Geography GCSE - Rivers and Flooding

Rivers and Flooding

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  • Created by: naomi
  • Created on: 17-04-12 19:40

The Hydrological Cycle

  • Precipitation: All water released from clouds, such as rain, snow, hail, sleet and fog.
  • Surface runoff: Water flowing across the suface. The water may be in a channel, such as a river or stream, ir it may be overland flow when it makes its way across a field or down a roadway.
  • Interception: When water collects on objects such as leaves or flat roofs.
  • Infiltration: When water soaks into soil
  • Throughflow: When water soaks into soil and seeps through it towards a river or the sea.
  • Percolation: The downward movement of water through soil into rocks.
  • Groundwater flow: The movement of water below the water table. Water which is stores underground is called groundwater.
  • Evaporation: When water which is heated by the Sun becomes vapour and rises into the atmosphere. This may take place over land or sea.
  • Transpiration: All plants lose water through their leaves. Transpiration is when this water returns to the atmosphere where it evaportates.
  • Evapotranspiration: The terms for the processes of both evaportation and transpiration.
  • Condensation: When water bapour i cooled and turns into water droplets to form clouds.
  • Water table: The upper level of saturated ground. It is not flat, the level is closer to the surface in winter when there is plenty of rain. 
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The Drainage Basin

The drainage basin is a system; it is the area of land drained by a river and its tributaries. Large rivers such as the Nile or Mississippi have vast drainage basins. The boundary of a drainage basin is called the watershed. This is usually a range of hills or mountains. Rain falling beyond the watershed will flow into another river and is part of another drainage basin. 

A drainage basin can also be represented in a systems diagram.  This shows how water is held in stores and how it is transferred between stores. 

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Causes of Flooding

Physical Causes of Floods:

Heavy rain falling over a long period of time can cause saturation of the ground and a rise in the water table. If this happens no more rain can soak into (infiltrate) the ground and rivers become so full that they overflow their banks.

Heavy rain over a short period of time can cause floods particularly if the rain is intense and falls on hard ground which does not allow water to infiltrate. This can happen if there has been hot weather in the days or weeks before the rainstorm which had baked the ground. Rainfall cannot soak into the ground so it flows quickly into rivers which rise rapidly and overflow their banks resulting in a flash flood.

Rapid snow melt in later winter and spring may cause flooding as the amount of water flowing into the river is simply too much for the river to hold.

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Causes of Flooding - Human

People's activities which can cause flooding:

River management: Main aim of river management = to reduce the likelihood of flooding, it can lead to an increased risk of flooding. e.g. straightening a river channel and lining it with concrete = an area further downstream which is not managed - greater risk of flooding because water reaches it more rapidly & overwhelms the channel. Artificial embankments (levees) built alongside a river to keep water in the river channel can be breached, and this can cause severe flooding, as happened in New Orleans after Hurricane Katrina in 2005.

It is likely that flooding will increase in the future as a result of climate change caused by global warming. Changing patterns of rainfall and the increased number of storms will increase flood risk.

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The Storm Hydrograph

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The Storm Hydrograph

  • Storm Hydrograph: shoes how river discharge changes over a short period after a rainstorm. Storm hydrographs show how a river responds to heavy rainfall so they can be used to predict floods and help planners to decide about flood prevention strategies.
  • Discharge is the amount of water in a river.

A hydrograph is divided into two parts: base flow, which is water entering the stream from ground storage, and storm flow, which is the water from the recent rainstorm.

  • Base flow remains fairly constant but storm flow changes, so it is this part of the graph which shows how the river level changes a result of a rainstorm.
  • The ascending limb shows how river discharge increases and reaches peak discharge (the highest amount of water in the river). the time between peak rainfall and peak discharge is known as the lag time.
  • The descending limb shows how river levels return to normal.

The shape of a hydrograph can vary. The steeper the ascending limb and the shorter the lag time, the more likely the river is to flood because the amount of water in the river is rising quickly. A hydrograph with a longer lag time shows a river that is less likely to flood.

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Flood Control

Hard engineering: In the past the usual approach to flood control was the use of hard engineering methods. These are usually more expensive to install and to maintain. Hard engineering can also spoil the environment and impact on wildlife, and may also make rivers less attractive.

Integrated river management: Today, the approach is likely to be one of integrated river management, which emphasises the importance of sustainable solutions to the problems of flooding. This approach usually has some hard engineering but also includes natural flood control methods (sometimes called soft engineering). Integrated river management takes into account the impact on the lives of people and on the environment as well as the cost of the scheme.

New technology - river levels can now be carefully monitored, so flooding can be predicted & warnings given to people living in areas likely to be flooded. In Britain, this is the job of the Environment Agency. Risk of flooding can also be reduced by:

  • Planting trees (afforestation) in the upper catchment. However, this changes the appearance and ecosystem of the area.
  • Building a dam and reservoir. Very effective way of controlling flooding, & new lake can be used for recreation & for generating hydroelectricity. But it's expensive. Leads to loss of land & displaces a lot of people.
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Flood Control - Hard Engineering

Hard Engineering:

  • Building embankments, also called 'dykes' or 'levees'. These raised banks increase the capacity of the channel and keep floodwaters in the river.
  • Channelisation: Straightening and deepening the river. Water flows more quickly along a straight channel, reducing the risk of flooding. However, areas further downstream may be at greater risk because the floodwaters reach them more quickly. The river will revert to its natural course unless the new channel is reinforced continually.
  • Constructing dams and reservoirs in the upper sections of the river to trap water which can be released slowly, reducing the risk of flooding.
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Flood Control - Soft Engineering

  • Setting embankments back from the channel and avoiding building on the floodplains to allow flooding around the river
  • Planting water-loving plants, such as willow and alder. This helps to lower the water table and increases the amount of wildlife.
  • Digging a flood relief channel so the river can cope with increased discharge when necessary. In normal conditions, the meandering river course is maintained.
  • Providing general maintenance for rivers to keep the water flowing, for example cleaning rubbish and debris from the channel, cutting back trees and repairing river banks.
  • Dredging rivers to remove silt, sand and gravel to enlarge the channel and increase the amount of water it can hold.

The natural approach enhances the environment and can provide access for people to enjoy the river and its surroundings. Natural solutions to flood control are less expensive to implement and maintain than are hard engineering solutions, but nevertheless, the Environment Agency does not have enough funding to do as much maintenance work as it would like.

Natural or soft engineering approaches are usually a more sustainable way to protect areas from floods.

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Flood Control - Flood Warning

More than 5 million people in England and Wales live in properties that are at risk of flooding. One of the jobs of the Environment Agency is to provide information for these households and to issue warnings. 

It uses the latest technology to monitor rainfall and river levels 24 hours a day and uses this information to decide if flood warnings should be issued.

The Environment Agency provides guidance on its web site and through leaflets. this guidance includes the following advice:

  • Check the details of your insurance policy and add cover if necessary.
  • Make an emergency kit and keep your mobile phone charged.
  • Prepare a supply of sandbags if floods are forecast.
  • Move all electrical goods and valuables to high shelves or upstairs if floods are imminent.
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River Processes - Erosion

Erosion: Rivers erode in four main ways:

  • Hydraulic action: The power of running water undercuts the banks and erodes the bed.
  • Abrasion or corrasion: Rocks and pebbles carried by the river crash against the sides and bed of the river, scraping away material.
  • Attrition: Rocks and pebbles carried by the river bang into each other and break up into smaller pieces.
  • Solutions or corrosion: Some minerals from rocks are dissolved and carries away by the river, e.g. calcium carbonate

The upper course of the river

Rivers usually rise in hills or mountains. The steep gradient of the river means that erosion, particularly downward erosion, is the dominant process. The river flows quickly towards the sea, carrying large amount of sediment downstream. The river's course is not straight. It flows around interlocking spurs of higher land. There is a lot of bedload and many large rocks are angular in shape. Waterfalls and rapids are a common feature in this part of the river.

As waterfalls erode, they move slowly upstream, leaving a steep gorge on the lower side of the falls.

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River Processes - Transport

Transport: Rivers transport an enormous load. This material can be carried in four ways:

  • Traction: Boulders and rocks are dragged or rolled along the river bed
  • Saltation: Smaller-sized particles, such as pebbles or sand, are bounced along the bed
  • Suspension: Fine particles of silt of clay are held in the river
  • Solution: Minerals are dissolved in the river water

The middle and lower course of the river

As the river flows through lower-lying land it flows faster and with less turbulence. Compared with the upper course, this section has a gentler gradient and higher discharge. The bedload of smaller and rounder. Lateral (sideways) erosion is more important than downward erosion, but deposition also occurs. Features such as meanders, oxbow lakes, floodplains and levees develop. 

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River Processes - The Middle and Lower Course of t

The river forms gentle curves or meanders

  • The fastest water (current) on the outside of the bed causes erosion
  • Slower water on the inside of the meander means deposition occurs. A sand bank or slip-off slop develops.
  • River banks on the outside of the curve become steeper, forming a river cliff.

Meanders grow much larger

  • Meanders cause lateral (sideways) erosion across the floodplain
  • River cliffs or bluffs begin to merge, forming a cliff line
  • Land around the river floods and silt deposits accumulate

Meander is cut off from the main channel, usually during a flood. An oxbow lake is formed

  • Meander neck is broken during a flood
  • The river now has a new course, straight ahead now the meander has been cut off
  • The oxbow lake will eventually evaporate and disappear
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River Processes - Deposition


A river drops its load when it no longer has enough energy to carry it. The larger, heavier material is deposited first, and this is seen higher up the river. Pebbles, gravel, sand and silt are deposited in the middle and lower course. the dissolved load of not deposited but is carried out to sea.

The mouth of the river

As the river enters the sea, it tends to form a wide funnel shape, called an estuary. This part of the river is tidal and has a mixture of fresh water and salt water. Deposition of fine silt and clay around the estuary can form extensive mudflats and salt marshes.

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River Severn Case Study

The River Severn is 220 miles long and is Britain's longest river. It rises in the Cambrian Mountains of mid Wales and flows into the Severn Estuary and Bristol Channel. It has several major tributaries, including the River Avon and River Teme, and a number of large towns and cities lie along its route. 

The River Severn floods regularly for a number or reasons:

  • The Cambrian Mountains where the river rises is an area of high rainfall
  • It has a large catchment area and several large tributaries channel water into the River Severn
  • It flows across impermeable rock for much of its lenght so water cannot infiltrate and runs rapidly into the river
  • In recent years towns by the river have grown and floodplains have been used to new buildings, increasing surface runoff.
  • Flood defences in some places (such as Shrewsbury) increase the flood risk in other areas further downstream.
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Case Study: Causes of a river flooding in an MEDC

River Severn: July 2007 Floods

Causes: During the summer of 2007 many parts of Britain were flooded.

  • The floods along the River Severn were caused by heavy rain over a prolonged period resulting in the surrounding land becoming saturated and causing increased surface runoff. 
  • The river channel could not contain all the water and it overtopped its banks. 
  • However, flooding was probably made worse by flood defence schemes that have been erected in some areas and by building on the floodplain, which would otherwise provide a natural overspill area for water from the river.

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Case Study: Effects of a river flooding in an MEDC

River Severn: July 2007 Floods

Effects: The floods caused enormous damage to properties along the Severn, particularly in Upton, Tewkesbury and Gloucester. Three people died and thousands had to leave their homes. Many houses and businesses were badly damaged by rising floodwaters and, in places, sewerage and drainage systems collapsed. Roads and railway lines were closed the stranded people had to be recued by the emergency services. A water treatment centre at Tewkesbury and an electricity substation at Gloucester were closed, leaving people without water or electricity for several days. 

In the longer term, some people were still unable to return to their homes more than a year later. Farmers who lost their crops had no income and nothing to invest in their farms for the future, and some shops and businesses lost so much stock they had to close. Although insurance claims helped many to recover from the floods not everybody was adequately insured against all the damage.

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Case Study: Management of a river flooding in an M

River Severn: July 2007 Floods

Management: The Environment Agency is responsible for the UK's flood defences, although its budget is controlled by the government. The River Severn has extensive flood protection schemes in place and there are plans to improve these in the future.

In Shrewsbury the river has been straightened to allow the water to flow through the town more quickly. Parts of the town are protected by both embankments and flood walls and when floods are predicted removable barriers can be put in place. More flood walls and removable barriers are planned.

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Case Study: Managemtn of a river flooding in an ME

River Severn: July 2007 Floods


Upton on Severn is currently protected by temporary flood barriers that are put in place when river levels start to rise. However, in July 2007 the barriers did not arrive in time because they were delayed by hold-ups on the motorway, allowing flood waters to rise and inundate the town. A £3.6 million scheme has been proposed which includes building a wall running 400 metres alongside the river. Although residents know that a permanent solution to the flooding problem must be found this plan is controversial, as many do not want a permanent structure that will take away the riverside views which many visitors come to enjoy. 

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Case Study: Management of a river flooding in an M

River Severn: July 2007 Floods

Tewkesbury: has no engineered flood defences and relies on flood warnings provided by the Environment Agency. Since the devastation of 2007 local people have asked for proper flood defences to be put in place but at the moment there are no plans for this. Between now and 2026, just under 15,000 new homes are scheduled to be built in and around Tewkesbury. If these are built on the floodplain the flood risk for the town is likely to increase. 

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Some rivers have a delta. This forms when silt accumulates at the mouth of the river, especially during flooding, and the sea is unable to remove it. There are three types of delta:

  • Arcuate or fan-shaped e.g. the Niger (Nigeria)
  • Cuspate e.g. the Ebro (Spain)
  • Bird's foot e.g. the Mississippi (USA)

How the Nile Delta has changed

Since the completion of the Aswan High Dam in 1970 huge quantities of silt have been trapped behind the dam in Lake Naser. Consequently, not enough silt reaches the delta. This has the following serious consequences:

  • The delta is getting smaller and people have lost their homes and land
  • Valuable nutrients are lost and fish stocks have gone down
  • The soil is less fertile without the annual replenishment of sily. Farmers must now buy artificial fertilisers for their land. These are expensive and can cause pollution. 
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This was so useful, I missed this topic in class and this just helped so much :)  thanks!

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