Rivers and flooding
Learn how water reaches a river, read a storm hydrograph and compare ways to manage flooding.
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Part 1 of 3
Follow the water
A drainage basin is the area drained by a river and its tributaries. Its watershed divides it from neighbouring basins. Rainfall supplies water; interception, soil and groundwater store some of it. Water leaves through river flow and evapotranspiration.
Infiltration moves water into soil. Surface runoff travels over the land. Impermeable surfaces reduce the area available for infiltration; drainage can deliver water rapidly to a channel. Initial soil wetness and rainfall intensity also matter.
Worked example
A fictional 2 km² catchment receives 20 mm of rain. Convert 20 mm to 0.020 m and 2 km² to 2,000,000 m². Rainfall volume = 0.020 × 2,000,000 = 40,000 m³. This is rainfall input, not river discharge: some water is stored or leaves by other routes.
Common mistake: Rainfall depth is not discharge. Discharge is a volume per second, and its timing depends on the catchment.
Part 2 of 3
Read the storm response
A hydrograph plots discharge over time. Find the rainfall peak and discharge peak before measuring lag time. Use the time axis, not the apparent distance between the peaks.
A higher, earlier discharge peak can indicate faster delivery of storm water. One hydrograph cannot isolate the cause: compare rainfall, catchment size and initial conditions.
Worked example
Rainfall peaks at hour 3; discharge peaks at hour 8. Lag time = 8 − 3 = 5 hours. If discharge rises from 4 to 16 m³/s, the increase is 12 m³/s and the percentage increase is 12 ÷ 4 × 100 = 300%.
Common mistake: A discharge of 16 m³/s is 400% of 4 m³/s, but the increase is 300%.
Part 3 of 3
Choose and evaluate management
Storage, leaky barriers and floodplain space can delay runoff. Defences can reduce local exposure. Warnings and preparedness help people respond. A measure may protect one area while leaving others exposed. Some risk remains even where there is protection.
Ask how the measure performs during a flood, who it protects and what happens downstream. A defence can be overtopped; storage can fill.
Worked example
A fictional channel receives 60,000 m³ over two hours: average flow is 60,000 ÷ 7,200 = 8.33 m³/s. Spreading the same volume over four hours gives 4.17 m³/s. These averages illustrate timing; they do not calculate the actual peak of a flood.
Common mistake: A lower average does not by itself establish a lower peak. You need to know how discharge changes throughout the flood.
Try a different resource
River hydrographs
Blue bars show rainfall in the hour ending at each labelled time. The green line shows discharge at that time. Peak lag is the difference between the hours of maximum rainfall and maximum discharge.
Scroll the diagram sideways to see the full resource.
| Hour | Rainfall in preceding hour (mm) | Discharge (m³/s) |
|---|---|---|
| 0 | 0 | 5 |
| 1 | 4 | 5 |
| 2 | 11 | 5 |
| 3 | 6 | 5 |
| 4 | 0 | 7 |
| 5 | 0 | 13 |
| 6 | 0 | 19 |
| 7 | 0 | 13 |
| 8 | 0 | 7 |
| 9 | 0 | 5 |
| 10 | 0 | 5 |
| 11 | 0 | 5 |
| 12 | 0 | 5 |
Resource description
Hour 0: 0 mm rainfall, 5 m³/s discharge; Hour 1: 4 mm rainfall, 5 m³/s discharge; Hour 2: 11 mm rainfall, 5 m³/s discharge; Hour 3: 6 mm rainfall, 5 m³/s discharge; Hour 4: 0 mm rainfall, 7 m³/s discharge; Hour 5: 0 mm rainfall, 13 m³/s discharge; Hour 6: 0 mm rainfall, 19 m³/s discharge; Hour 7: 0 mm rainfall, 13 m³/s discharge; Hour 8: 0 mm rainfall, 7 m³/s discharge; Hour 9: 0 mm rainfall, 5 m³/s discharge; Hour 10: 0 mm rainfall, 5 m³/s discharge; Hour 11: 0 mm rainfall, 5 m³/s discharge; Hour 12: 0 mm rainfall, 5 m³/s discharge
Try the model
Case studies
Slowing the flow at Pickering · Somerset floods, 2013–14
These pages provide selected evidence. Check the whole case requirement with your teacher.
Read further
- www.usgs.gov — source or course reference
- www.sqa.org.uk — source or course reference
- www.sqa.org.uk — source or course reference
- qualifications.pearson.com — source or course reference
- www.ocr.org.uk — source or course reference
- www.ocr.org.uk — source or course reference
- www.eduqas.co.uk — source or course reference
- www.eduqas.co.uk — source or course reference
Drainage basins as systems · Flood management and residual risk
Course links
AQA GCSE Geography · Pearson Edexcel GCSE Geography B · National 5 Geography · Higher Geography · Pearson Edexcel GCSE Geography A · OCR GCSE Geography A · OCR GCSE Geography B · Eduqas GCSE Geography A · Eduqas GCSE Geography B
Next topics
Water cycles: budgets, residence time and feedback · Geographical enquiry: from question to justified conclusion
Selected teaching sequence; linked cases cover bounded interventions rather than complete course cases.
Scottish links identify reusable parts of the topic. Scottish landforms, soil, weather and disease requirements need additional course-specific teaching.