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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
What is the problem with shields equation
For rocks that stick out or for different sizes the entrainment value will be different
26
What sort of grain sizes was shields using
All the same size
27
What happens with gravel in most gravel beds
Beds become organised
28
What is it called when gravel beds become organised
Pavement/armour layer
29
What is coarser is gravel bed rivers surface or subsurface
Surface, small grains fall through to the sub surface
30
What is petrusion
When large gravel a stick up into the flow
31
What is the name for when small grains stop and are then carryed on in transport
Winnowing
32
What is it called when small particles are out of the way of the flow
Hiding
33
What is equal mobility an expression of
Relitive size
34
What is important in terms of the particle and relation to others around it
Size of the particle relative to those around it
35
When happens when larger particles trap other ones
No particle moves until the biggest one is moved, equal mobility
36
What is the consensus now which sheilds work
Was important, but there are now more accurate studies that better show entrainment
37
What did Powell and ashworth show about shear stress
Not one fixed number for critical dimensionless shear stress, between 0.0096 and 0.067
38
What did Andrews say is the most important thing to consider about grain Size
Relative grain size is more important
39
What value does tbh e pretending grain have in Andrews equation
Di
40
What value does the surrounding particles have in Andrews equation
D50
41
If there is perfect equal mobility what would rhe shear stress component equal
-1
42
What is the equation for entrainment according to Andrews
Critical dimensionless shear stress= 0.0834(Di/D50) -0.872
43
If there is no overlap between shear stress and shear stress required for movement what will happen
Nothing, no movement of particles
44
How do we get entrainment in a river system
Shear stress needs to over lap with shear stress required for entrainment
45
What is transport
The volume of sediment moved by each transport event
46
What is qb
Bed load discharge, about of sediment in transport
47
What cancels out level values of shear stress
Bed load pulses
48
What does Reid et al 1985 show us about shear stress
Not a simple relationship between that and pulses
49
What did Gomez and church find out
Looked at 12 sediment transport equations and found that none were particularly accurate best was bagnold
50
For hydraulically smooth boundaries what drag dominates
Viscous drag
51
For hydraulically rough boundaries what drag dominates
Form drag
52
Is there any flow seperation around a grain in laminar flow
No
53
is there any flow separation around a grain in rough flow
yes
54
What does the level of effective drag depend on
Depends on the shape and position of the grain on the bed
55
Which sediment grains move close to the bed
more than 0.1mm
56
Why do larger grains near the bed move slowly
Intermittent collision with the bed
57
What is the name for the larger particles that are in contact with the bed while moving
bed load
58
What is necessary to continue movement of the bed load
Upward dispersive force that must be exactly balanced by the weight of the moving grains (Bagnold 66 and 73)
59
What is Saltation
Jumping, the dominant mode of bed load transport, with rolling and sliding only at the threshold of entrainment
60
How does jumping occur
as particles are forced over a bed with increasing velocity, they often are forced in to the air through contact with other rocks
61
What does the mean height of the bed-load zone control
roughness height
62
Who came up with the idea of shear stress and shear stress needed for entrinmant graph
Grass 1970
63
what is the sediment transport rate
the amount of sediment that can be moved past a given width of flow in a given time
64
What does sediment transport rate control
the formation of bedforms such as ripples and dunes
65
How can the bed-load transport rate be expressed
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
How do grains stay in transport motion
The fluid must exert a mean downstream force to maintain steady motion

Year 3 Rivers And Sediment (12 decks)

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