A.1.3: Fluvial processes in rivers Flashcards
Erosion
The wearing away of rock and soil found along the river bed and banks.
The breaking down of rock particles being carried downstream by the river
Why does erosion occur?
- Water moves because of the changing gradient. Therefore, it has the potential energy to carry out work.
- It happens because of dynamic equilibrium. The channel bed will adapt to transport its load.
- The dynamic changes downstream as a result of aspects in the channel that varyā
1. Depth
2. Wetted perimeter
3. Channel roughness
4. Load size
Abrasion (corrasion)
The wearing away of bed and banks by sediment carried in the flow of the river
When the load carried by the river repeatedly hits the bed or banks dislodging particles into the flow of the river
Attrition
Erosion caused when rocks and boulders transported by the river bump into each other and break up into smaller pieces
When stones carried by the river knock against each other gradually making the stones smaller and less rounded
Hydraulic action
- The processes by which the water itself compresses pockets of air in cracks in the bed or banks.
- The pressure may cause the crack to widen, breaking off rock.
The force of the water hitting the river bed and banks. This is the most effective when the water is moving fast and when there is a lot of it.
Solution
Soluble particles are dissolved into the river
When the river flows over limestone or chalk, the rock slowly dissolved. This is because it is soluble in mildly acidic river water
Factors that affect erosion
Load - the heavier and sharper the load the greater the potential for erosion
Velocity - the greater the velocity the greater the potential for erosion
Geology - soft, unconsolidated rocks such as sand and gravel are easily eroded. Clay however is not, this is because it is so small it tends to be cohesive.
Gradient - increased gradient increases the rate of erosion.
P.H. - rates of solution are increased when the water is more acidic.
Human Impact - deforestation, dams and bridges interfere with the natural flow of a river and frequently end up increasing the rates of erosion.
Transporation: Traction
Heavy rocks and boulders are rolled along the river bed.
Happens most in times of flood, when the current is strongest
Transporation: Saltation
Small stones and pebbles are ābouncedā along the river bed.
Saltation can take place when the river is less than that needed for āTractionā to take place.
Transportation: Suspension
Very small particles of sand or clay that are āsuspendedā in the water.
These particles will āsettleā if kept in a jar of water overnight and the water will look clear
Transportation: Solution
Takes place when material is dissolved in the water, it is invisible and does not colour the water.
- Occurs often in limestone and does not colour the water.
- Occurs often in limestone landscapes where the water if very acidic.
- Some pollutants like weedkiller are also held in solution in the water
Hjulstrom Curve
- Illustrates the relationship between velocity and grain size
- Illustrates what processes will happen to different sized sediment at various velocities
(Used to determine whether a river will erode, transport or deposit sediment)
Deposition
Where a rivers velocity (speed) decreases so it no longer has the energy to transport its load
Causes of deposition
- A shallowing of gradient which decreases velocity and energy
- A decrease in the volume of water in the channel
- An increase in the friction between the water and the channel.
- A river regime varies seasonally. Some have a wet season where the velocity is greater, depth increases and width increases. Some rivers have a very high flow in spring after the snow melts. BUT, erosion and deposition can vary due to human influence.
