1. Analysis of Hydrographs
Construction
And
Analysis of Hydrographs
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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)
= cross sectional area
rivers mean (average) velocity X
(at a particular point in its course)
Storm Hydrographs
Show the change in discharge caused by a period of rainfall

3. Construct & Analyse Hydrographs ?
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
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4. Storm (flood) Hydrographs
Construction
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Storm (flood) Hydrographs

5. Flood Hydrograph 3 2 Discharge (m3/s) 1 0 12 24 36 48 30 72
Basin lag time
Peak flow
Flood Hydrograph 3
Rising limb
Overland flow
Recession limb 2 mm
Discharge (m3/s) 4
Through flow 1 3 2
Base flow
Hours from start of rain storm

6. Hours from start of rain storm
The discharge of the river is measured in cumecs - this stands for cubic metres per second 3 2 1
Discharge (m3/s)
Hours from start of rain storm

7. Rainfall shown in mm, as a bar graph3 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

8. Discharge in m3/s, as a line graph2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

9. Rising limb
The normal (base) flow of the river starts to rise when run-off, ground and soil water reaches the river. 3
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

10. Peak flow
Peak flow
Maximum discharge in the river, the time when the river reaches its highest flow 3
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

11. shows that water is still reaching the river but in decreasing amounts
Recession limb
Peak flow
shows that water is still reaching the river but in decreasing amounts 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

12. The time it takes for the water to find its way to the river
Basin lag time
Basin lag time
Peak flow
The time it takes for the water to find its way to the river 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

13. Normal discharge of the river
Base flow
Basin lag time
Peak flow
Normal discharge of the river 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Base flow
Hours from start of rain storm

14. + = Overland flow Through flow Storm Flow 3 2 Discharge (m3/s) 1
Basin lag time +
Peak flow 3
Through flow =
Rising limb
Overland flow
Recession limb 2
Storm Flow mm
Discharge (m3/s) 4
Through flow 1 3 2
Base flow
Hours from start of rain storm

15. Volume of water reaching the river from surface run off
Overland flow
Through flow
Volume of water reaching the river from surface run off
Volume of water reaching the river through the soil and underlying rock layers

16. Analysis
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17. Interpretation of Storm Hydrographs
Basin lag time
You need to refer to:
Peak flow 3
Rising Limb
Rising limb
Base flow
Through flow
Overland flow 2
Recession limb mm
Recession Limb
Discharge (m3/s) 4 1 3
Lag time 2
Rainfall Intensity
Hours from start of rain storm
Peak flow compared to Base flow
Recovery rate, back to Base flow

18. Some Factors influencing
Storm Hydrographs
Area
Slope
Rock Type
Land Use
Soil
Precipitation / Temp
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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. These influencing factors will: Change throughout the rivers course
Remember!
These influencing factors will:
Influence each other
Change throughout the rivers course