Lecture 3 Flashcards
What is the Law of the wall method
Shear stress indicator used for one point, predicts flow velocity using logarithmic chart
What proportion of the flow is used to measure the shear stress
Bottom 15-20%
If velocity and log depths are plotted together what should it be shown as
A straight line
How is the shear stress calculated
From the gradient of the line in the graph
If water is moving together with little deviation what will there be less of
Shear, rate of change will be quite low
Even in calm flows how is the flow structure organised
In grouped bands
What are areas in the flow that are travelling slower called
Low speed streaks
Why are low speed steaks important in turbulence creation
They can lead to bursting effects due to rolling on the bed
What happens at low speed zones
Flow starts to roll over
What is the shape of the low speed roll when it lifts up into the flow
Horseshoe
What is the term when water is ejected up into the flow
Bursting
Where is the highest velocity in ejection
Middle of the flow as it’s getting lifted up
What is flow called when it falls down towards the bed
Sweeping flow
What does sweeping flow give us
Higher shear stress
What does bursting do sediment wise
Brings sediment to the main flow of the channel
What is created in larger topographic beds
Higher and low pressure zones
What happens when eddies become big enough
They move out into the flow
What is the name of the instabilities that are created when two different flow structures meet
Kelvin helmholtz instabilities
What do kelvin helmholtz create
Boils which go up into the flow
What is the strouhal relationship
Dimensionless number which shows the frequency of eddy shedding, as the dune changes size so does the shedding
Where can you get flow separation in a planform sense
Round a bend in a river especially in right bends
In terms of direction which way does U normally go in the river
Downstream
What effects variability in flow scales
Time
What helps us to understand how turbulent a flow is
Variation away from the mean
What are the negatives to the reach average method
Broad brush estimate which has nothing on the spatial resolution
Which river see the most sediment yields
Low rainfall of 250mm to 350mm a year
When does sediment yield reach a minimum
Rainfall over 750mm, due to vegetation cover
Why are gravel bed rivers irregular on the bed
Presence of morphological features occurring at different spatial scales
What are the 3 scales of the river bed
Grain scale, microtopographic scale and the large scale bed undulations
What did Brayshaw say was the percentage coverage of pebble clusters
10-20%
What are the 3 sections of turbulent reactions over pebble clusters
Acceleration,shedding, upwelling
what are flows called that do not vary in velocity or cross sectional area
uniform flows
What is the name for when water accelerates due to movement into a narrower cross section
convective acceleration
What are flows called that vary in cross sectional area and velocity vary
non-uniform flows
What are non-uniform decelerating convection associated with
deposition of sediment
how can we define forces in river science
the rate of change of momentum
What is a body force
force that acts from distance on everything and every part of a substance (gravity)
What is a surface force
act by direct contact between the surface of a substance and the medium applying the force
What is hydrostatic pressure
pressure at a point caused by the mass of water and air being pushed down by gravity
Where will we find the highest hydrostatic forces on the banks of rivers
where the river reach is the deepest
what creates the buoyancy force on grains
larger hydrostatic pressure on the top than on the bottom
What are the curved swirling motions in turbulence fluids
turbulent eddies
What causes a kelvin Helmholtz instability
difference between rates of velocity
Why does water flow slower at the bed than in the main flow channel
friction at the base of the flow
What gradient is created when water decelerates towards the bed
velocity gradient
What is the name of the layer where a velocity gradient exists as well as shear stress
boundary layer
What is the name of flow where there is no velocity gradient or shear stress
free or external stream
What is the name of the thin less than a mm thick layer where flow behaves as if it was laminar
viscous sublayer
What are the 3 sections of the boundary layer
outer, inner and viscous sublayer
what motions are created in the viscous sublayer
Taylor-Gortler vortices
What do Taylor-Gortler Vortices create
flow streaks where a common flow between the vortices is seen
What is another name for the inner layer
buffer layer
what is the size of the inner layer in relation to the viscous sub layer
3-7 times the size
What forms in the inner layer
intense small scale turbulence eddies
What is flow like in the inner layer
transitional between pseudo-laminar and turbulent
What happens to low speed streaks after they form
they oscillate and rise rapidly into the outer zone where they mix with other eddies
What happens in the space left behind by the rising ow speed streak
filled by a falling sweep eddy
In rougher beds what happens to the bursting process
much more rapid and sudden
What happens to the velocity profile during a bursting event
bed velocity speeds up
what is the structure of horseshoe bursting events
a primary hairpin, with multiple subsidiary hairpins
what is the point where the shear layer reconnects to the bed
re-attachment point
what does three dimensional structure of flow patterns influence
local bed scour, sediment transport and downstream bar growth
what do channel junctions control
downstream flow mixing. sediment transfer, bar evolution and channel change (mclellend)
what is the basic planform shape of river junctions
Y shaped or, |- shaped
when two channels first meet, what is the nature of the river channel in a Y channel
balanced
what will happen to the Y shape junction downstream of connection
flow will find a new thalweg and recirculation zones will change
