What this lesson teaches
I can apply fieldwork to fluvial flood risk and mitigation strategies.
Syllabus 9173, Fieldwork. Students should devise geographical questions or hypotheses and follow through the fieldwork in the following areas: Fluvial flood risk and strategies to mitigate it.
Make a guess
Students visit a canal on a dry, sunny day and find the water far below the top of the banks. Does this show flood risk is low?
- Yes, because canals rarely flood.
- No. One dry visit cannot show storm conditions.
- Yes, because the water is well below the banks.
- No, because flood risk cannot be studied in the field.
Show the answer
No. One dry visit cannot show storm conditions.
Normal flow says little about how high the water rises in heavy rain. Rainfall, water-level and past flood records are also needed.
Combine evidence from the channel and catchment with rain and flood records, because one visit cannot show every storm.
Flood risk combines the chance of flooding with who or what is exposed and how vulnerable they are. A river next to many homes may threaten more people than an equally flood-prone river through empty land. Decide whether you are studying runoff, channel capacity, exposure or a management strategy.
Map land cover, slopes, drains and buildings. Safely observe the channel, and use secondary rainfall, river level and flood records. Note dates, because measurements after a dry spell do not show storm conditions.
With a teacher's approval and safe conditions, you can estimate discharge: Q = A x v, channel area times mean velocity. Split an uneven channel into sections and add them up. A surface float does not give the true mean velocity, which affects accuracy.
Judge a strategy against its aim. A detention pond may lower the peak at one point but still leave homes exposed in extreme storms. Compare effects upstream and downstream.
- Flood risk
- Chance of flooding plus exposure and vulnerability.
- Discharge
- Q = A x v: area times mean velocity.
- Safety
- Use records instead of entering dangerous water.
Step by step
Define risk and place
Which stretch, catchment and people or assets?
Combine scales and dates
Link channel data with catchment and records.
Evaluate the strategy
What does it manage, and where does risk remain?
Worked example: A discharge estimate
A measured cross-section has an area of 4 square metres and a mean velocity of 0.5 metres per second. Discharge is about 2 cubic metres per second. This shows flow at that place and time, not the channel's flood capacity.
Watch out for this
No flooding during the visit means low flood risk.
One visit may miss rare events. Use past floods, rainfall and exposure.
Check your understanding
What most weakens the claim that a detention pond removed flood risk?
- Only one small storm was observed after it was built.
- The study used both primary and secondary sources of data.
- The pond stores some of the runoff from the estate.
Show the answer
Only one small storm was observed after it was built.
Yes. One small storm cannot test extreme events.