Rivers

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Rivers revision for EDEXCEL A GCSE Geography Unit 2 Natural Environment

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Rivers

  1. 1. Rivers Thursday 22nd May
  2. 2. Long Profile of a River
  3. 3. Watershed – An area of high ground which separates water flowing to different drainage basins and rivers Source – The place where the river originates Tributary – A small stream or river that joins onto another larger river Confluence – where two rivers meet. Meander – a U shaped bend in the river Flood Plain – a flat area of land that a river drains onto during times of high discharge Mouth – where the river meets the sea or lake. Also called a delta or an estuary
  4. 4. The upper course of a river has very steep slopes. This is because the river moves very fast down hill and erodes the rock away quickly. As you can see from the pictures it forms a V-shaped valley. Where the hills of the valley overlap they are called interlocking spurs. At this point the river channel is very narrow and has lots of large, sharp rocks in it. Other small rivers called tributaries join the main river in the upper course and there are often waterfalls.
  5. 5. In the middle course of the river the slopes are not as steep as the river has come out of the high ground (medium slope). The valley sides are also not as steep and the river channel begins to get wider and deeper. Stones become rounder as they are eroded and the river begins to curve causing meanders.
  6. 6. The lower course has very gentle slopes and the valley becomes very flat and wide with flood plains either side of the river. The river channel is very deep and wide with very rounded stones. There are many meanders at this point as well as oxbow lakes and it is sometimes tidal. The mouth of the river can be found in the lower course and huge amounts of sediment are often deposited here.
  7. 7. 7 Erosion…again! • Attrition – where material is moved along the bed of a river, collides with other material, and breaks up into smaller and smaller pieces. • Abrasion – fine material rubs against the river bank. The bank is worn away, by a sort of sandpapering action. • Solution – some rocks forming the banks and bed of a river are dissolved by acids in the water • Hydraulic action – the sheer force of water hitting the banks of the river.
  8. 8. Erosion and deposition on a meander 1) In a meander water is fastest on the outside of the bend. 2) The erosion processes of attrition and abrasion cause undercutting and a river cliff. 3) On the inside of the river bend, water flows more slowly. 4) Because the water has less energy, rocks and silt carried along by the river drop out of the flow and are deposited as a river beach.
  9. 9. Fast moving water on the outside of the bend High velocity (fast moving) water erodes the river bank creating a river cliff Water moves slower round the inside of the bend As the water has a lower velocity it has less energy and the rocks it was carrying get deposited on the inside of the bend as a river beach
  10. 10. How a waterfall forms A waterfall will form when there are two layers of rock. One layer is hard rock and more resistant to erosion and one which is soft and eroded easily. A plunge pool is formed at the bottom of the waterfall in the soft rock. The soft rock underneath the hard rock gets worn away, undercutting the hard rock. Abrasion and attrition help to make the plunge pool deeper. Eventually the hard rock becomes too far undercut and the overhang of hard rock breaks off due to gravity. The waterfall retreats upstream. This process repeats over and over for thousands of years. As the waterfall continues to retreat a steep sided gorge is formed
  11. 11. How a levee is formed on a flood plain
  12. 12. Impermeable surfaces!!!!!!! A lot of flooding events are to do with rocks being impermeable. This means water cannot soak into the ground and so runs over the surface entering the river much faster! Most of the building materials used in towns and cities are impermeable meaning flooding is much more likely when a river is in a built up area.
  13. 13. Effects of Flooding People • Loss of belongings • Damage to property • Disruption to transport • Disease and illness (LIC) • Insurance claims (HIC) • Contamination of water supplies and loss of services e.g. gas • Crops and animals lost • Death Environment • Landslides • Soil contamination by sewage • Vegetation destroyed • Animals drowned • Loss of wildlife habitats • Soil erosion
  14. 14. Predicting Flooding • Forecasting means that you use information from previous years to predict where and when certain rivers will flood. • Many rivers, especially those that have a history of flooding are monitored by the Environment Agency. If river levels rise to potentially dangerous levels then they are able to warn people to evacuate to a safer place. • The flood warnings have three stages as shown in the diagram on the right. These warnings are broadcast on the weather forecast and people can also look up there area online to check that they are not at risk. • The Environment Agency also produces maps which can be accessed online to show areas that are at risk of flooding. Computers can also be programmed to automatically monitor risk and predict flooding.
  15. 15. Preparing for Floods
  16. 16. Planning for Floods There is no point in governments and agencies having warnings and plans of action in place if they do not let local people know about the flood risk and what they should do. This is where education is extremely important. Governments try to educate people by: • Sending leaflets through the post • Advertising in newspapers and on TV • Posting information on websites • Offering telephone helpline numbers • Having flood drills In the USA this is the advice given for before, during and after a flood • Store drinking water in clean bathtubs and containers • Keep a stock of food that requires no cooking or refrigeration • Keep a first aid kit on hand • Assemble a disaster kit that includes: first aid kid, canned food and a can opener, bottled water, rubber boots, rubber gloves, battery powered radio, torch and extra batteries.
  17. 17. Flood ManagementHard Engineering Soft Engineering Embankments (levees) – high banks of soil built on or near the river banks to contain the increased amount of water during a flood. Channelisation – this involves making the channel of the river wider, deeper or straighter to allow more water to flow through Flood relief channels – extra channels are built next to rivers or leading from them so when the river is in flood the extra water flows into the channels rather than onto streets and houses. Washlands – areas on the flood plain that are deliberately not built on and allowed to flood. Afforesation – trees are planted in the drainage basin to increase interception and decrease overland flow so water enters the rivers more slowly. Land use zoning – the government chooses areas of land for different uses depending on how high their flood risk is. e.g. areas near the river are used for farming rather than building. Flood warning system – rivers are carefully watched and if the levels are rising, places downstream can be warned.
  18. 18. Afforesation – catches the rainfall before it enters the river Washlands – allow water to spill out of the river channel and flood before it reaches towns and cities Flood Relief Channel/Channelisation – happen upstream where it is easy to change the course of the river. Embankments – happen next to towns and cities to protect property Flood Warning and land use zoning – only useful if it is in a town or a city.
  19. 19. Boscastle – a flooding and river management case study Causes of the flood • 200mm of rain fell in 24hrs • V shaped valley = lots of runoff • Three rivers (Jordan, Paradise and Valancy) converge the middle of in Boscastle. • The ground was saturated by heavy rainfall over the previous week. • Thunderstorms stayed over Boscastle for 6 hrs
  20. 20. Effects of the flood • The flooding affected 58 properties and 1000 people • 84 wrecked cars were in the harbour, with another 32 out at sea. • Cost North Cornwall District Council up to £2 million. • A dramatic decrease in tourist numbers immediately after the flood led to the town being described by business leaders as a ‘tourist ghost town’. Boscastle – a flooding and river management case study
  21. 21. Boscastle – a flooding and river management case study Protection and Prevention • In 2005, a new £800,000 flood defence scheme opened in Boscastle which includes a new, larger culvert (storm drain) on the river Jordan which will be able to carry double the amount of floodwater. • In order to increase its capacity the river has been widened and deepened and the lower bridge replaced. • A new car park has been built which is raised and set back from the river and a new tree management scheme has been introduced to reduce the chances of tree blockage. • Flood defence works together with the regeneration scheme cost £10 million and were officially opened in October 2008.

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