Processes of transport and sedimentary structures - Flows, Sediment and Bedforms - Current ripples
14 important questions on Processes of transport and sedimentary structures - Flows, Sediment and Bedforms - Current ripples
What are turbulent sweeps?
Irrgeulartities in flow in the viscous sublayer that move grains by rolling or saltation and create local clusters of grains
How does a current ripple begin to form? Start with turbulent sweeps and end with crest and lee side.
What is a through of a current ripple and how is it formed?
At the flow attachment point there are
increased stresses on the bed, which result in erosion
and the formation of a small scour, the trough of the
ripple.
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When viewed from above, 3 current ripple kinds can be distinguished, what are they and how can they be distinguished?
How can straight ripples form into linguoid ripples?
Name a morphological difference between straight and linguoid ripple crests
Straight and linguoid ripple crests create different patterns of cross-lamination in 3D.
What is the difference in formation between planar cross-lamination and trough cross-lamination? Which are caused by straight, sinuous and linguoid ripples?
A perfectly straight ripple would generate cross-laminae
that all dipped in the same direction and lay in the
same plane: this is planar cross-lamination
Sinuous and linguoid ripples have lee slope surfaces that
are curved, generating laminae that dip at an angle to
the flow as well as downstream. As linguoid ripples
migrate, curved cross-laminae are formed mainly in
the trough-shaped low areas between adjacent ripple
forms resulting in a pattern of trough cross-lamination
What are starved ripples and how are they preserved?
If there is a fixed amount of sand available the ripple will migrate
over the surface as a simple ripple form, with erosion in the troughs matching addition to the crests. These starved ripple forms are preserved if blanketed by mud.
What happens if the current is adding more sand particles
than it is carrying away (to the ripple formation)?
The amount of sand deposited
on the lee slope will be greater than that removed
from the stoss side. There will be a net addition of sand
to the ripple and it will grow as it migrates, but most
importantly, the depth of scour in the trough is reduced leaving cross-laminae created by earlier migrating ripples preserved. In this way a layer of cross-laminated sand is generated.
What are climbing ripples and how are they formed?
When the rate of addition of sand is high there will be no net removal of sand from the stoss side and each ripple will migrate up the stoss side of the ripple form in front.
What is necessary for climbing ripples to form and what do they indicate because of that?
Climbing ripples are therefore indicators of rapid sedimentation as their formation depends upon the addition of sand to the flow at a rate equal to or greater than the rate of downstream migration of the ripples.
(When the addition of sediment from the current exceeds the forward movement of the ripple, deposition will occur on the stoss side as well as on the lee side. )
What is required for the formation of current ripples?
- moderate flow velocities over a hydrodynamically smooth bed
- Sands in which the dominant grain size is less than 0.6mm (coarse sand grade) because bed roughness created by coarser sand creates turbulent mixing, which inhibits the smallscale flow separation required for ripple formation.
- Small scale flow separation
Why is the formation of current ripples independent to water depth, in contrast to subaqueous dunes and wave ripples?
Because ripple formation is controlled by processes
within the viscous sublayer.
Mention the dimensions of current ripples (eg wavelength, max height, ratio height-wavelength and ratio wavelength to grain size)
Current ripples can be up to 40mm high and the wavelengths (crest to crest or trough to trough distances) range up to 500mm. The ratio of the wavelength to the height is typically between 10 and 40. There is some evidence of a relationship between the ripple wavelength and the grain size, approximately 1000 to 1. It is important to note the upper limit to the dimensions of current ripples and to emphasise that ripples do not ‘grow’ into larger bedforms.
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