What does the river energy depend on?
3
- Water mass - more mass, more energy - gravity.
- The height of the river above its base level - gives it a source of potential energy.
The higher the source, the more energy it has. - Channel steepness- controls speed of the water - determines how much kinetic energy there is.
What is the wetted perimeter?
Overall length of the bed and banks that the river is in contact with.
How can river energy be lost?
2
Internally - through turbulence flow within the flow of the river.
Externally - through contact with bed and banks
What is abrasion?
Eroded pieces of rock in the water scrape and rub against the river beds, removing the material.
What is hydraulic action?
Power of moving water.
The pressure of the water breaks rock particles away from the bed and banks.
What is corrosion?
The minerals in the rock are dissolved by weak acids (caused by CO2) in the water and are carried away in solution.
What types of rocks does corrosion affect?
Rocks that contain carbonates e.g limestone and chalk.
What is attrition?
Eroded rocks smash into each other and break into smaller fragments.
Smooth and rounded rock edges.
What is cavitation?
Air bubbles in turbulent waters implode causing shockwaves that break pieces of rock off the bed and banks.
What is the information for vertical erosion?
3
- Upper course - the river is attempting to cut down to its base level, which is usually sea level.
- Causes steep-sided valleys.
- Mainly abrasion and hydraulic action.
What is the information for lateral erosion?
4
- Middle to lower course.
- Valley floor lies closer to sea level.
- Lots of energy here - used to laterally widen the valley as the river meanders.
- Mainly hydraulic action.
What is traction?
Large boulders are rolled along the river bed by water moving downstream.
When does traction occur?
+ EXAMPLE
Times of high discharge (and high energy levels).
EXAMPLE: Boscastle flood August 2004 - River Valency.
What is saltation?
Small stones bounce along the river bed.
What is suspension?
Very small particles (sand/silt) are carried along by the flow of the water.
Material also picked up, as well as carried.
What is solution? (transport)
Dissolved minerals are transported within the mass of the moving water.
What does river capacity mean?
The amount of material it can carry - the total volume of the load.
How is river capacity related to river velocity?
A river’s capacity increases according to a third power of its velocity.
What does river competence mean?
The diameter of the largest particle that the river can carry for a given velocity.
When does deposition occur?
5
- River gradient reduction e.g enters a lake.
- Reduced discharge e,g during a dry spell.
- Shallow water e.g inside of a meander.
4, Increase in load size. - The river floods - reduced velocity on the floodplain.
What does the Hjulstrom Curve show?
The link between river velocity and river competence.
What is the first relationship that the Hjulstrom Curve shows?
3
- Sand particles are picked up by lower critical erosion velocities than silt or clays.
- The smaller silt and clay are harder to pick up (entrain) as they stick together.
- They lie on the river bed and offer less resistance to water flow than larger particles.
What is the second relationship that the Hjulstrom Curve shows?
2
- Once picked up, particles can be carried at lower velocities than those required to pick them up.
- Larger particles - only a small difference between the critical erosion velocity and the settling velocity - deposited very soon after being picked up.
What is the third relationship that the Hjulstrom Curve shows?
Clays and silts are only deposited at very low velocities.
Some clay particles may never be deposited on the river bed.
