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Year 3 Rivers And Sediment > Lecture 6 > Flashcards

Lecture 6 Flashcards

1
Q

What does sediment not just move as

A

Just individual grains

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2
Q

What is a bed form

A

A morphological feature generates by sediment movement

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3
Q

What are the two main theories for how flat beds move into bed forms

A

Bed defects and wave instability

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4
Q

What does the bed defect theory link to

A

What we know about turbulence and flow structure

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5
Q

What creates the initial defect in the stream bed

A

High shear stress on an area of bedform

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6
Q

What happens when the sweep stops moving with sediment

A

creates a small pile of sediment

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7
Q

What happens once the defect is created

A

high pressure upstream, low pressure down stream

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8
Q

What zone is created downstream of the newly created high pressure

A

A low pressure recirculation zone

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9
Q

What does the change in velcocity between the high pressure and low pressure give us

A

a shear layer

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10
Q

When the shear layer attaches to the bed what does this create

A

a reattachment points, area of new erosion

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11
Q

What is the concept of wave instability

A

substances of two different densities and speeds move over each other like kelvin helmholtz instabilities

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12
Q

What is the diagram used to show bed forms, shear stress and thier diameter

A

bed form phase diagram

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13
Q

What are the proposed 4 zones to look at in terms of the bed forms phase diagram

A

sand sub, sand super, gravel sub, gravel super

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14
Q

What is the other name for bed forms which are in the super critical flow

A

Upper flow regime

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15
Q

what are the main bed form types in sub critical flow on sand beds

A

Ripples and dunes

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16
Q

What are ripples restricted by

A

grain size

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17
Q

What are dunes restricted by

A

flow depth

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18
Q

What is the average height of ripples

A

3cm

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19
Q

What are ripples controlled by

A

flow right near the bed of the channel.

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20
Q

What size do dunes start at

A

4cm

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21
Q

How long can dunes grow to be

A

100’s meters

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22
Q

The dunewave length is how much times of the depth

A

5-7 times

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23
Q

How tall is the dune in relation to the depth

A

6-10th of the depth

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24
Q

What can part of the variability be down to in terms of dunes

A

hard to measure, different rivers show different values

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25
Q

What has been found out in terms of dune size in river systems

A

dunes have been seen to be larger that expected in small streams, and smaller than expected in larger streams.

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26
Q

Why might deeper channles have smaller dunes

A

higher shear stress=more erosion of the bed

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27
Q

Are dunes thought to be symetrical or asymetrical in river systems

A

asymentrial

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28
Q

What is the steeper side known as

A

Lee 20-35 degrees

29
Q

What is the shallower dunes face known as

A

Stoss 5-10 degrees

30
Q

What is the bottom part of the dune called

A

trough

31
Q

What is the top of the dune called

A

crest

32
Q

how do we work out the amplitude

A

trough height to crest height

33
Q

How can the crest line be described from a planform viewpoint

A

Sinous crestline

34
Q

What else is common with dunes in terms of sediment size

A

can see sediment sorting

35
Q

what are the 3 forms of movement in dune form

A

fine sediment ricurculation, coarse avalanches, sediment overshoot of shear layer

36
Q

What happens in high sediment transport rates

A

strong sorting, coase sediment avalanches with lots of fine sediment in ricurculation, may create regressive ripples

37
Q

What happens in low sediment transport rates

A

all the sediment moves together, poor sorting

38
Q

how can we see lee face deposition

A

can be preserved in the dunes as lines

39
Q

What happens when each dune scours out the next one down stream

A

Nothing preserved, brand new dune

40
Q

How do we get lee faces that are preserved

A

need a certain degree of aggredation

41
Q

When you get a high amount of agredation what can happen to dune bed forms

A

can stack on each other and be draped on one another

42
Q

How much of the lee face is generally preserved

A

1/3

43
Q

How can we work out the original dune height from whats left

A

multiply by 3, from this we can work out the depth

44
Q

What are the most common bed form features

A

dunes

45
Q

What information can we get from a dune form

A

flow direction flow depth, rate of agredation and sediment transport rate

46
Q

What sand forms are found in super critical flow

A

anti dunes

47
Q

What are anti dunes best defined by

A

standing waves

48
Q

Are there any flow circulation zones in anti dunes

A

no, super critical flow

49
Q

Which side of the anti dunes is erosional

A

the lee side

50
Q

Which way does an anto dune migrate

A

up stream

51
Q

What is a pebble cluster

A

when grains of sediment get trapped behind a larger pebble or rock

52
Q

What is the larger particle known as in a pebble cluster

A

key stone

53
Q

What zone is created after the pebble cluster

A

recirculation zone that traps finer particles

54
Q

How much of the river bed has pebble clusters

A

10%

55
Q

how are bed load sheets formed

A

much broader areas of the sub critical flow

56
Q

how large is the leading edge of a bed load sheet

A

just 2-3 grains thick

57
Q

what is formed in super critical gravel zones

A

transverse ribs

58
Q

What is a tranvserse rip

A

an alternating pattern of stripes, perpendicular to the flow

59
Q

What is a bed configuration

A

as assemblage of bed forms in a particular area

60
Q

When do ripples form

A

at the threshold of bed load motion

61
Q

What size must grains be under to form ripples

A

0.7mm

62
Q

where do ripples not occur

A

in mud, as it is soon entrained from the bed

63
Q

what causes the initiation and growth of ripples

A

turbulent variation in near bed flow velocity

64
Q

What is the maximum length of a ripple

A

60cm

65
Q

What is a lower plane bed

A

just above the threshold for motion, transitional flows

66
Q

What size do grains need to exceed to form dunes

A

0.1mm

67
Q

Why can ripples not form in coarse sediments

A

the bed instability related to local disruption of a viscous sub layer does not exist

68
Q

What does differential transport of bed load grains result in

A

size segregation of grain sizes

69
Q

Where are pebble clusters common

A

poorly sorted cobble clusters

Year 3 Rivers And Sediment (12 decks)

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