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Flooding is one of the most significant natural hazards in the UK. Understanding why rivers flood and what happens when they do is essential for managing flood risk and protecting people and property. This lesson examines the physical and human causes of flooding, the effects on people and the environment, and how hydrographs are used to analyse flood events.
A flood occurs when a river's discharge exceeds the capacity of its channel, causing water to overflow onto the surrounding floodplain. Floods can be caused by natural processes, human activity, or (most commonly) a combination of both.
| Cause | How It Increases Flood Risk |
|---|---|
| Prolonged rainfall | Soil becomes saturated — it cannot absorb any more water. Additional rainfall flows over the surface as surface run-off directly into rivers, rapidly increasing discharge. |
| Intense rainfall | Very heavy rain falls faster than the soil can absorb it, even if the ground was previously dry. Surface run-off is high. |
| Snowmelt | A rapid thaw following heavy snowfall releases large volumes of water into rivers over a short period. |
| Impermeable rock | Rocks like granite and clay do not allow water to pass through. Rainfall runs off the surface quickly rather than soaking in, leading to rapid increases in river discharge. |
| Steep slopes | Water runs off steep slopes quickly, reaching the river channel in a short time. This produces a rapid rise in discharge. |
| Saturated or frozen ground | If the ground is already waterlogged (from previous rain) or frozen, it cannot absorb more water. Run-off is increased. |
| Thin soils | Thin, rocky soils in upland areas absorb less water, increasing run-off. |
| Drainage basin shape | A circular drainage basin delivers water to the river from all directions simultaneously, producing a rapid peak in discharge. An elongated basin delivers water more gradually. |
| High drainage density | Many tributaries funnel water into the main river quickly, increasing discharge. |
| Tidal surges | In coastal and estuarine areas, a high tide combined with low atmospheric pressure and onshore winds can push seawater upstream, blocking river outflow and causing flooding. |
Human activities have significantly increased flood risk in many parts of the UK.
| Cause | How It Increases Flood Risk |
|---|---|
| Urbanisation | Buildings, roads, car parks, and pavements create impermeable surfaces that prevent infiltration. Rainwater runs rapidly into drains and then into rivers. Urban areas can produce run-off rates 5-10 times higher than rural areas. |
| Deforestation | Trees intercept rainfall on their leaves (interception), absorb water through their roots (uptake), and release it gradually through transpiration. Removing trees removes these natural buffers, increasing the speed and volume of run-off. |
| Agricultural practices | Ploughing compacts subsoil, reducing infiltration. Draining wetlands removes natural water storage. Overgrazing compacts soil. |
| Channel straightening | Artificially straightening a river channel speeds up the flow, delivering flood water downstream faster. This may protect the straightened area but increases flood risk downstream. |
| Building on floodplains | Floodplains are naturally designed to flood — they act as natural storage for excess water. Building on them reduces storage capacity and puts people and property at risk. |
| Climate change | Increasing global temperatures are leading to more intense rainfall events in the UK, increased winter precipitation, and potentially more frequent storms — all increasing flood risk. |
| Drains and sewers | Urban drainage systems channel water directly into rivers, bypassing the natural infiltration process and reducing lag time. During extreme events, the drainage system can be overwhelmed, causing surface water flooding. |
Exam Tip: In exam questions about flood causes, the best answers link physical and human causes together. For example: "The flood was caused by intense rainfall (physical) falling on an area that had been heavily urbanised (human), meaning the impermeable surfaces prevented infiltration and dramatically increased surface run-off, leading to a rapid rise in river discharge."
A flood hydrograph (also called a storm hydrograph) is a graph that shows how a river's discharge changes over time during and after a rainfall event. It is one of the most important diagrams in GCSE Geography.
| Component | Description |
|---|---|
| Rainfall bar chart | Shown at the top of the graph (often as blue bars); shows the amount and timing of rainfall |
| Discharge line | The curve showing river discharge (m³/s) over time |
| Base flow | The normal, day-to-day discharge of the river before the storm (fed by groundwater) |
| Rising limb | The part of the curve where discharge is increasing as rainwater reaches the river |
| Peak discharge | The maximum discharge reached during the flood event |
| Falling limb (recession limb) | The part of the curve where discharge is decreasing as the flood subsides |
| Lag time | The time delay between the peak rainfall and the peak discharge — this is the most important measurement on the graph |
| Lag Time | Meaning | Likely Conditions |
|---|---|---|
| Short lag time | Water reaches the river quickly → high flood risk | Impermeable surfaces, steep slopes, urbanisation, deforestation, intense rainfall |
| Long lag time | Water reaches the river slowly → lower flood risk | Permeable rock, gentle slopes, vegetation, rural land use, less intense rainfall |
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