The causes and impacts of flooding
Heavy and prolonged Rainfall
Flooding occurs when a river's discharge is extremely high. The high level of discharge leads to the river breaking it's banks and spilling over onto the flood plain. The two main causes of flooding are prolonged rain and heavy rain.
During the winter of 2014 parts of the UK were left isolated for months following one of the wettest winters on record. In January 2014 parts of the UK received more than double the average rainfall for the month. Data obtained by the BBC from the Met office showed that the south east and central southern England had suffered wettest January since records began in 1910.
Follow the link below to the BBC news webpage for the full report for more details including the impacts the flooding had, and the possible reasons behind the prolonged rainfall.
http://www.bbc.co.uk/news/uk-25944823
Prolonged rainfall - after a long period of rainfall the ground becomes saturated and is unable to absorb anymore water. Once the ground reaches saturation point any further rainfall will not be able to infiltrate the ground, this leads to increased surface runoff, which in turn leads to an increase in the level of river discharge.
Heavy rainfall - during heavy rainfall such as, the rain that comes down during a thunderstorm, rain can fall so rapidly that it is unable to infiltrate the ground, this leads to a rapid increase in surface runoff, and can lead to a flash flood (rapid rise in river discharge).
Snow melt (thawing snow) and melting ice can lead to a massive increase in a river's discharge. In Mountainous regions such as the Himalayas, snow melt contributes to annual flooding events.
Physical factors of a drainage basin that contribute to flooding
Sparse vegetation and deciduous tree
Sparse vegetation - vegetation intercepts rainfall, this prolongs the time it takes for rainfall to reach the ground, and flow into a river channel. When vegetation is sparse in a drainage basin, hardly any rainfall is intercepted, more rainfall lands directly on the ground and reaches the river, which in turn increases river discharge.
Deciduous trees - shed their leaves in the autumn. This means that during the winter deciduous trees do not have any leaves, the effects are very similar to the effects of sparse vegetation, very little rainfall is intercepted, more rainfall lands on the ground and reaches the river, increasing discharge.
Impermeable ground
Clay soils and some types of rock such as granite and shale are impermeable which means that they do not allow surface water to infiltrate (pass through) them. Lack of infiltration leads to increased surface runoff which in turn increases river discharge.
The ground can also become impermeable when it is baked hard during hot summer weather, or when it is frozen during the winter. When the ground is baked solid due to heat or frozen solid due cold weather, water is unable to infiltrate the soil, surface runoff increases which leads to increased river discharge.
Circular drainage basins
In a circular drainage basin all of the points in the basin are similar distances from the river. This means that water draining from the river will enter the drainage basin within a short time frame, this increases the rate of discharge.
High drainage density
Drainage basins that have a high drainage density (lots of streams) drain quickly, and have short lag times. The multiple streams that feed the main river channel allow water to flow quickly from the streams into the main river channel which increases the rate of discharge.
Steep slopes
Water will reach the main river channel quickly in drainage basins that have steep-sided valleys. The steeper the gradient the quicker water will flow and the greater the rate of discharge will be.
Human factors - the effects caused by human activity
Construction, deforestation, agriculture and flood management strategies are just four of the ways that humans have altered drainage basins. Human activity can increase the frequency and severity of flooding.
Urbanisation
Impermeable materials such as concrete and tarmac which humans use in the construction of urban areas result in rapid surface runoff. Surface runoff is quickly diverted along man-made drainage systems which feed into rivers. Both of these reduce lag time and increase discharge.
Deforestation
When trees and plants are cleared from an area both interception and evapotranspiration are reduced, due to this a greater volume of water reaches the river channel and river discharge goes up.
Deforestation also leaves the soil loose and exposed to the elements. This results in the soil being eroded by rainwater and carried into the river, where it raises the river bed. This results in the river channel having a reduced capacity and increases the risk of flooding because the river will break its banks sooner due to it not being able to carry as much water as it could previously.
Flood management strategies
If flood management strategies fails, for example, if a dam fails they can actually make flooding worse. In the case of a dam failing the massive amount of water that would be released at one time would result in an enormous increase in discharge and could have catastrophic effects.
Agriculture
Overgrazing has the same effect as deforestation because it reduces vegetation, which reduces interception and evapotranspiration, and increases discharge. Overgrazing and ploughing can also increase soil erosion and its associated problems (see deforestation above).
Climate change
It is now well known and rarely disputed that human activity has, and is, contributing to climate change. Climate change has been linked to, increased rainfall and, an increase in stormy weather. The result of this is that in some areas we are seeing an increase in flooding, which is having devastating effects on people around the world.
Flooding occurs when a river's discharge is extremely high. The high level of discharge leads to the river breaking it's banks and spilling over onto the flood plain. The two main causes of flooding are prolonged rain and heavy rain.
During the winter of 2014 parts of the UK were left isolated for months following one of the wettest winters on record. In January 2014 parts of the UK received more than double the average rainfall for the month. Data obtained by the BBC from the Met office showed that the south east and central southern England had suffered wettest January since records began in 1910.
Follow the link below to the BBC news webpage for the full report for more details including the impacts the flooding had, and the possible reasons behind the prolonged rainfall.
http://www.bbc.co.uk/news/uk-25944823
Prolonged rainfall - after a long period of rainfall the ground becomes saturated and is unable to absorb anymore water. Once the ground reaches saturation point any further rainfall will not be able to infiltrate the ground, this leads to increased surface runoff, which in turn leads to an increase in the level of river discharge.
Heavy rainfall - during heavy rainfall such as, the rain that comes down during a thunderstorm, rain can fall so rapidly that it is unable to infiltrate the ground, this leads to a rapid increase in surface runoff, and can lead to a flash flood (rapid rise in river discharge).
Snow melt (thawing snow) and melting ice can lead to a massive increase in a river's discharge. In Mountainous regions such as the Himalayas, snow melt contributes to annual flooding events.
Physical factors of a drainage basin that contribute to flooding
Sparse vegetation and deciduous tree
Sparse vegetation - vegetation intercepts rainfall, this prolongs the time it takes for rainfall to reach the ground, and flow into a river channel. When vegetation is sparse in a drainage basin, hardly any rainfall is intercepted, more rainfall lands directly on the ground and reaches the river, which in turn increases river discharge.
Deciduous trees - shed their leaves in the autumn. This means that during the winter deciduous trees do not have any leaves, the effects are very similar to the effects of sparse vegetation, very little rainfall is intercepted, more rainfall lands on the ground and reaches the river, increasing discharge.
Impermeable ground
Clay soils and some types of rock such as granite and shale are impermeable which means that they do not allow surface water to infiltrate (pass through) them. Lack of infiltration leads to increased surface runoff which in turn increases river discharge.
The ground can also become impermeable when it is baked hard during hot summer weather, or when it is frozen during the winter. When the ground is baked solid due to heat or frozen solid due cold weather, water is unable to infiltrate the soil, surface runoff increases which leads to increased river discharge.
Circular drainage basins
In a circular drainage basin all of the points in the basin are similar distances from the river. This means that water draining from the river will enter the drainage basin within a short time frame, this increases the rate of discharge.
High drainage density
Drainage basins that have a high drainage density (lots of streams) drain quickly, and have short lag times. The multiple streams that feed the main river channel allow water to flow quickly from the streams into the main river channel which increases the rate of discharge.
Steep slopes
Water will reach the main river channel quickly in drainage basins that have steep-sided valleys. The steeper the gradient the quicker water will flow and the greater the rate of discharge will be.
Human factors - the effects caused by human activity
Construction, deforestation, agriculture and flood management strategies are just four of the ways that humans have altered drainage basins. Human activity can increase the frequency and severity of flooding.
Urbanisation
Impermeable materials such as concrete and tarmac which humans use in the construction of urban areas result in rapid surface runoff. Surface runoff is quickly diverted along man-made drainage systems which feed into rivers. Both of these reduce lag time and increase discharge.
Deforestation
When trees and plants are cleared from an area both interception and evapotranspiration are reduced, due to this a greater volume of water reaches the river channel and river discharge goes up.
Deforestation also leaves the soil loose and exposed to the elements. This results in the soil being eroded by rainwater and carried into the river, where it raises the river bed. This results in the river channel having a reduced capacity and increases the risk of flooding because the river will break its banks sooner due to it not being able to carry as much water as it could previously.
Flood management strategies
If flood management strategies fails, for example, if a dam fails they can actually make flooding worse. In the case of a dam failing the massive amount of water that would be released at one time would result in an enormous increase in discharge and could have catastrophic effects.
Agriculture
Overgrazing has the same effect as deforestation because it reduces vegetation, which reduces interception and evapotranspiration, and increases discharge. Overgrazing and ploughing can also increase soil erosion and its associated problems (see deforestation above).
Climate change
It is now well known and rarely disputed that human activity has, and is, contributing to climate change. Climate change has been linked to, increased rainfall and, an increase in stormy weather. The result of this is that in some areas we are seeing an increase in flooding, which is having devastating effects on people around the world.
The effects flooding has on, people, the economy, and the environment.
Flooding has numerous impacts which affect the environment and every living organism on the Earth. The impacts of flooding can be far reaching; flooding can affect people socially, economically, and seriously affect their health, and has led to thousands of deaths around the world.
Social impacts are usually greater in poorer countries. There are a number of reasons for this which include; flood defences being of poorer quality and not being maintained to the same standard, people not being able to evacuate as easily as those from richer countries, poor quality buildings which are more likely to collapse than those in richer countries, and inferior sanitation systems meaning there's a greater risk of contamination and disease.
Absolute economic impact - usually greater in richer countries, than in poorer countries, this is because richer countries have more high value buildings and infrastructure than poorer countries do.
Relative economic impact - is higher in poorer countries, although the value of buildings, infrastructure, and crops are lower in poorer countries, damage to them affects the economy, and results in less money being available to aid the countries recovery.
The current situation in Pakistan (September 2014) demonstrates the impact flooding can have on peoples live. The current situation has led to more than 700,000 people having to flee their homes and is said to be the worst flooding that Kashmir has seen in decades.
Use the following link to read more about the flood and the effects it is having on peoples' lives in the Punjab Province of Pakistan.
http://www.bbc.co.uk/news/world-asia-india-29137789
Listed below are just some of the possible social, economic and environmental impacts that flooding.
Social impacts
Death - People and animals can be killed.
Drinking water can become contaminated with sewage that is being carried in the floodwater, and when there is of a lack of clean drinking water people are at risk from diseases such as dysentery and diarrhoea.
Floodwater can result in people becoming homeless when their homes are inundated or damaged by floodwater and sewage. This can also result in the loss of personal possessions many of which will have sentimental value and will be irreplaceable.
Economic impacts
Flooding can lead to the temporary, or permanent closure of a business, due to the premises being inundated with water and/or the power supply being affected.
The cost of cleaning up after a flood can be very high. Rescue work and property repairs after a flood tend to be costly, and usually lead to an increase in insurance premiums.
Additional costs, and loss of business due to closure, repairs, and increased insurance premiums following flooding ,can mean, some businesses are forced to shut down permanently, resulting in an increase in unemployment.
The public transport network, roads and bridges can be destroyed causing further disruption and problems for people living in the area.
Food prices can also be affected by flooding. Flooding can destroy crops which can inevitably lead to an increase in the price of food.
Environmental impacts
Negative
Contaminated floodwater can pollute rivers, lakes and streams etc. with sewage and rubbish
Flooding leads to increased erosion of riverbanks
Positive
Deposition of sediment carried in the floodwater on the floodplain increases the fertility of the land.
Flooding can create wetlands such as marshes and provide habitats for numerous species.
The following link takes you to the flood-world website which gives further detail on the causes and impacts of flooding. The site focuses on the countries of Bangladesh and The Netherlands to show how various countries have been affected by flooding and show the impacthuman activity can have on drainage basins.
http://water104website.weebly.com/index.html
Flooding has numerous impacts which affect the environment and every living organism on the Earth. The impacts of flooding can be far reaching; flooding can affect people socially, economically, and seriously affect their health, and has led to thousands of deaths around the world.
Social impacts are usually greater in poorer countries. There are a number of reasons for this which include; flood defences being of poorer quality and not being maintained to the same standard, people not being able to evacuate as easily as those from richer countries, poor quality buildings which are more likely to collapse than those in richer countries, and inferior sanitation systems meaning there's a greater risk of contamination and disease.
Absolute economic impact - usually greater in richer countries, than in poorer countries, this is because richer countries have more high value buildings and infrastructure than poorer countries do.
Relative economic impact - is higher in poorer countries, although the value of buildings, infrastructure, and crops are lower in poorer countries, damage to them affects the economy, and results in less money being available to aid the countries recovery.
The current situation in Pakistan (September 2014) demonstrates the impact flooding can have on peoples live. The current situation has led to more than 700,000 people having to flee their homes and is said to be the worst flooding that Kashmir has seen in decades.
Use the following link to read more about the flood and the effects it is having on peoples' lives in the Punjab Province of Pakistan.
http://www.bbc.co.uk/news/world-asia-india-29137789
Listed below are just some of the possible social, economic and environmental impacts that flooding.
Social impacts
Death - People and animals can be killed.
Drinking water can become contaminated with sewage that is being carried in the floodwater, and when there is of a lack of clean drinking water people are at risk from diseases such as dysentery and diarrhoea.
Floodwater can result in people becoming homeless when their homes are inundated or damaged by floodwater and sewage. This can also result in the loss of personal possessions many of which will have sentimental value and will be irreplaceable.
Economic impacts
Flooding can lead to the temporary, or permanent closure of a business, due to the premises being inundated with water and/or the power supply being affected.
The cost of cleaning up after a flood can be very high. Rescue work and property repairs after a flood tend to be costly, and usually lead to an increase in insurance premiums.
Additional costs, and loss of business due to closure, repairs, and increased insurance premiums following flooding ,can mean, some businesses are forced to shut down permanently, resulting in an increase in unemployment.
The public transport network, roads and bridges can be destroyed causing further disruption and problems for people living in the area.
Food prices can also be affected by flooding. Flooding can destroy crops which can inevitably lead to an increase in the price of food.
Environmental impacts
Negative
Contaminated floodwater can pollute rivers, lakes and streams etc. with sewage and rubbish
Flooding leads to increased erosion of riverbanks
Positive
Deposition of sediment carried in the floodwater on the floodplain increases the fertility of the land.
Flooding can create wetlands such as marshes and provide habitats for numerous species.
The following link takes you to the flood-world website which gives further detail on the causes and impacts of flooding. The site focuses on the countries of Bangladesh and The Netherlands to show how various countries have been affected by flooding and show the impacthuman activity can have on drainage basins.
http://water104website.weebly.com/index.html
Calculating the frequency of floods
Fortunately large magnitude floods are not frequent events, except in places where annual events such as monsoons or snow melt take place. Smaller magnitude floods happen more frequently. The most common cause of large magnitude floods are heavy or prolonged periods of rain. Through keeping records, of when flood events have taken place over many years, we are now able to predict the frequency of large magnitude floods.
Flood events are predicted using the Flood Return Interval which is abbreviated to RI.
If a 2m flood occurred in 1745, then in 1847 and again in 1944 the Return Interval (RI) of a 2m flood is approximately every 100 years.
Whereas, large magnitude floods are rare, and may have a Return Interval (RI) of around one hundred years, smaller magnitude floods tend to happen more frequently, they have short Return Intervals (RI) and may occur once every one or two years.
Fortunately large magnitude floods are not frequent events, except in places where annual events such as monsoons or snow melt take place. Smaller magnitude floods happen more frequently. The most common cause of large magnitude floods are heavy or prolonged periods of rain. Through keeping records, of when flood events have taken place over many years, we are now able to predict the frequency of large magnitude floods.
Flood events are predicted using the Flood Return Interval which is abbreviated to RI.
If a 2m flood occurred in 1745, then in 1847 and again in 1944 the Return Interval (RI) of a 2m flood is approximately every 100 years.
Whereas, large magnitude floods are rare, and may have a Return Interval (RI) of around one hundred years, smaller magnitude floods tend to happen more frequently, they have short Return Intervals (RI) and may occur once every one or two years.
Sources
AQA (2012): AS & A2 Geography complete revision and practice all in one book
bbc.co.uk/news
water104website.weebly.com
AQA (2012): AS & A2 Geography complete revision and practice all in one book
bbc.co.uk/news
water104website.weebly.com