Hydrograph explanation and animation
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  • 1. Construction And Analysis of Hydrographs © Microsoft Word clipart © Microsoft Word clipart
  • 2. Hydrograph Record of River Discharge (the level of water flowing down a river channel) over a period of time, they show how certain rivers respond to a rainstorm. River Discharge (the level of water flowing down a river) (is calculated) Storm Hydrographs Show the change in discharge caused by a period of rainfall = cross sectional area rivers mean (average) velocity X (at a particular point in its course)
  • 3. 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
  • 4. Construction Of Storm (flood) Hydrographs © Microsoft Word clipart
  • 5. 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
  • 6. The discharge of the river is measured in cumecs - this stands for cubic metres per second 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s)
  • 7. 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
  • 8. 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
  • 9. 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 normal (base) flow of the river starts to rise when run-off, ground and soil water reaches the river.
  • 10. 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, the time when the river reaches its highest flow
  • 11. 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 shows that water is still reaching the river but in decreasing amounts
  • 12. 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 The time it takes for the water to find its way to the river
  • 13. 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
  • 14. 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 Overland flow Through flow + = Storm Flow
  • 15. Volume of water reaching the river from surface run off Overland flow Through flow Volume of water reaching the river through the soil and underlying rock layers
  • 16. Analysis © Microsoft Word clipart
  • 17. Interpretation of Storm Hydrographs
    • Rainfall Intensity
    • Rising Limb
    • Recession Limb
    • Lag time
    • Peak flow compared to Base flow
    • Recovery rate, back to Base flow
    You need to refer to: Basin lag time 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 Base flow Through flow Overland flow
  • 18. Some Factors influencing Storm Hydrographs
    • Area
    • Slope
    • Rock Type
    • Land Use
    • Soil
    • Precipitation / Temp
    © Microsoft Word clipart
  • 19. Area
    • Large basins receive more precipitation than small therefore have larger runoff
    • Larger size means longer lag time as water has a longer distance to travel to reach the trunk river
    Area Rock Type Soil Slope Land Use Precipitation / Temp
  • 20. Slope
    • Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
    Area Rock Type Soil Slope Land Use Precipitation / Temp
  • 21. Rock Type
    • Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb
    Area Rock Type Soil Slope Land Use Precipitation / Temp
  • 22. Soil
    • Infiltration is generally greater on thick soil
    • The more infiltration occurs the longer the lag time and shallower the rising limb
    Area Rock Type Soil Slope Land Use Precipitation / Temp
  • 23. Land Use
    • Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag
    • In wooded areas, trees intercept/absorb the precipitation, creating a shallow rising limb and lengthening the time lag
    Area Rock Type Soil Slope Land Use Precipitation / Temp
  • 24. Precipitation & Temperature
    • Short intense rainstorms can produce rapid overland flow and steep rising limb
    • If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off
    • Snow on the ground can act as a store producing a long lag time and shallow rising limb . Once a thaw sets in the rising limb will become steep
    Area Rock Type Precipitation / Temp Soil Slope Land Use
  • 25. Remember! These influencing factors will:
    • Influence each other
    • Change throughout the rivers course