Geography - Rivers Revision AQA

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Erosion

Vertical Erosion - Deepens the river valley at the upper course making it V-shaped

Lateral Erosion - Widens the river valley in the middle and lower courses

Hydraulic Action - The sheer force of the water braking the rocks away from the river channel

Abrasion - When rocks are picked up by the river and scrape against the river bed, causing it to erode

Attrition - When rocks are picked up by the river and smash into each other, breaking into smaller fragments. The edges of the rock also become rounded

Solution - The river water dissolves soluble rocks

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Tranportation

The material that a river has eroded is transported downstream in four ways:

Traction - Large particles like boulders are pushed along the river bed by the force of the water

Saltation - Pebble-sized particles are bounced along the river bed by the force of the water

Suspension - Small particles like solt and clay are carried along by yhe water

Solution - Soluable materials dissolve in water and are carried along

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Deposition

Deposition is when a river drops the eroded material. It happens when a river slows down.

Deposition can occur when:

  • The volume of water in the river falls
  • The river reaches its mouth
  • The water is shallower on the inside bend
  • The amount of eroded material increases
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Erosion Landforms

Waterfall:

  • Waterfalls form when a river flows over an area of hard rock followed by an area of soft rock
  • The softer rock is eroded more than the hard rock creating a 'step' in the river
  • As water goes over the step it erodes more and more of the softer rock
  • A steep drop is eventually created, which is called a waterfall

Gorge:

  • Gorges form from waterfalls
  • The hard rock is eventually undercut by erosion. It become unsupported and collapses
  • The collapsed rocks are swirled around the foot of the waterfall where they erode the softer rock by abrasion and creates a plunge pool
  • Overtime more undercutting causes more collapses. The waterfall will retreat leaving behind a steep sided gorge

Interlocking spurs - Occurs in the upper course of a river where the most erosion is vertical which creates steep sided v-shaped valleys. The river has to wind around the hillsides that stick out in their path. Hillsides that interlock are called interlocking spurs

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Erosional and Depositional Landforms

Meanders:

  • In the middle or lower course, rivers develop meanders
  • The flow of water is faster on the outside bend because the river channel is deeper
  • So more erosion happens on the outside bend forming river cliffs
  • The flow of water is slower on the inside bend because the river channel is shallower
  • So eroded material is deposited on the inside bend as it has no energy to carry the load forming slip-off slopes

Ox-Bow Lakes:

  • Are formed from meanders
  • Erosion causes the outside bends of a meander to get closer until there's only a small bit of land left between the bends (called a river neck)
  • The river breaks through this land, usually through a flood
  • The river flows along the shortest course
  • Deposition eventually cuts of the meander forming an onx-bow lake
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Depositional Landforms

Flood Plain:

  • The flood plain is the wide valley floor on either side of a river which occasionally gets flooded
  • When a river floods onto the flood plain, the water slows down and deposits the eroded material which makes the flood plain bigger
  • Meanders move along floodplains making it wider

Levees - Levees are natural embankments along the edges of a river channel. During a flood, eroded material is deposited over the whole flood plain. The heaviest material is dropped closest to the river channels its the first thing to get dropped once a river slows down.Overtime that material builds up creating leavees along the edges of the channel

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River Discharge

River Discharge is the volume of water flowing in a river

Peak discharge - The highest discharge in the period of time you are looking at

Lag Time - The delay between peak rainfall and peak discharge. Lag time happens because most rainwater does not land directly in the river channel

Rising Limb - The increase in river discharge as rainwater flows into the river

Falling Limb - The decrease in river discharge as the river returns to its normal level

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Factors affecting River Discharge

Factors that increase discharge:

  • High rainfall causes more runoff and a shorter lag time
  • Intense rainfall causes more runoff and a shorter lag time
  • Impermeable rock means water can't infiltrate into the rock so there's more runoff and a shorter lag time
  • Hot, dry conditions and freezing conditions both result in hard ground, so there's more runoff and a shorter lag time
  • Steep slopes cause more runoff and a shorter lag time

Factors that decrease discharge:

  • Low rainfall causes less runoff and a longer lag time
  • Light rainfall causes less runoff and a longer lag time
  • Permeable rock means water can infiltrate through the pore spaces so there's less runoff and a longer lag time
  • Mild conditions result in hard ground so water can infiltrate into it causing less runoff and a longer lag time
  • Gentle slopes cause less runoff and a longer lag time
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Flooding

Flooding - Occurs when a river carries so much water that it cannot be contained by its banks so it overflows onto the floodplain

Physical factors affect flooding:

  • Prolonged rainfall - After long periods of rain, the soil becomes saturated and water can not infiltrate. This increases the runoff into rivers and increases discharge quickly which can cause a flood
  • Heavy rainfall - Heavy rainfall means there is a lot of runoff. This increases the discharge quickly which can cause a flood
  • River Valley - If the river valley is V-shaped it means there is a steeper slope which increase the runoff and discharge quickly which can cause a flood
  • Snow melt - When snow and ice melt it means a lot of water goes in to the river in a short space of time. This increases discharge quickly which can cause a flood

Human factors affect flooding:

  • Deforestation - Trees can catch water and there leaves and take in in through their roots. This means cutting down trees increases the volume of water that reaches the river which increases discharge and makes a flood much more likely
  • Building Construction - Building are usually made by impermeable materials. These materials increase runoff which will increase discharge and make a flood more likely
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Flood Management - Hard Engineering

Hard engineering - man made structures built to control the flow of rivers and reduce flooding

 ( Blue = Strategy   Red = What is it?   Green = Benefits    Purple = Disadvantages )

Channel Straightening - The rivers course is straightened by cutting out meanders by building artificial straight channels - Water moves out of the area more quickly because it doesn't travel as far which reduces the risk of a flood - Flooding may happen downstream of the straightened channel instead as the water is carried there faster

Man-made levees - Man-made embankments along both sides of the river - The embankments mean that the river channel can hold more water which reduces the risk of flooding. there also quite cheap to make - If the levees fall it can cause catastrophic flooding

Dams and reservoirs - Dams are walls built across the river normally in the upper course. reservoir is a artificial lake formed behind a dam - Reservoirs store water and release it slowly which reduces the risk of flooding. they can be used to generate hydroelectric power - Dams are very expensive to build

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Flood Management - Soft Engineering

Soft Engineering - Schemes set up using knowledge of a river and its processes to to reduce the effects of flooding

 ( Blue = Strategy   Red = What is it?   Green = Benefits    Purple = Disadvantages )

Flood Warnings - People are warned about possible flooding through TV, radio etc - The impact of flooding is reduced as warnings give people time to move possessions upstairs, put sandbags in position and evacuate - Warnings don't stop a flood from happening and people might not even have access to them

Preparation - Buildings are modified to reduce the damage of a flood and people can make plans of what to do in a flood - The impact of flooding is reduced, buildings are less damaged and people know what to do in a flood - Can give people a false sense of security and its expensive to modify buildings

Flood plain zoning - Preventing buildings on flood plains - The risk of flooding is reduced, impermeable surfaces are not created and there are not any buildings to damage - Urban areas will become congested and some flood plains may already have been built on

Do Nothing

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Flooding Case Study - Rich UK - River Severn

Immediate Impacts:

  • 3 people lost their lives
  • 45000 people were left without power
  • The cost estimated at almost £6million

Long term impacts:

  • Many were distressed and suffered from depression
  • Many people remained in mobile homes for even a year later
  • Road Damage

Immediate Responses:

  • Emergency rest centres were set up for evacuated people
  • The RAF helped with air lift
  • Sandbags were delivered to protect from further flooding

Long term responses:

  • Temporary mobile home were set up
  • Government increased the spending on risk management for flooding
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Flooding Case Study - Poor Bangladesh

Immediate Impacts:

  • 60% of Bangladesh was submerged including 40% of the capital Dhaka
  • 600 deaths
  • 30 million homeless

Long term impacts:

  • The deaths toll rose to 750 with disease
  • Dhaka's airport was flooded as well as roads and railways
  • Damage estimated at $7billion

Within Bangladesh responses:

  • Food, medicine and blankets provided made difficult with transport problems
  • Local communities had to rebuild their homes

International:

  • United Nations appealed for donations but only 20% arrived 2 months later
  • More appropriate flood warnings in place
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Managing UK Water

The demand for is increasing. Over the past 25 years, the amount of water used by people has gone up by 50% and the UK's population is predicted to increase by around 10 million in the next 20 years

One way to manage the demand is to transfer water from areas of surplus to areas in need

This has problems:

  • The dams and aqueducts that are needed are very expensive
  • It could affect the wildlife that lives in the rivers
  • There may be political issues

Another way is to build more reservoirs to store more water. However this can involve flooding settlements and relocating people

Fixing leaky popes would mean less water is lost during transfer

People can reduce the amount of water they use at home e.e taking showers

Water companies want to install water meters so they are more likely to be careful how much they use as they pay for every drop

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Managing UK Water

The demand for is increasing. Over the past 25 years, the amount of water used by people has gone up by 50% and the UK's population is predicted to increase by around 10 million in the next 20 years

One way to manage the demand is to transfer water from areas of surplus to areas in need

This has problems:

  • The dams and aqueducts that are needed are very expensive
  • It could affect the wildlife that lives in the rivers
  • There may be political issues

Another way is to build more reservoirs to store more water. However this can invovle flooding settlements and relocating people

Fixing leaky popes would mean less water is lost during transfer

People can reduce the amount of water they use at home e.e taking showers

Water companies want to install water meters so they are more likley to be careful how much they use as they pay for every drop

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Managing Water Case Study - Kielder Dam

Advantages:

  • Its a multi purpose schemes not only used for water supply but for flood control, HEP and tourism
  • The tourism industry boosts the regions economy with the addition of the Kielder marathon in the last couple of years

Disadvantages:

  • Dams and aqueducts are very expensive 
  • A variety of wildlife habitats were lost
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Drainage Basins

A drainage basin is the area of land drained by a river. The part of the hydrological cycle that happens on land goes on in drainage basins

Inputs: Precipitation

Flows: Surface runoff; Channel flow; Infiltration; Through flow; Groundwater flow; Percolation

Stores: Channel storage; Groundwater storage; Interception storage; Surface storage

Outputs:Evaporation; Transpiration; River flow into the sea

Drainage basins are separated by a boundary called a watershed which are ridges of high land with water falling either side, moving into separate watersheds

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