Theme 2
1. Weather & Climate
Physical factors affecting patterns of global climate
From the icy cold of the Arctic to the tropical heat and intense rainstorms of the equator, different regions of the world have very different and distinctive climates.
The picture to the left shows how the solar heating of the Earth varies with latitude
The picture to the left shows how the solar heating of the Earth varies with latitude
Climate graphs
Describing a climate graph
Each climate graph has four features that you need to describe. Study the graph and ask yourself:
1. What is the total amount of rainfall? This calculated by adding all of the values for the rainfall bars together. 2. Are there distinctive wet or dry seasons? If so, when are they, and how long does each last? 3. What is the annual temperature range? This is the difference between the hottest and coldest times of the year. 4. Does the temperature show a distinctive seasonal pattern? If so, at what time of year are the hot and cold seasons? | Describing the climate of Reykjahlid, Iceland -Maritime climate
1. Reykjahlid has a rather dry climate with a total annual rainfall of only around 430mm.
2. Between December and April the precipitation is very low and, because temperatures are below freezing, would fall as snow. Monthly rainfall in the summer is slightly higher than in winter. 3. The annual temperature range is around 15 degrees Celsius which is large 4. The months from June to August are cool, with temperature just below 10 degrees Celsius. The months from December to March are cold with temperatures in the range 0to -4 degrees Celsius during this winter period |
Describing the climate of Gabon, Africa - Tropical climate
1. Gabon has a rather wet climate with a total annual rainfall of around 2500mm.
2. Between June and August the precipitation is very low. Monthly rainfall in the winter is much higher than in summer.
3. The annual temperature range is around 4 degrees Celsius which is very small
4. The months from June to August are cooler, with temperature just above 24 degrees Celsius. The months from December to April are
hot with temperatures in the range 28 to 27 degrees Celsius during this summer period
2. Between June and August the precipitation is very low. Monthly rainfall in the winter is much higher than in summer.
3. The annual temperature range is around 4 degrees Celsius which is very small
4. The months from June to August are cooler, with temperature just above 24 degrees Celsius. The months from December to April are
hot with temperatures in the range 28 to 27 degrees Celsius during this summer period
How does altitude affect temperature?
Mountainous regions are much colder than the lowlands. Temperatures decrease by 1 degree Celsius for every 100m in height. This is because solar radiation (heat from the sun) passes directly through the atmosphere, heating the Earth's surface. Warm air rises from the Earth's surface as convection currents. As the air rises it cools.
How does the sea affect temperatures?
Ocean currents are able to transfer heat from warm latitudes to cooler ones. The west coast of Britain is kept warmer in winter than other places in similar latitudes by one such current of warm water, the North Atlantic Drift (also known as the Gulf Stream). The sea is able to retain its heat in winter and cools down very slowly. Places towards the centre of Europe and so further away from the sea have much colder winters. For example Plymouth and Prague are at the same latitude (50 degrees North), but have very different winter temperatures. The warming effect of the sea has a big impact on coastal regions, giving them what is called a maritime climate. Places with a maritime climate tend to have quite a lot of precipitation, mild (rather than cold) winters and warm (rather than hot) summers.
High pressure systems and continental climate
Around the edge of the Arctic region between 60 degrees North and 70 degrees North, are large parts of the North American, European and Asian land masses. The climate here is characterised by severe winters and relatively short, cool summers. The most extreme temperatures are experienced in places far from the sea such a Norilsk in Russia. Here, the average winter temperature is -32 degrees Celsius whereas summer temperatures reach a comparatively high 15 degrees Celsius. This enormous temperature range (of 47 degrees Celsius) is due to the fact that continents heat up and cool down very quickly. This feature of the climate of large land masses is known as continentally.
During the winter, temperatures in Russia and Canada can fall to -20 degrees Celsius and below. This extremely cold ground chills the air above it. The cold air sinks, pressing down on the ground creating a high pressure system known as an anticyclone. Anticyclones are characterised by clear, cloudless skies. Any heat given to the ground by the weak winter sunshine is quickly lost in the cloudless night sky.
During the winter, temperatures in Russia and Canada can fall to -20 degrees Celsius and below. This extremely cold ground chills the air above it. The cold air sinks, pressing down on the ground creating a high pressure system known as an anticyclone. Anticyclones are characterised by clear, cloudless skies. Any heat given to the ground by the weak winter sunshine is quickly lost in the cloudless night sky.
Maritime climate: Iceland - the effects on its people
We have seen how the land masses of Canada and Russia quickly lose their heat in the Arctic winter. By comparison, oceans cool down very slowly. So ocean currents are able to transfer heat from warm latitudes to cooler ones. Iceland's coastline is kept warm by one such current of warm water, the Gulf Stream. This brings warm water across the Atlantic from the tropics. This warm water heats the air above it and gives Iceland's coastal regions a maritime climate which is warmer and wetter than other places at similar latitudes.
How does Iceland's climate affect its people?
Iceland's climate has always been a challenge to the Icelandic people. Snow in winter closes many roads and some are not passable until May. However, the Gulf Stream prevents ice from forming in coastal waters so fishing boats can leave port throughout the year. See the mind map below for how Iceland's climate affects its people:
How does Iceland's climate affect its people?
Iceland's climate has always been a challenge to the Icelandic people. Snow in winter closes many roads and some are not passable until May. However, the Gulf Stream prevents ice from forming in coastal waters so fishing boats can leave port throughout the year. See the mind map below for how Iceland's climate affects its people:
Features of Gabon's tropical climate
Tropical climates are found within 15 degrees of latitude either side of the equator. The climate close to the equator (within in 5 degrees of latitude) is hot throughout the year. No matter what time of year, the sun at midday s always high overhead and there are no seasonal variations like the winter and summer that we experience in the K.
The climate of tropical regions is dominated by the tropical rain belt (or ITCZ). For example, in the Amazon region (which is within 5 degrees of the equator) there is between 1500mm and 2000mm of rainfall a year. London, by comparison, has an average of 593mm of rainfall each year. Regions between 5 degrees and 15 degrees of latitude either side of the equator have a seasonal pattern of rainfall with distinct wet and dry seasons.
Why does it rain so much in the tropics? In the heat of the tropics, large air masses are constantly warmed by the hot ground below. this creates massive zones of low pressure. these air masses are unstable, meaning that warm air is rising within them.
The climate of tropical regions is dominated by the tropical rain belt (or ITCZ). For example, in the Amazon region (which is within 5 degrees of the equator) there is between 1500mm and 2000mm of rainfall a year. London, by comparison, has an average of 593mm of rainfall each year. Regions between 5 degrees and 15 degrees of latitude either side of the equator have a seasonal pattern of rainfall with distinct wet and dry seasons.
Why does it rain so much in the tropics? In the heat of the tropics, large air masses are constantly warmed by the hot ground below. this creates massive zones of low pressure. these air masses are unstable, meaning that warm air is rising within them.
Types of rainfall
Frontal rainfall
When two different air masses meet usually a cold and a warm front. Condensation will occur which will cause clouds to form and ultimately rain.
Relief rainfall
Clouds are heavy when full of rain. When they hit mountains they are unable to rise above them. The clouds have to rain (drop their water) to rise. Once they have dropped their water can pass over the mountains. you will often find on rainy windward slope and one dry leeward slope.
Convectional rainfall
1. The sun heats up the ground
2. Water evaporates into the sky
3. The water condenses and forms clouds
4. It rains and often results in thunderstorms. This process repeats everyday in tropical climates.
2. Water evaporates into the sky
3. The water condenses and forms clouds
4. It rains and often results in thunderstorms. This process repeats everyday in tropical climates.
Depressions
These storms affect the UK throughout the year and bring wet and windy weather to the UK.
Warm air (Tropical maritime) migrating north from the tropics meets cold dense air (Polar Maritime) migrating South from the Polar region.
The warm air is undercut by the advancing cold air and because it has more energy and is less dense is forced to rise upwards at a COLD FRONT
Ahead of this, warm air advances into cold air and is also forced to rise above this denser cold air at a WARM FRONT.
At both fronts air is rising so cooling and condensation take place which eventually results in rain AT BOTH FRONTS.
The rising air creates low pressure at the earth’s surface at the centre of the storm. Air rushes in from higher pressure areas around the depression giving the high winds we often associate with depressions. Eventually the cold front catches up with the warm front and an OCCLUDED FRONT is created. View an animated sequence of the life cycle of depression from the Met Office (at the bottom of the page)
Warm air (Tropical maritime) migrating north from the tropics meets cold dense air (Polar Maritime) migrating South from the Polar region.
The warm air is undercut by the advancing cold air and because it has more energy and is less dense is forced to rise upwards at a COLD FRONT
Ahead of this, warm air advances into cold air and is also forced to rise above this denser cold air at a WARM FRONT.
At both fronts air is rising so cooling and condensation take place which eventually results in rain AT BOTH FRONTS.
The rising air creates low pressure at the earth’s surface at the centre of the storm. Air rushes in from higher pressure areas around the depression giving the high winds we often associate with depressions. Eventually the cold front catches up with the warm front and an OCCLUDED FRONT is created. View an animated sequence of the life cycle of depression from the Met Office (at the bottom of the page)
The weather in a depression
Left you can see the weather a depression brings to the UK. Note that this is a cross section through the depression. As this storm passes from the West of the UK to the East what changes would occur in our weather?
Anticyclones
In contrast to depressions, anticyclones only involve one type of air mass which usually cover large areas and do not have any fronts. They are high pressure systems in which the air moves downwards towards the earth's surface. As the air descends, the molecules become compressed, the pressure increases and it warms. When air is warming, any moisture in the atmosphere is evaporated so no clouds can form. The sky is clear. Anticyclones can be very large, typically at least 3,000 km wide. Once they become established, they can give several days of settled weather. Winds are very gentle or even calm in an anticyclone, move clockwise, and this is shown on a synoptic chart by wide spaced isobars. Look here at an excellent site showing an animation of how air moves in an anticyclone
British anticyclone weather
In Britain in summer an anticyclone will mean heat waves during the day. At night, however, as there are no clouds, heat will be quickly lost. The ground will cool sufficiently to cause condensation of water vapour in the descending warm air and mist or heavy dew may form. This will clear quickly in the morning sun. After a few days, a layer of hot air builds up at ground level, which eventually will give rise to thunderstorms, ending the anticyclone.
In winter the longer nights combined with clear skies leads to intense cooling of the land. There is an increased risk of dew, frost and thicker, more extensive fog patches which may be slow to clear or even persist.Under very calm conditions, both frost and fog may persist for several days. An anticyclone's very stable conditions and little air movement means that pollution is trapped at low levels, resulting in very poor air quality such as smogs.
British anticyclone weather
In Britain in summer an anticyclone will mean heat waves during the day. At night, however, as there are no clouds, heat will be quickly lost. The ground will cool sufficiently to cause condensation of water vapour in the descending warm air and mist or heavy dew may form. This will clear quickly in the morning sun. After a few days, a layer of hot air builds up at ground level, which eventually will give rise to thunderstorms, ending the anticyclone.
In winter the longer nights combined with clear skies leads to intense cooling of the land. There is an increased risk of dew, frost and thicker, more extensive fog patches which may be slow to clear or even persist.Under very calm conditions, both frost and fog may persist for several days. An anticyclone's very stable conditions and little air movement means that pollution is trapped at low levels, resulting in very poor air quality such as smogs.
The movement of air from areas of high pressure to areas of low pressure
Low pressure systems - formation of tropical storms
The figure shows how cyclones form. The green arrows show where warm air is rising. The red arrows indicate where cool air is sinking.
• Tropical cyclones form only over warm ocean waters near the equator.
• To form a cyclone, warm, moist air over the ocean rises upward from near the surface. As this air moves up and away from the ocean surface, it leaves is less air near the surface. So basically as the warm air rises, it causes an area of lower air pressure below.
• Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then this new “cool” air becomes warm and moist and rises, too. And the cycle continues…
• As the warmed, moist air rises and cools the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the ocean surface.
• As the storm system rotates faster and faster, an eye forms in the centre. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.
When the winds in the rotating storm reach 39 mph (63 kmph), the storm is called a “tropical storm”. And when the wind speeds reach 74 mph (119 kmph), the storm is officially a
“tropical cyclone” or hurricane. Tropical cyclones usually weaken when they hit land, because they are no longer being “fed” by the energy from the warm ocean waters. However, they often move far inland, dumping many centimeters of rain and causing lots of wind damage before they die out completely.
• Tropical cyclones form only over warm ocean waters near the equator.
• To form a cyclone, warm, moist air over the ocean rises upward from near the surface. As this air moves up and away from the ocean surface, it leaves is less air near the surface. So basically as the warm air rises, it causes an area of lower air pressure below.
• Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then this new “cool” air becomes warm and moist and rises, too. And the cycle continues…
• As the warmed, moist air rises and cools the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the ocean surface.
• As the storm system rotates faster and faster, an eye forms in the centre. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.
When the winds in the rotating storm reach 39 mph (63 kmph), the storm is called a “tropical storm”. And when the wind speeds reach 74 mph (119 kmph), the storm is officially a
“tropical cyclone” or hurricane. Tropical cyclones usually weaken when they hit land, because they are no longer being “fed” by the energy from the warm ocean waters. However, they often move far inland, dumping many centimeters of rain and causing lots of wind damage before they die out completely.
Case study 7: Cyclone Nargis, Burma
The weather event:
Violent storms that form overwarm seas above 27 degrees Celsius. They are areas of intense low pressure around which very strong winds
over 200km/h and heavy rain rotate.
The centre of the storm is a calm ‘eye’ of sinking air. The winds produce wave surges 7 metres high, which flood low-lying areas like
the Irrawaddy Delta.
Once in a while, a tropical thunderstorm grows and grows, becoming a giant hurricane
How it affected people:
– 130,000 people died - Farmers
– Buildings damaged – 800,000 homes destroyed
– 260,000 people displaced – including fishermen and
children could not go to school
– Shortages of food as the land was flooded
– Spread of disease due to sewage – water sources become
polluted
How it affected the environment:
– Rice fields were flooded in the Irrawaddy Delta – damage to farmer’s livelihoods
– 2008 & 2009 harvests of rice were destroyed
– Strong winds up to 135 mph
– Storm surge of 7.6m
– Heavy rainfall & flooding
– Farmland, livestock, fisheries & animal habitats were all destroyedResponses to the hazard:People were rescued
Clearing of flood debris
Giving people fresh water, tents and food
Building of a new flood levee
Violent storms that form overwarm seas above 27 degrees Celsius. They are areas of intense low pressure around which very strong winds
over 200km/h and heavy rain rotate.
The centre of the storm is a calm ‘eye’ of sinking air. The winds produce wave surges 7 metres high, which flood low-lying areas like
the Irrawaddy Delta.
Once in a while, a tropical thunderstorm grows and grows, becoming a giant hurricane
How it affected people:
– 130,000 people died - Farmers
– Buildings damaged – 800,000 homes destroyed
– 260,000 people displaced – including fishermen and
children could not go to school
– Shortages of food as the land was flooded
– Spread of disease due to sewage – water sources become
polluted
How it affected the environment:
– Rice fields were flooded in the Irrawaddy Delta – damage to farmer’s livelihoods
– 2008 & 2009 harvests of rice were destroyed
– Strong winds up to 135 mph
– Storm surge of 7.6m
– Heavy rainfall & flooding
– Farmland, livestock, fisheries & animal habitats were all destroyedResponses to the hazard:People were rescued
Clearing of flood debris
Giving people fresh water, tents and food
Building of a new flood levee
Drought: When extreme weather is a hazard
The extreme heat of August 2003 caused suffering and discomfort for millions of people across Europe. The heat caused heatstroke and dehydration especially among elderly people. it is estimated that 30,000 people died in the extreme heat/ France was the worst hit. French doctors estimate that he heatwave caused 14,000 deaths. on a normal August day about 50 people are admitted to hospital suffering from heat exhaustion. in August 2003 that number rose to 500 and the hospitals struggled to cope. In addition, the heatwave also used several forest fires in southern Europe. All in all, the extreme heat is estimated to have caused £1 billion of damage.
The map to the left shows temperature anomalies in Europe in August 2003.
The map to the left shows temperature anomalies in Europe in August 2003.
2. Ecosystems
What are ecosystems?
An ecosystem is a community of plants and animals and the environment in which they live. Ecosystems contain both living (biotic) and non-living (abiotic) parts. The living parts includes such things as plants, insects and birds which depend on each pther for food. Plants may also depend on insects and birds for pollination and seed dispersal. The non-living part of an ecosystem includes such things as the climate, soils and rocks, This non-living environment provides nutrients, warmth, water and shelter for the living parts of the ecosystem.
The global distribution pattern of biomes
Climate is such an important factor in influencing the natural vegetation and wildlife of a region that biomes (the largest-scale ecosystems) broadly match the world's climate zones.
Where do we find ecosystems?
Investigating the relationships between climate and ecosystems in the tropical rainforest
Biomes such as the tropical rainforest have climate conditions have to promote rapid plant growth. Tropical rainforest trees grow quickly and reach a height of 40m or more. Other biomes, like the tundra, have very slow growing plants that never grow more than a few centimetres high. The differing growth rates of the plants in these biomes can be explained by factors such as the amount of water. These factors all depend on either climate or latitude.
The importance of 'so what....'.
To get extra marks you need to explain ideas to the fullest extent that you can. For example:
In equatorial regions the temperature is constantly above 25 degrees Celsius so plants can grow all year and grow quickly.
There is plenty of sunlight overhead so plants grow straight and tall.
There is plenty of water, sunshine and nutrients so a wide variety of plants are able to grow. This allows a wide diversity of insects, birds and animals.
The importance of 'so what....'.
To get extra marks you need to explain ideas to the fullest extent that you can. For example:
In equatorial regions the temperature is constantly above 25 degrees Celsius so plants can grow all year and grow quickly.
There is plenty of sunlight overhead so plants grow straight and tall.
There is plenty of water, sunshine and nutrients so a wide variety of plants are able to grow. This allows a wide diversity of insects, birds and animals.
Convectional rainfall in the rainforest
Nutrient cycles also depend on climate
Plants need minerals containing nitrogen and phosphates. These nutrients exist in rocks, water and the atmosphere. The plants take them from the soil releasing them back in to the soil when the plant dies. This process forms a continuous cycle. The figure to the left represents nutrient stores and flows in the rainforest ecosystem.
How do ecosystem process benefit people?
Sadly logging, oil exploration, intensive farming and over-fishing are all damaging natural ecosystems. But does it really matter if there are fewer forests and less wildlife? After all, farming and fishing provide use with food, jobs and wealth.
Ecosystems provide key services:
Scientists argue that ecosystems should be protected and not just for their scientific value. They argue that ecosystems provide people with a number of essential service which they describe as key services. Furthermore, they say that these key services have financial value. They include:· Maintaining a steady supply of clean water to rivers
· Preventing soil erosion
· Reducing the risk of river floods
· Providing natural materials such as timber for building, or plants for medicinal use; 75% of the world’s population still rely on plant extracts to provide then with medication
· Providing food stuffs such as honey, fruit and nuts
Key services provided by different ecosystems:
· Tropical rainforests – support thousands of plants and wild animals that contain chemical that may be useful to agriculture or medicine
· Coniferous (boreal or taiga) forests – provide people with the opportunity to develop recreation or tourism businesses
· Mangrove forests – provide a safe environment for fish to spawn and juvenile fish to mature, so helping to maintain fish stocks
· Peat bogs/moors – act as huge stores of carbon dioxide, so helping to regulate the greenhouse effect
· Tropical coral reefs – inspire a sense of awe and wonder in human beings
· Sand dunes – act as natural coastal defences against storm surges, strong winds and coastal floods
Ecosystems provide key services:
Scientists argue that ecosystems should be protected and not just for their scientific value. They argue that ecosystems provide people with a number of essential service which they describe as key services. Furthermore, they say that these key services have financial value. They include:· Maintaining a steady supply of clean water to rivers
· Preventing soil erosion
· Reducing the risk of river floods
· Providing natural materials such as timber for building, or plants for medicinal use; 75% of the world’s population still rely on plant extracts to provide then with medication
· Providing food stuffs such as honey, fruit and nuts
Key services provided by different ecosystems:
· Tropical rainforests – support thousands of plants and wild animals that contain chemical that may be useful to agriculture or medicine
· Coniferous (boreal or taiga) forests – provide people with the opportunity to develop recreation or tourism businesses
· Mangrove forests – provide a safe environment for fish to spawn and juvenile fish to mature, so helping to maintain fish stocks
· Peat bogs/moors – act as huge stores of carbon dioxide, so helping to regulate the greenhouse effect
· Tropical coral reefs – inspire a sense of awe and wonder in human beings
· Sand dunes – act as natural coastal defences against storm surges, strong winds and coastal floods
Tropical rainforests regulate water supply
The figure below shows how rainforests play an essential role in the regional water cycle of tropical areas. The forests acts as a store for water in between rainfall events. After a rainstorm it is thought that about 80% of the rainfall is transferred back to the atmosphere by evaporation and transpiration. this moisture condenses forming rain clouds for the next rainstorm. So rainforests are a source of moisture for future rainfall events.
At least 200 million people live in the world's tropical rainforests. This includes the tribal groups, or indigenous peoples, of the rainforest. Many more people live downstream of the rivers that leave these forests. The forest maintains a constant and even supply of water to these rivers. If the rainforest water cycle were to be broken then the water supply of many millions of people could be put at risk. The total amount of water flowing in the rivers would be reduced and the supply would be become more uneven with periods of low water supply punctuated by sudden flooding.
Conservationists argue that we need to place a greater value on these key services than on the value of the tropical timber alone. The benefits of a clean and regular water supply can be measured in financial terms. Rebuilding homes after a river flood can also be measured financially. The conservationists argue that these key services are more valuable in the long term than the short-term profits gain from logging.
At least 200 million people live in the world's tropical rainforests. This includes the tribal groups, or indigenous peoples, of the rainforest. Many more people live downstream of the rivers that leave these forests. The forest maintains a constant and even supply of water to these rivers. If the rainforest water cycle were to be broken then the water supply of many millions of people could be put at risk. The total amount of water flowing in the rivers would be reduced and the supply would be become more uneven with periods of low water supply punctuated by sudden flooding.
Conservationists argue that we need to place a greater value on these key services than on the value of the tropical timber alone. The benefits of a clean and regular water supply can be measured in financial terms. Rebuilding homes after a river flood can also be measured financially. The conservationists argue that these key services are more valuable in the long term than the short-term profits gain from logging.
How does the structure of the rainforest prevent soil erosion and flooding?
The canopy of the rainforest act rather like an umbrella: intercepting rainfall and preventing much of it from falling directly to the ground. Raindrops from a tropical storm hit the ground with great force, causing soil erosion. But drip and stem flow have much less force, so a continuous canopy prevents soil erosion.
The effects of clearing the rainforest are shown below:
Food webs and chains
In any ecosystem, animals need to eat to survive - and whatever is eaten is part
of the food chain. In the oak woodland, it works like this:
oak leaf ------------> caterpillar ---------------> wood mouse --------------> fox
(The arrow means: 'it gets eaten by' and shows a flow of energy in
the ecosystem)
Often several consumers eat the same type of food. So, for
example, caterpillars and aphids (a type of fly) both feed on oak leaves.
Individual food chains then link up to form a food web.
The diagram above shows a food web
of the food chain. In the oak woodland, it works like this:
oak leaf ------------> caterpillar ---------------> wood mouse --------------> fox
(The arrow means: 'it gets eaten by' and shows a flow of energy in
the ecosystem)
Often several consumers eat the same type of food. So, for
example, caterpillars and aphids (a type of fly) both feed on oak leaves.
Individual food chains then link up to form a food web.
The diagram above shows a food web
How are ecosystems managed?
A case study of logging in the Solomon Islands
The Solomon Islands re a large group of islands in the Pacific ocean. The natural ecosystem of these mountainous islands is tropical rainforest. The World bank estimates GNI (Gross National Income) per person o be $730, making this the poorest country in the Pacific region. he country has one of the highest malaria rates in the world and infant mortality is high. Standards of education also need to be improved and adult literacy is relatively low compared with other Pacific countries. The country's economic and social development was crippled by fighting between different ethnic groups between 2000 and 2003. Since hen the government has struggled to create economic growth.
How has the tropical rainforest ecosystem been traditionally used?
More than 80% of the Solomon Islands are subsistence farmers or fishermen. This means they only produce enough food to feed their own families and do not make much profit from their work. The islands are heavily forted and most communities are located around the coastline. The rainforest is still an important resource for villagers. They use it to gather foodstuffs such as fruit, nuts an honey. Thy also collect leaves, berries and bark to make traditional medicine. For many communities the forest is also an important source of timber not only for building a repairing their homes, but also to build their ocean-going fishing canoes.
Logging and agri-business
Is logging sustainable?
The second largest island in the Solomon's is Santa Isabel. the communities in North Isabel sold logging rights to a Malaysian TNC/MNC. This meant that the and was still owned by the community but the logging company paid the community for the right to log timber for a fixed period of time. They made various promises to protect the environment during logging. However the TNC/MNC broke their promises and their poor logging practices have resulted in severe soil erosion, silting-up of rivers, and flooding.
Commercial logging firms such as this NC/MNC make profit if they work quickly. They use bulldozers to reach the valuable trees. For every tree cut for its timber, t is estimated that 40 or more are destroyed by the heavy machinery. This process destroys trees that have fruit, nuts or medicinal value to the villagers. The villagers have received payments from the TNC/MNC but this amount to only about 1% of the value of the timber.
Deforestation damages wildlife habitats and often leads to problems of soil erosion. In many cases the logging companies are acting illegally. Illegal logging practices include:
Commercial logging firms such as this NC/MNC make profit if they work quickly. They use bulldozers to reach the valuable trees. For every tree cut for its timber, t is estimated that 40 or more are destroyed by the heavy machinery. This process destroys trees that have fruit, nuts or medicinal value to the villagers. The villagers have received payments from the TNC/MNC but this amount to only about 1% of the value of the timber.
Deforestation damages wildlife habitats and often leads to problems of soil erosion. In many cases the logging companies are acting illegally. Illegal logging practices include:
- Cutting trees without permission
- Cutting trees close to rivers where soil erosion can the lea to flooding
- Ignoring the rights of local land owners
- Paying bribes to local officials
- Non-payment of taxes
Could logging be sustainable?
Logging can provide a better income for local people and not cause long-term damage to the environment. This can be achieved in various ways:
Only a few trees are felled. If only two trees per hectare are felled every ten years, a rainforest will eventually recover
Saplings are planted to replace cut trees
Local people fell the tree and process the timber on site using small portable tools
The Isabel Sustainable Forestry Management Project is one small example. It was funded by aid (450,000 Euros) given by the European Union (EU) I the mid 1990s. The scheme created skilled labour for local people. Trees are carefully felled to avoid damage to trees of fruit or medicinal value. The timber is then cut into planks in the forest using a portable sawmill. This means that large machines are not needed. It also mans that local people add value to the timber, so more profit is retained by the village. This method of processing the timber means that the community keeps about 40% of the finished value of the timber. The project was successful in protecting 17,000 hectares of forest. But the amount of timber produced has been very small.
Only a few trees are felled. If only two trees per hectare are felled every ten years, a rainforest will eventually recover
Saplings are planted to replace cut trees
Local people fell the tree and process the timber on site using small portable tools
The Isabel Sustainable Forestry Management Project is one small example. It was funded by aid (450,000 Euros) given by the European Union (EU) I the mid 1990s. The scheme created skilled labour for local people. Trees are carefully felled to avoid damage to trees of fruit or medicinal value. The timber is then cut into planks in the forest using a portable sawmill. This means that large machines are not needed. It also mans that local people add value to the timber, so more profit is retained by the village. This method of processing the timber means that the community keeps about 40% of the finished value of the timber. The project was successful in protecting 17,000 hectares of forest. But the amount of timber produced has been very small.
Sustainable forest management in Central America
Deforestation creates a major problem for wildlife: the forest becomes fragmented. As clearings get bigger the wildlife is restricted to isolated fragments o forest that are separated by farm land. The animals become trapped in islands of forest surrounded by an ocean of farm land.
The governments of Central America (also know as Mesoamerica) are co-operating with each other in an ambitious conservation project. They want to create a continuous wildlife corridor through the length of Central America. The corridors will be created by planting strips of forest to connect the remaining fragment of forest together. The project is called the Mesoamerican Biological Corridor and involves all seven governments of Central America, plus Mexico.
The governments of Central America (also know as Mesoamerica) are co-operating with each other in an ambitious conservation project. They want to create a continuous wildlife corridor through the length of Central America. The corridors will be created by planting strips of forest to connect the remaining fragment of forest together. The project is called the Mesoamerican Biological Corridor and involves all seven governments of Central America, plus Mexico.
Debt-for-nature swap
Mesoamerica is a biodiversity hotspot. It only amounts to 1% of the world's land surface, but it is estimated to contain 7% of the world's terrestrial (land-based) species. Western governments are encouraging conservation in this region by offering debt-for-nature swaps. Under these arrangements, the Central American governments agree to spend money conserving ecosystems and wildlife. In return, the Western governments agree to reduce the amount of money that is owned to them. One debt-for-nature swap was made between Costa Rica and the USA. In 2007 Costa Rica agreed to spend $26 million on conservation projects. In exchange, the US government and two non-governmental organisations (NGOs) agreed to buy back a similar amount of Costa Rica's debt.
Ecotourism in Costa Rica
The government and businesses in Costa Rica have also encouraged the growth of ecotourism. These are small-scale tourist projects that create money for conservation as well as creating local jobs. It is estimated that 70% of Costa Rica;' tourists visit the protected environments. In 2000 Costa Rica earned $1.25 billion from ecotourism. One successful example is the creation of a canopy walkway through a small, privately owned part of the Monteverde reserve. Tourists are charged $45 to climb up into the canopy and walk along a rope bridge, the longest of which is 300m long.
3. Desertification
The issue of desertification?
What is desertification?
Desertification is the process by which environments beomce more like desert. Over a period of years, the amount of natural begetation decreases and the soil is exposed to the hot sun. when it rains, the rainwater runs over the surface of the soil, rather than soaking down into it, and the soil can be wahshed away. The soil becomes degraded or worn out. It's harder to grow crops and food shortages and water shortages may both become more commmon.
Desertification is a serious issue that affects over 1 billion people around the world. It affects large parts of North America, Africa, Central Asia and Australia, so it affects people in countries at different levels of economic development. However, its most serios effects are on those people who already live in poverty, because desertification makes it even harder for them to make a lving from the land. It is estimated that 90% of the people who are affected by desertification live in the world's poorer countries and that US $42 billion worth of income is lost due to desertification every year.
Desertification is a serious issue that affects over 1 billion people around the world. It affects large parts of North America, Africa, Central Asia and Australia, so it affects people in countries at different levels of economic development. However, its most serios effects are on those people who already live in poverty, because desertification makes it even harder for them to make a lving from the land. It is estimated that 90% of the people who are affected by desertification live in the world's poorer countries and that US $42 billion worth of income is lost due to desertification every year.
Map of desertification
Unpredictable patterns of rainfall
Regions that have low rainfall totals each year are at most risk from desaertification. The Sahel region of Africa is one such region. The Sahel has a long dry season of nine months, followed by a wet season of rainfall for three months. The total amount of rainfall over these three months is similar to the total amount of rainfall in Cambridge in a year. However, these wet seasons have become unpredictable, with short periods of heavy rainfall running off the land and failing to soak down into soil where it is needed to recharge the soil moisture and rock aquifers.
Seasonal rainfall patterns in the savannah
The regions that suffer from desertification are regions which have a tropical climate with a long dry season followed by a short rainy season. Suring the rainy season, rainfall is due to a combination of frontal and convectional rainfall. These regions naturally have a savannah ecosystem of grasses, shrubs and trees. The trees are scattered. They do not form a continuous canopy like that of a tropical rainforest. However, the trees, shrubs and grasses all protect the soil from erosion. In regions where the trees and shrubs have been cut down or burnt, the process of desertification has been rapid. Therefore, it seems that the process of desertification is caused, at least in part by poor management of the land.
Case study 9 - Causes, effects and management of desertification, Sahel Region Africa
Some useful videos:http://www.bbc.co.uk/learningzone/clips/burkina-faso-desertification/4322.html
http://www.bbc.co.uk/learningzone/clips/burkina-faso-desertification/4322.html
http://www.bbc.co.uk/learningzone/clips/burkina-faso-large-scale-impact-of-desertification/4324.html
http://www.bbc.co.uk/learningzone/clips/burkina-faso-local-impact-of-desertification/4323.html
http://www.bbc.co.uk/learningzone/clips/desertification-expansion-of-the-sahara-desert/1498.html
http://www.bbc.co.uk/learningzone/clips/desertification-and-climate-change/1501.html
http://www.bbc.co.uk/learningzone/clips/expansion-of-the-sahara-desert/1500.html
http://www.bbc.co.uk/learningzone/clips/burkina-faso-desertification/4322.html
http://www.bbc.co.uk/learningzone/clips/burkina-faso-large-scale-impact-of-desertification/4324.html
http://www.bbc.co.uk/learningzone/clips/burkina-faso-local-impact-of-desertification/4323.html
http://www.bbc.co.uk/learningzone/clips/desertification-expansion-of-the-sahara-desert/1498.html
http://www.bbc.co.uk/learningzone/clips/desertification-and-climate-change/1501.html
http://www.bbc.co.uk/learningzone/clips/expansion-of-the-sahara-desert/1500.html
Human causes
Physical causes
Is there a link between poverty and desertification?
Ghana
Ghana is a tropical country in West Africa. Ghana covers an area od 238,000 square kilometres, making it very similar in size to the UK (244,00 sq kilometres). While all parts of hana have a hot tropical climate, the maount of rainfall varies significantly from north to south. Northern Ghana is the driest part of the country, with a dry season that can last up to eight months of the year. The natural vegetation in this dry zone is grassland and savanna woodland. In recent years, huge numbers of trees from these environments have been felled to increase the size of farms or to use as firewood. Most rural people cook using wood or charcoal on open stoves. People in the cities buy wood from the countryside for cooking too. As a conseqence of this damage to the vegetation, soil erosion has beomce a serious issue. Deforestation may evene be contributiing to local climate change by reducing the amount of water that can return back into the atmosphere through evapotranspiration.
How do differences in quality of life affect people in Ghana?
The northern regions of Ghana face severe problems such as poverrty, lack of job opportunities (especailly for women), and a lack of safe drinking water. The region has a harsh climate and farming in an unreliable way of making a living. The lack of decent roads and public transport makes it difficult forn rural families to get to local towns to visit friends, go to the shops, or get medical attention. There is a severe shortge of teachers in the northern regions of Ghana.in rural northern Ghana, the infant mortality rate (IMR) is twice as high as in urban areas in the south. Malaria, acute respiratiry infections, diarrhoea, malnutrition and measles are still the five main causes of death in young children.
Is poor land management the cause of desertification in Ghana?
Farming in the savannah region of Ghana is a mixture of crop growing and animal grazing/ farmers keep goats and cattle for both their milk and meat. Crops are grown using a traditional bush fallow system. Scrub vegetation is removed by slashing and burning. Crops such as maize, root crops and vegetables are grown between one and three years. The land is then abandoned for between eight and fifteen years. This is known as the fallow period. During this fallow period, the natural shrubs grow back. Leaves from the shrubs decompose the soil, replacing organic fibre and nutrients that have been taken out by farming. This system is sustainable as long as the fallow period remains long enough. However, in some villages the fallow period is now only two to three years. This does not give the soil enough time to recover. It loses its organic content and its structure becomes dusty. This means that the soil is at risk of erosion from both wind and rainfall.
Is commercial farming to blame for desertification and food shortages?
In recent years, many European Trans National Companies (TNCs) have either bought or leased land in Africa to grow crops. This means that land is converted from the traditional bush fallow system by large agricultural businesses (known as agri-businesses) that usually grow a single crop in very large fields. Some agri-businesses grow biofuel crops (crops that are then processed for their natural oils). These oils are then used in biofuels that replace diesel in European cars. One such crop is jatropha. A large number of foreign TNCs, including Argolis (Italy) and ScanFuel (Norway) have been buying or leasing land in Ghana over the last ten years. It is estimated that 5 million hectares (an area the size of Denmark) is now used for commercial farming by foreign agri-businesses in this way and that as much as 37% of all of Ghana's cropland is now used to grow jatropha. Some see this an important way for Ghana to earn foreign income. In the past, Ghana earned most its income from the export of tropical timbers. This led to a rapid loss of tropical rainforest during the period 1950-1980. Growing commercial crops such as jatrpha should be more sustainable.
4. Rivers
The drainage basin hydrological cycle
Diagram
The drainage basin is the catchment area from which a river system obtains its water.
An imaginary line called the watershed delimits one drainage basin from another. The watershed generally follows a ridge of high land; any rain falling on the other side of the ridge will eventually flow into another river in the adjacent drainage basin.
The drainage basin hydrological system is an open ysem with inputs and outputs.
An imaginary line called the watershed delimits one drainage basin from another. The watershed generally follows a ridge of high land; any rain falling on the other side of the ridge will eventually flow into another river in the adjacent drainage basin.
The drainage basin hydrological system is an open ysem with inputs and outputs.
Inputs into the drainage basin include:
· Energy from the sun for evaporation
· Precipitation - rain & snow
Outputs move moisture out of the drainage basin and include:
· Evaporation and transpiration from plants (collectively called evapo-transpiration)
· Runoff into the sea
· Water percolating deep into underground stores where it can be effectvely lost from the system
Stores of water
· On the surface - glaciers, lakes, rivers, puddles
· Vegetation stores water by interception and plants
· The soil can hold water
· Groundwater is stored in permeable rocks
Transfers and flows- moves water through the system and enable inputs of water to be processed from one store to another.
· Transfers include stemflow, infiltration, through flow and groundwater flow.
· Energy from the sun for evaporation
· Precipitation - rain & snow
Outputs move moisture out of the drainage basin and include:
· Evaporation and transpiration from plants (collectively called evapo-transpiration)
· Runoff into the sea
· Water percolating deep into underground stores where it can be effectvely lost from the system
Stores of water
· On the surface - glaciers, lakes, rivers, puddles
· Vegetation stores water by interception and plants
· The soil can hold water
· Groundwater is stored in permeable rocks
Transfers and flows- moves water through the system and enable inputs of water to be processed from one store to another.
· Transfers include stemflow, infiltration, through flow and groundwater flow.
Channel processes
The work of a river involves three main processes: erosion, transportation and deposition. At any one time the dominant process operating within the river depends on the amount of energy available. This is governed by the velocity of the flow and the amount of water flowing within the channel (discharge).
Erosion - the processes
Abrasion
The scraping, scouring and rubbing action of materials carried along by a river (load). Rivers carry rock fragments in the flow of the water or drag them along the bed, and in doing so wear away the banks and bed of the river channel.
Hydraulic action
This is caused by the sheer power of moving water. It is the movement of loose unconsolidated material due to the frictional drag of the moving water on sediment lying on the channel bed.
Corrosion
This is most active on rocks that contain carbonates, such as limestone and chalk. The minerals in the rock are dissolved by weak acids in the river water and carried away in solution.
Attrition
This is the reduction in the size of fragments and particles within a river due to the processes describe above. The fragments strike one another as well as the river bed. They therefore become smoother, smaller and more rounded as they move along the river channel.
The scraping, scouring and rubbing action of materials carried along by a river (load). Rivers carry rock fragments in the flow of the water or drag them along the bed, and in doing so wear away the banks and bed of the river channel.
Hydraulic action
This is caused by the sheer power of moving water. It is the movement of loose unconsolidated material due to the frictional drag of the moving water on sediment lying on the channel bed.
Corrosion
This is most active on rocks that contain carbonates, such as limestone and chalk. The minerals in the rock are dissolved by weak acids in the river water and carried away in solution.
Attrition
This is the reduction in the size of fragments and particles within a river due to the processes describe above. The fragments strike one another as well as the river bed. They therefore become smoother, smaller and more rounded as they move along the river channel.
Transport
River energy not used for erosion is not lost through friction can be used to transport a river's load. A river obtains its load from two main source:
· Material that has been washed, or has fallen, into the river from the valley sides
· Material that has been eroded by the river itself from the bed or banks
A river transports its load in four main ways (see diagram below)
· Traction– large stones and boulders are rolled along the river bed by water moving downstream. This mainly happens during periods of high discharge and consequently high energy levels
· Saltation– small stones bounce or leap-frog along the channel bed. This process is associated with relatively high energy conditions. Small particles may be thrust up from
the bed of the river only to fall back to the bottom again further downstream. As these particles land they in turn dislodge other particles upwards, causing more such bouncing movement to take place
· Suspension– very small particles of sand and silt are carried along by the flow of the river. Such material is not only carried but it is also picked up, mainly through the
turbulence that exists within the river. Suspension normally contributes the largest proportion of sediment to the load of the rover. The suspended load is the main cause of the brown appearance of many rivers and streams
· Solution– dissolved minerals are transported with the mass of moving water
· Material that has been washed, or has fallen, into the river from the valley sides
· Material that has been eroded by the river itself from the bed or banks
A river transports its load in four main ways (see diagram below)
· Traction– large stones and boulders are rolled along the river bed by water moving downstream. This mainly happens during periods of high discharge and consequently high energy levels
· Saltation– small stones bounce or leap-frog along the channel bed. This process is associated with relatively high energy conditions. Small particles may be thrust up from
the bed of the river only to fall back to the bottom again further downstream. As these particles land they in turn dislodge other particles upwards, causing more such bouncing movement to take place
· Suspension– very small particles of sand and silt are carried along by the flow of the river. Such material is not only carried but it is also picked up, mainly through the
turbulence that exists within the river. Suspension normally contributes the largest proportion of sediment to the load of the rover. The suspended load is the main cause of the brown appearance of many rivers and streams
· Solution– dissolved minerals are transported with the mass of moving water
Deposition
A river deposits when there is a decrease in its level of energy, it is no longer competent to transport its load, Deposition usually occurs when:
· There is a reduction in the gradient of the river, for example when it enters a lake
· The discharge is reduced, such as during and after a dry spell of weather
· There is shallow water, for example on the inside of a meander
· There is an increase in the calibre (size) of the load. This may be due to a tributary bringing in larger particles, increased erosion along the river’s course, or a landslide
into the river
· The river floods and overtops its banks, resulting in a reduced velocity on the floodplain outside the main channel
In general, the largest fragments are the first to be deposited, followed by successively smaller particles, although the finest particles may never be deposited. This pattern of deposition is reflected in the sediments found along the course of a river. The channel of upland rivers are often filled with large boulders. Gravels, sands and silts can be carried further and are often deposited further downstream. Sands and silts are deposited on the flat floodplains either side of the river in its lower course.
· There is a reduction in the gradient of the river, for example when it enters a lake
· The discharge is reduced, such as during and after a dry spell of weather
· There is shallow water, for example on the inside of a meander
· There is an increase in the calibre (size) of the load. This may be due to a tributary bringing in larger particles, increased erosion along the river’s course, or a landslide
into the river
· The river floods and overtops its banks, resulting in a reduced velocity on the floodplain outside the main channel
In general, the largest fragments are the first to be deposited, followed by successively smaller particles, although the finest particles may never be deposited. This pattern of deposition is reflected in the sediments found along the course of a river. The channel of upland rivers are often filled with large boulders. Gravels, sands and silts can be carried further and are often deposited further downstream. Sands and silts are deposited on the flat floodplains either side of the river in its lower course.
River discharge
River discharge is defined as the volume of water passing a measuring point or gauging station in a river in a given time. It is measured in cubic metres per second (cumecs).
The storm hydrograph
The storm hydrograph (shown to the left) shows variations in a river's discharge over a short period of time, usually during a rainstorm. The starting and finishing level show the base flow of the river. As storm water enters the drainage basin the discharge rises, shown by the rising limb, to reach the peak discharge, which indicates the highest flow in the channel. The receding limb shows the fall in the discharge back to the base level. The time delay between maximum rainfall amount and peak discharge is the lag time.
Factors affecting a river's discharge
Rock and soil type
· Permeable rocks ad soils (such as sandy soils) absorb water easily, so surface run-off is rare
· Impermeable rock and soils (such as clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
· Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
Land use
· In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and joins rainwater from
other drains – eventually spilling into the river
· In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge
· Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
Rainfall
· The amount and type of rainfall will affect a river’s discharge
· Antecedent rainfall is rain that has already happened. It can mean that the ground has become saturated. Further rain will then flow as surface run-off towards the river
· Heavy continual rain, or melting snow, means more water flowing into the river
Relief
· Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more likely.
Weather conditions
· Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river.
· High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
· Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in
· Permeable rocks ad soils (such as sandy soils) absorb water easily, so surface run-off is rare
· Impermeable rock and soils (such as clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
· Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
Land use
· In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and joins rainwater from
other drains – eventually spilling into the river
· In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge
· Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
Rainfall
· The amount and type of rainfall will affect a river’s discharge
· Antecedent rainfall is rain that has already happened. It can mean that the ground has become saturated. Further rain will then flow as surface run-off towards the river
· Heavy continual rain, or melting snow, means more water flowing into the river
Relief
· Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more likely.
Weather conditions
· Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river.
· High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
· Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in
Landforms of fluvial erosion and deposition
Waterfalls
Formation of a waterfall:
1.Waterfalls are found in the upper course of a river. They usually occur where a band of hard rock lies next to soft rock. They may often start as rapids. 2. As the river passes over the hard rock, the soft rock below is eroded (worn away) more quickly than the hard rock leaving the hard rock elevated above the stream bed below. 3. The 'step' in the river bed continues to develop as the river flows over the hard rock step (Cap Rock) as a vertical drop. 4. The drop gets steeper as the river erodes the soft rock beneath by processes such as abrasion and hydraulic action. A plunge pool forms at the base of the waterfall. 5. This erosion gradually undercuts the hard rock and the plunge pool gets bigger due to further hydraulic action and abrasion. Eventually the hard cap rock is unsupported and collapses. The rocks that fall into the plunge pool will continue to enlarge it by abrasion as they are swirled around. A steep sided valley known as a gorge is left behind and as the process continues the waterfall gradually retreats upstream. |
Meanders
Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff. In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope or point bar. The water in a meander flows in a corkscrew like movement as it moves from the inside of the bend towards the outside of the bend. This is called helicoidal flow.
Remember - a meander is asymmetrical in cross-section (see diagram above). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).
Remember - a meander is asymmetrical in cross-section (see diagram above). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).
Oxbow lakes
As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and
narrower.
Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander usually during a flood event when the energy in the river is at its highest. The water now takes its
shortest route rather than flowing around the bend.
Deposition gradually seals off the old meander bend forming a new straighter river channel.
Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.
Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up,
the feature left behind is known as a meander scar.
narrower.
Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander usually during a flood event when the energy in the river is at its highest. The water now takes its
shortest route rather than flowing around the bend.
Deposition gradually seals off the old meander bend forming a new straighter river channel.
Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.
Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up,
the feature left behind is known as a meander scar.
Levees
In its middle and lower courses, a river is at risk from flooding during times of high discharge. If it floods, the velocity of the waterfalls as it overflows the banks. This results in deposition, because the competence of the river is suddenly reduced. It is usual for the coarsest material to be deposited first, forming small raised banks (levees) along the sides of the channel. Subsequent floods increase the size of these banks and further deposition on the bed of the river also occurs. This means that the river, with channel sediment build-up, now flows at a higher level than the floodplain. For this reason, the authorities sometimes strengthen levees and increase their height.
Floodplains
Floodplains are created as a result of both erosion and deposition, although the accumulation of river deposits suggests that they are predominately depositional features. They are relatively flat areas of land either side of the river, which form the valley floor in the middle and lower courses of the river. They are composed of alluvium - river deposited silts and clays. Over time, a floodplain becomes wider and the depth of sediment accretions increases. The width of the floodplain is determined by the amount of meander migration and lateral erosion that has taken place. Lateral erosion is most powerful just downstream of the apex of the meander bend. Over time, this results in the migration of meanders, leaving their scars clearly visisble on the floodplain. Interlocking spurs are eventually removed by lateral erosion in the middle course, leaving behind a bluff line and widening the valley. the depth of the alluvial deposits depends partly on the amount of flooding in the past, so floodplain creation is linked to extreme events. Over time, point bars and old meanders scars become incorporated into the floodplain, adding to the alluvial deposits. These become stabilised by vegetation as the meanders migrate and abandon their former courses.
Deltas
A delta is a feature of deposition, located at the mouth of a river as it enters a sea or lake. Deposition occurs as the velocity and sediment-carrying capacity of the river decrease on entering the lake or sea, and bedload and suspended material are dumped. Flocculation occurs as fresh water mixes with seawater and clay particles coagulate due to chemical reaction. The clay settles on the river bed.
Deltas form only when the rate of deposition exceeds the rate of sediment removal. In order for a delta to form the following conditions are likely to be met:· The sediment load of the river is very large, as in the Mississippi and Nile rivers
· The coastal are into which the river empties its load has a small tidal range and weak currents. This means that there is limited wave action and, therefore, little transportation of sediment after deposition has taken place. This is a feature of the Gulf of Mexico and the Mediterranean Sea
Deltas can be described according to their shape. The most commonly recognised is the characteristic arcuate delta, for example the Nile delta, which has a curving shoreline and a dendritic pattern of drainage. Many distributaries break away from the main channel as deposition within the channel itself occurs causing the river to braid. Longshore drift keeps the seaward edge of the delta relatively smooth in shape. The Mississippi has a bird's food delta. Fingers of deposition build out into the sea along the distributaries' channels, giving the appearance from the air of a bird's claw. A cuspate delta is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea currents or longshore.
Deltas form only when the rate of deposition exceeds the rate of sediment removal. In order for a delta to form the following conditions are likely to be met:· The sediment load of the river is very large, as in the Mississippi and Nile rivers
· The coastal are into which the river empties its load has a small tidal range and weak currents. This means that there is limited wave action and, therefore, little transportation of sediment after deposition has taken place. This is a feature of the Gulf of Mexico and the Mediterranean Sea
Deltas can be described according to their shape. The most commonly recognised is the characteristic arcuate delta, for example the Nile delta, which has a curving shoreline and a dendritic pattern of drainage. Many distributaries break away from the main channel as deposition within the channel itself occurs causing the river to braid. Longshore drift keeps the seaward edge of the delta relatively smooth in shape. The Mississippi has a bird's food delta. Fingers of deposition build out into the sea along the distributaries' channels, giving the appearance from the air of a bird's claw. A cuspate delta is pointed like a cup or tooth and is shaped by gentle, regular, but opposing, sea currents or longshore.
Causes of flooding
Physical causes of flooding
Flooding occurs when a river's dicharge exceeds the capacity of its channel to carry that discharge. the river overflows its banks. Flooding may be caused by a number of natural causes or physical factors:
· Excessive levels of precipitation occurring over a prolonged period of time. This eventually leads to saturation of the soil. When the water table reaches the ground surface, there is increased overland flow or runoff
· Intensive precipitation over a short period of time particularly when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case
· The melting of snow particularly when the subsoil is still frozen, so that infiltration capacity is reduced
· Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes bring abnormally large amounts of precipitation
The nature of the drainage basin has an influence on the likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during time of heavy rainfall.
· Excessive levels of precipitation occurring over a prolonged period of time. This eventually leads to saturation of the soil. When the water table reaches the ground surface, there is increased overland flow or runoff
· Intensive precipitation over a short period of time particularly when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case
· The melting of snow particularly when the subsoil is still frozen, so that infiltration capacity is reduced
· Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes bring abnormally large amounts of precipitation
The nature of the drainage basin has an influence on the likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during time of heavy rainfall.
Key terms
Hazard: a natural event that threatens life and property. A disaster is the realisation of the hazard. Flooding is an example of a natural hazard
River management: River basins are subject to strategies designed to prevent flooding and to ensure that there is an adequate supply of water The YouTube video shows the flooding at Boscastle, Cornwall, August 2004 |
The impact of human activities on flooding
Urbanisation
More people are living in towns and cities
Population growth and urbanisation has led to demand for land to build on - floodplains are flat and are food for housing Concrete and tarmac, used for roads and pavements as they are impermeable, preciptation cannot infiltrate so gets into the river much more quickly Less interception as trees and plant matter is removed so precipitation gets into the river much more quickly. Often surface water is channelled directly into drains and sewers, so precipitation reaches the river much more quickly. Bridges over rivers can constrict rivers, slow discharge and reduce the carrying capacity of the river. | Deforestation
In poor countries rapid deforetation has taken place.
Land is now used for framing, settlement and mining etc. With no trees there is a greater risk of soil erosion as the preciptation is not intercepted. Flood damage is greatest near the mouth of a river because wide,flat floodplains are most susceptible to damage. The volume of water is greatest here because many tributaires have joined the river. | River Management
The main aim of river management is to reduce the likelihood of flooding. However, in some circumstances it can actually increase the risk:
Bangladesh: flood embankments have built along some river channels. They are designed to increase river capacity but at times have prevented floodwater draining back into the rivers Farakka Dam, India: Lots of rainfall, meant the lake behind the dam could burst. The floodgates of the dam were opened. This stopped the dam from bursting but it greater increased the discharge of the river in Bangladesh. This coincided with the normal floods and made the severity much worse |
Climate change
Global warming has been blamed fr what some claim is an increasing frequency of flooding. There is evidence that average sea temperatures have risen and this rise has been blamed for the increasing frequency and severity of tropical revolving storms in the Caribbean. Such storm bring heavy rainfall and storm surges along the coastlines of countries lying in their path. In spring 2005, scientist reported that average sea temperatures were 3 degrees Celsius above normal and predicted that the 2005 hurricane season in the Caribbean and southern states of the USA would be particularly savage. This proved to be the case. notable hurricanes included Katrina, which led to the flooding of New Orleans.
It is predicted that global warming will result in reduced rainfall in some areas, but in other, such as western European rainfall totals might increase. higher temperatures will result in increased evaporation over the seas and oceans, leading to greater precipitation. Such an increase will inevitably cause more rivers to flood, particularly since most floodplains have become heavily urbanised over the last two centuries.
Global warming could lead to the melting of the polar ice caps. One major consequence of this would be a rise in sea level, so floodplains lying close to the present sea levels would be at risk from flooding. The major deltas of the world, such as those of the Nile, the Mississippi and the Ganges-Brahmaptura, would be at particular risk.
It is predicted that global warming will result in reduced rainfall in some areas, but in other, such as western European rainfall totals might increase. higher temperatures will result in increased evaporation over the seas and oceans, leading to greater precipitation. Such an increase will inevitably cause more rivers to flood, particularly since most floodplains have become heavily urbanised over the last two centuries.
Global warming could lead to the melting of the polar ice caps. One major consequence of this would be a rise in sea level, so floodplains lying close to the present sea levels would be at risk from flooding. The major deltas of the world, such as those of the Nile, the Mississippi and the Ganges-Brahmaptura, would be at particular risk.
Hard and Soft engineering: which is the better option?
Hard engineering strategies involve the use of technology in order to control rivers, while soft engineering, adopts a less intrusive form of management, seeking to work alongside natural processes. Hard engineering approaches tend to give immediate results and the river but are expensive. However, in the future, they may make problems worse or create unforeseen ones. Soft engineering is much cheaper and offers a more sustainable option as it does not interfere directly with the river’s flow.
What’s more important?
In the aftermath of the 2009 Cumbrian floods, local people were angry that more hadn’t been done to prevent them. They accused the authorities of ‘putting salmon before people’ after their earlier request to lower the river bed by 3 metres in Cockermouth had been turned down because it might harm fish stocks.The cost of protection
Professor Samuels advises the government on managing rivers. He said ‘It is technically possible to defend places like Cockermouth against extreme events, but only by building huge walls and embankments along the river, which would cost billions and alter the character of the town. For most people, that would be unacceptable as the floods.’
Flood defence on the River Waal -http://www.bbc.co.uk/learningzone/clips/flood-defence-dyke-construction-on-the-river-waal/3253.html
River management the River Mississippi -
http://www.bbc.co.uk/learningzone/clips/river-management-the-mississippi/3078.html
Hard Engineering
Hard engineering involves building structures to defend places from floodwater. Dams and reservoirs exert a huge degree of control over a river. The natural flow of water is prevented by a dam (often a concrete barrier across the valley), water fills the area behind it and is released or held depending on circumstances such as current and expected rainfall. Dams and reservoirs are normally constructed as part of a multi-purpose project rather than with just a single aim in mind.
Hard engineering strategies involve the use of technology in order to control rivers, while soft engineering, adopts a less intrusive form of management, seeking to work alongside natural processes. Hard engineering approaches tend to give immediate results and the river but are expensive. However, in the future, they may make problems worse or create unforeseen ones. Soft engineering is much cheaper and offers a more sustainable option as it does not interfere directly with the river’s flow.
What’s more important?
In the aftermath of the 2009 Cumbrian floods, local people were angry that more hadn’t been done to prevent them. They accused the authorities of ‘putting salmon before people’ after their earlier request to lower the river bed by 3 metres in Cockermouth had been turned down because it might harm fish stocks.The cost of protection
Professor Samuels advises the government on managing rivers. He said ‘It is technically possible to defend places like Cockermouth against extreme events, but only by building huge walls and embankments along the river, which would cost billions and alter the character of the town. For most people, that would be unacceptable as the floods.’
Flood defence on the River Waal -http://www.bbc.co.uk/learningzone/clips/flood-defence-dyke-construction-on-the-river-waal/3253.html
River management the River Mississippi -
http://www.bbc.co.uk/learningzone/clips/river-management-the-mississippi/3078.html
Hard Engineering
Hard engineering involves building structures to defend places from floodwater. Dams and reservoirs exert a huge degree of control over a river. The natural flow of water is prevented by a dam (often a concrete barrier across the valley), water fills the area behind it and is released or held depending on circumstances such as current and expected rainfall. Dams and reservoirs are normally constructed as part of a multi-purpose project rather than with just a single aim in mind.
Hard engineering: The Three Gorges Dam, China
The Three Gorges dam was constructed at Yichang on the River Yangtse. The capacity of the reservoir should reduce the risk of flooding downstream from a 1-in-10-year event to a 1-in-100-year event. Not only will this benefit over 15 million people living in high-risk flood areas, it will also protect over 25,000ha of farmland. The dam is already having a positive impact on flood control, navigation and power generation, but it has caused problems. The Yangtse used to carry over 500 million tonnes of silt every year. Up to 50% of this is now deposited behind the
dam, which could quickly reduce the storage capacity of the reservoir. The water in the reservoir is becoming heavily polluted from shipping and waste discharged from cities. For example, Chongquig pumps in over 1 billion tonnes of untreated sewage per year. Toxic substances from factories, mines and waste tips submerged by the reservoir are also being released into the reservoir.
Most controversially, at least 1.4 million people were forcibly moved from their homes to accommodate the dam, reservoir and power stations. These displaced people were promised compensation for their losses, plus new homes and jobs. Many have not yet received this, and newspaper articles in China have admitted that so far over $30 million of the funds set aside for has been taken by corrupt local officials.
dam, which could quickly reduce the storage capacity of the reservoir. The water in the reservoir is becoming heavily polluted from shipping and waste discharged from cities. For example, Chongquig pumps in over 1 billion tonnes of untreated sewage per year. Toxic substances from factories, mines and waste tips submerged by the reservoir are also being released into the reservoir.
Most controversially, at least 1.4 million people were forcibly moved from their homes to accommodate the dam, reservoir and power stations. These displaced people were promised compensation for their losses, plus new homes and jobs. Many have not yet received this, and newspaper articles in China have admitted that so far over $30 million of the funds set aside for has been taken by corrupt local officials.
Soft engineering
Soft engineering involves adapting to flood risks, and allowing natural processes to deal with the rainwater. It is a strategy that accepts the
natural processes of the river and seeks to work with it to reduce the effects of folding rather than attempting to gain control of it. A conscious decision can be made to ‘do nothing’ but simply to allow natural events to happen, even if this involves the risk of flooding. In some poorer areas of the world, this is a necessary approach. In richer areas, it could mean money is set aside in years when flooding does not occur to provide relief after the event. However, there are many more positive approaches that can be adopted to reduce the risk of flooding without exerting a major force over the river and its processes.
natural processes of the river and seeks to work with it to reduce the effects of folding rather than attempting to gain control of it. A conscious decision can be made to ‘do nothing’ but simply to allow natural events to happen, even if this involves the risk of flooding. In some poorer areas of the world, this is a necessary approach. In richer areas, it could mean money is set aside in years when flooding does not occur to provide relief after the event. However, there are many more positive approaches that can be adopted to reduce the risk of flooding without exerting a major force over the river and its processes.
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Case Study 10 & 11 (Coming soon)
5. Coastal processes
What processes are associated with the sea?
Waves provide the force that shapes our coastline. Waves are created by friction between wind and the surface of the sea. Stronger winds make bigger waves. Large waves also need time and space in which to develop. So larger waves need the wind to blow for a long time over a large surface area of water. The distances over which a wave has developed is known as fetch, so the largest waves need strong wind and a long fetch.
The water in a wave moves in circular motion. A lot of energy is spent moving the water up and down. So waves in deep water have little energy to erode the coastline. However, as a wave enters shallow water near the shore its motion changes. The water below the surface is slowed by friction with the sea bead while the water at the surface surges forward freely. It is this forward motion of the breaking wave that causes erosion.
Video showing wave formation - http://www.bbc.co.uk/learningzone/clips/understanding-wave-formation/4018.html
The water in a wave moves in circular motion. A lot of energy is spent moving the water up and down. So waves in deep water have little energy to erode the coastline. However, as a wave enters shallow water near the shore its motion changes. The water below the surface is slowed by friction with the sea bead while the water at the surface surges forward freely. It is this forward motion of the breaking wave that causes erosion.
Video showing wave formation - http://www.bbc.co.uk/learningzone/clips/understanding-wave-formation/4018.html
The above diagram shows you what happens when a wave moves closer towards the coast and crashes onto a beach.
Crest - this is the top part of the wave and eventually topples onto the beach.
Swash - this is the water that rushes up the beach
Backwash - this is the water that flows back towards the sea
Crest - this is the top part of the wave and eventually topples onto the beach.
Swash - this is the water that rushes up the beach
Backwash - this is the water that flows back towards the sea
Types of waves
There are two types of waves; constructive and destructive waves.
Constructive waves:
· Constructive waves are waves that surge up the beach and have a powerful swash
· They carry large amounts of sediment and 'construct' the beach making it more extensive
· Main characteristics: They have a strong swash and a weak backwash and are smaller in height compared to destructive waves.
· They are waves which contain a small amount of energy.
Destructive waves
· Destructive waves are named because they 'destroy' the beach.
· When the waves hit the beach they rear up and smash down onto the beach.
· There is very little swash when the wave breaks but has a powerful backwash.
· The backwash removes the sediment which leads to the 'destruction' of the beach
· Main characteristics: They have a strong backwash, lots of energy and are waves which are high in height
Constructive waves:
· Constructive waves are waves that surge up the beach and have a powerful swash
· They carry large amounts of sediment and 'construct' the beach making it more extensive
· Main characteristics: They have a strong swash and a weak backwash and are smaller in height compared to destructive waves.
· They are waves which contain a small amount of energy.
Destructive waves
· Destructive waves are named because they 'destroy' the beach.
· When the waves hit the beach they rear up and smash down onto the beach.
· There is very little swash when the wave breaks but has a powerful backwash.
· The backwash removes the sediment which leads to the 'destruction' of the beach
· Main characteristics: They have a strong backwash, lots of energy and are waves which are high in height
Processes of erosion
Coastal erosion
Erosion – use the acronym CASH Corrasion – (abrasion) is caused by large waves hurling beach material against the cliff Attrition – is when waves cause rocks and boulders on the beach to bump into each other and to break up into small particles Solution – (corrosion) is when salts and other acids in seawater slowly dissolve a cliff Hydraulic pressure (power/action) – is the force of waves compressing air in cracks in a cliff One final process of erosion is: Abrasion: this is the 'sandpapering' effect of pebbles grinding over a rocky platform, often causing it to become smooth. Coastal transportation · Traction– rolling stones along the sea floor (needs the most energy) · Saltation– sand-sized particles bounce along the sea floor in a ‘leap frog’ movement · Suspension– silt and clay-sized particles are carried within the water flow· · Solution– some minerals dissolve in the water (this needs the least energy) |
Evidence of coastal erosion
How are coastal processes affected by geology?
The rate of coastal erosion depends on a number of factors including rock type and rock structure. The cliffs at Happisburgh in Norfolk are between 6 and 10m high. They are made pf loosely compacted layers of sand, silt and clay deposited at the end of the last ice age. These sedimentary rocks are not very resistant. Erosion by waves at the toe of the cliff causes large sections of the cliff to slump and collapse onto the beach where it is easily washed away. Slumping is speeded up when rain water flows over the upper slope, eroding gulleys.
Very useful BBC videos:
Formation of landforms - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-and-landforms/9966.html
Wave-cut platforms and headlands/bays - http://www.bbc.co.uk/learningzone/clips/wave-cut-platforms-and-headland-erosion/4021.html
Weathering and erosion - http://www.bbc.co.uk/learningzone/clips/weathering-erosion-and-coastal-features/4022.html
An example of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-dorset/3244.html
An examle of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-coastal-processes-and-landforms/3245.html
More landforms - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-west-wales/3084.html
Cliff slumping - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-blowholes-and-cliff-slumping/3085.html
Very useful BBC videos:
Formation of landforms - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-and-landforms/9966.html
Wave-cut platforms and headlands/bays - http://www.bbc.co.uk/learningzone/clips/wave-cut-platforms-and-headland-erosion/4021.html
Weathering and erosion - http://www.bbc.co.uk/learningzone/clips/weathering-erosion-and-coastal-features/4022.html
An example of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-dorset/3244.html
An examle of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-coastal-processes-and-landforms/3245.html
More landforms - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-west-wales/3084.html
Cliff slumping - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-blowholes-and-cliff-slumping/3085.html
Headlands and bays
Cliffs rarely erode at an even pace. Sections of cliff that are particularly resistant to erosion stick out to form headlands. Weaker sections of coastline that are more easily eroded form bays.
· Form where there are alternating outcrops of resistant outcrops of resistant(harder) and less resistant (softer). ·
· Destructive waves erode the soft rock move quickly to form bays·
· The harder rock is more resistant and are left protruding into the sea·
· The headlands protect the adjacent (next by) bays from destructive waves
· As the headlands protect the bays sand is deposited to form a beach. At the headlands there often wave-cut platforms and notches.
· Form where there are alternating outcrops of resistant outcrops of resistant(harder) and less resistant (softer). ·
· Destructive waves erode the soft rock move quickly to form bays·
· The harder rock is more resistant and are left protruding into the sea·
· The headlands protect the adjacent (next by) bays from destructive waves
· As the headlands protect the bays sand is deposited to form a beach. At the headlands there often wave-cut platforms and notches.
Cliffs and wave-cut platforms
· Wave erosion is greatest when large waves break against the foot of the cliff ·
· The waves undercut the foot of the cliff to form a wave-cut notch ·
· Over time the notch enlarges the until the cliff above it is unsupported and collapses
· The gentle sloping expanse of rock marking the foot of the retreating cliff is called a wave-cut platform
· Wave-cut platforms are exposed at low tide but hidden at high tide
· The waves undercut the foot of the cliff to form a wave-cut notch ·
· Over time the notch enlarges the until the cliff above it is unsupported and collapses
· The gentle sloping expanse of rock marking the foot of the retreating cliff is called a wave-cut platform
· Wave-cut platforms are exposed at low tide but hidden at high tide
Headlands and bays
· Many cliff faces are not solid rock and have many cracks and fissures
· Waves make the cracks bigger through erosion (hydraulic action, abrasion and corrosion) to form a cave
· The cave gets bigger and forms an arch
· The arches collapse and to form a stack
· The stack is eroded at its base and forms a stump
· Waves make the cracks bigger through erosion (hydraulic action, abrasion and corrosion) to form a cave
· The cave gets bigger and forms an arch
· The arches collapse and to form a stack
· The stack is eroded at its base and forms a stump
Beaches and sand dune processes
Beaches are dynamic environments. In other words, the energy of the wind and waves is constantly moving sediment around and changing the shape of the beach. When waves approach the beach at an angle, some of the sediment is tranpsorted along the coastline in a process known as longshore drift. Sediment is also moved up and down a beach due to swash and backwash.
Useful weblinks:
BBC Class Clips video - Blakeney Point
BBC Class Clips video - Kaitorete Spit, New Zealand
BBC Scotland video about spits, bars and tombolos
BBC Bitesize - Spits
BBC Bitesize - Tombolos
Useful weblinks:
BBC Class Clips video - Blakeney Point
BBC Class Clips video - Kaitorete Spit, New Zealand
BBC Scotland video about spits, bars and tombolos
BBC Bitesize - Spits
BBC Bitesize - Tombolos
Longshore Drift
Video showing how Longshore Drift works - http://www.bbc.co.uk/learningzone/clips/landforms-created-by-longshore-drift-and-coastal-deposition/9965.html
Waves approach the beach in the same direction as the wind
When the wave breaks, swash carries material up the beach at the same angle as the wind
The backwash carries the material straight back down the beach under gravity·
This process slowly moves material along the coastline
Waves approach the beach in the same direction as the wind
When the wave breaks, swash carries material up the beach at the same angle as the wind
The backwash carries the material straight back down the beach under gravity·
This process slowly moves material along the coastline
Landforms of deposition
Beaches
Video showing all of the coastal landforms of deposition - http://www.bbc.co.uk/learningzone/clips/depositional-coastlines/4023.html
· They are usually found in sheltered bays between two headlands
· The headlands protect the area from erosion
· Low constructive waves deposit material on the shore
· Gradually a beach is built up
· Material on a beach is well sorted – the biggest pebbles are nearest the land with the smallest nearest the sea
· The larger the material the steeper the beach – a pebble beach is steeper than a sandy beach
· They are usually found in sheltered bays between two headlands
· The headlands protect the area from erosion
· Low constructive waves deposit material on the shore
· Gradually a beach is built up
· Material on a beach is well sorted – the biggest pebbles are nearest the land with the smallest nearest the sea
· The larger the material the steeper the beach – a pebble beach is steeper than a sandy beach
Spits
http://www.geographyalltheway.com/ks3_geography/coasts/imagesetc/deposition.swf- an amazing animation which explains the formation of a
spit!!
Video showing the formation of a spit - http://www.bbc.co.uk/learningzone/clips/the-formation-of-spits/3246.html
A spit is an area of sand or shingle that has been transported by longshore drift and then deposited as the coastline has changed direction. It is attached to the land at one end. It is a depositional landform. Hurst Castle Spit in Hampshire is a very famous example.
Where the coastline changes direction, sediment is deposited in the same direction as the original coastline (i.e. in line with the prevailing wind direction). Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up. The material is deposited in the deeper water offshore until the ridge is built above the level of the sea. Drift continues along the seaward side of the spit extending it further down the coast while salt marsh develops in the slow-moving water on the landward side.
Spits can become a permanent feature. This happens when the prevailing wind picks up sand from the beach and blows it inland across the spit to form sand
dunes. These dunes will then be colonised by vegetation, which stabilises them. It is common for a salt marsh to develop in the sheltered area of water behind the
spit. Water is trapped behind the spit, creating a low energy zone. As the water begins to stagnate, mud and marsh begin to develop behind the spit.
A spit may grow out across a river estuary. Where the spit is crossing a river mouth, the river will be diverted so that it follows the coastline for some miles before reaching the sea.
spit!!
Video showing the formation of a spit - http://www.bbc.co.uk/learningzone/clips/the-formation-of-spits/3246.html
A spit is an area of sand or shingle that has been transported by longshore drift and then deposited as the coastline has changed direction. It is attached to the land at one end. It is a depositional landform. Hurst Castle Spit in Hampshire is a very famous example.
Where the coastline changes direction, sediment is deposited in the same direction as the original coastline (i.e. in line with the prevailing wind direction). Where there is a break in the coastline and a slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and gradually a ridge is built up. The material is deposited in the deeper water offshore until the ridge is built above the level of the sea. Drift continues along the seaward side of the spit extending it further down the coast while salt marsh develops in the slow-moving water on the landward side.
Spits can become a permanent feature. This happens when the prevailing wind picks up sand from the beach and blows it inland across the spit to form sand
dunes. These dunes will then be colonised by vegetation, which stabilises them. It is common for a salt marsh to develop in the sheltered area of water behind the
spit. Water is trapped behind the spit, creating a low energy zone. As the water begins to stagnate, mud and marsh begin to develop behind the spit.
A spit may grow out across a river estuary. Where the spit is crossing a river mouth, the river will be diverted so that it follows the coastline for some miles before reaching the sea.
Bars
Bars can form in several ways:
(a) a spit grows the whole way across a bay
(b) a sandbank develops offshore, parallel to the shore, and is moved towards the coastline by the waves and wind until it joins the mainland
Slapton Sands is an example of a bar. The lagoon of water than has formed on the landward side of the bar is called Slapton Ley.
A tombolo is formed where a spit joins an island to the mainland. An example is the Isle of Portland which is joined to the mainland by a shingle ridge known as Chesil Beach.
(a) a spit grows the whole way across a bay
(b) a sandbank develops offshore, parallel to the shore, and is moved towards the coastline by the waves and wind until it joins the mainland
Slapton Sands is an example of a bar. The lagoon of water than has formed on the landward side of the bar is called Slapton Ley.
A tombolo is formed where a spit joins an island to the mainland. An example is the Isle of Portland which is joined to the mainland by a shingle ridge known as Chesil Beach.
Tombolos
A tombolo is a ridge of beach material (typically sand), built by wave action, that connects an island to the mainland. Tombolos are often formed where a spit continues to grow by long shore drift, joining land to an offshore island.
Some Tombolos continue to build up as waves are refracted around the island at the end. This happens because when the waves near the island they are slowed down by the shallow water surrounding it. As a result they are then refracted or “bend” around the island to the opposite side as they approached. The wave pattern created by this water movement causes a convergence of long shore drifting on the opposite side of the island.
The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up the beach shoreline, known as a spit, will connect with an island and form a tombolo.
Some Tombolos continue to build up as waves are refracted around the island at the end. This happens because when the waves near the island they are slowed down by the shallow water surrounding it. As a result they are then refracted or “bend” around the island to the opposite side as they approached. The wave pattern created by this water movement causes a convergence of long shore drifting on the opposite side of the island.
The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up the beach shoreline, known as a spit, will connect with an island and form a tombolo.
Why are sea levels rising?
Sea levels are rising. Scientists in Amsterdam in the Netherlands, began taking measurements of sea level in 1700 and similar readings were started in Liverpool in 1768. readings taken in Europe and the USA over the last 100 years prove that sea levels have risen by around 180mm (an average of 1.8mm per year). This rise is largely due to climate change. Higher temperatures mean two things:
Warm water expands slightly in volume, so as the oceans get warmer they also get slightly higher.
The ice sheets that cover large parts of Antarctica and Greenland are melting. As the ice melts, water that has been trapped as ice for tens of thousands of years flows into the oceans.
Warm water expands slightly in volume, so as the oceans get warmer they also get slightly higher.
The ice sheets that cover large parts of Antarctica and Greenland are melting. As the ice melts, water that has been trapped as ice for tens of thousands of years flows into the oceans.
Why are some coastlines more at risk than others?
As well as the general rise in sea levels of 1.8mm per year, there are local factors that mean some coastlines are more at risk than others. This is because some coasts are sinking or subsiding. Subsidence can be caused by more than one factor:
River estuaries and deltas sink under their own weight. A river delta, such as that of the Mississippi in the USA, is made of millions of tonnes of loosely compacted sediment and water. As more sediment is deposited, the particles become more compressed and the water is squeezed out. Parts of the city of New Orleans, USA are subsiding 28mm per year.
In some parts of England the crust has been sinking ever since the ice melted 10.000 years ago at the end of the last Ice Age. Northern parts of the UK were covered by thick layers of heavy ice and the crust was pressed down. When the ice melted, the crust in this part of the UK began to rise slowly. At the same time, the southern part of the UK began to sink. This processes is called postglacial rebound. The subsidence due to rebound is about 2mm per year in the south
River estuaries and deltas sink under their own weight. A river delta, such as that of the Mississippi in the USA, is made of millions of tonnes of loosely compacted sediment and water. As more sediment is deposited, the particles become more compressed and the water is squeezed out. Parts of the city of New Orleans, USA are subsiding 28mm per year.
In some parts of England the crust has been sinking ever since the ice melted 10.000 years ago at the end of the last Ice Age. Northern parts of the UK were covered by thick layers of heavy ice and the crust was pressed down. When the ice melted, the crust in this part of the UK began to rise slowly. At the same time, the southern part of the UK began to sink. This processes is called postglacial rebound. The subsidence due to rebound is about 2mm per year in the south
The effects of sea level rises
In the UK, East Anglia is likely to be hardest hit and this threatens coastal defences and natural ecosystems. Elsewhere in the world, vast areas of low-lying coastal plains such as Bangladesh and whole chains of islands such as the Maldives and Tuvalu could disappear. More than 70% of the world's population live on coastal plains so the effects of sea-level rises are going to be devastating.
The effects of sea-level rise in Norfolk: · Settlements such as King's Lynn may be under threat as sea levels rise. · Valuable agricultural land (the Fens) will be at greater risk of flooding · The Norfolk Broads are a popular tourist destination bringing £5 million+ to the local area · As sea levels rise, erosion rates are likely to increase, threatening coastal settlements such at Overstand and Happisburgh. Current sea defences will need strengthening, which will be expensive · In 1953 East Anglia suffered terribly from a storm surge, which killed 300 people. People are worried that such an event may occur again · The Thames Barrier currently protects buildings worth £80 billion. It will probably need to be replaced in the next 30 to 50 years, · As sea levels rise, large areas of the Lower Thames estuary will be at risk from flooding, affecting housing, industry and farmland · Low-lying mudflats and marshes in Essex are particularly vulnerable to sea-level rise. · Areas of salt marsh are being squeezed between sea walls and rising sea · Some 22% of East Anglia's salt marshes could be lost by 2050 · In some places, managed retreat (a controversial political decision) will breach sea walls to allow deliberate flooding so that salt marshes can reform How are coasts managed?
The usual way to manage coastlines has been through a combination of hard and soft engineering strategies. Hard engineering means building structures that prevent erosion and fix the coastline in place. Wide beaches soak up a lot of wave energy and are a natural defence against coastal erosion. Soft engineering strategies mimic this by encouraging natural deposition to take place along the coastline.
Holderness Coast - Case Study 12 (or Barton-on-Sea)What you need to know:
For a named area of coastline you need to be able to:
Name an area of coastline Describe and explain the processes which act on the coastline Describe the landforms that occur on the coastline Describe and explain the attempts made to manage the coastline Where is the Holderness coastline?
Holderness is a lowland region of England that lies between the chalk hills of the Worlds and the North Sea. It is part of the East Riding of Yorkshire.
The Holderness Coast is one of Europe's fastest eroding coastlines. The average annual rate of erosion is around 2 metres per year. The main reason for this is because the bedrock is made up of till. This material was deposited by glaciers around 12,000 years ago. What processes are operating on the coastline?
The Holderness Coastline is made up of soft boulder clays (tills) left after the retreat of the Devensian ice sheets about 12 000 years ago. They can be seen on the coast, being rapidly eroded by the sea. To look at, they are a mass of brown clay containing pieces of rock (erratic) brought here by the glaciers from Scandinavia, Scotland, the Lake District and Northeast England. These soft, recent deposits sit on a platform of chalk which slopes away gently to the east.
Erosion of the Holderness cliffs is a cyclic, four stage process: 1.The soft Boulder Clay cliffs become saturated with rain water and lose their strength. 2.The cliff is too steep and fails either as a block of material or as a slurry slide 3.Cliff failure reduces the angle and prevents further erosion but … 4.Large waves from the North East remove the debris in longshore drift to the Southand the cliff oversteepens, rain falls and the cycle begins again. In summary all of the major processes of erosion occur. The landforms on the Coastline
There are various landforms on the coastline - you will already be aware of their formation - I suggest you revise these carefully.
Landforms listed below: Headland - Flamborough Head Small Cliffs - at Mappleton Beaches - the whole way down Spit - at Spurn point Why does coastal management create controversy?
It is natural that people who live near the coast expect to be protected from erosion and flooding. However, the usual strategies of hard and soft engineering are expensive and, in the face of rising sea-levels, may not be sustainable. Planner now believe that a mixture of engineering strategies and coastal land-use zoning is needed to manage coastlines in the future. These zones will include some urban areas where fuure building will be prevented and other where homes will be rebuilt further inland and coastal retreat will be managed. The idea of moving urban areas further inland is now being considered by scientists and some politicians in Australia but many residents of coastal towns, such as Byron Bay in New South Wales, are unhappy.
  | The effects on Papua New Guinea
· Loss of homes
· Loss of land for growing crops and rearing animals which could result in starvation · As the sea-level rises the fresh water table becomes contaminated resulting in a reduction of fresh water - dehydration · Loss of indigenous cultures · Coral reefs die due to changes in sea-level |
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