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.
Storm hydrograph of a flashy basin: a short lag time and high peak push discharge above bankfull, so the river floods. Mitigation aims to lower and delay this peak.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
What raises and lowers river flood risk
| Factor | Raises flood risk | Lowers flood risk |
|---|---|---|
| Rainfall | Intense storms or rapid snowmelt | Steady, light rain that soaks in |
| Relief (land shape) | Steep slopes speed run-off downhill | Gentle relief spreads and slows the flow |
| Geology & soils | Impermeable rock / saturated soil → run-off | Permeable rock and deep soil let water infiltrate |
| Basin shape | Round, compact basin concentrates flow fast | Long, thin basin spreads the flow out in time |
| Human actions | Urbanisation (concrete, drains), deforestation, floodplain building | Afforestation, zoning, washland storage, defences |
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 vs soft engineering for flood mitigation
| Approach | How it works | Strengths | Weaknesses |
|---|---|---|---|
| Dam + reservoir (hard) | Stores flood water, releases it slowly | Strong control of the peak; water + power | Very costly; silting; displaces people upstream |
| Levees / flood walls (hard) | Raise channel capacity so it carries more | Cheap-ish; protects a defined area | Can fail catastrophically; raises risk downstream |
| Channel straightening (hard) | Speeds water through quickly | Clears one reach fast | Pushes the flood peak downstream |
| Afforestation (soft) | Trees intercept rain and slow run-off | Cheap, sustainable, adds habitat | Slow to work; needs a lot of land |
| Floodplain zoning (soft) | Keeps housing off the most flood-prone land | Very cheap; removes risk at source | Hard to enforce; no use once built on |
| River restoration (soft) | Re-meandering + wetlands store water upstream | Natural storage; biodiversity gain | Limited against the very largest floods |
Key terms to use precisely
- Flood — when river discharge exceeds bankfull and water spills onto the floodplain.
- Flood risk — the probability of a flood multiplied by the harm (damage, lives) it would cause.
- Hard engineering — built structures that control the river (dams, levees, straightening, flood walls).
- Soft engineering — natural / planning approaches (afforestation, zoning, river restoration, warnings).
- Levee (embankment) — a raised bank that increases channel capacity so more discharge is carried before overtopping.
- Floodplain zoning — land-use planning that keeps housing off the most flood-prone land.
- Risk transfer — when a defence protects one reach but pushes a higher flood peak downstream.
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.
Three illustrative mitigation schemes
- A large upstream dam (hard) — a reservoir on a flood-prone river stores the surge and releases it slowly, sharply lowering the downstream peak — but at huge cost and with people displaced.
- Managed flood storage on a lowland river (soft + hard mix) — upstream washlands are deliberately flooded to hold water back, protecting the town below; cheaper and more natural, but limited against the very largest floods.
- Floodplain zoning + warnings in a delta city (soft) — zoning, raised housing, embankments and early-warning radio cut deaths even though property damage stays high — soft measures can save lives where building defences is unaffordable.
✍️ IB-style questions
For a small drainage basin: (i) identify one channel modification and explain how it can raise flood risk; (ii) identify a different channel modification and explain how it can mitigate flooding.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Evaluate the effectiveness of two or more different approaches to flood mitigation.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick self-check
Tap each card to reveal the answer.
When does a river flood? When discharge rises above bankfull, so the channel cannot hold all the water and it spills onto the floodplain.
What two kinds of factor control flood risk? Physical factors (rainfall, relief, geology, basin shape) and human factors (urbanisation, deforestation, floodplain building). Flood risk = probability × harm.
Difference between hard and soft engineering? Hard = built structures that hold or speed the water (dams, levees, straightening). Soft = working with nature and planning (afforestation, zoning, restoration, warnings) — cheaper and more sustainable but slower.
How can a levee both protect and harm? A levee raises channel capacity so the protected town floods less, but the river then carries more water faster to the next town downstream, where the flood peak is higher — risk is transferred, not removed.
What does a top [10] Evaluate answer need? Two or more contrasting measures linked to the flood peak, named schemes, a weighing of effectiveness vs cost and side-effects, and a clear judgement on which works best and where.
🎯 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).