Lecture 6

Question 1

What does sediment not just move as

Answer 1

Just individual grains

Question 2

What is a bed form

Answer 2

A morphological feature generates by sediment movement

Question 3

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

Answer 3

Bed defects and wave instability

Question 4

What does the bed defect theory link to

Answer 4

What we know about turbulence and flow structure

Question 5

What creates the initial defect in the stream bed

Answer 5

High shear stress on an area of bedform

Question 6

What happens when the sweep stops moving with sediment

Answer 6

creates a small pile of sediment

Question 7

What happens once the defect is created

Answer 7

high pressure upstream, low pressure down stream

Question 8

What zone is created downstream of the newly created high pressure

Answer 8

A low pressure recirculation zone

Question 9

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

Answer 9

a shear layer

Question 10

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

Answer 10

a reattachment points, area of new erosion

Question 11

What is the concept of wave instability

Answer 11

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

Question 12

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

Answer 12

bed form phase diagram

Question 13

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

Answer 13

sand sub, sand super, gravel sub, gravel super

Question 14

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

Answer 14

Upper flow regime

Question 15

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

Answer 15

Ripples and dunes

Question 16

What are ripples restricted by

Answer 16

grain size

Question 17

What are dunes restricted by

Answer 17

flow depth

Question 18

What is the average height of ripples

Answer 18

3cm

Question 19

What are ripples controlled by

Answer 19

flow right near the bed of the channel.

Question 20

What size do dunes start at

Answer 20

4cm

Question 21

How long can dunes grow to be

Answer 21

100’s meters

Question 22

The dunewave length is how much times of the depth

Answer 22

5-7 times

Question 23

How tall is the dune in relation to the depth

Answer 23

6-10th of the depth

Question 24

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

Answer 24

hard to measure, different rivers show different values

Question 25

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

Answer 25

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

Question 26

Why might deeper channles have smaller dunes

Answer 26

higher shear stress=more erosion of the bed

Question 27

Are dunes thought to be symetrical or asymetrical in river systems

Answer 27

asymentrial

Question 28

What is the steeper side known as

Answer 28

Lee 20-35 degrees

Question 29

What is the shallower dunes face known as

Answer 29

Stoss 5-10 degrees

Question 30

What is the bottom part of the dune called

Answer 30

trough

Question 31

What is the top of the dune called

Answer 31

crest

Question 32

how do we work out the amplitude

Answer 32

trough height to crest height

Question 33

How can the crest line be described from a planform viewpoint

Answer 33

Sinous crestline

Question 34

What else is common with dunes in terms of sediment size

Answer 34

can see sediment sorting

Question 35

what are the 3 forms of movement in dune form

Answer 35

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

Question 36

What happens in high sediment transport rates

Answer 36

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

Question 37

What happens in low sediment transport rates

Answer 37

all the sediment moves together, poor sorting

Question 38

how can we see lee face deposition

Answer 38

can be preserved in the dunes as lines

Question 39

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

Answer 39

Nothing preserved, brand new dune

Question 40

How do we get lee faces that are preserved

Answer 40

need a certain degree of aggredation

Question 41

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

Answer 41

can stack on each other and be draped on one another

Question 42

How much of the lee face is generally preserved

Answer 42

1/3

Question 43

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

Answer 43

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

Question 44

What are the most common bed form features

Answer 44

dunes

Question 45

What information can we get from a dune form

Answer 45

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

Question 46

What sand forms are found in super critical flow

Answer 46

anti dunes

Question 47

What are anti dunes best defined by

Answer 47

standing waves

Question 48

Are there any flow circulation zones in anti dunes

Answer 48

no, super critical flow

Question 49

Which side of the anti dunes is erosional

Answer 49

the lee side

Question 50

Which way does an anto dune migrate

Answer 50

up stream

Question 51

What is a pebble cluster

Answer 51

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

Question 52

What is the larger particle known as in a pebble cluster

Answer 52

key stone

Question 53

What zone is created after the pebble cluster

Answer 53

recirculation zone that traps finer particles

Question 54

How much of the river bed has pebble clusters

Answer 54

10%

Question 55

how are bed load sheets formed

Answer 55

much broader areas of the sub critical flow

Question 56

how large is the leading edge of a bed load sheet

Answer 56

just 2-3 grains thick

Question 57

what is formed in super critical gravel zones

Answer 57

transverse ribs

Question 58

What is a tranvserse rip

Answer 58

an alternating pattern of stripes, perpendicular to the flow

Question 59

What is a bed configuration

Answer 59

as assemblage of bed forms in a particular area

Question 60

When do ripples form

Answer 60

at the threshold of bed load motion

Question 61

What size must grains be under to form ripples

Answer 61

0.7mm

Question 62

where do ripples not occur

Answer 62

in mud, as it is soon entrained from the bed

Question 63

what causes the initiation and growth of ripples

Answer 63

turbulent variation in near bed flow velocity

Question 64

What is the maximum length of a ripple

Answer 64

60cm

Question 65

What is a lower plane bed

Answer 65

just above the threshold for motion, transitional flows

Question 66

What size do grains need to exceed to form dunes

Answer 66

0.1mm

Question 67

Why can ripples not form in coarse sediments

Answer 67

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

Question 68

What does differential transport of bed load grains result in

Answer 68

size segregation of grain sizes

Question 69

Where are pebble clusters common

Answer 69

poorly sorted cobble clusters