Geography - AS Rivers AQA

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Erosion Procceses

Hydraulic action - Sheer force of the water breaks rock particles away from the bed and banks meaning this is strongest in rapids and waterfalls.

Abrasion (corrasion) - Eroded pieces of rock in the river scrape and rub against the bed and banks, removing material. 

Attrition - Eroded rocks smash into each other and break into smaller fragments. The edges of the rocks also get rounded when they rub together. Attrition doesn't erode the bed and banks, it just makes the already eroded rocks smaller and more rounded.

Cavitation - Air bubbles in turbulent water explode causing shockwaves that break pieces of rock of the bed and banks.

 Corrosion - Dissolving of rock by chemical processes which reacts with the rocks, breaking them down. 


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Transportation Processes

Solution - Substances that can dissolve are carrier along in the water

Suspension - Fine material like slit and clay is whipped up by turbulence and carrier along in the water 

Saltation - Larger particles like pebbles or gravel are too heavy to be carrier by suspension. Instead, the force of the water caused them to bounce along the river bed

Traction - Very large particles like boulders are too heavy to be bounced by saltation. Instead these particles are rolled along the bed by the force of the water

 

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Deposition Processes

Reduced rainfall causes lower discharge, which means the river slows down and has less energy

Increased evaporation or abstraction (removing water for human use) also causes lower discharge

Friction e.g. In shallow areas and close to the river banks

 When  the river is forced to slow down e.g. Before a narrow section of the channel

Energy is lost when the river meets the sea

The River floods and exceeds bank full resulting in reduced velocity on the floodplain


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Long Profile

In the Upper course, the gradient is steep (V-shaped) and the river is high above sea level, which gives it lots of potential energy

As the gradient decreases in the Middle course, potential energy is converted into kinetic energy (movement) - the river gain velocity

In the Lower course, the river has little potential energy, but lots of kinetic energy - it flows faster

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Channel Characteristics on Velocity

The velocity and discharge of a river increase as you go downstream from source to mouth
River velocity is influenced by gradient, discharge and channel characteristics - the shape and roughness

 

  1. The more energy a river has available for erosion and transportation, the more efficient it is. An efficient river will have a high velocity, high discharge and little friction
  2. Efficiency is measured by hydraulic radius. The larger the hydraulic radius, the more efficient a river is. The hydraulic radius is the channels cross section area divided by the length of the wetted perimeter (length of banks and bed of the river in contact with water). Contact between them creates friction, which increases energy loss and slow down the river. A larger hydraulic radius means that a smaller proportion of the water is in contact with the wetted perimeter. So friction is lower, which reduces energy loss, increasing velocity and discharge
  3. Channel roughness can also effect efficiency. Large load on the bed increases the wetted perimeter and causes more friction, reducing the efficiency and velocity of the river. Channel roughness is greatest in the upper course, so although the gradient is steep, the river loses a lot of energy to friction, so velocity is at its lowest here. In the lower douse, the bed and banks are smooth, so theres less friction and less energy is lost, so velocity is at its highest here
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River Landforms - Waterfall

  1. Waterfalls form when a band of hard rock (igneous rock) meets soft rock (sedimentary rock) and the soft rock erodes more than the hard rock, creating a ‘step’ in the river
  2. Water that flows over that step speeds up due to a lack of friction as it drops over the step. This gives the water greater erosive power, causing further erosion of the softer rock and undercutting of the hard rock
  3. This leaves an overhand of the hard rock and is it is undercut, it can collapse. A deep plunge pool is carved out by abrasion at the bottom of the waterfall as the bits of collapsed rock are swirled around by turbulence
  4. Over time, more undercutting causes more collapse and the waterfall will retreat, leaving behind a steep sided gorge
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River Landforms - Potholes

  1. Potholes are circular hollows drilled into the hard bedrock by turbulent high velocity flow
  2. Active vertical abrasion occurs along the bed producing potholes, especially on fast flowing rivers with strong eddy currents
  3. The eddy currents are produced by unevenness in the hard rock created by differential river erosion
  4. Eddying creates a shallow hollow that may become occupied by pebbles or stones
  5. The swirling of the pebbles creates turbulence which deepens the hollow (vertical erosion) into a pothole. This type of abrasion is known as ‘drilling’
  6. Adjacent potholes may join to create one large pothole which adds to the process of vertical erosion
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River Landforms - Rapids

  1. Rapids are a series of stepped steep gradients causing an increase in water flow and turbulence
  2. Rapids occur when the bed material is highly resistant to the erosive power of the river but there is still some differential erosion of harder rock so sections of the rock become exposed at the surface as steps
  3. The river then falls vertically over these steps which are separated by small plunge pools which the size of them are increased by hydraulic action and vertical abrasion
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River Landforms - Meanders

  1. Meanders form where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals
  2. As the river is deeper in pools, it is more efficient and has more erosive power. Energy is lost over a riffle because of friction
  3. The spacing between the riffles and pools cause the rivers flow to become uneven and maximum flow to be concentrated on the outside bend of the river. This maximum flow is known as thalweg
  4. Turbulence increases in and around pools as the water speeds up, so the water begins to twist and coil, known as helicoidal flow, which spirals from bank to bank between pools
  5. The helicoidal flow causes more erosion and deepening of the pools and causes eroded material to be deposited on the inside bend, known as the point bar, where the river loses energy
  6. The combination of erosion and deposition exaggerates the bends until large meanders are formed. The combined processes also creates the meanders distinctive asymmetric cross-section
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River Landforms - Oxbow Lake

  1.  An Oxbow Lake is a horseshoe shaped lake separated from an adjacent river
  2. Erosion is greatest on the outer bank of a meander, and with deposition on the inner bank, the neck of the meander becomes increasingly narrower as continuous erosion brings the outer banks together
  3. During periods of high discharge and high velocity water flow, such as floods, the river cuts through the neck 
  4. This original meander sis then abounded in favour of a straighter, faster route downstream
  5. Deposition then sealed of the meander ,creating an ox-bow lake
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River Landforms - Flodplains

  1. Rivers have the most energy at bank full and usually carry huge amount of suspended load
  2. Any further rise causes it to flood, caring the sediment onto the adjacent flat land
  3. Efficiency and energy is reduced because of an increase in friction, so the river loses velocity and deposits the load. As the flood water disappears, the suspended load (alluvium) is deposited and left on the land
  4. After many floods layers of alluvium builds up creating a floodplain. Over years, lateral and downstream migration of meanders also adds new flat land to the floodplain in terms of old point bar deposits. 
  5. Continual lateral erosion on the outside bend of a meander leads to a collapse of the river cliff, then subsequently lots of deposition on the next inside bend, creating a build up of point bar deposits showing that floodplain formation is a dynamic process
  6. The edge of a floodplain is often marked by a slope called the bluff line
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River Landforms - Levees

  1. Levees are natural, raised embankments formed when a river breaks it's banks
  2. During a flood, material is deposited across the whole floodplain as the river loses velocity and energy due to an increased friction with the land
  3. The heaviest material is dropped first, closest to the river, creating a large embankment on the side of the river bank
  4. Over time, this material builds of on a river bank, creating a levee  
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Main Causes of Flooding

Flooding occurs when the discharge of a river is so high, that the river exceeds its banks onto the floodplain

  1.  A major cause is prolonged rainfall. After a long period of rain, the ground becomes saturated so it can't absorb any more water. Any further rainfall can't infiltrate, which increases surface runoff, which increases discharge
  2. Heavy rainfall can lead to rapid surface runoff if the rainfall is too intense for infiltration to occur. This can lead to a sharp rise in river discharge called a flash flood
  3. Melting snow and ice can also lead to a huge increase in a rivers discharge
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Physical factors that increase the risk of Floodin

Sparse vegetation or deciduous trees:

  •  Sparse vegetation in the drainage basin means little rainfall is intercepted, so more rain reaches the ground. This increases the volume of water reaching the river through through-flow and surface runoff
  • Deciduous trees have to leaves in winter, which has the same effect as sparse vegetation - little rainfall is intercepted, meaning more rainfall reaches the river

Impermeable ground:

  • Some rocks such as granite are impermeable - they don't allow infiltration of surface water. This heavily increases surface runoff, which increases discharge rapidly
  • If the ground has been naked hard by the heat of the summer, or it's frozen, the same thing happens - water cannot infiltrate, increasing surface runoff and discharge

Seep slopes:

  • If the drainage basin has steep sided valleys, water will reach the river channel much faster because water flows more quickly on steep slopes because of the steep gradient. This increases discharge quickly
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Human factors that increase the risk of Flooding

Urbanisation:

  •  They have large areas of impermeable Tarmac and concrete, so surface runoff is rapid
  • Gutters and drains quickly take runoff to rivers

Deforestation:

  • Clearing trees and plants reduces interception. This increases the volume of water that reaches the channel, which increases discharge
  • It also leaves the soil loose. The soil is eroded by rainwater and carried to the river which raises the river bed and reduces the capacity, so it takes less water for the river to flood

Flood management:

  • Can actually make flooding worse, like if a Dam breaks, they release large amounts if water, therefore giving a huge increase in discharge

Climate change:

  • Climate change could cuase storms which consequently increase the likelihood of flooding
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Hard Engineering

Hard engineering defences are man made structures that reduce flooding. However they can have many disadvantages including:

  1. They’re expensive to build and maintain. Therefore, poorer countries often cannot afford these flood defences
  2. Floods happen less often, but when they do, they can be disastrous meaning if for example, a dam breaks, then a hug he amount of water will flood the land
  3. Natural processes are disturbed e.g. crops don't get fertile soils
  4. Some people think there a visual disturbance
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Soft Engineering

Soft engineering defences use knowledge of the whole river basin and its processes, to try to work with nature. They have many advantages including:

  1. Cheaper to maintain than hard engineering so poorer countries are able to afford these defences
  2. They can improve recreational opportunities such as fishing
  3. More attractive than hard engineering
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