Factors affecting flood risk

Physical factors:
- Heavy precipitation: exceeds infiltration capacity, causing surface runoff.
- Prolonged precipitation: saturates the soil, reducing infiltration and increasing runoff.
- Sudden snowmelt or storms: can cause flash flooding
- Geology: impermeable rocks (e.g. granite or clay) prevent infiltration and increase runoff, while permeable rocks (e.g. sandstone or chalk) allow water to soak in, reducing flood risk.
- Steep slopes: cause water to flow rapidly downhill reducing infiltration time and increasing flood risk.
- Vegetation: dense flora increases interception and transpiration, reducing runoff, whereas sparse or absent vegetation leads to erosion and faster surface flow.
Human factors:
- Land use: bare soil from agriculture or deforestation transfers water quickly, and ploughing slopes create tunnels (streams downhill).
- Urbanisation: lack of vegetation and impermeable surfaces (e.g. concrete, tarmac) results in overland flow and little absorption.
- River management: channel straightening, levees, embankments, dams, and reservoirs can all help manage or divert flood risks.
- Climate change: leads to more frequent and intense storms, increasing rainfall and wetter seasons, increasing the likelihood of floods.

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