation basin and
length of depositional lobe
Interlinked
- Depth of blowout and size of depositional lobe Sand budget within the blowout not being lost as an output
- Mid-blowout width and depositional lobe length
- Depth of blowout and size of depositional lobe Sand budget within the blowout not being lost as an output
- HESP 2002
- Length of deflation basin and
length of depositional lobe
Interlinked
- saucer Subject to deflation = erosion
- Shapes can change
Not true dunes – features formed from
pre-deposited sands
- •saucer-,cup-
or trough-shaped depression or hollow formed by wind erosion on a pre-existing
sand deposit
- 3. foredune
- 1.Large volumes of sand blown in from the beach moving in land
- •Crescentic
or hairpin shaped sand accumulation
- •Horns point upwind (opposite of barchans)
- •Occur in association with vegetation, esp. on trailing limbs •Can develop from blowouts
- •Horns point upwind (opposite of barchans)
- •Crescentic
or hairpin shaped sand accumulation
- 2. blowouts
- 1. parabolic
- DUNE TYPE
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