Topic 2 Flashcards
where is soil forming processes dominant?
flat hilltops
with increasing ____, colluvial and mass wasting is dominant
gradient
colluvial footslope
where material accumulates and deposits
what happens if we had no rivers, regarding transport?
there would be no transport over longer distances. for example, on hill slopes, sediment is created and then transported down to rivers.
3 points on channel initiation (diffusive processes)
- Transport rate is proportional to hill slope
- Involve sediment movement without concentrated flow of water wind or ice
- Result in filling of depressions and smoothing of relief
What are two examples of channel initation?
Rainsplash and sheetwash
Rainsplash
raindrop impacts on bare soils cause disaggretion of soil, tiny craters and downslope sediment transport
Sheetwash
overland flow thats not concentrated in discrete channels, occurs when rainfall > infiltration capacity, can move loose material down gentle slopes. but can’t erode channels
Where are diffusive processes dominant ?
areas of shallow slope
Hortonian model
- suggests there’s a critical point where from sheetwash (or Hortonian overland flow) starts eroding bed and initiates channel development.
- moving downslope, volume of water flowing increases and the driving force increases until it can move sediment.
- subparallel rills form downslope which become dominants and a first order stream forms
Channel Initiation (Advective Processes)
Material is moved with the fluid and in direction of fluid motion. Once water is focused into channels, networks evolve
Types of advective processes
Rills and gullies
Rills
sheetflows concentrate and cuts small parallel channels these may form and join to make a gully.
Gullies
deeper V shaped channels carved by concentrated run off
Hydraulic geochemistry
statistical relationships between the channel form/dimensions and Q
what is flooding?
flow that exceeds channel banks and occupies the floodplain
what are the two critical stages of flooding?
bankfull discharge and annual mean flood
megafloods
transport high amount of material but occur less frequently
low flows
incapable of transporting a lot of sediment
so what type of flow intensity allows for sediment transport?
moderate flows
effective discharge
discharge that transports the most sediments. this is due to the fact that it transports a moderate amount of sediment and occurs relative frequently
velocity deforms in 4 ways
- with distance from the bed
- Across the stream
- Downstream
- With time
velocity increases towards ___ in a stream and decreases with ___
the centre, depth
turbulent
most common in nature. molecules fulfill random paths as they move downwards that provokes a lot of mixing.
laminar
not common in nature. water molecules flow along parallel path, little vertical variation, no mixing.
why does velocity vary in short time scales due to turbulence?
turbulence makes water flow in eddies more turbulence velocity leads to more velocity fluctuation
Reynolds number
compares driving (velocity, hydraulic) and resisting forces (viscous)
Froude number
compares inertial forces (velocity) and gravitational forces (weight of water)
total resistance has 3 main components
free surface resistance, channel resistance, boundary resistance
free surface resistnace
loss of energy due to surface waves & abrupt surface gradient changes
channel resistance
due to undulations in the channel beds and banks & changes in planform and cross section
boundary resistance
due to individual clasts or bed forms
___ elements all cause turbulence that resists flow
Resisting
relative roughness
ratio of water depth to particle size. so larger particles and shallower flow increases roughness
most of the energy in a stream is dissipated due to
flow resistance
stream competence
- maximum particle size that can be transported
- increases with velocity because competence is a function of boundary shear stress
stream capacity
theoretical maximum mass of sediment a stream is able to transport
stream load
amount of sediment actually carried
sediment transport processes will depend on the ____
river reach
3 flavours of sediment transport
solution, suspended load, bedload
solution
material that is dissolved. Travels at same velocity of fluid. Precipitates in lakes and oceans.
suspended load
fine-grained sediment transported in the water column and supported by turbulence. Travels a little slower than water. May stay suspended for longer distances without deposition.
Bedload
sediment transported along the bed by rolling and sliding (traction) and bouncing (saltation).
wash material
very small particles that once entrained stay in suspension and are not found in the bed. may form a large part of floodplain deposits.
bed material load
sediment that occurs in the bed (includes sediment that was transported as bedload and in suspension but deposited in the bed).
entrainment
initiation of grain moment balance of forces (gravity, drag, lift).
gravity
keeping the material at erst
drag
exerted by the flowing water
lift
velocity gradient above the particle creates pressure gradients
critical shear velocity (critical near bed velocity)
represents the relationship between velocity and entrainment. describes the entrainment “threshold”
critical shear stress
force responsible for entrainment and transport. the dragging force at the onset of particle motion tangential to the bed. signifies the down slope component of the fluid weight exerted as particle motion begins (a geomorphic threshold).
shields parameter
a dimensionless approach for determining critical shear stress. Divides critical shear stress by the submerged weight of the particle to be transported. Grain size diameter affects how much the particle will protrude up into the turbulent part of the flow.
True or False: If bed shear stress > critical shear stress, grains on the bed begin to move
True
critical stream power
Bagnold prosed that entrainment and transportation of bedload can be analyzed in terms of specific unit stream power.
what does the Rouse equation give us?
sediment concentration in the water column relative to a sediment concentration at the top of the bedload layer
how do we get suspended flux per unit width?
sediment transport rate can be predicted by multiplying concentration at any height by the velocity of the flow. Thus, to get suspended flux per unit width, we sum up vertically
where does deposition occur
under lower current velocities and shear stresses due to the time it takes particles to settle.
4 types of limitations of predictive models
spatial and temporal averages, turbulence neglected, measurement difficulty in natural streams, variable particle and bed characteristics
erodibility
tensile strength of the rock controls the erodibility of the rock
