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This lesson examines the causes, types and impacts of flooding at different scales, using case studies from contrasting global contexts. It addresses Edexcel A-Level Geography (9GE0) Paper 1, Topic 5, Enquiry Question 2, focusing on how flooding occurs within the hydrological system and its consequences for people and the environment.
A flood occurs when water overflows onto land that is not normally submerged. Flooding is a natural part of the hydrological cycle — floodplains are, by definition, areas designed by natural processes to accommodate excess water. However, human activities and settlement on floodplains have made flooding one of the most costly and deadly natural hazards.
| Type | Description | Typical Duration |
|---|---|---|
| Fluvial (river) flood | River discharge exceeds channel capacity; water spills onto the floodplain | Days to weeks |
| Pluvial (surface water) flood | Intense rainfall overwhelms drainage capacity in urban areas; no river involved | Hours |
| Coastal flood | Storm surge, high tides and/or wave overtopping inundate coastal lowlands | Hours to days |
| Flash flood | Extremely rapid onset flooding, typically in steep, small catchments or urban areas | Minutes to hours |
| Groundwater flood | Water table rises above ground level, usually in chalk/limestone areas after prolonged wet periods | Weeks to months |
| Cause | Mechanism | Example |
|---|---|---|
| Intense rainfall | Rainfall intensity exceeds infiltration capacity → overland flow → rapid channel response | Boscastle 2004: 75 mm fell in 2 hours (200-year event) |
| Prolonged rainfall | Sustained rainfall saturates soil → all subsequent rain runs off → rivers rise steadily | UK winter 2013–14: wettest December–February in 250 years of records |
| Snowmelt | Rapid warming after heavy snowfall releases large volumes of meltwater | European floods, March 2006 (Danube, Elbe) |
| Convective storms | Intense, localised thunderstorms produce extreme rainfall rates (>100 mm/hr) | Toll Bar, South Yorkshire, 2007: 90 mm in 90 minutes |
| Tropical cyclones | Extreme precipitation + storm surge. Hurricane Harvey (2017) dropped 1,539 mm of rain on southeast Texas over 4 days | Houston flooding 2017: 203,000 homes damaged |
| Atmospheric rivers | Narrow corridors of concentrated moisture transported from the tropics. Can deliver rainfall equivalent to 7–15 average-size rivers. | California, January 2023 |
Human activities significantly increase flood risk by modifying drainage basin characteristics.
| Human Factor | How It Increases Flood Risk |
|---|---|
| Urbanisation | Impermeable surfaces increase surface runoff by 200–500%; storm drains speed up delivery to channels; lag time reduced |
| Deforestation | Removes interception (25–40% of rainfall); reduces transpiration; soil degradation reduces infiltration → increased and faster runoff |
| Channel modification | Straightening channels increases velocity, transferring flood peak downstream faster; bank reinforcement prevents natural floodplain storage |
| Floodplain development | Building on floodplains reduces natural storage capacity and puts people and property at risk |
| Agricultural drainage | Field drains speed up water transfer from fields to rivers; removes natural wetland storage |
| Climate change | Intensification of hydrological cycle: more extreme rainfall events; ~7% more moisture per °C of warming; rising sea levels increase coastal flood risk |
| Dam failure | Catastrophic release of stored water (e.g. Banqiao Dam, China, 1975 — killed ~171,000 people) |
Exam Tip: When explaining human causes of flooding, always link the activity to a specific hydrological process (e.g., urbanisation reduces infiltration, which increases surface runoff, which increases peak discharge and reduces lag time). This process-based reasoning is essential for top marks.
| Category | Impact |
|---|---|
| Area flooded | ~37% of the country's area submerged at peak inundation (July 2020) |
| Displacement | 5.4 million people displaced; 2.4 million homes damaged or destroyed |
| Deaths | ~270 deaths (relatively low due to improved early warning systems) |
| Agriculture | 1.5 million acres of cropland submerged; rice harvest losses estimated at $500 million |
| Infrastructure | 5,500 km of roads damaged; 300+ bridges destroyed |
| Health | Waterborne diseases (cholera, diarrhoea, typhoid) increased; 1.5 million people lacked access to clean water |
| Education | 4,000+ schools damaged; millions of school days lost |
| Economy | Total damage estimated at $12 billion (~5% of GDP) |
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