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Flooding 2 Causes
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Flooding 2 Causes

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  • 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. 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. Starter activity…. • Precipitation • Interception • Surface storage • Infiltration • Soil moisture • Percolation • Groundwater • Transpiration • Evaporation • Surface run off • Through flow • Groundwater flow • Water table
  • 4. • Textbook page 110 task 2
  • 5. • Discharge • Drainage basin • Storm hydrograph • Flashy • Relief • Impermeable • Porous • Pervious • Deforestation • urbanisation
  • 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. So what factors affect river discharge?
  • 8. Factors affecting discharge •Amount and type of rainfall • Soil •Temperature • Land Use • Slope •Previous weather conditions • Relief •Drainage Density •Rock Type
  • 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. • 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. 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. 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. 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. 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.

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