IGCSE Rivers and GCSE Rivers


Specification:

2.2.2 River processes
Candidates should be able to:
• Demonstrate an understanding of the work of a river in eroding, transporting and depositing. Reference should be made to the erosional processes of hydraulic action, corrasion, corrosion (solution) and attrition. River transport should include the processes of traction, saltation, suspension and solution. Reasons why and where in a river’s course deposition takes place should be studied. It should be realised that the effectiveness of the river processes concerned will vary according to the volume and velocity of the running water and the nature of the load (boulders, pebbles, sand and silt) which, in turn, will be affected by the bedrock along the course of the river.
• Describe and explain the landforms associated with these processes.
A study should be made of the following:
Forms of river valleys – long profile and shape in cross section, rapids, waterfalls, potholes, meanders, oxbow lakes, deltas, levées and flood plains.

2.4 Interrelationships between the natural environment and human activities
Candidates should be able to:
• Demonstrate an understanding that the natural environment presents hazards and offers opportunities for human activities. Reference should be made to the hazards posed by volcanic eruptions, earthquakes, tropical storms, flooding and drought. Reference to the opportunities and problems posed for people could be incorporated when studies are made of the natural environment, for example the advantages and difficulties offered by river flood plains and deltas.


Key River Terminology

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Source: The start of the river, normally found in mountainous areas.

Mouth: The end of the river, this is normally where a river enters the sea, but it can be where it enters a lake.

Tributary: A small river that flows into a bigger river

Confluence: Where two rivers join/meet.

Estuary: The section of the river near the mouth that is tidal.

Drainage basin (catchment area): The area of land that drains into one river and its tributaries. A drainage basin is known as an open system because water can be added and lost.

Watershed: The dividing line between two drainage basins.

River Long Profile: The long profile is the course the river takes from its source to its mouth. The long profile is often split into upper course and lower course (and sometimes middle course as well). The upper course is near the source and is normally found in mountainous areas. The lower course is near the mouth and on much flatter ground nearer sea level. Some rivers may have their mouth in lakes or wetland areas e.g. the Okovanga in Botswana, but this is unusual.

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Bed: the bottom of the river channel.

Bank: The sides of the river channel. A river has two banks.

Wetted perimiter: The length of the bed and the banks in contact with the river.

Channel: The route course (between bed and banks) that a river flows. The flow of the river is often described as channel flow.

Thalweg: The fastest part of the river. The thalweg is always near the middle of the river channel, where there is least friction.

REMEMBER why rivers are so rich - two banks!
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River Processes (Erosion, Transportation and Deposition)


Erosion: the process of wearing something away. The river erodes in four main ways; attrition, corrosion, corrasion and hydraulic action.

Corrasion (abrasion): The process of a rivers' load crashing and rubbing into a rivers' banks and bed causing pieces to break off.

Corrosion (solution): The process of water dissolving a rivers' load as well as its bed and banks.

Hydraulic action: Water and air getting into cracks in a rivers banks and bed causing erosion through increased pressure.

Attrition: Load crashing into each other in a river. This normally happens with suspended load.

Remember in the upper course near the source there is more vertical erosion and in the lower course near the mouth, there is more horizontal (lateral) erosion.

This is because near the source a rivers' load is bigger and more angular and therefore less likely to be suspended in a rivers flow. Instead it will bounce and crash into the bed, causing vertical erosion. However, nearer the mouth load is smaller and smoother and therefore more likely to be suspended and therefore more likely to crash in the banks, causing horizontal erosion.

Because of erosion a rivers' load tends to get smaller and smoother as you move from the source to the mouth.
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From: Geography Course Companion - IB Diploma


Transportation: The river can transport material when it has excess energy. The river transports in four main ways; traction, saltation, solution and suspension. Material carried by a river is know as load.

Traction: The process of large pieces of load rolling along a river bed.

Saltation: The process of load bouncing a long a river bed.

Suspension: The process of smaller pieces of load being carried in a rivers flow.

Solution: The processed of dissolved pieces of material being transported in a solution.

Sometimes a fifth form of transportation is also mentioned; flotation.

Flotation: When material is transported on the surface of the river.
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Load: Material that is transported by a river. If the material is being transported along the bed, it is known as bedload, load transported in a rivers flow is often called suspended load.

Deposition: When a river does not have enough energy, it will start depositing its loads. Deposit means to put something down.

The Bradshaw Model

The Bradshaw model attempts to show very simply how different characteristics of a river change from source to mouth. The left of the model represents the source and the right side the mouth.

Velocity: Is the measure of the speed and direction of an object (to be very simple - the speed the river is travelling). Velocity is normally measured in m/s (metres a second). It is commonly believed that the velocity of rivers is faster in the upper course. However, this is not true, the velocity of the river actually increases as you near the mouth, as shown in the Bradshaw model to the right. This because the cross-section of the river is smaller near the source so there is greater friction and also the discharge is smaller. Nearer the mouth the discharge of the river increases as does the rivers cross-section meaning less energy is lost to friction. We measure velocity with the formula: speed = distance/time.

Discharge: Is the amount of water being carried by a river. Discharge is measured as cubic metres per second (CUMECS). We calculate discharge by using the formula discharge = cross-section x velocity.

Cross-section: The cross-section of the river is the width of the river times the depth of the river. A river's cross-section will get bigger as it moves from the source to the mouth. Because a river's depth can vary we normally take several depth readings and calculate an average.

Gradient: Gradient means how steep something is. The gradient of the river will normally get less step as it travels from the source to the mouth.
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The Hydrological Cycle


The Hydrological Cycle (also known as the water cycle): This is the continuous movement of water through the land, oceans and atmosphere. The hydrological cycle is known as closed system because water cannot be added or lost.
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Open System: This is a system where water can be added or lost. A drainage basin is known as open system because water can be added in the form of precipitation and lost in the form of evaporation and transpiration

Closed System: This is a system where water can not be added or lost. The hydrological cycle is known as a closed system because it includes all the water on our planet and no water can be added to it, or lost from it.

Inputs: When water is added to a system.
Transfers: When water is moving with in a system.
Stores: When water is stationary within a system.
Outputs: When water leaves (exits) a system.
  • Precipitation: Any moisture that falls from the sky e.g. rain or snow.
  • Infiltration: When water travels from the surface of the earth into the ground beneath.
  • Surface run-off (overland flow): When water travels across the surface of the earth
  • Channel flow: Water that is travelling in rivers or streams.
  • Stem flow: When intercepted water then travels down the branches and trunks of vegetation.
  • Percolation: When water travels from unsaturated ground into saturated ground.
  • Groundwater flow: The movement of water through saturated ground.
  • Throughflow: The movement of water through unsaturated ground.
  • Canopy drip: Intercepted water dripping off vegetation onto the ground.
  • Interception: When an object (building, tree) stops precipitation reaching the ground beneath.
  • Surface storage: Any water that is held on the surface of the earth e.g. lake or pond. Some surface stores like puddles may only be temporary.
  • Soil-moisture storage: Water that is stored below the surface in unsaturated ground.
  • Groundwater storage: Water that is stored in saturated ground.
  • Evaporation: Liquid water from surface stores and rivers turning into water vapour (gas).
  • Transpiration: Liquid water evaporating from vegetation.

These two terms are sometimes collectively known as evapotranspiration.

  • River discharge: Eventually most rivers enter the sea and discharge the river's flow into the sea.

Other Key Terms


Saturated soil: Soil that can hold no more water.

Unsaturated soil: Soil that still has space between its pores and can hold more water.

Water table: The line between saturated and unsaturated soil. The water table can go up and down depending on the amount of rainfall and the amount of water being used.

Condensation: When water vapour condenses and turns into water droplets i.e. clouds.

Permeable: A surface that will allow water to pass through it.

Non-permeable: A surface that will not allow water to pass through it. This means that water can not infiltrate.

Porous: An object (or ground) that can hold water i.e. it has pores where water can be stored.

Non-porous: An object (or ground) that can not hold water i.e. it does not have any pores
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Systems Diagram


Because the hydrological cycle is very complicated, it is often shown as a systems diagram. A system diagram removes all the physical features e.g. valleys, rivers, mountains and represents the cycle in a series of couloured boxes and arrows.

Normally on a systems diagram, inputs, outputs, stores and transfers are represented by different colour boxes e.g.

INPUTS = BLUE
OUTPUTS = RED (brown in the diagram to the right)
TRANSFERS = YELLOW
STORES = GREENN

The boxes are then connected by arrows to show the different routes water can take from being added to a system and removed from it.

Have a go at making your own systems diagram by downloading and using the worksheet below.


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Human Impacts on the Hydrological Cycle


Whenever you are asked to talk about human impacts on the hydrological cycle, you should try and include as much river geographical terminology as possible. Below are some explanations of human impacts trying to incorporate geographical terminology.

Deforestation: When humans cut down trees, there is less interception and therefore less canopy drip and stem flow. This means that more precipitation falls directly onto the surface increasing the number of temporary surface stores and surface run-off. Deforestation tends to increase the risk of flooding because water reaches the ground and rivers quicker, causing the ground to become saturated and rivers to flood.

Urbanisation and construction: When houses and roads are built, impermeable surfaces are created, this means there is less infiltration and more surface run-off. However, buildings can also intercept precipitation and humans can build artificial drains which can reduce flooding by redirecting water away from vulnerable areas. Also urban areas (towns and cities) use large amounts of water in showers, toilets, swimming pools, gardens, washing machines and dishwashers.

Reforestation and afforestation: This is basically the opposite of deforestation. When humans plant trees, the amount of interception increases. This means that water reaches the ground more slowly because stem flow and canopy drip slow the movement of water downwards. Planting trees can reduce flooding because precipitation reaches the ground and rivers over a longer period giving the ground time to absorb excess water.

Irrigation: Irrigation is the watering of the ground. Irrigation is used when there is insufficient precipitation and the ground is too arid to grow crops.

Agriculture: Agriculture is the biggest user of water, they use large amounts of water to irrigate crops. Agriculture can therefore reduce the amount of water in rivers and under the ground (groundwater storage - aquifers). Agriculture can also pollute water through its use of fertilisers and pesticides and through animal waste run-off. Fertilisers can cause algae to grow in lakes and rivers which can damage ecosystems (eutrophication - see IGCSE and GCSE Energy, Water and the Environment).

Industry: Like agriculture, industry is a big user of water therefore reducing the amount of water in rivers and under the ground. Industry can also be a big polluter as well. Chemicals and metals that are spilt or dumped into rivers can damage ecosystems and make water harmful for humans to drink.

Transport: Transport like ferries obviously need ports to be built which can change infiltration rates and surface run-off. They can also cause pollution through oil spills and the noise of propellers can disturb marine life.

Dams: Dams create artificial surface stores (reservoirs). They can also reduce the velocity and discharge of rivers by regulating the amount of water released. Because they can regulate the amount of water released they also reduce the risk of flooding.

River Landforms

River landforms can be categorised in two different ways. They can be classified either by the processes that made them (erosion, deposition, erosion and deposition) or where they are found (upper course, middle course or lower course)

Upper Course

Middle Course

Lower Course

  • Waterfalls
  • Rapids
  • Gorges
  • V-shaped valleys
  • Interlocking spurs
  • Meanders
  • Oxbow lakes
  • Levees
  • Braided rivers
  • Deltas
  • Floodplains
  • Meanders
  • Oxbow lakes

EROSIONAL LANDFORMS

DEPOSITIONAL LANDFORMS

EROSIONAL AND DEPOSITIONAL LANDFORMS

  • Waterfalls
  • Gorges
  • Rapids
  • Potholes
  • V-shaped valleys
  • Interlocking spurs
  • Deltas
  • Levees
  • Braided Rivers
  • Meanders
  • Oxbow lakes
  • Floodplains
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Waterfalls, Rapids, Potholes, V-shaped Valleys and Interlocking Spurs (Erosional Landforms)


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Benefits from living near waterfalls

  • Beauty
  • Tourism (Niagara, Victoria, Angel, Iguazu)
  • Jobs related to tourism e.g. guide, hotelier
  • Possible HEP potential

Possible Problems of living near waterfalls

  • Danger from fast flowing water (drowning)
  • Problems of creating transport links across or near falls (gorge)
  • Overcrowding an pollution from becoming a honeypot location
  • Navigation problems up and down river
Waterfall: Waterfalls are where water descends vertically. Waterfalls are usually created by a change in rock type. As the river moves from hard rock to soft rock, erosion increases creating a waterfall.

As the water falls over the waterfall it erodes the river bed and the bottom of the waterfall. This makes a plunge pool and causes some undercutting. The undercutting creates an overhang which will eventually become too heavy to be supported and collapse into the plunge pool. The whole process then starts again, which means the waterfall is constantly retreating upstream towards the source. As the waterfall retreats it leaves behind a gorge.

Gorge: A gorge is a deep sided valley left behind when a waterfall retreats.



Rapids

Rapids are sections of rough turbulent (white water) water. They are normally in a river's upper course and are formed when you get layers of hard and soft rock. The layers of soft rock erode quicker than the layers of hard rock. This makes the bed of the river uneven creating rough turbulent water.

Rapids can become popular with tourists who want to raft down them. Some of the most popular rafting rivers are the Nile in Uganda and the Zambezi in Zimbabwe.

However, rapids can cause problems with transport, ships can not travel up and down rapids and it is very hard to build bridges across them. They are also extremely dangerous if people fall into them.
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Potholes: Potholes are holes found in the river bed.
Load carried by the river is washed around in a circular
motion causing vertical erosion.

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V-shaped Valley and Interlocking Spurs

V-shaped valleys are found in a river's upper course where vertical erosion is dominant. The river will erode (cut) downwards while the sides are weathered (attacked by the weather e.g. rain, wind and the sun). The weathered material will then fall into the river and be transported away leaving a v-shaped valley.

Interlocking spurs are the areas of the valley (hills) that stick out into the river forcing it to meander around them.
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Deltas, Braided Rivers and Levees (depositional landforms)


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Deltas: When a river meets the sea its velocity suddenly reduces. This reduction in velocity means that much of the rivers load is deposited at the mouth of the river. If the river deposits quicker than the sea can erode then a delta starts to develop. Deposition can mean that the main river channel gets blocked, forcing the water to find alternative routes to the sea by making distributaries. Deltas have very fertile soil and tend to be excellent for farming. There are three types of delta; arcuate, cuspate and bird's foot.

Distributary: A distributary is a small river or stream that breaks off from the main river in deltas.

Flocculation: This is the process of particles (load/sediment) joining together. Salt can cause flocculation, so when freshwater meets the sea (saltwater) particles are likely to flocculate. If they flocculate they become heavier and are more likely to be deposited.
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ARCUATE DELTA e.g. River Nile: They are called arcuate deltas because the outside edge has a very regular shape, just like the arc of a circle.
BIRD'S FOOT DELTA e.g. Mississippi River: They are called bird's foot deltas because they look a bit like a birds foot.
CUSPATE DELTA e.g. Ebro Delta: Cuspate delta have been shaped by opposing currents of fairly equal force.
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ADVANTAGES OF LIVING IN MEKONG DELTA

DISADVANTAGES OF LIVING IN MEKONG DELTA

  • Fertile land for farming, especially crops like rice
  • Plenty of water for fishing or aquaculture. This can feed families or provide an economic income.
  • Abundant drinking water
  • Water for cooking, washing, etc.
  • Tourism opportunities e.g. Mekong Delta
  • Houses can be built on water and are then transportable and they do not flood
  • Good transport links, possible to trade
  • Hard to make transport links by roads - cost of bridges
  • Areas are at constant risk of flood
  • Vulnerable to rising sea levels
  • Water is often dirty and polluted (industry, agriculture, sewers)
  • Mosquitoes are attracted to water (Malaria, Dengue)
  • Storm surges from tropical storms
  • Cost of building defences
Braided River: Braided rivers usually occur on rivers with variable flow. When the river flow is high the river has excess energy and can transport large amounts of load. However, when the flow is low, the river is forced to deposit its load. The deposited material forces the reducing flow into small channels creating a braided river. The small islands created by deposition are known as eyots.
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Levees: Levees can be natural or manmade. They form on the edge of the river channel. When a river floods the velocity of the river travelling over the flooded floodplain suddenly falls. Because of the fall in velocity (and subsequent reduction in surplus energy) the river immediately deposits its largest load creating embankments that line the river channel. Over time (and after many flood events) these embankments begin to grow in size and become more stable. These are sometimes strengthened by humans or completely made by humans to try and reduce flooding.

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Meanders, oxbow lakes and floodplains (erosional and depositional landforms)


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Meanders: A meander is simply a bend in the river. If a river is very bendy it is said to be sinuous. A meander starts to form when the thalweg moves to one side of the river channel. This causes greater erosion on one side of the channel and deposition on the other. Over time the erosion and deposition will cause the river to bend. Meanders are constantly moving and over long periods of time widen the floodplain.

Slip-off slope (point bar): This is formed on the side of the river with greatest deposition (the inside of the meander). It is simply deposited material that forms a small beach like area and gently sloping bank.

River cliff: This happens on the side of the river channel with greatest erosion (the outside of the meander). It creates a steep sided bank which often leads to undercutting of the bank.

Thalweg: The thalweg is simply where the channel flow is quickest.
Oxbow lakes: Oxbow lakes are created when two meanders connect (join). The river will often finally connect the two meanders during a flood event when the river is more powerful.The thalweg then shifts to the centre of the river (and does not travel around the old meander) causing deposition on the outside of the river channel cutting off the old meander and creating an oxbow lake.

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Floodplain: Floodplains are simply the areas of land that rivers flood onto when they exceed bankfull discharge. Floodplains are created by the constant movement of meanders. Because horizontal erosion is more dominant in a rivers lower course, floodplains tend to be a lot wider in a river's lower course. If you live on a floodplain you are at risk of being flooded. However, they also make great farmland because of alluvial deposits.

Alluvium: Mineral rich load that is deposited on floodplains in times of flood. Alluvium is essential to keep farmland fertile.

Bankfull discharge: This is when the river channel is full and can not hold any more water. If the river exceeds bankfull discharge then it floods.

Bluff Line: The outer limits of the floodplain. The bluff line is found just before the gradient of the valley sides start to increase.

Strand line: This is the line of material left behind after a river has flooded. The strand line is found on the outer limit of the flood somewhere on the floodplain. A strand line normally consists of pieces of wood and litter.
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Flood Protection Techniques


A flood is a when a river exceeds its bankfull discharge and floods onto its floodplain. There are numerous techniques that are used to try and reduce flooding. The techniques can be roughly divided into hard and soft engineering.

Hard engineering: This is building physical and permanent structures usually out of concrete and/or metal. Hard engineering solutions are usually more expensive, often ugly to look at, but can be very effective at preventing floods.

Levees are embankments built next to the river channel. The simple idea of levees is to increase the cross-sectional area of the river channel. This means that the river channel is able to hold more water before reaching bankfull discharge and are therefore less likely to flood.
Flood barriers are very expensive to build so therefore are only used to protect important urban areas. Flood barriers when closed form a physical barrier across the river channel. The flood barrier below is in London and is actually designed to stop storm surges flowing up the river from the sea.
Flood control channels are concrete channels that normally contain no water. They may run above or below the surface and only get used when the normal river is about to reach bankfull discharge. If a river is about to reach bankfull discharge then water will be diverted into the flood control channel.
Dams are probably the most recognisable of all flood defences and one of the most expensive to build. Dams often have multiple purposes, but one of those purposes is to regulate a river's flow so that the river never exceeds bankfull discharge.
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Soft engineering: This working with nature to reduce flooding. Soft engineering solutions can be cheaper and less visually intrusive.

Reforestation or afforestation is simply planting trees. Trees can be planted anywhere in a river's floodplain. The purpose of planting trees is to try and increase the lag time (the period of time between peak rainfall and peak discharge) of a river. Trees increase the lag time by intercepting precipitation and slowing its movement to rivers. By slowing the movement of water to the river means it will experience smaller peaks in discharge.
Controlled Flooding as the name suggests, it is allowing a river to flood. However, instead of letting the river flood anywhere a river is allowed to flood on land with low economic value. This means that area of farmland maybe allowed to be flooded in order to protect more important land uses like settlements and sites of industry.
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Temporary Techniques: Some defences are only erected or put in place when a river is at risk of flood. Some places only have temporary defences because permanent ones are too expensive or too disruptive i.e. block entry to buildings or road.

Sandbags are a very simple form of defence. Sandbags can be used to place around buildings to protect them from flood waters or used to increase the channel size so that the river can hold more water.
Temporary Flood barriers can be placed in front of doorways, around entire buildings or across streets. The idea of temporary flood barriers is to stop flood water damaging property.
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Land use planning (zoning): Many countries around the world have mapped areas that are at risk of flooding. If an area is at risk of flooding then low value land uses like farming will be built. If an area is safe from flooding then higher value land uses like houses and industry maybe built.

Boscastle Floods 2004


Boscastle is located in the county of Cornwall which is situated in the SW of the UK. Boscastle is a small village that only had 888 residents in 2001. A flash flood occurred on the Valency River in Boscastle on the 16th August 2004. The flood was extremely severe and was a 1 in 400 year event. Despite the devastating nature of the flood there was amazingly no deaths or serious injuries. This was largely due to the rescue efforts of 7 helicopters who airlifted 150 people to safety.

However, about 100 cars, 5 caravans, 6 buildings and several boats were washed into the sea; approximately 100 homes and businesses were destroyed; trees were uprooted and debris was scattered over a large area.

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Key words:

Flash floods: A flood that arrives with little or no notice. They are extremely dangerous because people have no time to prepare.

Impermeable: Rock or ground that water cannot infiltrate into.

Topography / Relief: The shape of the land e.g. is it flat or is it hilly

Gradient: How steep or gentle something is.

Saturated (waterlogged): Ground that all the pore space is full so that it cannot adsorb anymore water.

Flash flooding - BBC article
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PHYSICAL CAUSES OF BOSCASTLE FLOOD

HUMAN CAUSES OF BOSCASTLE FLOOD

  • The Valency valley has a very steep gradient
  • The steep valley sides caused very quick surface run-off
  • The ground around the river was saturated. The saturation was caused by heavy rain leading up the 16th August
  • The weather on the 16th August was extremely hot, causing convectional rainfall
  • Over 12cm of rain fell in a couple of hours
  • The narrow valley, small river channel and fast surface run-off caused the river to reach bankfull discharge very quickly
  • Much of the valley rock is impermeable
  • There were no flood barriers in Boscastle
  • The sewer and drainage system in Boscastle were very old so they became full very quickly
  • A bridge in the centre of Boscastle caused material to get stuck behind. This caused a temporary dam causing the river to flood around the village
  • Boscastle was built in a narrow valley next to a river.
  • Residents had never experienced a major flood event so were unprepared.
  • Although Boscastle is only a small urban area, urbanisation can contribute to flooding. Urbanisation creates impermeable surfaces which stops infiltration and increases surface run-off into rivers.
Dozens rescued from flash floods - BBC article

Rebuilding Efforts after the 2004 Boscastle Flood


Since the 2004 flood, many things have been done to reduce the chance of flooding in the future. Some of the major improvements are:

2004
  • Buildings searched for victims
  • Debris removed from roads and river
  • Improved drainage built
  • Temporary bridge installed
2005
  • Shops and restaurants reopen
  • Water and electricity restored
  • Defences improved
  • Damaged buildings rebuilt
2006
  • Work to widen and straighten river
  • Car park raised
2007
  • Drainage further improved
  • Pipes for new sewers installed
  • New bridge installed
2008
  • Old bridge demolished
  • Harbour pavement concreted.

Bangladesh: Bangladesh is located in South Asia. On three sides it is surrounded by India and on the fourth is the Bay of Bengal. Bangladesh has a population of 156 million people, a population density of 1099 square kilometres and a GDP per capita of only $1900.
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Because of various human and physical factors, Bangladesh regularly suffers from floods. Below are some of the main causes:

Human Causes
  • Deforestation reducing interception and increasing surface run-off
  • Urbanisation reducing infiltration and increasing surface run-off
  • Population growth increasing population density
  • Growth of informal settlements on marginal land (floodplains)
  • Dams in India causing sedimentation of rivers (reducing cross-section)
  • Poorly maintained flood defences
  • Poor transport and communication links (don't cause floods but prevent the population from being warned and evacuated)
  • Lack of weather warnings (again don't cause, but reduce warning times and give residents less chance to escape)
  • Global warming is causing more snow to melt in the Himalayas, it is causing sea levels to rise and it is increasing the frequency and magnitude of cyclones.

Physical Causes
  • Three major rivers flow through Bangladesh, the Ganges, the Brahmaputra and the Meghna
  • The south of the country is very low lying and is basically one big floodplain
  • 70% of Bangladesh is less than 1 metre above sea level
  • Snow melt in Himalayas during Spring and Summer increases river discharge
  • Bangladesh experiences the monsoon season (tropical rains) every year from June to September
  • Cyclones can hit Bangladesh from the Bay of Bengal causing storm surges
  • 10% of Bangladesh is covered in lakes and rivers
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Bangladesh cyclone kills hundreds - BBC article

Flash floods kills Bangladeshi livelihoods - BBC article

20 million at risk from rising sea levels in Bangladesh - BBC article

The Effects of the 1998 Flood

  • Over 57% of Bangladesh's land surface was flooded
  • Over 1,300 people were killed
  • 7 million homes were destroyed making 25 million homeless
  • Spread of water borne diseases like typhoid
  • Shortage of clean water and food - many rice paddies flooded (2 million tonnes of rice lost)
  • 0.5 million cattle and poultry lost
  • Roads and bridges damaged
  • Total costs estimated at about $1 billion
Flood Management in Bangladesh

In 1989 the government of Bangladesh began working with a number of international agencies to produce the national Flood Action Plan (FAP). The plan contained 26 proposals that were hoped would reduce the problem of flooding in the future.
Short Term Management

  • Boats used to rescue victims stranded on buildings and patches of dry land
  • Emergency water and food supplies distributed
  • Food supplied for surviving animals
  • Aid accepted from foreign countries
  • Tents and blankets provided for homeless
  • Basic repairs to houses and sewer systems
Long Term Management

  • Reduced rates of deforestation in the Himalayan foothills
  • The building of seven large dams - cost up to $40 million and construction up to 40 years
  • The building of 5000 flood shelters
  • The building of 350km of levees (embankments)
  • Create flood water storage areas
  • Developing flood warning scheme
There are some advantages of flooding in Bangladesh including:
  • Recharging of groundwater stores
  • Alluvium is deposited on floodplains which is good for farming
  • Water is added to rice paddies which is good for rice
  • Deposition from increased river discharge can make new land
  • Pollutants can be washed off the land.


Dams and Reservoirs


Dam: A barrier placed across a river. A dam is usually made from reinforced concrete.

Reservoir: The artificial store (lake) that develops behind a dam.

Multipurpose scheme: Dam are often built not just to provide renewable energy, but also to prevent flooding and to create a store of water, hence they are multipurpose schemes.

Locks: An enclosed area with gates at either end, that allow boats to move up or down a dam. If a boat wishes to move down a dam, them it moves into the lock and the gates are closed. Water is then drained out of the lock and the boat drops down to the height of the water at the bottom of the dam.

HEP: Hydroelectric power. Electricity that is generated by the force of descending (falling) water.

Navigation: The process of plotting a route up a river, across a sea, etc. Although the dams create an artificial barrier that makes navigation harder, if locks are built to pass the dam, then navigation for ships is usually much simpler in the reservoir behind the dam.

Three Gorges Dam


The Three Gorges Dam is built on the Yangtse River in Central China. The dam is the largest HEP in the world and is massive in scale. The dam 2,335m wide, 101 metres high and 115m thick. It has 34 generator with an electricity capacity of 22,500MW. The reservoir behind the dam will be 660km long and have a surface area of 1,045km2. Although the dam is effectively a wall across the river, the navigation has been improved by building a series of 5 locks alongside the dam.

The site of the Three Gorges Dam was chosen over 15 other sites based on the following advantages:
  • The valley floor was wide and low making building easy
  • The Yangtse has huge HEP potential
  • The bedrock is firm with limited crack making a firm foundation
  • Although in a seismically active (earthquake region) there have been few earthquakes near the site of the dam
  • There was a small island making construction of a diversion channel easy during construction
  • Workers could easily be moved to the site
  • The relocation of people was not really an issue in Communist China
external image three_gorges_dam_1.jpg

ADVANTAGES OF THREE GORGES DAM

DISADVANTAGES OF THREE GORGES DAM

  • The Three Gorges Dam has become a tourist attraction with people visiting the dam
  • The dam produces clean renewable energy. It was initially thought that it would meet 10% of China's energy needs, but this figure has fallen as China has developed.
  • Improved navigation and trade as far as Chongqing - Chongqing is now one of the fastest growing cities in the world.
  • The amount of flooding has reduced downstream of the dam.
  • Water is stored behind the dam that can be used for drinking, farming, etc.
  • The dam created a lot of jobs during building and there are now permanent jobs operating and maintaining the dam
  • The dam created a sense of national pride because it is the largest dam in the world.
  • The Yangtse freshwater dolphin has become extinct, mainly due to increased river traffic and pollution
  • The sturgeon and alligator are severely threatened because of pollution and disruption to breeding patterns
  • The dam cost an estimated $39 billion to build
  • The dam is built near a fault line so is vulnerable to earthquakes
  • 1.3 million people had to be relocated as the reservoir flooded
  • Many archeological sites were covered in water.
  • Farmland downstream of the dam will become less fertile as less alluvium is deposited
Upstream: Means above the dam and towards the source.

Downstream: Means below the dam and towards the mouth.

CHANGES IN RIVER UPSTREAM OF DAM

CHANGES IN RIVER DOWNSTREAM OF DAM

  • A large artificial store will be created behind the dam
  • The gradient of the river will be reduced
  • The velocity of the river will be reduced
  • Evaporation rates behind the dam will increase because there is a large surface area of water
  • The discharge of the river downstream will be reduced
  • The discharge downstream will become more regular
  • There will be less load (and therefore alluvium) in the river downstream
  • The velocity of the river will be reduced downstream