Erosion and Transportation

Erosion Processes

         Erosion is the wearing away and removal of material from the wetted perimeter of the channel. Water flowing down a channel has kinetic energy, which is used for erosion.

         All material transported by the river is known as the load.

         Under flood conditions, when greater velocities are experienced hydraulic action, abrasion, and attrition all operate more effectively and so the effectiveness of erosion will be significantly increased.

         Erosion can take place in four ways:-

1.       Hydraulic action

-          Moves material in the channel through the impact of the moving water, and its frictional drag on the particles on the bed.

-          Usually it can only remove small particles such as sand and fine gravel.

-          As velocity increases, there is more turbulence, lifting more sand grains from the bed.

-          River banks are often undermined by hydraulic action, eventually they collapse into the river.

2.     Abrasion

-          Occurs when particles already being transported by the river hit, or are dragged along, the rock bed.

-          The action is similar to a hammer chipping the rock, or a file smoothing the surface and producing very small particles, which are easily transported.

-          The larger the particles in the load the quicker the erosion.

-          Small particles smooth and polish the surface of the bed.

-          This is the most effective method of erosion, normally responsible for most of the downcutting of a river channel.

-          Abrasion is ineffective if there is very little load in solid form.

-          More abrasion takes place during storm flow conditions than at low flow.

-          Potholes are a common feature of rapid abrasion. They are cylindrical holes drilled into the rock by turbulent high velocity flow. Vertical eddies may be strong enough to rotate a small pebble, grinding a depression in the rock. Very large potholes are found underneath glaciers where water velocities are high.

3.     Solution

-          This is mostly on Carbonate rocks, i.e. Limestone.

-          Carbonic acid dissolves the rock and carries it away in solution.

-          Almost all rocks are soluble to some extent.

4.     Attrition

-          The reduction in size of the particles in transport as they strike one another or the channel bed.

-          Thus a particle moving downstream undergoes a progressive reduction in size.

-          The sharp edges and angles of the particle become more rounded, as they are exposed.

-          Upstream parts of a river have large angular sediment, and downstream parts have fine rounded particles. But this is a generalisation, angularity and size of particles depend upon their lithology, the length of time it has been in the river and the distance travelled. Pebbles may also be rounded by other things such as waves or glaciers.

Transportation

Material which has been added to the load of the river by erosion is transported in three ways:-

1.     Suspension

-          Some particles (i.e. Silt and Clay) are small enough to be held up by turbulence in the channel, and form the suspended load.

-          The more turbulent the water the larger the particles which can be transported in suspension.

2.     Traction and Saltation

-          Larger particles (i.e. sand and gravel) roll and side (traction) or bounce (saltation) along the bed of the river under the hydraulic force of the moving water.

-          The particles moving close to or along the bed or the river are known as bed load.

3.     Solution

-          The dissolving of rocks adds to the rivers load, and is known as the dissolved load.

Velocity and Transportation

As the riverís velocity is increased the discharge rises.

Discharge [Q] = cross sectional area [A] X Velocity [V]

(m3/sec)††††††††††††††††††††††††††† (m2)††††††††††††† (m/s)

-          The higher velocity enables the river to carry larger particles. The competence of a river is measured by the largest transported particle.

-          The greater the volume of water flowing down a channel enables a greater load to be carried, so the capacity of the river is increased.

-          Hjulstrom shows the relationship between particle size, channel velocity, and erosion, transportation and deposition.

-          Transport requires much less energy than erosion.

-          So, once a particle is eroded from the channel bed, it can be carried a long distance even if velocity falls. Eventually velocity will be so slow that the particle is deposited.

-          Small particles require disproportionately larger velocities to raise them from the channel bed.

-          This is due to the cohesion (attraction) of the particles towards each other.

-          Also, as the small particles lie on the channel bed, they offer less resistance to water flow than larger particles, and therefore need a more energetic steam to lift them.

-          Velocity of flow determines the size and amount of material that can be transported. So particles that constitute part of the bed load under low flow may be carried along the channel as part of the suspended load in times of flood.

-          The increase in turbulence creates stronger vertical eddies, which support the larger load.

-          Many rivers in flood have a colouration due to the load, normally brown. But rivers from glacial snouts are milky white or grey, rivers from deserts are red.

Bibliography

††††††††††† - Clowes and Comfort p112- 115

- Waugh p72-73