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Sedimentology and Stratigraphy > (TA) Flows, Sediment and Bedforms > Flashcards

(TA) Flows, Sediment and Bedforms Flashcards

1
Q

What is a bedform? Give examples

A

The non-static metamorphological feature formed by the interaction of flow and cohesionless sediment on a bed. Produces the sedimentary structure. I.e. Dunes, Ripples, Ergs

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2
Q

What is a sedimentary structure?

A

Internal structure to a bedform

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3
Q

What kind of information can a sedimentary structure provide? (3)

A

Current strength, flow depth, direction of sediment transport

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4
Q

What are the three layers of flow depth?

A

Viscous sublayer, Boundary layer, free stream

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5
Q

What is the “free stream”?

A

Not affected by boundary effects

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6
Q

What is the “boundary layer”?

A

Velocity begins to decrease due to friction with the bed

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7
Q

What is the “viscous sublayer”?

A

Reduced turbulence, usually <1mm thick, thickness decreases with increasing flow velocity yet is independent of flow depth

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8
Q

What defines the smoothness/roughness of the bed?

A

Relationship between thickness of viscous sub-layer and size of grains on bed

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9
Q

What is the technical definition of “hydraulically smooth”?

A

All particles are within the viscous sublayer

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10
Q

What is the technical definition of “hydraulically rough”?

A

Particles project up through the viscous sublayer

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11
Q

Why is the viscous sublayer important?

A

Roughness/smoothness fundamental for the formation of various bedforms

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12
Q

What are the fundamentals for the formation of current ripples?

A

Turbulent sweet moves grains by rolling/saltation to create local clusters in the viscous sublayer - build up to create channels of increased flow.

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13
Q

What is the separation point of a bedform?

A

Point at the top of the bedform where flow detaches

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14
Q

What is the attachment point of a bedform?

A

Point at the bottom of a bedform where the flow reattaches causing increased turbulence and erosion

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15
Q

What is the separation bubble?

A

The area on the lee side of a new bedform which is in the shadow of the bedform, and underneath the flow passing over

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16
Q

How does the Bernoulli effect come into play in the formation of a ripple?

A

Expansion of flow over the top of the ripple causes an increase in pressure and transport is suddenly reduced, causing deposition on the lee side

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17
Q

What is the maximum critical slope angle for sand?

A

30 degrees

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18
Q

What is cross-laminae?

A

Sedimentary structure formed by a ripple bedform, from the movement of the detachment and attachment surfaces

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19
Q

Name three forms of current ripples seen in plan view

A

Straight crested ripple, sinuous crested ripple, linguoid ripple

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20
Q

What is an “linguoid ripple”?

A

An unconnected arculate ripple

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21
Q

How does the plan view of ripples tend to evolve?

A

From straight crested ripples to linguoid with time and higher velocities

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22
Q

What type of bedform is likely to form trough cross-lamination?

A

Linguoid ripples/sinuous crested ripples

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23
Q

What type of bedform is likely to form planar cross-lamination?

A

Straight crested ripples

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24
Q

What is the name of a ripple with a fixed amount of sand?

A

Starved ripple

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25
Do starved ripples migrate?
Yes
26
How can starved ripples be preserved?
If blanketed by mud
27
What happens in cases of net addition of sand to a ripple?
Grows as it migrates, and the depth of the scour in the trough will be reduced
28
What happens when the rate of sand addition to a ripple is particularly high?
Migration of ripple up the ripple in front - climbing ripples
29
What does it mean if the angle of climb in climbing ripples is steeper?
High rate of deposition
30
What does it mean if the angle of climb in climbing ripples is shallow?
Moderate rate of deposition
31
What size must the dominant grain size be to form current ripples?
<0.6 mm diameter - coarse sand
32
Why is there a restriction on how large a grain size can be to form current ripples?
Larger than coarse sand causes bed roughness which induces turbulent mixing
33
What is the relationship between current ripple formation and water depth?
Independent of each other - can be few cm to km's - other bedforms are water depth dependant, however.
34
What is the maximum height of a current ripple?
40mm high
35
What is the maximum length of a current ripple wavelength?
500mm
36
What is the typical ratio of wavelength to height in a current ripple?
Somewhere between 10 and 40
37
What does the limit of current ripple dimensions mean in terms of processes?
Current ripples do not grow into larger bedforms
38
What controls flow separation and reattachment points in dune bedforms?
Water depth
39
What would be a typical dune height in a water depth of a few m's?
10's cm high
40
What would be a typical dune height in a water depth of 10's m's?
m's high
41
What is the typical thickness of a cross-bed?
>1cm thick
42
What occurs when a roller-vortex appears in a bedform?
Reverse flow up the lee side
43
What is it called when the basal contact of a cross-bed is angular?
Planar cross bedding
44
What is it called when the basal contact of a cross-bed is curved?
Tangential cross bedding
45
Planar cross bedding is often ___, tangential cross bedding is more often ___
Tabular, trough
46
What are counter flow ripples?
When a roller-vortex is well developed, small ripples form on the base of the lee side of the dune heading back up to the peak
47
What type of dune bedform signifies higher velocity flow?
Sinuous crested and Isolated dunes
48
What is the wavelength range of dunes?
600mm to 10's m
49
What is the height range of dunes?
10's mm to 10's m
50
What is the grain size range of dunes?
Fine gravel to fine sand
51
Are dunes better developed in finer grain sizes or coarser grain sizes? Why?
Better in coarser | Increased suspended load, suppresses turbulence, flow separation does not occur
52
What is the main constraint on dune formation?
Sustained flow
53
What is required for cross-bedding to occur in a dune?
Migration of dune which requires a long lived flow
54
What are the four facies that dunes are usually found within
River channels, Deltas, Estuaries, Shallow marine environment
55
What is a bar form?
A subaqueous bedform which is the next scale up from a subaqueous dune
56
How do we classify a bar form?
It's position in the channel - side/alternate bars and mid-channel bars
57
What is the geological name for "horizontal layering due to flow"?
Plane bedding
58
What does plane bedding consist of?
Planar lamination
59
What does the viscous sub-layer look like when planar laminations are forming?
Grain sizes are larger than the viscous sub-layer, the bed is rough and ripple formation does not occur
60
__ size sand particles generate planar lamination at low velocities. As speeds increase, dune bedforms are generated. At very high velocities in __ to __ size sand particles, planar lamination is observed again
Coarse, Fine to Coarse
61
How would one define a "plane bed"?
Well defined planar laminations which are 5 to 20 grains thick
62
What are primary current lineations?
Ridges parallel to flow direction which are formed by sweeps of grains within the viscous sublayer
63
When are primary current lineations subdued? (2)
When the bed surface is rough, or in coarser sands
64
What is the name for the diagram which shows sediment size (x) vs mean flow velocity (y)?
Hjulstrom diagram/curve

Sedimentology and Stratigraphy (19 decks)

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