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Year 3 Rivers And Sediment > Lecture 9 > Flashcards

Lecture 9 Flashcards

1
Q

What controls the diversity of channel shape

A

Bed load, slope, stream power, bank strength

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2
Q

How does clay and silt change river shape

A

More suspension, therefore fewer bar form, more cohesive structures less bank erosion

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3
Q

If everything is moving as suspended load what channel will form

A

Straight, low sinuosity

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4
Q

What will increasing discharge lead to

A

Increasing stream power

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5
Q

Why can’t we think of stream power seperatly

A

Works in relation with other factors

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6
Q

What factor is said to be largely erroneous

A

Discharge regularity

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7
Q

How does slope effect river shape

A

Links with shear stress, can create more erosion

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8
Q

What is required for gravel bar formation

A

Higher shear stress, more strean power

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9
Q

What can be said about braided river channels

A

They are in continuum

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10
Q

What might one river system have

A

Multiple channel systems

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11
Q

How can river channels change temporally

A

They can evolve and move across time going through different channel phases

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12
Q

What is a river called where there is more than 1 channel belt

A

Anabranching rivers

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13
Q

What is a anastamosed channel

A

Multiple belts but fixed, not wandering

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14
Q

What defines a wandering river system

A

Weaker banks, still anabrancjmhing but avulsion and channel movement takes place

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15
Q

Where will we get highly braided channels

A

High stream power and smallish grains

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16
Q

Which channel features can be found across all types of channel

A

Ripples, dunes, antidunes ect

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17
Q

What determines bar form size

A

Length proportional to local channel width and height proportion to channel depth

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18
Q

What is the difference in bars between braided and meander

A

Meander are attached to the bank, point bar

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19
Q

What is river sinuosity

A

Valley distance and river distance, channel length divided by valley length

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20
Q

Above what value gives us a braided river

A

1.5

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21
Q

Where does the unit bar come from

A

An amalgamation of dune forms

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22
Q

Which part of the uni bar is highest

A

The end, build up of sediment

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23
Q

Where does erosion occur in a bar system

A

On the banks if banks are erodable

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24
Q

Where is deposition in meander unibars

A

Over lower slower flow inside of the bend

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25
Q

At high river stage what can form on the river bars

A

Cross bar channels

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26
Q

When do cross bar channels form

A

When flow is dropping, as water is draining off if the bar

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27
Q

What is bar push

A

Inner bar deposition

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28
Q

What is bank pull

A

Outer bank deposition

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29
Q

What does bar push do

A

Builds inner bar which deflects flow to other bank

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30
Q

How does bank pull work

A

Bank erosion is fast which pulls channel with it

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31
Q

What is the area of fastest flow in the river

A

Thalweg

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32
Q

How do meanders tend to migrate

A

Downstream

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33
Q

What has happened to pool after downstream meander migration

A

Pool is filled in

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34
Q

What fills in the pool

A

Point bar

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35
Q

Where are the most coarse particles in the flow

A

Deepest bit of the pool

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36
Q

What is the grain size of the bar

A

Fine material

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37
Q

From the bed up how does material change

A

Fining up, big at the bottom, fine at the top

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38
Q

What are each time step of downstream meander movement called

A

Lateral accretion surfaces

39
Q

What are the 4 main ways that meander bends move

A

Expansion, translation, wavelength change, rotation

40
Q

What is translation

A

Same size of meander moves downstream, both wavelength or amplitude change, 50% of all migration

41
Q

What is meander expansion

A

Right at the bend apex, migrates away from the river

42
Q

In an expanding meander what is different

A

Point bar preserved on both sides

43
Q

What is paleohydraulics

A

Empirical relationships between meandering river parameters and flow discharge

44
Q

How can we infer meander wavelength

A

Mean annual discharge

45
Q

What is chute cutoff

A

Flow goes across the point bar at high discharge, can create new dominate channels

46
Q

What is the name for the bar that stops the flow of the original channel

A

Plug channel

47
Q

What sediment are plug bars made of

A

Very fine grained sediment

48
Q

What is neck cutoff

A

Meander cuts across a close section of a meander creating an “oxbow” lake

49
Q

What is radius of curvature

A

Radius of circle that fits on the band

50
Q

What is outer bank deposition

A

Tight bends, change of direction difficult for flow to get round, bar forms on outside of the bend

51
Q

What grains on the inside of the bend is outer bank deposition

A

Much finer

52
Q

why is an alluvial channel distinctive (bridge)

A

it has finite width and depth

53
Q

What does the sediment boundary control (bridge)

A

Both water flow and sediment transport

54
Q

What does the river bed evolve towards (bridge)

A

A statistically constant geometry composed of bars that’s are in steady equilibrium with flow and sediment transport

55
Q

What is an alternate bar

A

bar occurring on altering sides of the river channel

56
Q

What migration rate is faster, compound, unit or point bars

A

Unit

57
Q

What have mobile unit bars been known as

A

free bars

58
Q

What are the main controls of alluvial rivers geometry

A

Flow and sediment

59
Q

How can flow change river geometry

A

High flows can move and shape sediment, past the capabilities of normal channel flow

60
Q

What is channel forming discharge

A

A single discharge measure that represents the flood discharge needed for change in geometry of river channels (bridge)

61
Q

What is mean size of bed sediment proportional to

A

Channel slope

62
Q

What is channel roughness related to

A

Grain size and stream power

63
Q

What must sediment transport rate equal for channel equalibrium

A

Sediment supply rate

64
Q

What happens If sediment supply rate is reduced

A

Degradation will occur

65
Q

What happens to the shape of a river when valley slope is increased

A

Width and braiding increases

66
Q

What controls sediment transport rate

A

Shear stress and stream power

67
Q

What is the common myth of discharge variability

A

Variability is greater for braided rivers, not true

68
Q

How does vegetation stop bank erosion

A

Helps stop bank cut and bar surfaces given time for development

69
Q

What other factors does hayashi and okazaki say predict channel patterns

A

Froude number

70
Q

According to bettes and white if channel slope is equal to valley slope what will happen

A

Channel remains straight

71
Q

What will happen if channel slope is greater than valley slope

A

Aggregation will occur in non equalibrium

72
Q

What happens If the valley slope is greater than channel slope

A

Meander or braided river

73
Q

Which part of the meander bend has the largest sediment

A

deepest point, outside

74
Q

During rising flow levels where does erosion take place in meanders

A

thalwegs, confluence scours and upstream ends of bars, will gain deposition in falling flow

75
Q

What are meandering systems explained by

A

bend instability

76
Q

Why can meander bends be seen as dynamic

A

bends move through bank erosion (Van Dijk)

77
Q

What is erosion balanced by

A

Floodplain formation on the other side (Van Dijk)

78
Q

What is meander bend growth managed by

A

Neck cutoff

79
Q

What is the relationship between bars and bends

A

They ‘resonate’ at particular width depth ratios, leading to stationary alternate bars

80
Q

What is bend development limited by

A

non linear processes e.g cutoffs and channel adjustment

81
Q

When are neck cutoffs common

A

Rivers with high sinuosity

82
Q

When are chute cut offs common

A

Rivers with low sinuosity

83
Q

How can swale cause chute cut offs

A

Swales capture overbank flow and incise until river cut off

84
Q

How can aggradation cause a meander

A

build up of sediment reduces the rivers capacity for flow and so some water spills over lower banks

85
Q

How can embayment cause a meander

A

creates erosion upstream of the pointbar, leading to the creation of a channel

86
Q

What happens when the banks of the river are the same non cohesive sediment as the beds

A

the banks erode so that the river becomes wider and shallower

87
Q

How did the transverse bed slope change with the channel

A

it increased with decreasing bend radii, often deeper than expected due to over deepening in the bend

88
Q

What can stop a meandering channel turning into a braided river

A

rapid closure of abandoned channels

89
Q

when did single chute cutoffs occur (van Dijk)

A

as the bend migrated downstream

90
Q

what promotes the development of dynamic meandering

A

poorly sorted sediment, and dynamic in flow (in a flume, dijk)

91
Q

when will a sinuous channel stop migrating

A

a critical bend amplitude in reached, leading to cut off

92
Q

What is a meandering amplitude

A

Distance of the meander size between the meanders

93
Q

What is wavelength and radius scaled to

A

Stream size

94
Q

Why is the relationship between amplitude and width less constant

A

Because width controlled by bank erosion

Year 3 Rivers And Sediment (12 decks)

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