Lecture 11 Flashcards
How does a system stay balanced
Sediment supply must be equal to stream power otherwise either aggregation or degradation will take place
What geological feature can lead to high amounts of sediment
Techtonic
What can increase stream power
Slope, depth, rainfall, stream order
How does climate impact sediment and water
Sea level rise, more rainfall then more sediment,
How can vegetation impact river systems
More stable banks, water control and uptake
Where do we get little records of what a river is doing
Little sediment with little aggregation, no new locations, channel belt is stable
Where will we get high rates of preservation and sediment movement
High sediment with high channel aggregation
What is a key aspect of channel change
Sediment supply
What is the channel belt
The whole channel shape which the river runs in
What is an avulsion
A abrupt shift of the entire channel belt to a new location, inter avulsion period can be 1000s years
How long does it take for an avulsion to complete
May be almost instant, but may take longer for the whole channel to fill in or move
What is the hop length
Distance that the new channel shifted
What is avulsion length
distance that the channel belt is before is rejoins the channel
what is the hop length
size of the channel movement
what was the relationship between channel belt width and distance it was moving
almost a 1 correlation. 2.5 times channel belt width is how much it is moving over.
What is the set up
what would make the channel belt [perfect for switching to a new location
What is the trigger
The chance of the river avulsing after it has been set up to do so
what are the two theory for set up
superelevation and gradient advantage
what is super elevation
levee’s build up either side of the channel, channel starts to become perched above the flood plain.
what happens when the depth of the channel is the height of the land cover
channel depth above floodplain, when spills out all old channel is lost
What is gradient advantage
is there a tilt in the flood plain itself, water will move on the gradient, cutting a new channel
What is a trigger event
large flood which starts the avulsion
what can happen in really big floods
avulsion can take place even without set up
what is a key control on the avulsion processes
sediment supply.
Where do we get heigher avulsion frequency
more sediment.
What does bed load do to channels
build the channel up creating more channel height and therefore more avulsion chance
what does suspended sediment do to banks
builds up levees
How do we get a perched channel
some suspended sediment for levees and some bedload to build channel height
If you had little coarse sediment but lots of fine sediment what would happen
hinder avulsion, larger levees shallower channel
In south America what has seen to be linked with avulsions
la nina events cause avulsions, 51 in la nina only 5 with el nino
When can avulsions be linked climatically
high rainfall events
what were rivers like pre 450ma
no vegetation, braided rivers were much more avulsive with no vegetation growth
When was the older rhine-meuse avulsions
8000ya, nearer to the coast
what drives the original avulsions
sea level rise, reduce the slope, sediment deposition
What was the second avulsion stage caused by
subsidence of the crust, more low lying area
What is the rhine avulsion duration
325 years
If sea levels wall what happens to the land
erosion and incision
How was the bridge karssenberg
based on the rhine system, numerical system model
what is sediment transport a function of
the discharge
How will channel incision change river shape
more sediment released if more incsion
What can happen to the original channel after evolution
single channels can create multiple channels
what are the two probability factors that lead to channel avulsion
Ps, birfurcation, super elevation,
Pd, probability of a yearly flood discharge
If you’ve had one avulsion what is the probability of another one happening
it drops
What can happen to channel belt size
they can widen
How does the model know which channel to give more water
slope
What happens if we get lots of connected channels together
can find aquifers below the soil
what is base level?
the lower limit to which a river can erode its valley (powell 1875)
what are the 3 river planform types
straight, braided, meander
what is the equation for stream power
water density x gravity x discharge x slope
What is unit stream power
stream power divided by width
What are the 5 basic degrees of freedom in a channel system
Slope, width, depth, platform, bed roughness
What is the efficient transfer of water
Hydraulic efficiency
What is the primary movement process of larger bed load
Saltation
What is the typical longprofile of a river
Concave from source to mouth
What does curved channel result in
Super elevation of the waters surface higher at the outside of the bend
Other than a flood, how else can avulsions occur
water build up after downstream blockage
How much has the po river in Italy moved in 3000 years
50km
how much has the yellow river moved laterally over 4200 years
800km
what happens when avulsive rivers stay separate and both channels filled over a large length of time
they are termed anastomosed
when do inter avlusion periods decrease
when base sea level rises
what is the average inter avulsion period of the river kosi
4-44 years
how does the flow of the kosi river change
gradual transfer of flow form one channel to another
how were some of the po’s avulsions created
man made movement of channels
when does avulsion frequency increase
with increasing deposition rate in the channel belt
how might tectonic activity effect avulsions
may occur as a response to an individual tectonic event., or gradual tectonic shift
tectonically, where to avulsing channels concentrate
in areas of tectonic subsidence
how are belts seen over time
seen to be stacked on top of each other laterally
What must the diversion of a channel to a new system result in
channel incision
what did some scientists use dune movement to estimate
bed load movement
what did kostachuk find about lagging with river dunes structure
dune height and length lagged behind changes in channel velocity leading to a hysteresis effect
in migrating dunes, how much trapped sediment was suspension
95%
