Causes Of River Floods


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Causes Of River Floods

  1. 1. Causes of river floods Hydrographs + flooding
  2. 2. Flood hydrographs <ul><li>Answer the Qs. </li></ul><ul><li>What does a hydrograph show? [1mark] </li></ul><ul><li>What is the ‘lag time’? [1mark] </li></ul><ul><li>River discharge is measured in what unit? [1] </li></ul><ul><li>If a hydrograph has a short lag time what does that mean? [1mark] </li></ul><ul><li>What does ‘impermeable’ mean? [1mark] </li></ul>
  3. 3. Answers <ul><li>What does a hydrograph show? [1mark] </li></ul><ul><li>- how river discharge responds to rainfall (it shows rainfall, discharge and links the two) </li></ul><ul><li>What is the ‘lag time’? [1mark] </li></ul><ul><li>- the waiting time between peak rainfall + peak river discharge </li></ul><ul><li>River discharge is measured in what unit? [1] </li></ul><ul><li>- Cumecs (cubic metres per second) </li></ul><ul><li>If a hydrograph has a short lag time what does that mean? [1mark] </li></ul><ul><li>- That water gets to the river very quickly so surface runoff must be high / water is not infiltrating </li></ul><ul><li>What does ‘impermeable’ mean? [1mark] </li></ul><ul><li>- A surface that water cannot infiltrate (soak in to), e.g. conrete </li></ul>
  4. 4. The Flood / Storm Hydrograph <ul><li>Hydrographs are used to show the relationship between precipitation and discharge (the amount of water passing in the river channel) . </li></ul><ul><li>Hydrographs are used to predict whether a river is likely to flood . </li></ul><ul><li>Discharge is measured in Cumecs (cubic metres of water per second). </li></ul><ul><li>Lag Time is the delay between peak precipitation and peak river discharge. </li></ul><ul><li>Short Lag Times means water enters the river system very quickly following rainfall </li></ul><ul><li>Long Lag Times means water takes a longer time to enter the river system following rainfall </li></ul>
  5. 5. Lag times <ul><li>So a lag time changes depending on the characteristics of the river basin </li></ul><ul><li>What kinds of things might cause rainfall to enter the river more slowly or more quickly? (Tip – think about factors that slow down infiltration or increase surface run-off) </li></ul><ul><li>Short lag times = are caused by steep slopes , impermeable rock, sparse vegetation and a small drainage basin. These mean surface run-off is high so water enters the river quickly following precipitation. </li></ul><ul><li>Long lag times = are caused by gentle slopes, permeable rock, dense vegetation and a large drainage basin. These mean surface run-off is lower, there is more interception and infiltration so water takes longer to get to the river channel. </li></ul>
  6. 6. Lag times This could be for an urban location, quick surface runoff and less infiltration This could be for a rural location, slow surface runoff and more infiltration
  7. 7. Lag times <ul><li>Lag times are short if water cannot infiltrate or is not intercepted, it means water from precipitation is getting in to the river flow very quickly + there is more surface runoff Could be an urban drainage basin </li></ul><ul><li>Lag times are longer if water can infiltrate or gets intercepted, it means water is taking longer to enter the river channel + there is less surface runoff Could be a rural drainage basin </li></ul>
  8. 8. Questions <ul><li>P76/77. </li></ul><ul><li>Which hydrograph do you think is for a rural location? </li></ul><ul><li>Which is urban? </li></ul><ul><li>Why? </li></ul>
  9. 9. What are the causes of flooding? <ul><li>Make a copy of the diagram of causes of flooding on page 76 </li></ul><ul><li>Colour classify each cause into either physical or human . </li></ul><ul><li>Complete activities 2-4 </li></ul><ul><ul><li>2. For 2 marks </li></ul></ul><ul><ul><li>3. For 3 marks </li></ul></ul><ul><ul><li>4. For 2 marks </li></ul></ul>
  10. 10. Answers <ul><li>2. The UK’s population is still rising, e.g. from migration, and new housing is needed. There is little space in the UK and land is cheap on floodplains so people build here. </li></ul><ul><li>3. Using permeable surfaces in urban areas, ploughing across hills instead of up and down, planting trees, etc. </li></ul><ul><li>4. The lag time is shorter and the peak discharge higher in urban areas when compared to rural areas. This is because there is less infiltration in urban areas and more surface runoff. </li></ul>
  11. 11. Causes of Flooding <ul><li>Precipitation - Heavy rainfall over a long period creates saturated soil = less infiltration = more surface run-off </li></ul><ul><li>Flash Floods - Hot dry areas where the land is baked hard causes flooding if there is an intense burst of heavy rainfall - water cannot infiltrate and results in rapid runoff. </li></ul><ul><li>Snowmelt - When temperatures rise and snow melts the stored precipitation is released as runoff because infiltration will be low as the ground is still frozen. </li></ul><ul><li>Deforestation - When trees are cut down this reduces interception, transpiration and storage by the trees. This results in increased surface runoff. </li></ul><ul><li>Urbanisation - When land is urbanised vegetation is removed and the land is covered in concrete and tarmac. These surfaces are impermeable = increase surface runoff. </li></ul>Basically, anything that reduces infiltration and increases surface runoff
  12. 12. What are the impacts of river flooding?
  13. 13. What are the impacts of river flooding?
  14. 14. Poster task <ul><li>In pairs or groups, create a poster to show what you have learnt about rivers. </li></ul><ul><li>Include: </li></ul><ul><li>- Types of erosion </li></ul><ul><li>- Upper / middle / lower course features </li></ul><ul><li>- Landforms (meanders, oxbow lake, waterfalls, floodplain, delta) – how they form </li></ul><ul><li>- Causes of flooding </li></ul><ul><li>- Effects of flooding </li></ul>This will be GRADED! You will each get a photocopy to keep
  15. 15. LEDC case study <ul><li>For the exam, you need to know one MEDC case study and one LEDC case study </li></ul><ul><li>LEDC vs MEDC, what does it mean? </li></ul><ul><li>Your case studies for river flooding: </li></ul><ul><li>- MEDC (Yorkshire + your hw case) </li></ul><ul><li>- LEDC (Mozambique) </li></ul>