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Year 3 Rivers And Sediment > Lecture 3 > Flashcards

Lecture 3 Flashcards

1
Q

What is the Law of the wall method

A

Shear stress indicator used for one point, predicts flow velocity using logarithmic chart

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2
Q

What proportion of the flow is used to measure the shear stress

A

Bottom 15-20%

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3
Q

If velocity and log depths are plotted together what should it be shown as

A

A straight line

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4
Q

How is the shear stress calculated

A

From the gradient of the line in the graph

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5
Q

If water is moving together with little deviation what will there be less of

A

Shear, rate of change will be quite low

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6
Q

Even in calm flows how is the flow structure organised

A

In grouped bands

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7
Q

What are areas in the flow that are travelling slower called

A

Low speed streaks

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8
Q

Why are low speed steaks important in turbulence creation

A

They can lead to bursting effects due to rolling on the bed

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9
Q

What happens at low speed zones

A

Flow starts to roll over

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10
Q

What is the shape of the low speed roll when it lifts up into the flow

A

Horseshoe

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11
Q

What is the term when water is ejected up into the flow

A

Bursting

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12
Q

Where is the highest velocity in ejection

A

Middle of the flow as it’s getting lifted up

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13
Q

What is flow called when it falls down towards the bed

A

Sweeping flow

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14
Q

What does sweeping flow give us

A

Higher shear stress

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15
Q

What does bursting do sediment wise

A

Brings sediment to the main flow of the channel

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16
Q

What is created in larger topographic beds

A

Higher and low pressure zones

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17
Q

What happens when eddies become big enough

A

They move out into the flow

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18
Q

What is the name of the instabilities that are created when two different flow structures meet

A

Kelvin helmholtz instabilities

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19
Q

What do kelvin helmholtz create

A

Boils which go up into the flow

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20
Q

What is the strouhal relationship

A

Dimensionless number which shows the frequency of eddy shedding, as the dune changes size so does the shedding

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21
Q

Where can you get flow separation in a planform sense

A

Round a bend in a river especially in right bends

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22
Q

In terms of direction which way does U normally go in the river

A

Downstream

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23
Q

What effects variability in flow scales

A

Time

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24
Q

What helps us to understand how turbulent a flow is

A

Variation away from the mean

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25
Q

What are the negatives to the reach average method

A

Broad brush estimate which has nothing on the spatial resolution

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26
Q

Which river see the most sediment yields

A

Low rainfall of 250mm to 350mm a year

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27
Q

When does sediment yield reach a minimum

A

Rainfall over 750mm, due to vegetation cover

28
Q

Why are gravel bed rivers irregular on the bed

A

Presence of morphological features occurring at different spatial scales

29
Q

What are the 3 scales of the river bed

A

Grain scale, microtopographic scale and the large scale bed undulations

30
Q

What did Brayshaw say was the percentage coverage of pebble clusters

A

10-20%

31
Q

What are the 3 sections of turbulent reactions over pebble clusters

A

Acceleration,shedding, upwelling

32
Q

what are flows called that do not vary in velocity or cross sectional area

A

uniform flows

33
Q

What is the name for when water accelerates due to movement into a narrower cross section

A

convective acceleration

34
Q

What are flows called that vary in cross sectional area and velocity vary

A

non-uniform flows

35
Q

What are non-uniform decelerating convection associated with

A

deposition of sediment

36
Q

how can we define forces in river science

A

the rate of change of momentum

37
Q

What is a body force

A

force that acts from distance on everything and every part of a substance (gravity)

38
Q

What is a surface force

A

act by direct contact between the surface of a substance and the medium applying the force

39
Q

What is hydrostatic pressure

A

pressure at a point caused by the mass of water and air being pushed down by gravity

40
Q

Where will we find the highest hydrostatic forces on the banks of rivers

A

where the river reach is the deepest

41
Q

what creates the buoyancy force on grains

A

larger hydrostatic pressure on the top than on the bottom

42
Q

What are the curved swirling motions in turbulence fluids

A

turbulent eddies

43
Q

What causes a kelvin Helmholtz instability

A

difference between rates of velocity

44
Q

Why does water flow slower at the bed than in the main flow channel

A

friction at the base of the flow

45
Q

What gradient is created when water decelerates towards the bed

A

velocity gradient

46
Q

What is the name of the layer where a velocity gradient exists as well as shear stress

A

boundary layer

47
Q

What is the name of flow where there is no velocity gradient or shear stress

A

free or external stream

48
Q

What is the name of the thin less than a mm thick layer where flow behaves as if it was laminar

A

viscous sublayer

49
Q

What are the 3 sections of the boundary layer

A

outer, inner and viscous sublayer

50
Q

what motions are created in the viscous sublayer

A

Taylor-Gortler vortices

51
Q

What do Taylor-Gortler Vortices create

A

flow streaks where a common flow between the vortices is seen

52
Q

What is another name for the inner layer

A

buffer layer

53
Q

what is the size of the inner layer in relation to the viscous sub layer

A

3-7 times the size

54
Q

What forms in the inner layer

A

intense small scale turbulence eddies

55
Q

What is flow like in the inner layer

A

transitional between pseudo-laminar and turbulent

56
Q

What happens to low speed streaks after they form

A

they oscillate and rise rapidly into the outer zone where they mix with other eddies

57
Q

What happens in the space left behind by the rising ow speed streak

A

filled by a falling sweep eddy

58
Q

In rougher beds what happens to the bursting process

A

much more rapid and sudden

59
Q

What happens to the velocity profile during a bursting event

A

bed velocity speeds up

60
Q

what is the structure of horseshoe bursting events

A

a primary hairpin, with multiple subsidiary hairpins

61
Q

what is the point where the shear layer reconnects to the bed

A

re-attachment point

62
Q

what does three dimensional structure of flow patterns influence

A

local bed scour, sediment transport and downstream bar growth

63
Q

what do channel junctions control

A

downstream flow mixing. sediment transfer, bar evolution and channel change (mclellend)

64
Q

what is the basic planform shape of river junctions

A

Y shaped or, |- shaped

65
Q

when two channels first meet, what is the nature of the river channel in a Y channel

A

balanced

66
Q

what will happen to the Y shape junction downstream of connection

A

flow will find a new thalweg and recirculation zones will change

Year 3 Rivers And Sediment (12 decks)

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