Lecture 9 Flashcards
What controls the diversity of channel shape
Bed load, slope, stream power, bank strength
How does clay and silt change river shape
More suspension, therefore fewer bar form, more cohesive structures less bank erosion
If everything is moving as suspended load what channel will form
Straight, low sinuosity
What will increasing discharge lead to
Increasing stream power
Why can’t we think of stream power seperatly
Works in relation with other factors
What factor is said to be largely erroneous
Discharge regularity
How does slope effect river shape
Links with shear stress, can create more erosion
What is required for gravel bar formation
Higher shear stress, more strean power
What can be said about braided river channels
They are in continuum
What might one river system have
Multiple channel systems
How can river channels change temporally
They can evolve and move across time going through different channel phases
What is a river called where there is more than 1 channel belt
Anabranching rivers
What is a anastamosed channel
Multiple belts but fixed, not wandering
What defines a wandering river system
Weaker banks, still anabrancjmhing but avulsion and channel movement takes place
Where will we get highly braided channels
High stream power and smallish grains
Which channel features can be found across all types of channel
Ripples, dunes, antidunes ect
What determines bar form size
Length proportional to local channel width and height proportion to channel depth
What is the difference in bars between braided and meander
Meander are attached to the bank, point bar
What is river sinuosity
Valley distance and river distance, channel length divided by valley length
Above what value gives us a braided river
1.5
Where does the unit bar come from
An amalgamation of dune forms
Which part of the uni bar is highest
The end, build up of sediment
Where does erosion occur in a bar system
On the banks if banks are erodable
Where is deposition in meander unibars
Over lower slower flow inside of the bend
At high river stage what can form on the river bars
Cross bar channels
When do cross bar channels form
When flow is dropping, as water is draining off if the bar
What is bar push
Inner bar deposition
What is bank pull
Outer bank deposition
What does bar push do
Builds inner bar which deflects flow to other bank
How does bank pull work
Bank erosion is fast which pulls channel with it
What is the area of fastest flow in the river
Thalweg
How do meanders tend to migrate
Downstream
What has happened to pool after downstream meander migration
Pool is filled in
What fills in the pool
Point bar
Where are the most coarse particles in the flow
Deepest bit of the pool
What is the grain size of the bar
Fine material
From the bed up how does material change
Fining up, big at the bottom, fine at the top
What are each time step of downstream meander movement called
Lateral accretion surfaces
What are the 4 main ways that meander bends move
Expansion, translation, wavelength change, rotation
What is translation
Same size of meander moves downstream, both wavelength or amplitude change, 50% of all migration
What is meander expansion
Right at the bend apex, migrates away from the river
In an expanding meander what is different
Point bar preserved on both sides
What is paleohydraulics
Empirical relationships between meandering river parameters and flow discharge
How can we infer meander wavelength
Mean annual discharge
What is chute cutoff
Flow goes across the point bar at high discharge, can create new dominate channels
What is the name for the bar that stops the flow of the original channel
Plug channel
What sediment are plug bars made of
Very fine grained sediment
What is neck cutoff
Meander cuts across a close section of a meander creating an “oxbow” lake
What is radius of curvature
Radius of circle that fits on the band
What is outer bank deposition
Tight bends, change of direction difficult for flow to get round, bar forms on outside of the bend
What grains on the inside of the bend is outer bank deposition
Much finer
why is an alluvial channel distinctive (bridge)
it has finite width and depth
What does the sediment boundary control (bridge)
Both water flow and sediment transport
What does the river bed evolve towards (bridge)
A statistically constant geometry composed of bars that’s are in steady equilibrium with flow and sediment transport
What is an alternate bar
bar occurring on altering sides of the river channel
What migration rate is faster, compound, unit or point bars
Unit
What have mobile unit bars been known as
free bars
What are the main controls of alluvial rivers geometry
Flow and sediment
How can flow change river geometry
High flows can move and shape sediment, past the capabilities of normal channel flow
What is channel forming discharge
A single discharge measure that represents the flood discharge needed for change in geometry of river channels (bridge)
What is mean size of bed sediment proportional to
Channel slope
What is channel roughness related to
Grain size and stream power
What must sediment transport rate equal for channel equalibrium
Sediment supply rate
What happens If sediment supply rate is reduced
Degradation will occur
What happens to the shape of a river when valley slope is increased
Width and braiding increases
What controls sediment transport rate
Shear stress and stream power
What is the common myth of discharge variability
Variability is greater for braided rivers, not true
How does vegetation stop bank erosion
Helps stop bank cut and bar surfaces given time for development
What other factors does hayashi and okazaki say predict channel patterns
Froude number
According to bettes and white if channel slope is equal to valley slope what will happen
Channel remains straight
What will happen if channel slope is greater than valley slope
Aggregation will occur in non equalibrium
What happens If the valley slope is greater than channel slope
Meander or braided river
Which part of the meander bend has the largest sediment
deepest point, outside
During rising flow levels where does erosion take place in meanders
thalwegs, confluence scours and upstream ends of bars, will gain deposition in falling flow
What are meandering systems explained by
bend instability
Why can meander bends be seen as dynamic
bends move through bank erosion (Van Dijk)
What is erosion balanced by
Floodplain formation on the other side (Van Dijk)
What is meander bend growth managed by
Neck cutoff
What is the relationship between bars and bends
They ‘resonate’ at particular width depth ratios, leading to stationary alternate bars
What is bend development limited by
non linear processes e.g cutoffs and channel adjustment
When are neck cutoffs common
Rivers with high sinuosity
When are chute cut offs common
Rivers with low sinuosity
How can swale cause chute cut offs
Swales capture overbank flow and incise until river cut off
How can aggradation cause a meander
build up of sediment reduces the rivers capacity for flow and so some water spills over lower banks
How can embayment cause a meander
creates erosion upstream of the pointbar, leading to the creation of a channel
What happens when the banks of the river are the same non cohesive sediment as the beds
the banks erode so that the river becomes wider and shallower
How did the transverse bed slope change with the channel
it increased with decreasing bend radii, often deeper than expected due to over deepening in the bend
What can stop a meandering channel turning into a braided river
rapid closure of abandoned channels
when did single chute cutoffs occur (van Dijk)
as the bend migrated downstream
what promotes the development of dynamic meandering
poorly sorted sediment, and dynamic in flow (in a flume, dijk)
when will a sinuous channel stop migrating
a critical bend amplitude in reached, leading to cut off
What is a meandering amplitude
Distance of the meander size between the meanders
What is wavelength and radius scaled to
Stream size
Why is the relationship between amplitude and width less constant
Because width controlled by bank erosion
