Lecture 6 Flashcards
What does sediment not just move as
Just individual grains
What is a bed form
A morphological feature generates by sediment movement
What are the two main theories for how flat beds move into bed forms
Bed defects and wave instability
What does the bed defect theory link to
What we know about turbulence and flow structure
What creates the initial defect in the stream bed
High shear stress on an area of bedform
What happens when the sweep stops moving with sediment
creates a small pile of sediment
What happens once the defect is created
high pressure upstream, low pressure down stream
What zone is created downstream of the newly created high pressure
A low pressure recirculation zone
What does the change in velcocity between the high pressure and low pressure give us
a shear layer
When the shear layer attaches to the bed what does this create
a reattachment points, area of new erosion
What is the concept of wave instability
substances of two different densities and speeds move over each other like kelvin helmholtz instabilities
What is the diagram used to show bed forms, shear stress and thier diameter
bed form phase diagram
What are the proposed 4 zones to look at in terms of the bed forms phase diagram
sand sub, sand super, gravel sub, gravel super
What is the other name for bed forms which are in the super critical flow
Upper flow regime
what are the main bed form types in sub critical flow on sand beds
Ripples and dunes
What are ripples restricted by
grain size
What are dunes restricted by
flow depth
What is the average height of ripples
3cm
What are ripples controlled by
flow right near the bed of the channel.
What size do dunes start at
4cm
How long can dunes grow to be
100’s meters
The dunewave length is how much times of the depth
5-7 times
How tall is the dune in relation to the depth
6-10th of the depth
What can part of the variability be down to in terms of dunes
hard to measure, different rivers show different values
What has been found out in terms of dune size in river systems
dunes have been seen to be larger that expected in small streams, and smaller than expected in larger streams.
Why might deeper channles have smaller dunes
higher shear stress=more erosion of the bed
Are dunes thought to be symetrical or asymetrical in river systems
asymentrial
What is the steeper side known as
Lee 20-35 degrees
What is the shallower dunes face known as
Stoss 5-10 degrees
What is the bottom part of the dune called
trough
What is the top of the dune called
crest
how do we work out the amplitude
trough height to crest height
How can the crest line be described from a planform viewpoint
Sinous crestline
What else is common with dunes in terms of sediment size
can see sediment sorting
what are the 3 forms of movement in dune form
fine sediment ricurculation, coarse avalanches, sediment overshoot of shear layer
What happens in high sediment transport rates
strong sorting, coase sediment avalanches with lots of fine sediment in ricurculation, may create regressive ripples
What happens in low sediment transport rates
all the sediment moves together, poor sorting
how can we see lee face deposition
can be preserved in the dunes as lines
What happens when each dune scours out the next one down stream
Nothing preserved, brand new dune
How do we get lee faces that are preserved
need a certain degree of aggredation
When you get a high amount of agredation what can happen to dune bed forms
can stack on each other and be draped on one another
How much of the lee face is generally preserved
1/3
How can we work out the original dune height from whats left
multiply by 3, from this we can work out the depth
What are the most common bed form features
dunes
What information can we get from a dune form
flow direction flow depth, rate of agredation and sediment transport rate
What sand forms are found in super critical flow
anti dunes
What are anti dunes best defined by
standing waves
Are there any flow circulation zones in anti dunes
no, super critical flow
Which side of the anti dunes is erosional
the lee side
Which way does an anto dune migrate
up stream
What is a pebble cluster
when grains of sediment get trapped behind a larger pebble or rock
What is the larger particle known as in a pebble cluster
key stone
What zone is created after the pebble cluster
recirculation zone that traps finer particles
How much of the river bed has pebble clusters
10%
how are bed load sheets formed
much broader areas of the sub critical flow
how large is the leading edge of a bed load sheet
just 2-3 grains thick
what is formed in super critical gravel zones
transverse ribs
What is a tranvserse rip
an alternating pattern of stripes, perpendicular to the flow
What is a bed configuration
as assemblage of bed forms in a particular area
When do ripples form
at the threshold of bed load motion
What size must grains be under to form ripples
0.7mm
where do ripples not occur
in mud, as it is soon entrained from the bed
what causes the initiation and growth of ripples
turbulent variation in near bed flow velocity
What is the maximum length of a ripple
60cm
What is a lower plane bed
just above the threshold for motion, transitional flows
What size do grains need to exceed to form dunes
0.1mm
Why can ripples not form in coarse sediments
the bed instability related to local disruption of a viscous sub layer does not exist
What does differential transport of bed load grains result in
size segregation of grain sizes
Where are pebble clusters common
poorly sorted cobble clusters
