River channel processes and landforms Flashcards
Erosion
Wearing away and removal of material from the river bank and beds
It takes up the most energy
Transportation
Moving of sediment
Deposition
Occurs when a river is no longer competent or has lost the capacity to carry its load
Any reduction in river velocity will mean material being deposited, starting with the coarsest sediment as this requires much energy to remain in suspension
It takes up the least energy
River’s energy
Around 95% of a river’s energy is lost to friction
Energy is spent on flow, then transport, then finally erosion
Energy levels in a river vary spatially and temporally
What the amount of energy in a river depends on
The vertical distance to sea level at any point in a river’s course (potential energy)
The volume and velocity of river flow - as they flow they convert potential energy to kinetic energy
Hydraulic Action
The force of air and water on the sides of rivers and in cracks
Evorsion (type of hydraulic action)
The sheer force of water eroding the bank and the bed by swirling vortices of water
Cavitation (type of hydraulic action)
The turbulent water hits current river banks and pushes water into cracks
The air in cracks is compressed, pressure increases and in time the rock will collapse
Most dominant form of hydraulic action
Abrasion (Corrasion)
The rubbing or scouring of the bed and banks by sediment carried along the river
Can vary from fine particles kept in suspension by turbulent flow to heavier boulders rolled along at times of bankfull flow
Major method by which rivers erode both vertically and horizontally
Potholes
Turbulent eddies in the current swirl pebbles around to form potholes that are hollow in the river bed and pebbles are likely to become trapped
Attrition
Reduction in size of the sediment particles as they collide with each other, the bed and banks
Pieces of sediment become smaller and more rounded as they move downstream, so more common to find rounder, smaller rocks downstream and coarser, angular rocks upstream
Solution / Corrosion
Process is independent of river discharge and velocity
Occurs when rocks dissolve in the water and are carried away
Most common when the rocks in the channel are carbonate (limestone, chalk)
3 types of load
Suspended load, bed load and dissolved load
Hydraulic Radius
Area / wetted perimeter
The higher the HR, the more efficient the river is as less energy is lost through friction
Upper course has lowest HR, lower course has highest HR
Saltation
When pebbles, sand and gravel (bedload) are lifted up by currents and bounced along the bed in a hopping motion
Traction
When largest boulders and cobbles (bedload) roll or slide along the bed
Suspension
Very fine sand particles such as clay and silt (suspended load) are dislodged and carried by turbulence in a fast flowing river
Solution
Water flowing within a river channel contains acids (e.g. Carbonic from precipitation) which dissolve the load such as limestone in running water and removed in solution
Soil particle sizes
Clay < 0.002mm
Silt = 0.002mm - 0.05mm
Sand = 0.05mm - 2mm
Gravel = 2mm - 5mm
Pebbles = 5mm - 75mm
Cobbles + Boulders =75mm - 200mm
River competence
Maximum size of load a river is capable of transporting
River capacity
Total volume of sediment a river can transport
River calibre
Measurement of the long axis of sediment in a river
Factors influencing Capacity, Competence and Calibre
Velocity - at low velocity only fine particles may be transported and at high velocity heavier material can be moved
Discharge - related to velocity and also depends on channel size
An increase of 10 times in discharge raises the transporting power of a stream by 10^3 or 10^4 times
When deposition occurs
Sudden reduction in gradient
River enters a lake or the sea
Discharge reduced following a drought
Shallower water
Sudden increase in sediment volume
River overflows its banks
Flocculation