Combine channel and catchment evidence with rainfall and flood records; one visit cannot represent every storm.
Flood risk combines the chance of inundation with exposure and vulnerability. A river beside many homes may threaten more people than an equally flood-prone channel through unoccupied land. Define whether the investigation concerns runoff, channel capacity, exposure or a management strategy.
Map land cover, slopes, drainage connections and the location of buildings. Use safe observations of channel features and suitable secondary rainfall, river-level and flood records. Record the dates: measurements after a long dry period do not describe storm conditions.
Where teacher-approved conditions and methods allow, channel cross-sectional area and mean velocity help estimate discharge: Q = A x v. Divide an uneven channel into subsections and add their discharges. A surface float does not directly measure the mean velocity through the whole depth; method assumptions affect accuracy.
Evaluate a strategy against a stated aim: detention may reduce peak flow at one point yet leave some properties exposed during an extreme event. Compare upstream and downstream effects, storm size, maintenance and land-use changes. Use safe secondary data instead of entering water during dangerous conditions.
Step by step
Define risk and location
State the reach, catchment and people or assets being considered.
Combine scales and dates
Link channel observations to catchment land cover and historical rainfall or flood records.
Evaluate the strategy
Ask which conditions it manages, where risk remains and whether it shifts elsewhere.
Worked example: An illustrative discharge estimate
A measured cross-section has area 4 square metres and mean velocity 0.5 metres per second. Estimated discharge is 2 cubic metres per second. This describes flow at that location and time; it does not establish the channel's maximum flood capacity.
Watch out for this
No flooding during the visit means low flood risk.
A short visit may miss rare events. Use past floods, rainfall, catchment conditions, exposure and strategy limits.
Check your understanding
What most weakens a claim that a detention pond eliminated flood risk?
- The investigation uses both primary and secondary sources.
- The pond stores some runoff.
- Only one small storm was observed after construction.