1. Flood Hydrographs How do we know if a river will flood?
How can we represent that graphically?

2. Essential question
How can we analyze flood risk?

3. Flooding clips http://www.youtube.com/watch?v=XLtTrGjyrBY

4. 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

5. Measuring river velocity with a current meter

6. 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

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

8. Activities: 1. Create your own flood hydrograph 2. label it 3
Activities: 1. Create your own flood hydrograph 2. label it 3. Analyse the two hydrographs and decide which is more likely to flood and why... Write your answer on the back of the poster

9. 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

10. 3 2 1
Discharge (m3/s)
Hours from start of rain storm

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

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. + = 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

19. 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

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

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

22. 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

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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. Quick Quiz: Answer in Pairs
What is a tributary?
Name three ways in which the worlds water is stored
From what source do we get most of the water we use?
What does confluence mean?
What is the term used to describe the downward flow of water through pores in the soil?
In the hydrological cycle – where does the energy that causes evaporation come from?
Explain how different types of rock and soil influence drainage
What types of drainage basin are more likely to flood? Explain.
Name one input in the drainage basin system
Name one store in the drainage basin system
Name an output in the drainage basin system
Explain what the basin lag time is
How does interception by vegetation influence the flood hydrograph?
How do concrete roads influence the flood hydrograph?
List three human factors that effect basin lag time
List three natural factors that effect basin lag time

36. Quick Quiz: Answer in Pairs
What is a tributary?
Name three ways in which the worlds water is stored
From what source do we get most of the water we use?
What does confluence mean?
What is the term used to describe the downward flow of water through pores in the soil?
In the hydrological cycle – where does the energy that causes evaporation come from?
Explain how different types of rock and soil influence drainage
What types of drainage basin are more likely to flood? Explain.
Name one input in the drainage basin system
Name one store in the drainage basin system
Name an output in the drainage basin system
Explain what the basin lag time is
How does interception by vegetation influence the flood hydrograph?
How do concrete roads influence the flood hydrograph?
List three human factors that effect basin lag time
List three natural factors that effect basin lag time

37. Factors that affect flood risk…
Natural & Human

38. The hydrograph – a reminder!
Key Fact: Shorter Lag Time + Steeper Rising Limb = Increased Flood Risk

39. Natural & Human Factors causing flooding:
With partner list all of the things that may increase flood risk in a drainage basin / river system…
Make one list of natural factors that may increase flood risk or cause flooding and another for human factors that may increase flood risk or cause flooding
Now try to decide what are the most important three factors in each list! – explain why each factor increases flood risk (think about hydrographs and lag time)
Share your answers with the class.

40. Natural & Human Factors causing flooding:
With partner list all of the things that may increase flood risk in a drainage basin / river system…
Make one list of natural factors that may increase flood risk or cause flooding and another for human factors that may increase flood risk or cause flooding
Now try to decide what are the most important three factors in each list! – explain why each factor increases flood risk (think about hydrographs and lag time)
Share your answers with the class.

41. Tasks:
With partner list all of the things that may influence flood risk in a drainage basin / river system…
Make one list of natural factors that may influence flooding human factors that may influence flood risk or cause flooding
Now try to decide what are the most important three factors in each list! – explain why each factor increases flood risk (think about hydrographs and lag time)
Share your answers with the class.

42. Natural Factors 1 Weather & Climate Precipitation type (Rain Vs Snow)
Length of storm in time (leading to saturation)
Intensity of rainfall (precipitation exceeds infiltration rate)
Seasonality
Temperature

43. Natural Factors 2 Surface Type & Geology Permeability of surface
Soil type
Rock Type

44. Natural Factors 3 Vegetation Type (trees, bushes, grasses)
Seasonal effects on vegetation (summer vs. winter)
Coverage (bare ground Vs Vegetation cover)
Interception & storage

45. Natural Factors 4 Relief (shape of the land) Shape of drainage basin
Size of drainage basin
Drainage density

46. Human Factors 1 Urbanization Surface permeability Drainage systems
Vegetation removal

47. Human Factors 2 Agriculture Changes to vegetation Animals
Erosion and runoff

48. Human Factors 3 Changes to the drainage basin
River management (dams, flood relief channels)
Flood prevention schemes (levees, dredging)
Straightening
Deforestation

49. Tasks 2: Now use your text book (New Wider World)
Read pages 280 – 281 to reinforce your understanding of these flood risk ideas
Now read the case study of flooding in Bangladesh p … make a mind map to represent this flood event – include all of the factors that led to this flooding… use one color for the natural factors and another color for the human factors.

50. reflection
Reflect on what you have learnt about flooding in the past two lessons
Things to mention:
Flood hydrographs and what they tell us
Natural and Human factors that influence flooding
If you have a question on this topic also write this