Processes of transport and sedimentary structures - Flows, Sediment and Bedforms - Dunes
13 important questions on Processes of transport and sedimentary structures - Flows, Sediment and Bedforms - Dunes
What is the definition of dunes, sometimes also called megaripples?
Beds of sand in rivers, estuaries, beaches and marine
environments also have bedforms that are distinctly
larger than ripples. The formation of dunes can be related to large-scale turbulence within the whole flow.
What are distinct similarities and differences between dunes and ripples?
Differences: Dunes are depth-dependent. The water depth controls the scale of the turbulent eddies in the flow and this in turn controls the height and wavelength of the dunes.
What are cross beds and do they form in ripples or dunes?
Migration of a subaqueous dune
results in the construction of a succession of sloping
layers formed by the avalanching on the lee slope and
these are referred to as cross-beds.
DUNES
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How is a roller vortex formed and does the roller vortex occur in dunes/ripples?
Flow separation creates a zone in front of the lee slope in which a
roller vortex with reverse flow can form.
Dunes
What are tabular cross-beddings and do they form in dunes or ripples?
Cross-beds bound by horizontal surfaces are sometimes referred
to as tabular cross-bedding.
Dunes
What happens to the roller vortex during high velocities? What type of morphology does this develop?
the base of the slip face that may be strong enough
to generate ripples (counter-flow ripples), which
migrate a short distance up the toe of the lee slope.
DUNES
What is trough cross-bedding, how is it formed and does it occur in dunes or ripples?
Trough cross-bedding formed by the migration of sinuous subaqueous dunes typically has asymptotic bottom contacts and an undulating lower boundary.
Elaborative version:
A further effect of the stronger/high velocity flow is the creation
of a marked scour pit at the reattachment point. The
avalanche lee slope advances into this scoured trough
so the bases of the cross-beds are marked by an undulating
erosion surface. The crest of a subaqueous dune
formed under these conditions will be highly sinuous
or will have broken up into a series of linguoid dune
forms.
DUNES
What are the order of magnitude of the sizes of dunes? Mention wavelengths, heights and what their size is in relation to ripples.
Dunes range in size from having wavelengths of about 600mm and heights of a few tens of millimetres to wavelengths of hundreds of metres and heights of over ten metres. The smallest are larger than the biggest ripples.
In which grain sizes do dunes form?
Dunes can form in a range of grain sizes from fine gravels to fine sands, but they are less well developed in finer deposits and do not occur in very fine sands or silts.
Why can dunes not form in very small sediment grains?
This grain size limitation is
thought to be related to the increased suspended load
in the flow if the finer grain sizes are dominant: the
suspended load suppresses turbulence in the flow and
flow separation does not occur
Mention two conditional requirements for dunes to develop?
- Turbulence
- Grain sizes larger than very fine sands or silts.
- Flow to be sustained long enough for a dune to build up
What is a conditional requirement for cross-bedding to develop in a dune?
short-lived flow events.
Mention the 4 environments in which dunes are most commonly encountered. Hint: where there are relatively strong, sustained flows.
- River channels
- Deltas
- Estuaries
- Shallow marine environments
where there are relatively strong, sustained flows
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