Lecture 10 Flashcards
What’s the main thing that discriminates a braided and a meander river
Multiple mid channel bars in a braided channel
What sides does a singular central unit bar deflect water to
Both sides, erosion on both banks
What happens to a single unit bar when flow goes either side of it
Can turn into a compound bar
Why are cross bar channels Important in braided systems
Can become the dominant channels due to widening and scouring of the channel
Barr migration is faster in meander or braided rivers
Braided
Can bars migrate even in flow with no flood events
Yes
How did the bar move on the jamuna river
Bar appeard, and doubled insize over the space of 2 years
What is one way that bars can grow
Accumulates other unitbars
Where do bars form
At a confluence site between multiple channels, creates scour
What does the scour zone create downstream
A supply of sediment
Is the flow on either sides of the bar normally even
No, one side normally sees more flow, creating a lopsided bar
How will flow be concentrated around a bar
In one channel
What does eneven channel flow cause around the bar
Elongation of one bar side
In parker et al how many bars could be tracked over 4 years
1
Do bars tracks from one year to the next, parker et al
No not in all cases, dynamic system
What is channel cut
Creation of new channels
Whag is channel fill
Abandonment of channels
When bars are moving how will that effect flow
Moving bars deflect flow differently
What can cause channel cut and fill
Bar shape, can stall and stop flows
When 2 under water bars merge together what do you get
A larger compound bar that sticks out of the water
What can happen with large compound bars with uni bar growth
Smaller unit bars can pile on the the compound bar side, adding sediment and size to the compound bar
Where can abandoned channels form
In the middle of bars, caused by abandoned cross bar channels
Where may cross bar channels also form
In the topographic low point between bar and compound bar
In the south Saskatchewan what bar formed in low flow
Large attached bar, simplified the channel down
What happens to braided systems during high flows
Channel cutting and new channels formed, many unit bars
In normal flow conditions what does the topography of braided rivers look like
Bell shaped curve, even
In a flood event what shape does the topographical chart represent
Uneven, sporadic
In rivers with strong banks, what does the system look like
Narrow with less unit bars due to less sediment entering the system and less width to move
Bars with weak vegetation growth are stable or unstable
Unstable
So with strong banks but weak vegetation growth what does the system look like
Dynamic and quick, unit bars always moving, compound bars do not survive for long
What are the key characteristics of a weak bank with low vegetation growth
Wide and unstable channel, many channels, with heavily divided flow
In a heavily venerated channel what is common
Fairly stable bars with the main channel acting like a meander system
What is laterally accretion
Bars can grow on the side of larger channels
What radar is used to see unit bar structure below the bed
Ground penetrating radar
How can we see bar movement in ground penetrating radar
Slip face movement
How can we see larger scale unit bar migration
Whole strands of sediment diagonally twisted together
How can high angle reflections show us bar migration
Can show movement and channel fill due to slip face accretion
How can gpr who channel truncation
Up down meander like shapes show channels that have cut through at different orientations
Do we get fining up in meander and braided channels
Yes
Do we only find lateral accretion surfaces in meander rivers
No, also on the side of braid bars
How do braided channels show lateral accretion
Braided channels are sinuous therefore they grow by sediment aggrading laterally
Is the process in meander fill and channel filling similar
Yes, toonen et al
What is the difference between braid bars and islands (Brice 1964)
Islands are not submerged at bankfull stage
Why do bars look different at different flow stages
different river depths produce different erosional characteristics
What is the difference between an anastomosing and braided channel
anastomosing channels are a lot longer, and are split of from the main river belt.
What is channel diversion associated with
high stage scouring of thalwegs
How does a channel become filled in
channel filled with sediment from the upstream end, downstream end blocked by moving channel bars
How will despositional unit scale vary
with the scale of the alluvial feature
What feature can be seen in a sandstone conglomerate body
complete channel belt
What feature can be seen in sets of inclined strata (storeys)
Individual channel bars and channel fills
What is depositional imprication
when deposits are aligned or orientated the same way
What is a paleocurrent
A geological indicator of flow direction
What does paleocurrent depend on
orientation of bedforms and sedimentary structure, what part of the channel bar fill is preserved
How do rivers self organise their planform pattern
feedback from bars, channels, floodplain and vegetation
What determins bar pattern
width-depth ratio
what 3 bar types can form in a point bar (Kleinhaus)
chute, tail and scroll
what 4 stages did kleinhaus create for braided rivers
highly braided, braided and meandering, meandering with scroll bars, laterally immobile no bars
what does vegetation affect (kleinhaus)
channel mobility
What can cause high sinuosity with vegetation in braided rivers
vegetation can stop the erosion of the point bar leading to outer bank erosion
according to carson, what 3 groups can river channels be separated in
bed load. mixed load and suspended load
