River/Channel Processes

Question 1

What are the three main processes?

Answer 1

Erosion, transportation and deposition.

Question 2

What does the dominant process depend on?

Answer 2

The amount of energy available.

Question 3

Why do rivers erode?

Answer 3

Because they possess energy.

Question 4

What do rivers total energy depend on?

Answer 4

1. The weight of the water (the greater the mass of the water, the more energy it will have due to the influence of gravity on its movement)
2. The height of the river above it base level (usually sea level- this gives it a source of potential energy and the higher the source of the river the more such energy it has)
3. The steepness of the channel ( this controls the speed of the river which determines how much kinetic energy it has)

Question 5

What can erosion affect?

Answer 5

Length, depth and width of a channel.

Question 6

What is headward erosion?

Answer 6

Headward erosion makes the river longer. It happens near a rivers source as through flow and surface runoff causes erosion at the point the water enters the river channel. (the valley head)

Question 7

What is vertical erosion?

Answer 7

vertical erosion deepens river channels, it happens in the upper stages of a river.

Question 8

What is lateral erosion?

Answer 8

Lateral erosion makes the river wider. It happens in the middle and lower stages of a river.

Question 9

Explain abrasion.

Answer 9

Abrasion is the scraping, scouring and rubbing action of materials carried along by a river (the load). Rivers carry rock fragments in the flow of water and drag them along the bed, which wear away the banks and bed of the channel. Abrasion is most effective in short turbulent periods when the river is at bankfull or in flood. During times when river levels are low, the load consists of small particles e.g. sand grains, these tend to smooth the surface of the river channel.

Question 10

Explain hydraulic action.

Answer 10

Caused by the pressure of moving water and movement of loose material due to the frictional drag f the moving water on sediment lying on the channel bed. As velocity increases, turbulent flow lifts a larger amount of sediment from the floor of the channel. Hydraulic action is effective i removing loose material in the banks of meanders, which can lead to under-cutting causing them to collapse. Hydraulic action is strongest in rapids or below waterfalls..

Question 11

What is corrosion?

Answer 11

Corrosion is most active on rocks that contain carbonates, such as limestone and chalk. The minerals in the rock are dissolved by weak acids in the water and carried away in solution.

Question 12

What is attrition?

Answer 12

The reduction in the size of fragments and particles within a river due to the processes described above. The fragments strike one another, as well as the river bed. They therefore become smoother, smaller and more rounded as they move along the channel. Larger, more angular fragments tend to be found upstream while small, more rounded fragments are found downstream.

Question 13

Why is energy lost?

Answer 13

Due to friction; internally through turbulence within the flow or externally through contact with the bed and banks of the channel.

Question 14

What is the wetted perimeter?

Answer 14

The overall length of the bed and the banks that the river is in contact with.

Question 15

What is transportation?

Answer 15

The process of eroded material being carried by the river..

Question 16

Where does the river obtain its load from?

Answer 16

Material that has been washed, or has fallen into the river from the valley sides. OR material that has been eroded by the river itself from the bed or banks.

Question 17

How many transport processes are they?

Answer 17

There are 4 main processes; traction, salutation, suspension and solution.

Question 18

Explain Traction.

Answer 18

Traction is the movement of large stones and boulders being rolled along the river bed by water moving downstream. This process operates at time of high discharge (and high energy)

Question 19

Explain Saltation.

Answer 19

The movement of small stones bouncing along the channel bed. This process is associated with quite high energy conditions. Small particles may be thrust up from the bed however fall back to the bottom further downstream. As the particles land they dislodge other particles upwards, causing more of this bouncing movement.

Question 20

What is suspension?

Answer 20

Very small particles of sand and silt are carried along by the flow of the river. Such material is not only carried but picked up by the turbulence in the water. Suspension, normally contributes the largest proportion of sediment to the load and is the cause of the ‘brown appearance’ of many rivers.

Question 21

What is solution?

Answer 21

This is the transportation of dissolved materials in the mass of the moving water.

Question 22

What is the capacity of the river?

Answer 22

A measure of the amount of material it can carry, the total volume of the load.

Question 23

What is the competence of a river?

Answer 23

The diameter of the largest particle that it can carry for a given velocity.

Question 24

When does deposition occur?

Answer 24

When there is a loss of energy.

Question 25

When does a river have a lose of energy?

Answer 25

1. A reduction in the gradient e.g. when it meets a lake
2. Reduction in rainfall and thus, reduction in discharge causing the river to slow down.
3. Where there is shallow water and therefore friction in areas like meanders.
4. When the river floods and overtops its banks, resulting in a reduced velocity on the flood plain outside the main channel.

Question 26

What does the Hjuliström curve show?

Answer 26

The relationship between the velocity of the river and the size of the particles that can be eroded, transported or deposited.

Question 27

What is the typical relationship the curve shows?

Answer 27

Velocity increases as discharge rises and generally this enables a river to pick up larger particles from the bed or banks of the channel. As velocity and discharge reduce, then particles are generally deposited according to their size, larger first.

Question 28

What is the critical erosion velocity curve?

Answer 28

This shows the minimum velocity needed for the river to pick up (erode) and transport particles of different sizes (in suspension or as bed-load) It takes a higher velocity to erode material than it does to just transport material.

Question 29

What is the mean settling velocity curve?

Answer 29

This shows the velocities at which particles of different sizes of deposited, e.g. the competence of the river at different velocities.

Question 30

Why can particles such as sand be eroded at lower velocities than finer particles such as silt or clay?

Answer 30

Silt and clay particles stick together more than sand, which means they are harder to dislodge, so it requires more energy to erode them.