Coastal Dunes Flashcards
What are snow dunes?
Wind blown snow can form dunes just like sand, and 11% of the Earth (mostly at the poles) is covered by them
Where can you find snow dunes?
On a coastal plain of the Arctic Ocean
Greenland, Antarctica, Alaska, etc.
What are the essential ingredients for a dune?
Sediment input from offshore to build a beach (sediment supply)
Availability of sediment on beach (lack of stabilisers)
Wind (transport capacity) to transport sediment from beach onshore
Space onshore for dunes to grow (lack of houses/cliffs)
Time for dunes to build between destructive storms
What type of beach is required for a dune?
High wave energy, wide surf zone, lots of bars Low and wide beach Least flow disturbance (flat beach) Large sediment availability Steepness reduces transport capacity
The angle the wind is blowing out compared where your beach is a vital dune formation consideration
Some way to get the sand from the intertidal zone (longshore transport and cross shore important), onto the beach and into the dune building area
Wind beach alignment crucial
Amount of beach exposure
Sediment movement processes
Saltation
Threshold wind speed required to entrain sediment
Very fine sediment more cohesive
Large sediment harder to entrain
Round quartz vs irregular shell fragments
Transport (dry vs. wet thresholds)
Wet sand is harder to entrain than dry sand
More cohesion so higher wind speed required
Meso-scale influences
An experiment to see how beach input to dunes varies over time:
Sediment availability- high tides and snow cover
The perfect conditions: when it is windy from the right direction, at low tide without any rain/snow
Strong winds often come with big waves, and/or rain
Moderate winds do the most work
Typical wind profile on a foredune
Wind speed up
Increase in wind speed close to dune crest
Particularly in larger dunes
Increased potential to transport sediment
Sometimes an Offshore wind is important
Recirculating wind within recirculation zone, or deflected from crest may be strong enough to transport sediment from the beach onto the dune.
Terrestrial laser scanning (TLS)
3D imprint of landscape High resolution (~1.2 mm) Surface topography Sand movement Vegetation
Sand transport on Protodune
Switch from erosion to deposition at 0.95m, upwind of crest
Reduced transport on strip lee; downwind
A bedform, only ~ 6 cm height can significantly influence transport!
Main coastal dune types
Foredunes (most common type) Blowouts (typically in association with foredunes) Parabolic dunes (more common on coasts with moderate to high wave/wind energy and sediment supply) Transgressive dunes (similar environments to parabolic, but limited vegetation e.g. aridity/cold)
Dunes have many uses
Natural resources (mining, water storage, crops)
Coastal protection
Tourism
Sand fences
Trap sand
Create new dunes, or fill in gaps Inexpensive and easy to construct
Different fence configurations
