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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
Q

Do starved ripples migrate?

A

Yes

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26
Q

How can starved ripples be preserved?

A

If blanketed by mud

27
Q

What happens in cases of net addition of sand to a ripple?

A

Grows as it migrates, and the depth of the scour in the trough will be reduced

28
Q

What happens when the rate of sand addition to a ripple is particularly high?

A

Migration of ripple up the ripple in front - climbing ripples

29
Q

What does it mean if the angle of climb in climbing ripples is steeper?

A

High rate of deposition

30
Q

What does it mean if the angle of climb in climbing ripples is shallow?

A

Moderate rate of deposition

31
Q

What size must the dominant grain size be to form current ripples?

A

<0.6 mm diameter - coarse sand

32
Q

Why is there a restriction on how large a grain size can be to form current ripples?

A

Larger than coarse sand causes bed roughness which induces turbulent mixing

33
Q

What is the relationship between current ripple formation and water depth?

A

Independent of each other - can be few cm to km’s - other bedforms are water depth dependant, however.

34
Q

What is the maximum height of a current ripple?

A

40mm high

35
Q

What is the maximum length of a current ripple wavelength?

A

500mm

36
Q

What is the typical ratio of wavelength to height in a current ripple?

A

Somewhere between 10 and 40

37
Q

What does the limit of current ripple dimensions mean in terms of processes?

A

Current ripples do not grow into larger bedforms

38
Q

What controls flow separation and reattachment points in dune bedforms?

A

Water depth

39
Q

What would be a typical dune height in a water depth of a few m’s?

A

10’s cm high

40
Q

What would be a typical dune height in a water depth of 10’s m’s?

A

m’s high

41
Q

What is the typical thickness of a cross-bed?

A

> 1cm thick

42
Q

What occurs when a roller-vortex appears in a bedform?

A

Reverse flow up the lee side

43
Q

What is it called when the basal contact of a cross-bed is angular?

A

Planar cross bedding

44
Q

What is it called when the basal contact of a cross-bed is curved?

A

Tangential cross bedding

45
Q

Planar cross bedding is often ___, tangential cross bedding is more often ___

A

Tabular, trough

46
Q

What are counter flow ripples?

A

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
Q

What type of dune bedform signifies higher velocity flow?

A

Sinuous crested and Isolated dunes

48
Q

What is the wavelength range of dunes?

A

600mm to 10’s m

49
Q

What is the height range of dunes?

A

10’s mm to 10’s m

50
Q

What is the grain size range of dunes?

A

Fine gravel to fine sand

51
Q

Are dunes better developed in finer grain sizes or coarser grain sizes? Why?

A

Better in coarser

Increased suspended load, suppresses turbulence, flow separation does not occur

52
Q

What is the main constraint on dune formation?

A

Sustained flow

53
Q

What is required for cross-bedding to occur in a dune?

A

Migration of dune which requires a long lived flow

54
Q

What are the four facies that dunes are usually found within

A

River channels, Deltas, Estuaries, Shallow marine environment

55
Q

What is a bar form?

A

A subaqueous bedform which is the next scale up from a subaqueous dune

56
Q

How do we classify a bar form?

A

It’s position in the channel - side/alternate bars and mid-channel bars

57
Q

What is the geological name for “horizontal layering due to flow”?

A

Plane bedding

58
Q

What does plane bedding consist of?

A

Planar lamination

59
Q

What does the viscous sub-layer look like when planar laminations are forming?

A

Grain sizes are larger than the viscous sub-layer, the bed is rough and ripple formation does not occur

60
Q

__ 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

A

Coarse, Fine to Coarse

61
Q

How would one define a “plane bed”?

A

Well defined planar laminations which are 5 to 20 grains thick

62
Q

What are primary current lineations?

A

Ridges parallel to flow direction which are formed by sweeps of grains within the viscous sublayer

63
Q

When are primary current lineations subdued? (2)

A

When the bed surface is rough, or in coarser sands

64
Q

What is the name for the diagram which shows sediment size (x) vs mean flow velocity (y)?

A

Hjulstrom diagram/curve

Sedimentology and Stratigraphy (19 decks)

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