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Year 3 Rivers And Sediment > Lecture 5 > Flashcards

Lecture 5 Flashcards

1
Q

What is the bed load

A

Coarser material down at the base of the river

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2
Q

What force resists the motion on bed forms

A

Gravity

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3
Q

What can the drag component on the bed also be known as

A

The shear stress force

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4
Q

What is the force that moves grain up into the flow

A

The list component

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5
Q

Why do we normally ignore the lift force

A

Smaller than the shear stress force and really hard to measure

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6
Q

What is the force acting parallel to the bed surface

A

Shear stress/drag FD

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7
Q

What else can cause the particle to lift up into the flow

A

Slight pressure difference

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8
Q

What is the pivot angle

A

Angle around the point of contact with the next downstream grain, angle at which it will get out of the bed

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9
Q

What is entrainment

A

First initial movement into the river stream

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10
Q

What is sediment transport

A

Sediment moving across the bed

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11
Q

What can flow entrainment also be seen as

A

The threshold needed to get to in order to move a grain

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12
Q

What is the name of the diagram used to determine when sediments move

A

Hjulstrom curve

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13
Q

What is the draw back of the hjulstrom curve

A

Does not consider the forces just velocity

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14
Q

Who came up with the first model for particle entrainment

A

Shields 1936

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15
Q

What was Sheilds looking at with his flume experiments

A

Force balance on the grain

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16
Q

Why does the grain size determine the force stopping motion

A

Bigger the grain heavier it is

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17
Q

What was main factors shields was looking at in ratio

A

Shear stress and grain size

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18
Q

What did sheilds find about the number between the forces

A

Was always the same number

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19
Q

What is the ratio according to shields

A

0.06

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20
Q

What relationship is there between the shear stress and grain size

A

Linear

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21
Q

What is critical shear stress

A

At the point of entrainment

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22
Q

What is pW

A

Water density

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23
Q

What is pS

A

Sediment density

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24
Q

What kind of entrainment is proposed by shields

A

Size selective entrainment

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25
Q

What is the problem with shields equation

A

For rocks that stick out or for different sizes the entrainment value will be different

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26
Q

What sort of grain sizes was shields using

A

All the same size

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27
Q

What happens with gravel in most gravel beds

A

Beds become organised

28
Q

What is it called when gravel beds become organised

A

Pavement/armour layer

29
Q

What is coarser is gravel bed rivers surface or subsurface

A

Surface, small grains fall through to the sub surface

30
Q

What is petrusion

A

When large gravel a stick up into the flow

31
Q

What is the name for when small grains stop and are then carryed on in transport

A

Winnowing

32
Q

What is it called when small particles are out of the way of the flow

A

Hiding

33
Q

What is equal mobility an expression of

A

Relitive size

34
Q

What is important in terms of the particle and relation to others around it

A

Size of the particle relative to those around it

35
Q

When happens when larger particles trap other ones

A

No particle moves until the biggest one is moved, equal mobility

36
Q

What is the consensus now which sheilds work

A

Was important, but there are now more accurate studies that better show entrainment

37
Q

What did Powell and ashworth show about shear stress

A

Not one fixed number for critical dimensionless shear stress, between 0.0096 and 0.067

38
Q

What did Andrews say is the most important thing to consider about grain Size

A

Relative grain size is more important

39
Q

What value does tbh e pretending grain have in Andrews equation

A

Di

40
Q

What value does the surrounding particles have in Andrews equation

A

D50

41
Q

If there is perfect equal mobility what would rhe shear stress component equal

A

-1

42
Q

What is the equation for entrainment according to Andrews

A

Critical dimensionless shear stress= 0.0834(Di/D50) -0.872

43
Q

If there is no overlap between shear stress and shear stress required for movement what will happen

A

Nothing, no movement of particles

44
Q

How do we get entrainment in a river system

A

Shear stress needs to over lap with shear stress required for entrainment

45
Q

What is transport

A

The volume of sediment moved by each transport event

46
Q

What is qb

A

Bed load discharge, about of sediment in transport

47
Q

What cancels out level values of shear stress

A

Bed load pulses

48
Q

What does Reid et al 1985 show us about shear stress

A

Not a simple relationship between that and pulses

49
Q

What did Gomez and church find out

A

Looked at 12 sediment transport equations and found that none were particularly accurate best was bagnold

50
Q

For hydraulically smooth boundaries what drag dominates

A

Viscous drag

51
Q

For hydraulically rough boundaries what drag dominates

A

Form drag

52
Q

Is there any flow seperation around a grain in laminar flow

A

No

53
Q

is there any flow separation around a grain in rough flow

A

yes

54
Q

What does the level of effective drag depend on

A

Depends on the shape and position of the grain on the bed

55
Q

Which sediment grains move close to the bed

A

more than 0.1mm

56
Q

Why do larger grains near the bed move slowly

A

Intermittent collision with the bed

57
Q

What is the name for the larger particles that are in contact with the bed while moving

A

bed load

58
Q

What is necessary to continue movement of the bed load

A

Upward dispersive force that must be exactly balanced by the weight of the moving grains (Bagnold 66 and 73)

59
Q

What is Saltation

A

Jumping, the dominant mode of bed load transport, with rolling and sliding only at the threshold of entrainment

60
Q

How does jumping occur

A

as particles are forced over a bed with increasing velocity, they often are forced in to the air through contact with other rocks

61
Q

What does the mean height of the bed-load zone control

A

roughness height

62
Q

Who came up with the idea of shear stress and shear stress needed for entrinmant graph

A

Grass 1970

63
Q

what is the sediment transport rate

A

the amount of sediment that can be moved past a given width of flow in a given time

64
Q

What does sediment transport rate control

A

the formation of bedforms such as ripples and dunes

65
Q

How can the bed-load transport rate be expressed

A

ib=Wub, ib is the bed load transport rate, W is the immersed weight of bed load grains and ub is the mean bed load grain

66
Q

How do grains stay in transport motion

A

The fluid must exert a mean downstream force to maintain steady motion

Year 3 Rivers And Sediment (12 decks)

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