1. What are floods and what causes them?
Specification Focus:
GEOG1 – Physical and human causes of flooding – two c/s of recent flooding events should be undertaken from contrasting areas of the world.
Learning Objectives:
To list and explain the causes of flooding by referring to the storm hydrograph and the water budge.

2. What is flooding?
Flooding occurs when a river’s discharge exceeds the capacity of its channel to carry that discharge and the river over flows its banks.
Various factors interact to cause flooding…
The causes of flooding can be divided into 3 categories:
Climatic (weather and climate – related causes)
Human (caused by people)
Physical (caused by features of the land)
Watch the clip about the flooding of the River Tay in what do you think were the main causes of this flood?

3. Natural Event or Natural Hazard?
If human and physical systems do not interact there is no hazard or disaster
e.g. a volcanic eruption on a remote unpopulated island is simply a natural event
A natural hazard is a natural event that threatens life and property
A disaster is the realisation of that hazard
e.g. flooding in a densely populated area

4. STARTER:
Make a copy of the Venn diagram below and write each cause of flooding in the correct place
Heavy rainfall
Rapid snowmelt
Climatic
Human
Dam failure
Saturated soil
Deforestation
Urbanisation
Arable farming involving ploughing
Dry baked soil
Steep slope
Impermeable rock
Extension: Can you think of 3 more causes to add to your diagram?
Physical

5. Physical Causes of FLOODING
Excessive levels of precipitation over a long time reduces infiltration capacity

6. Physical Causes of FLOODING
Intense precipitation over a short time in areas where the ground is baked hard from the sun

7. Physical Causes of FLOODING
Rapid snow melt when the soil is still frozen

8. Physical Causes of FLOODING
Climatic hazards e.g. cyclones in Bangladesh, hurricanes in the Gulf of Mexico bring abnormal amounts of precipitation.

9. Physical Causes of FLOODING
Nature of drainage basin – some are more likely to flood than others (remember the factors that cause flashy or delayed regimes)

10. Human and Climatic Causes of FLOODING
Urbanisation
Deforestation
River Management
Climate Change
Your task is to create an illustrated A4 poster to show how your factor cases flooding. You could do this as a cartoon strip or as a series of annotated sketches, or simply pictures with bullet pointed information underneath. Try to be creative and make it interesting! You should do it by hand but you can print out and stick in pictures from the internet if you’re really unsure how to draw something.
Once finished you should share your posters and explain to them how your factor causes flooding.
Think about how these factors can vary over time, space and scale, and think about which ones are most/least common/important. E.g. a hurricane vs urbanisation. Make sure they help each other out as there will be no teacher to help them.

11. Deforestation
Reduces interception and evapotranspiration, which increases run-off.
Loss of root systems may cause the soil to erode more rapidly.
Possible decrease in channel capacity due to increase in sediment carried.

12. Urbanisation
Highly impermeable surfaces, such as roads, roofs and pavements.
Some river channels may be straightened, leading to fast delivery of water downstream.
Smooth surfaces drained by a network of drains, gutters and sewers.
Natural river channels restricted by bridges or roadside facilities.

13. Coastal Influences High seasonal tides affect areas close to the sea.
Storm surges and tropical cyclones can result in high water levels in rivers and low-lying coastal areas.
Insufficient planning control allowing high density building along coastlines.

14. Climatological Intense precipitation reduces infiltration capacity.
Sudden increases in temperature can induce snowmelt, which can be made worse if frozen ground exists lower down, thus reducing infiltration rates.
Prolonged periods of rainfall produces saturated soil and increases overland flow.

15. Land Management Techniques/ Agriculture
Overgrazing can lead to less vegetation and can also compact the soil, increasing run-off.
Planning strategies may allow for building on low lying land prone to flooding.
Drainage systems, ditches and ploughing all reduce the lagtime for the water reaching the river channel.
Ploughing up and down a slope rather than along the contour increases soil erosion, which increases the rivers load.

16. Nature of the basin Infiltration rate depends on rock and soil type.
Shape of the basin, relief/gradient and vegetation cover affects how quickly runoff reaches the channel.

17. Homework: Notes and questions on pages 24-27
What have we learnt so far…?
 What caused the River Tay to flood in 1993?
Give 5 physical causes of flooding?
What are the other 2 causes of flooding apart from physical?
What type of floods tend to occur in very hot climates and why?
What’s the difference between a natural event and a natural hazard?
What’s the difference between a hazard and a disaster?
Homework: Notes and questions on pages 24-27

18. Why does Bangladesh flood?
Flooding in Bangladesh
Why does Bangladesh flood?
Split answers into Physical and Human causes
Bangladesh is a delta. The low level of the delta land means that the country is prone to flooding. 75% of Bangladesh is at or below just 10 metres in height.
Bangladesh also suffers regularly from substantial flooding caused by monsoon rains and melting snows from the Himalayas.
Source: BBC
Freshwater floods occur when a watershed receives so much water that it cannot drain into the soil quickly enough to take the water away. Bangladesh has three mighty rivers, the Ganges, the Jamuna and the Meghna.
Most floods follow heavy rain or melting snow, frozen ground and already high river levels. The floods in Bangladesh begin through a combination of heavy monsoon rains flooding the rivers and abnormally high tides in the Bay of Bengal preventing floodwater from running off the land and into the sea.
The likelihood of flooding can be increased by human activities too, such as too many trees being cut down, stripping the land bare of the vital top soil which slows and drains water. Also, In built-up areas with a lot of concrete roads and houses, there are fewer places for water to go and less soil for water to drain into. So planners in cities prone to flooding have to develop ways of channelling water to cope with potential floods.
Plenary - Get students to draw a table with human and physical causes of flooding and sort the text under the 2 headings. Or write a paragraph about each one.

19. Decreases the amount of infiltration.

20. Flooding caused by weather.

21. Because the steepness decreases the amount of infiltration and speeds up surface runoff.

22. Is the increase of the population living in urban areas.

23. By high levels of precipitation combined with snowmelt.