Flooding 1 Discharge And Hydrograph

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  • 1. Flooding 1 Key idea: The amount of water in a river fluctuates due to a number of reasons
  • 2. Lesson objectives • To know the terms: Flooding, hydrograph, rising limb, falling limb, lag time, drainage basin • To be able to describe the characteristics of a hydrograph
  • 3. Starter activity…. The area of land from which A DRAINAGE BASIN precipitation flows into a stream. DISCHARGE The amount of water in a river channel at a given time VELOCITY The speed of the river WATERSHED The boundary of a drainage basin
  • 4. 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
  • 5. Factors affecting discharge •Amount and type of rainfall • Soil •Temperature • Land Use • Slope •Previous weather conditions • Relief •Drainage Density •Rock Type
  • 6. Hydrographs Peak discharge Falling Rising limb limb storm flow normal (base) flow I = Lagtime Peak rainfall Show the change in discharge caused by a period of rainfall
  • 7. Storm hydrograph A graph which shows the relationship between precipitation and the level of a river
  • 8. APPROACH SEGMENT = base flow • The discharge of the river before it rains. RISING LIMB • Results from a rapid increase in rainwater reaching the river. LAG TIME • The difference between the time of the heaviest rainfall and the maximum level and /or discharge of the river. FALLING LIMB • Some rainwater is still reaching the river, but in decreasing amounts – throughflow and groundwater flow
  • 9. Why Construct & Analyse Hydrographs ? • To find out discharge patterns of a particular drainage basin Help predict flooding events, therefore influence implementation of flood prevention measures ©Microsoft Word clipart
  • 10. ph g ra Basin lag time d ro Peak flow y 3 H od limb Re lo ce Discharge (m3/s) F Rising ss Overland 2 io flow nl im mm b 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm
  • 11. Discharge (m3/s) 3 2 1 0 12 24 36 48 30 72 Hours from start of rain storm
  • 12. Rainfall shown 3 in mm, as a bar graph Discharge (m3/s) 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 13. Discharge in 3 m3/s, as a line graph Discharge (m3/s) 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 14. Rising limb The rising 3 flood water in the river li m b Discharge (m3/s) Rising 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 15. Peak flow Peak flow Maximum 3 discharge in the river limb Discharge (m3/s) Rising 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 16. Recession limb Peak flow Falling flood 3 water in the limb river Re Discharge (m3/s) ce Rising ss 2 ion mm li m b 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 17. Basin lag time Basin lag time Peak flow Time 3 difference between the limb Re ce peak of the Discharge (m3/s) Rising ss 2 io rain storm and nl im mm the peak flow b 4 1 3 of the river 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 18. Base flow Basin lag time Peak flow Normal 3 discharge of limb the river Re ce Discharge (m3/s) Rising ss 2 io nl im mm b 4 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm
  • 19. Basin lag time Peak flow 3 limb Re ce Discharge (m3/s) Rising ss Overland 2 io flow nl im mm b 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm
  • 20. Overland flow = surface run off Volume of water reaching the river from surface run off Through flow Volume of water reaching the river through horizontal movement of water through the soil and underlying rock layers
  • 21. • After a period of heavy rainfall, the discharge increases. • The lagtime is the difference in time between the peak of the rainstorm and the peak of the discharge. The shorter this is and the steeper the rising limb the greater the risk of flooding. • Complete task 1
  • 22. Plenary
  • 23. Homework Worksheet - task 2 a - c