Flooding 2 Causes


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Flooding 2 Causes

  1. 1. Hydrograph data to plot Thinking through Geog Flooding 2 Key idea: 2. Rivers flood due to a number of physical and human causes. 3. Flooding appears to be an increasingly frequent event
  2. 2. Lesson objectives • To know the terms: Flooding, hydrograph, rising limb, falling limb, lag time, drainage basin • To be able to explain the factors that cause flooding • To know the frequency and location of flood events in the UK over the past 20 years
  3. 3. Starter activity…. • Precipitation • Interception • Surface storage • Infiltration • Soil moisture • Percolation • Groundwater • Transpiration • Evaporation • Surface run off • Through flow • Groundwater flow • Water table
  4. 4. • Textbook page 110 task 2
  5. 5. • Discharge • Drainage basin • Storm hydrograph • Flashy • Relief • Impermeable • Porous • Pervious • Deforestation • urbanisation
  6. 6. A flood occurs when… …..the discharge is so great that all the water can no longer be contained within the channel so that the river overflows its banks
  7. 7. So what factors affect river discharge?
  8. 8. Factors affecting discharge •Amount and type of rainfall • Soil •Temperature • Land Use • Slope •Previous weather conditions • Relief •Drainage Density •Rock Type
  9. 9. Most floods occur because of the weather: Amount and type of rain • Prolonged rain – long and continuous periods of rainfall lead to saturated ground • With drizzle, there is time for water to infiltrate soil and underlying rock • Heavy rain which causes large amounts of run-off in a short period of time Temperature • High temperatures lead to greater evaporation and transpiration rates so river levels decrease • Snow melt – in spring a sudden increase in Textbook temperature which melts snow and ice. - In winter page 109 the water cannot seep into frozen ground • Previous weather conditions
  10. 10. • Rock type • Rock type determines how much water infiltrates and how much stays on the surface. • Relief • Relief affects the rate at which water runs off the land surface and into rivers. Steep slopes give faster run off due to gravity • Gentle slopes allow time for infiltration
  11. 11. Human activities can make the flood risk worse … • Building on a floodplain. Tarmac and concrete are impermeable which means that more water runs off the surface and faster. • Deforestation reduces interception by the trees and so more rainwater reaches the surface quicker increasing rates of surface run off
  12. 12. Your turn • Textbook page 110 task 3 • You are to produce a double page spread around the title ‘ The reasons river discharge fluctuates’ • In the central box give the meaning of ‘river discharge’ • Place the factors around the central title • Add information to each factor to explain how discharge is affected. • Illustrate each factor with diagrams, sketches or photos. • Your finished work should be informative, accurate, interesting, colourful and original
  13. 13. Which are physical and which are Plenary human factors? What factors increase The risk of flooding? Building in the drainage basin * Very wet soil *steep slopes * Many tributaries * Impermeable * Cutting * Hard dry soil rock down trees
  14. 14. Homework…. Card sort activity Cut out the two hydrographs – one for a ‘flashy’ river and a gentle graph Now cut out the individual cards and place them under the appropriate hydrograph.