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river channel processes & landforms (Landforms (Waterfalls - when a…
river channel processes & landforms
Types of Erosion
Abrasion ( corrasion)
The wearing away of the bed and bank by the lad carried by the river- Increases as the velocity increases.
Attrition
The wearing away of the load carried by the river - crates smaller, rounder particles
Hydraulic action
The force or air and water on the sides of rivers and in cracks
corrosion/solution
the removal of chemical ions- especially calcium
Deposition & sedimentation
Hjulstrom curve
The hjulstrom curve shows what work a river will do depending on its verloy & the size of the matreal present
Three important hjulstrom curve factors
1- smallest and largest parcels require high velocity to lift them E.G. parcels of 0.1mm to 1mm require velocities of 100mm/s where as 500mm/s to lift clay (0.01mm ) & gravel (2mm)
2 - higher velocity are required for entrainment than transport
3- when velocity falls below a certain level parcels with a settling verlicy will be deposited
causes of deposition
reduction in grant = deceased velocity
decrease in the amount of water in the channel
An increase in friction between water & chanel
River flow
Velocity and discharge
The
velocity
refers to the speed of a river - the
deeper the river the higher the velocity
The
discharge
is the rate of flow of a river at a pertalcer time - generally found by multiplying a cross secal area by the verlicy - expressed cumecs (cubic meters per second)
Hydraulic radius
is measure of a streams efficiency - it is caused by dividing the cross sectional area by the wetted perimeter ( the length of the bank in contact with water) - the higher it is the more efficient
paterns of flow
Laminar flow
- a smooth straight chanel with low velocity - allows for water to flow in sheets parallel to the chanel bed
Turbulent flow
- with there are high velocities & increased channel roughness - turbulence is associated with hydraulic action - vertical turbulence crates hollows in the bed
Helicoidal flow
- horizontal turbulence - a corkscrewing motion - associated with the presence of alternation pools & riffles in the chanel bed & where the river is carrying large amounts of material this may form meanders
Landforms
meanders
- helicoidal flow in the river causes erosion of the other banks & the formation of river cliffs. On the inner bank helicoidal flow leads to slip of slopes, this cases a meander
Oxbow lakes
- are a result of both erosion & deposition. Lateral erosion caused by leicordal fow is contrantred on the other deeper bank of the meander. during times of floodinging erotion incesed. The river breaks though & creates a new deeper chanel.In time the old meander is closed off by deposition to form a oxbow lake
Pools
- Are the deep parts of a meander - erosion
Riffles
- Are the sediments found in the strater sections of a meander - usually formed of coarse gravel
Waterfalls
- when a river spills over or a sudden change in gradient, undercutting rocks by hydraulic action & abrasion.The change in gradient is usually related to a band of resistant rocks.
1.
waterfalls are found in the upper valley & created by the erosion process. They ovvor when a band of hard rock (E.G. granite) overlies a a soft rock (E.G. sand stone)
2.
soft rock is is eroded faster, creating a plunge pool. potholing can occur where rocks are swept in deepening the plunge pool more though corrosion
3.
An over hand of hard rock forms, this is unstable & unsupported. This will collapse & the waterfall will retreat further back upstream
4.
This creates gorges which are steep sided river valleys. This process will repeat continuously. E.G. Highforce, The river tees.
Gorge
- is a deep narrow valley with very steep sides, currently occupied by a river or has been in the past.
They may have been formed as a result of:
retreat of waterfall
glacial overflow channeling
collapse of underground caverns in carboniferous limestone areas
surface runoff over limestone during a pre glacial period
Bluff
- is another term for river cliff floodplain is the part of the valley adjacent to the river channel - the river flows on it during a flood - zone of low relief & gentle gradient
Levees
- when a river floods sediment builds up on the edge of the chanel - flood water quickly lose verlichte & deposits coarse meterial
Flooding
Flooding
can occur when a river overflows its banks the main causes of floods is climate - where as condition that amplify flooding tent to be drenge bansen specific.
factors that intensify flooding:
rapid discharge in urban areas due to inoperable sfaces & increased dragne channels
urbanization/ urban growth
flood development - high risk of damage
bridges/ dams / obstructions
change in vegetation cover E.G. farming
river engineering works E.G. levees
Human induced climate change
Prediction
- some floods can be predicted spacticlly & temporally. Most floods occur on the floodplain close to the river but they can occur away from the river
Recurrence
- the recurrence interval refers to the regularity of a flood of a given size. A 100 year flood is a flood expected to occur once every 100 years
Forecasting & warning
- improved rheinfall & snowpack estimates & longer forecasts of rainfall. better gaging of rovers, convention of meteorological information & mapping of channels
