3. River Features Flashcards
What does the Hjulström Curve demonstrate?
The Hjulström Curve shows the relationship between river velocity and competence
What is Competence?
The competence is the maximum size of load (maximum particle size) a river is able to carry at a particular point
It shows the velocities at which sediment of a given size will normally be eroded, transported and deposited
What is Capacity?
Capacity is the total volume of material a river can transport
- A greater velocity is required to erode material compared to just transporting it
- Generally, higher velocities are needed to erode larger particles
- Deposition occurs when the velocity decreases - heaviest first
What is the velocity curve?
The velocity curve is the minimum velocity needed for the river to erode and transfer material of a different size
What is the setting velocity?
The setting velocity shows which size particles are dropped as the river velocity slows.
How does discharge affect the size and amount of the river’s load?
During times of high discharge and high velocity, the size and amount of the river’s load will increase considerably causing increased erosion within the channel.
Is coarser material more susceptible to deposition?
For coarser material, the boundary between transportation and deposition is narrow. A small drop in velocity is needed for deposition / sedimentation.
What features does fine clay have and how do these affect the Hjulström Curve?
Fine clay has high cohesive (sticky) properties and therefore needs a velocity similar to that of pebbles to initiate movement.
However, many individual minerals in clay are soluble so it stays in transport for a long time.
Describe the formation of a waterfall and gorge:
As water passes over alternating bands of soft and hard rock, it erodes the soft rock faster through abrasion and hydraulic action leading to the formation of a plunge pool and overhang.
Eventually, the overhand is eroded until it becomes unsupported and collapses, causing the waterfall to retreat and a steep-sided gorge to form. Attrition also causes the plunge pool to widen.
Describe the formation of rapids:
As water passes over alternating bands of soft and hard rock, it erodes the soft rock faster through abrasion and hydraulic action.
The water flow becomes turbulent as erosion of soft rock creates an uneven channel bed.
What are pools and riffles?
A pool is an area of deep water and greater erosion (energy build-up due to less friction).
A riffle is an area of shallow water created by deposition of coarse sediment.
How are meanders formed?
Meanders are formed because the thalweg, or line of fastest flow, swings from side to side as it passes around bends.
This causes erosion on the outside of the bend where the current is fastest and has more energy, and deposition on the inside of the bend where the current is slower.
Describe the formation of an oxbow lake:
Lateral erosion from the thalweg erodes the sides of the meander.
The meander continues to erode towards itself.
Eventually the meander erodes into itself forming a lake and a small island.
The change in the thalweg causes deposition to form land, sealing off the old meander and forming an oxbow lake.
What is a flood plain?
A flood plain is an area of flat land adjacent to the main river which experiences flooding during times of intense discharge.
What is a bluff?
A bluff is a steep, rounded slope formed by erosion from meanders. They usually define the edge of a flood plain.
