Key Idea: Topic 7.2 is about why rivers flood and how people try to reduce the damage. It pulls together three strands of micro 7.2.1: Why rivers flood: a flood is discharge above bankfull, and flood risk (probability × harm) is driven by both physical factors (rainfall, relief, geology, basin shape) and human factors (urbanisation, deforestation, building on floodplains). Hard vs soft engineering: hard approaches build structures to hold or speed the water (dams, levees, straightening); soft approaches work with nature and planning (afforestation, zoning, restoration, warnings). Judging mitigation: the best strategy lowers the flood peak or keeps people out of harm, and is judged across the whole basin — hard defences can transfer risk downstream. This is Option A (Freshwater) content, examined on Paper 1. SL students answer 2 options, HL 3 (same questions). Each option has a short structured part plus a [10] extended-answer essay marked on markbands.
🌊 Why rivers flood — physical and human factors
A river floods when discharge rises above bankfull — the channel cannot hold all the water, so it spills onto the floodplain. How likely and how damaging that is — the flood risk — depends on the natural setting AND on what people have done to the basin. The skill examiners test is reading a storm hydrograph (discharge against time after a storm): note the lag time and the peak discharge first, then explain the flood risk with one physical and one human factor.
[Diagram: geo-storm-hydrograph]
Tip: For a hydrograph question, read the figure first — quote the peak discharge with units and note the lag time. A short lag + high peak means a flashy basin and high flood risk. Then explain it with one physical factor (e.g. impermeable geology) and one human factor (e.g. urbanisation).
🏗️ Mitigation — hard vs soft engineering
Flood mitigation means the methods used to reduce flood risk — either by lowering the flood peak (storing or slowing water) or by keeping people and property out of harm's way (zoning, warnings). The two families are hard and soft engineering.
Hard defences often transfer risk. A levee that protects one town stops the river spreading onto that floodplain, so it carries more water faster to the next town downstream, where the flood peak is now higher. This is why examiners reward answers that judge a scheme across the whole basin, not just the protected reach.
🗺️ Real schemes to use in essays
A [10] mitigation essay needs named schemes as evidence. Keep three illustrative cases ready — one mostly hard, one mostly soft, and a mixed approach — so you can compare effectiveness, cost and side-effects.
✍️ IB-style questions
✅ Quick self-check
Tap each card to reveal the answer.
🎯 Highest-yield exam reminders
Exam Tips
- A flood = discharge above bankfull; flood risk = probability × harm.
- On a hydrograph: read the PEAK (with units) and the LAG TIME first — short lag + high peak = a flashy, high-risk basin.
- Explain a channel modification by linking it to the FLOOD PEAK / overtopping — never just name the structure.
- Hard engineering controls the peak but is costly and can TRANSFER risk downstream; soft engineering is cheaper and sustainable but slower.
- For the [10] essay, bring TWO+ contrasting measures plus a NAMED scheme and judge across the whole basin.
- Evaluate / To what extent must reach a clear JUDGEMENT — say which measure works best and under what conditions (city vs rural, rich vs poor, big vs small flood).