- The critical erosion velocity curve on the graph shows the minimum velocity needed for the river to pick up (erode) and transport particles of different sizes (in suspension or as bedload). It takes a higher velocity to erode material than it does to just transport material
- The mean settling velocity curve shows the velocities at which particles of different sizes are deposited. i.e it shows the competence of the river at different velocities.
Exam Questions
- Describe the 5 ways in which river erosion happens.
Hydraulic action is a form of erosion. The sheer force of the water erodes away the river bed. The second way is via Abrasion, where eroded pieces of rock erode in the water scrape away the river bed. The third method is through Cavitation. This is where air bubbles in turbulent stretches of water implode causing shockwaves which break pieces of rock off the banks and bed. The fourth way is via solution. This is where rock gets dissoloved by chemical process. Carbon dioxide dissolves in thw water to form a weak acid causing rock like limestone and chalk to break down. The last method is Attrition. This is where eroded rock smash into each other making rock smaller and more rounded off as they rub against each other.
Q2) Outline how the critical erosion velocity curve on a hjulstorm curve varies with particle size.
The critical erosion velocity curve shows the minimum velocity needed to pick up and transport particles of different sizes. Its lowest for sand (Particles between 0.1mm and 1mm), but it is higher for both larger (pebbles and rocks) and smaller particles (Silt and clay) because clay and silt require a faster velocity to be picked up and transported than sand as they tend to stick together more thus it takes more energy (velocity) to erode them. Obviously, heavier particles need more velocity and energy due to the fact that they are heavier so need a bigger push.
Q3) What is meant by the competence of a river?
When referring to competence we refer to the maximum particle size that can be transported.
The long profile and channel characteristics (Pages 8-9)
The long profile shows the rivers gradient from its source to the sea by showing the height of the river bed above the base level for the whole length of the river
The base level is the lowest point that the river can erode to.
The total amount of erosion and deposition along the full course of a river are balanced. But the rates of erosion and deposition change along the cource of a river, which can result in the formation of landforms like waterfalls (Where the rate of erosion is higher than the rate of deposition which make the profile uneven.
Because the total amount of erosion and deposition is balanced, the rate of erosion of landforms like waterfalls is equal to the rate of deposition which means that over time the long profile will change from being uneven to being a smooth curve. This is called the graded profile (but it hardly ever happens)
A rivers course can be split into three stages. The energy of a river varies in each stage.
- In the upper stage, the gradient is steep and the river is high above sea level, which gives It lots of potential energy (energy that can be converted)<e.g. source – mountain><landform – lake>
- As gradient decreases towards the middle stage, potential energy is converted to kinetic energy (movement) – the river gains velocity <Landform: Lake>
- In lower stage, river has little potential energy but lots of kinetic energy – it flows better <E.g. mouth: sea>
Channel characteristics affect velocity and discharge.
Velocity (kinectic energy) and discharge of a river increase as you go downstream from source to mouth. Discharge increases as tributaries and more surface runoff join the main channel. River velocity is influenced by gradient, discharge and channel characteristics – the shape and roughness.
- Most of rivers kinetic energy is used to overcome friction. The rest causes erosion
- The more energy a river has available for erosion and transportation, the more efficient it is
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An efficient river will have high velocity, high discharge and little discharge
The efficiency of a river is measured by hydraulic radius. The larger its hydraulic radius, the more efficient it is.
Channel roughness also affects efficiency. Protruding banks and large, angular boulders on the river bed increase the wetted perimeter and cause more friction. This reduces efficiency, velocity and discharge.
As channel roughness increases, so does turbulence (erratic swirling of the water in the main flow). Turbulent flow is more effective at picking up particles from the river bed than smooth flow – so turbulence causes greater erosion.
Channel roughness is greatest in the upper stages of the river. So even though the gradient is steep, the river loses a lot of energy to friction, so discharge and velocity are lowest here during normal conditions.
In the lower stages, the banks and bed of the river are smooth, so there is much less friction. This means that less energy is lost, so discharge ad velocity are the highest in this stage.
River processes can change as the river flows from source to mouth
Upper stage:
EROSION: Mainly vertical and by abrasion erosion occours. Erosion occours when there are high energy conditions (i.e. when velocity and discharge are high after heavy rain or ice melt). The rough channel causes turbulence and the large angular bedload is dragged along the river bed causing intense downward vertical erosion.
TRANSPORTATION: Mainly particles such as boulders carried by traction or saltation during high energy conditions.
DEPOSITION: Little occours – mainly largest particles deposited in the river bed as energy levels drop.
Middle Stage:
EROSION: Mainly lateral and by abrasion. Attrition of larger particles in this stage mean that sediment particles size decreases from source to mouth.
TRANSPORTATION: More material carried by suspension as particle size deceases. Some larger particles moved by saltation.
DEPOSTION: Sand and gravel are deposited across the flood plain as the river floods and friction reduces rivers energy
LOWER STAGE: EROSION: Although velocity and discharge are at its highest, there is less erosion because turbulence is lower and sediment particle size is reduced reducing abrasion. Some lateral erosion occurs during the formation of meanders.
TRANSPORTATION: MAINLY SMALLER PARTICLES LIKE SILT AND CLAY CARRIED BY SUSPENSION OR SUBTANCES CARRIED IN SOLUTION.
DEPOSITION: SMALLER PARTICLES LIKE SAND, SILT AND CLAY ARE DEPOSITED ON THE FLOOD PLAIN WHEN THE RIVER FLOODS AND IN THE RIVER MOUTH AS THE SEA ABSORBS THE RIVERS ENERGY.
(On the lower stage, the flat area is the flood plain)
Exam Questions
Q1: What does the long profile of a river show?
It shows the rivers gradient from its source to the sea
Q2: Describe the features of a lower stage river valley
They are wide, with gently sloping sides. There is a much wider flood plain caused by deposition
Q3: What does the hydraulic radius tell us about a river and how is it calculated?
It is calculated by the rivers cross sectional area divided by the wetted perimeter (the area that the water touches). It tells us how efficient a river is. For instance, the larger the hydraulic radius, the more efficient the river is.
Q4: Outline how the processes of erosion, transportation and deposition change along a rivers long profile:
#refer to book
River Landforms (Page 10 – 11)
(Define Fluvial: Caused by rivers and streams)
Waterfalls, Rapids and potholes are caused by fluvial erosion:
Waterfalls are usually found in the upper and middle courses of a river. They occur when harder rock e.g basalt, overlay softer rock e.g sandstone. The water flows over this rock, however, over time the soft rock becomes weaker so a ledge develops as the hard rock is still very stable. The water rushes over the ledge and erodes a plunge pool by abrasion and hydraulic action. Later on, the hard rock can not cope and it collapses into a plunge pool, where the debris helps speed up erosion. The process is repeated and the waterfall gradually repeats upstream carving out a gorge.
Potholes are small circular hollows in the river bed. They are formed by abrasion as turbulence swirls a rivers bedload round in a circular motion, causing it to rub and scrape out holes.
Rapids are relatively steep sections of river with turbulent flow where there are several sections of hard rock. They are like mini waterfalls.
Meanders are formed by combines erosion and deposition.
Meanders form where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of a river. The distance between pools is 5-6 times the width of the river bed.
Because the river channel is deeper in pools its more efficient so it has more energy. Energy is lost at the riffle because of friction.
The spacing and distance between riffles and pools causes the rivers flow to becomes uneven and maximum flow to become uneven and maximum flow to be concentrated on one side of the river
Turbelence increases in and around pools as the water speeds up, so the flow of water begins to twist and coil.
This causes corkscrew like currents in the river called helicodial flow, which spiral from bank to bank between pools.
The helicodal flow causes more erosion and deepening of the pools. It causes eroded material to be deposited on the inside of the next bend where the river looses energy.
The combination of erosion and deposition exaggerates the bends until large meanders are formed. The combined processes also create the meanders’ distinctive asymmetrical cross section.
Oxbow lakes are formed when the neck of the loop of a meander is broken through, often during flooding. Deposition dams off the loop leaving an oxbow lake.