L5 Factors Affecting River Levels

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  • 1. Factors influencing river levels Key idea: The amount of water in a river fluctuates due to a number of reasons NT pg 108 - 110
  • 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
    • To explain how the following factors affect discharge:
      • amount and type of rainfall
      • Temperature
      • previous weather conditions
      • relief
      • rock type (impermeable, permeable, porous and pervious)
      • Land use .
  • 3. Starter activity…. A DRAINAGE BASIN DISCHARGE VELOCITY WATERSHED The boundary of a drainage basin The amount of water in a river channel at a given time The area of land from which precipitation flows into a stream. The speed of the river
  • 4. Factors affecting discharge
    • Amount and type of rainfall
    • Temperature
    • Previous weather conditions
    • Relief
    • Drainage Density
    • Rock Type
    • Soil
    • Land Use
    • Slope
  • 5. FACTORS THAT AFFECT RIVER DISCHARGE ......Lag time is decreased so discharge increases
    • AMOUNT AND TYPE OF RAINFALL
    • Lots of rain and short heavy periods of rainfall means more run off
    PREVIOUS WEATHER After lots of rain soil may be saturated. More rain cannot infiltrate into the soil so surface run off increases. LAND USE Urban areas have drainage systems. Roads etc are form impermeable surfaces which increase run off. TEMPERATURE Hot, dry conditions and cold freezing conditions result in hardground which increases run off. ROCK TYPE Permeable rocks allow water to infiltrate pore spaces. Pervious rocks allow water to infiltrate between cracks Impervious rocks cause increased discharge RELIEF Steep slopes means more run off, lag time is decreased…
  • 6. Rising limb Falling limb Peak discharge Peak rainfall storm flow normal (base) flow Hydrographs I = Lagtime 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. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Flood Hydrograph
  • 11. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s)
  • 12. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) mm 4 3 2 Rainfall shown in mm, as a bar graph
  • 13. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) mm 4 3 2 Discharge in m 3 /s, as a line graph
  • 14. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb mm 4 3 2 Rising limb The rising flood water in the river
  • 15. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb mm 4 3 2 Peak flow Peak flow Maximum discharge in the river
  • 16. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb Recession limb mm 4 3 2 Peak flow Recession limb Falling flood water in the river
  • 17. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Basin lag time Time difference between the peak of the rain storm and the peak flow of the river
  • 18. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Base flow Normal discharge of the river
  • 19. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow
  • 20. Volume of water reaching the river from surface run off Overland flow = 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.
  • 22. 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.
  • 23. Plenary