Hydrographs show changes in river discharge over time in response to rainfall. They are constructed by measuring river discharge during and after storms. A typical hydrograph has a rising limb as discharge increases, peaks at maximum discharge, then declines on the recession limb. Analysis of hydrographs can predict flooding and inform flood prevention. Characteristics like basin area, slope, soil, land use, and rainfall patterns influence a hydrograph's shape and timing.

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