1. Weekly lesson objectives
Review the plotting and analysis of flood hydrographs
Develop your knowledge and understanding of the effect of global warming on flooding in the Brahmaputra river
Prepare and submit your binders for the quarterly binder check (minor – Thursday)
Complete the Hydrology Decision Making exercise post test (Medium)

2. Quick Quiz 1) On the hydrograph – show the peak flow/ discharge
2) On the hydrograph – show the rising limb
3) On the hydrograph – show the falling limb
4) On the hydrograph - What the maximum rainfall?
5) Explain what the basin lag time is
6) How does interception by vegetation influence the flood hydrograph?
7) How do tides influence the flood hydrograph?
8) How do concrete roads influence the flood hydrograph?
9) List three factors that effect basin lag time

3. Activity: create your own flood hydrograph with the information given, and answer the questions

4. Flood Hydrographs

5. Key words Discharge – Depends on the rivers velocity and volume
Velocity – is the speed of the river. It is measured in meters per second
Discharge is the velocity of a river times its volume. It’s the amount of water in a river passing a given point at a given time, measured in cumecs (cubic meters a second)

6. Hydrograph River Discharge Flood Hydrographs = cross sectional area X
Record of River Discharge over a period of time
River Discharge
= cross sectional area
rivers mean (average) velocity X
(at a particular point in its course)
Flood Hydrographs
Show the change in discharge caused by a period of rainfall

7. Measuring river velocity with a current meter

8. 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
©Microsoft Word clipart

9. Storm (flood) Hydrographs
Construction
©Microsoft Word clipart Of
Storm (flood) Hydrographs

10. 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

11. 3 2 1
Discharge (m3/s)
Hours from start of rain storm

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

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

14. Rising limb 3 2 Discharge (m3/s) 1 0 12 24 36 48 30 72
The rising flood water in the river (when the storm begins, the river does not respond immediately – the first water to reach the river is through surface runoff and throughflow) 3
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

15. Maximum discharge in the river (you can see this is after the rain)
Peak flow/discharge
Peak flow
Maximum discharge in the river (you can see this is after the rain) 3
Rising limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

16. Falling flood water in the river
Recession limb
Peak flow
Falling flood water in the river 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

17. Basin lag time
Basin lag time
Peak flow
Time difference between the peak of the rain storm and the peak flow of the river 3
Rising limb
Recession limb 2 mm
Discharge (m3/s) 4 1 3 2
Hours from start of rain storm

18. 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

19. + = 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

20. 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

21. Analysis
©Microsoft Word clipart

22. Note sharing activity
In groups – go around and get info of other students to complete your hydrograph analysis mind map

23. Factors influencing Flood Hydrographs
Paragraph: Describe the hydrograph and give
reasons for why it looks the way it does
Shape
Rock Type
Slope
Precipitation / Temp
Land Use
Factors influencing
Flood Hydrographs
Area
Tidal Conditions
Soil
Drainage Density
©Microsoft Word clipart

24. Factors influencing Flood Hydrographs
Shape
Rock Type
Porous
Slope
Precipitation / Temp
Land Use
Factors influencing
Flood Hydrographs
Area
Tidal Conditions
Soil
Drainage Density
©Microsoft Word clipart

25. Factors influencing Flood Hydrographs
Area
Shape
Slope
Rock Type
Soil
Land Use
Drainage Density
Precipitation / Temp
Tidal Conditions
©Microsoft Word clipart

26. 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
Falling 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

27. Here are some theoretical interpretations of influencing factors
BUT……
When interpreting hydrographs all factors must be considered together !
©Microsoft Word clipart

28. Area
Large drainage basins receive more precipitation than small therefore have larger runoff (area the water has to travel to reach the trunk river)
Larger size means longer lag time between peak rainfall and peak discharge as water has a longer distance to travel to reach the trunk river
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

29. Shape
Elongated drainage basin will produce a lower peak flow and longer lag time than a circular one of the same size – because the water takes longer to reach the river
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

30. Slope
Channel flow can be faster down a steep slope therefore steeper rising limb (because the water is moving faster – higher velocity) and shorter lag time
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

31. Rock Type
Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

32. Soil
Infiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layers
The more infiltration occurs the longer the lag time and shallower the rising limb
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

33. Land Use
Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag
Forests - intercepts the precipitation, creating a shallow rising limb and lengthening the time lag
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

34. Drainage Density
A higher density will allow rapid overland flow (because there are many streams)
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

35. 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 Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

36. Tidal Conditions
High spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow
Area Rock Type Drainage Density
Shape Soil Precipitation / Temp
Slope Land Use Tidal Conditions

37. These influencing factors will: Change throughout the rivers course
Remember!
These influencing factors will:
Influence each other
Change throughout the rivers course
©Microsoft Word clipart

38. A flood hydrograph

39. 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
Activity: Label your flood hydrograph

40. Activity: Interpret the graph and complete the following questions on your worksheet
a) Using the words below, copy and complete the following paragraph:
peak rainfall falling limb rising limb lag time peak discharge
The graph shows a steep ____________ as the flow of the river increases. When
the ______________ is reached a gentler _______________ records the gradual decrease in discharge as the rainwater drains away. The gap between _____________ and peak discharge is called the _________________. b)
i) After how many hours from the time of peak rainfall did the river reach its peak discharge? _________________
ii) For how many hours did the river flow above its normal level? ___________________
iii) Give two reasons why most rivers do not respond immediately to rainstorms.
c) Which of the following rivers is most likely to flood:River ___
River A A low discharge and lengthy lag time.
River B A high discharge and a short lag time.

41. Answers
a) Using the words below, copy and complete the following paragraph:
peak rainfall falling limb rising limb lag time peak discharge
The graph shows a steep rising limb as the flow of the river increases. When
the peak discharge is reached a gentler falling limb records the gradual decrease in discharge as the rainwater drains away. The gap between peak rainfall and peak discharge is called the lag time. b)
i) After how many hours from the time of peak rainfall did the river reach its peak discharge? 8 hours
ii) For how many hours did the river flow above its normal level? 21 hours
iii) Give two reasons why most rivers do not respond immediately to rainstorms.
Soil: Infiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layers
Tidal conditions: High spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow
c) Which of the following rivers is most likely to flood: River River B
River A A low discharge and lengthy lag time.
River B A high discharge and a short lag time.

42. Quick Quiz 1) On the hydrograph – show the peak flow/ discharge
2) On the hydrograph – show the rising limb
3) On the hydrograph – show the falling limb
4) On the hydrograph - What the maximum rainfall?
5) Explain what the basin lag time is
6) How does interception by vegetation influence the flood hydrograph?
7) How do tides influence the flood hydrograph?
8) How do concrete roads influence the flood hydrograph?
9) List three factors that effect basin lag time