Lecture 3

Question 1

What is the Law of the wall method

Answer 1

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

Question 2

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

Answer 2

Bottom 15-20%

Question 3

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

Answer 3

A straight line

Question 4

How is the shear stress calculated

Answer 4

From the gradient of the line in the graph

Question 5

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

Answer 5

Shear, rate of change will be quite low

Question 6

Even in calm flows how is the flow structure organised

Answer 6

In grouped bands

Question 7

What are areas in the flow that are travelling slower called

Answer 7

Low speed streaks

Question 8

Why are low speed steaks important in turbulence creation

Answer 8

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

Question 9

What happens at low speed zones

Answer 9

Flow starts to roll over

Question 10

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

Answer 10

Horseshoe

Question 11

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

Answer 11

Bursting

Question 12

Where is the highest velocity in ejection

Answer 12

Middle of the flow as it’s getting lifted up

Question 13

What is flow called when it falls down towards the bed

Answer 13

Sweeping flow

Question 14

What does sweeping flow give us

Answer 14

Higher shear stress

Question 15

What does bursting do sediment wise

Answer 15

Brings sediment to the main flow of the channel

Question 16

What is created in larger topographic beds

Answer 16

Higher and low pressure zones

Question 17

What happens when eddies become big enough

Answer 17

They move out into the flow

Question 18

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

Answer 18

Kelvin helmholtz instabilities

Question 19

What do kelvin helmholtz create

Answer 19

Boils which go up into the flow

Question 20

What is the strouhal relationship

Answer 20

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

Question 21

Where can you get flow separation in a planform sense

Answer 21

Round a bend in a river especially in right bends

Question 22

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

Answer 22

Downstream

Question 23

What effects variability in flow scales

Answer 23

Time

Question 24

What helps us to understand how turbulent a flow is

Answer 24

Variation away from the mean

Question 25

What are the negatives to the reach average method

Answer 25

Broad brush estimate which has nothing on the spatial resolution

Question 26

Which river see the most sediment yields

Answer 26

Low rainfall of 250mm to 350mm a year

Question 27

When does sediment yield reach a minimum

Answer 27

Rainfall over 750mm, due to vegetation cover

Question 28

Why are gravel bed rivers irregular on the bed

Answer 28

Presence of morphological features occurring at different spatial scales

Question 29

What are the 3 scales of the river bed

Answer 29

Grain scale, microtopographic scale and the large scale bed undulations

Question 30

What did Brayshaw say was the percentage coverage of pebble clusters

Answer 30

10-20%

Question 31

What are the 3 sections of turbulent reactions over pebble clusters

Answer 31

Acceleration,shedding, upwelling

Question 32

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

Answer 32

uniform flows

Question 33

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

Answer 33

convective acceleration

Question 34

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

Answer 34

non-uniform flows

Question 35

What are non-uniform decelerating convection associated with

Answer 35

deposition of sediment

Question 36

how can we define forces in river science

Answer 36

the rate of change of momentum

Question 37

What is a body force

Answer 37

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

Question 38

What is a surface force

Answer 38

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

Question 39

What is hydrostatic pressure

Answer 39

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

Question 40

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

Answer 40

where the river reach is the deepest

Question 41

what creates the buoyancy force on grains

Answer 41

larger hydrostatic pressure on the top than on the bottom

Question 42

What are the curved swirling motions in turbulence fluids

Answer 42

turbulent eddies

Question 43

What causes a kelvin Helmholtz instability

Answer 43

difference between rates of velocity

Question 44

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

Answer 44

friction at the base of the flow

Question 45

What gradient is created when water decelerates towards the bed

Answer 45

velocity gradient

Question 46

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

Answer 46

boundary layer

Question 47

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

Answer 47

free or external stream

Question 48

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

Answer 48

viscous sublayer

Question 49

What are the 3 sections of the boundary layer

Answer 49

outer, inner and viscous sublayer

Question 50

what motions are created in the viscous sublayer

Answer 50

Taylor-Gortler vortices

Question 51

What do Taylor-Gortler Vortices create

Answer 51

flow streaks where a common flow between the vortices is seen

Question 52

What is another name for the inner layer

Answer 52

buffer layer

Question 53

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

Answer 53

3-7 times the size

Question 54

What forms in the inner layer

Answer 54

intense small scale turbulence eddies

Question 55

What is flow like in the inner layer

Answer 55

transitional between pseudo-laminar and turbulent

Question 56

What happens to low speed streaks after they form

Answer 56

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

Question 57

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

Answer 57

filled by a falling sweep eddy

Question 58

In rougher beds what happens to the bursting process

Answer 58

much more rapid and sudden

Question 59

What happens to the velocity profile during a bursting event

Answer 59

bed velocity speeds up

Question 60

what is the structure of horseshoe bursting events

Answer 60

a primary hairpin, with multiple subsidiary hairpins

Question 61

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

Answer 61

re-attachment point

Question 62

what does three dimensional structure of flow patterns influence

Answer 62

local bed scour, sediment transport and downstream bar growth

Question 63

what do channel junctions control

Answer 63

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

Question 64

what is the basic planform shape of river junctions

Answer 64

Y shaped or, |- shaped

Question 65

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

Answer 65

balanced

Question 66

what will happen to the Y shape junction downstream of connection

Answer 66

flow will find a new thalweg and recirculation zones will change