Impermeable Geology: Rocks such as granite or clay prevent downward percolation of water. This naturally high runoff potential means basins with such geology are inherently more prone to flooding than those over chalk or sand.
Basin Morphology: Steep-sided valleys encourage rapid overland flow due to gravity. Similarly, circular drainage basins tend to have shorter lag times because water from all points in the basin reaches the main channel at roughly the same time.
Tectonic and Volcanic Triggers: Secondary causes include landslides blocking river channels (creating temporary dams) or volcanic heat melting sub-glacial ice, leading to catastrophic 'jökulhlaups' or glacial outburst floods.
Urbanisation: The replacement of vegetated surfaces with impermeable concrete and tarmac eliminates infiltration. Drainage systems and sewers are designed to move water to rivers as quickly as possible, which inadvertently increases peak discharge downstream.
Deforestation and Agriculture: Removing trees reduces interception and evapotranspiration, allowing more water to reach the soil. Agricultural practices like overgrazing and heavy machinery use compact the soil, reducing its porosity and increasing runoff.
Hard Engineering Mismanagement: While structures like dams and levees aim to control water, they can fail or displace the problem. Channelisation (straightening and lining rivers) increases flow velocity, which may protect one area but cause overwhelming flood peaks further downstream.
| Feature | Flash Flooding | Prolonged Flooding |
|---|---|---|
| Primary Cause | Intense, short-duration storms | Long-duration rainfall or monsoons |
| Lag Time | Very short (minutes to hours) | Longer (days to weeks) |
| Soil Condition | Often dry or overwhelmed by rate | Usually saturated to capacity |
| Predictability | Low; occurs with little warning | High; can be tracked via weather trends |
Environmental Benefits: Flooding is not purely negative; it recharges groundwater stores, replenishes soil nutrients through silt deposition, and triggers essential biological cycles for certain species.
Socio-Economic Costs: Negative impacts include the destruction of infrastructure, loss of crops leading to food insecurity, and long-term psychological trauma for displaced populations. Insurance costs often rise significantly in high-risk zones.
Hydrograph Analysis: Always look for the relationship between the rainfall bar chart and the discharge line. A narrow gap between the two indicates a high-risk, flashy catchment.
Human vs. Physical: When asked to evaluate causes, distinguish between 'primary' triggers (meteorological) and 'exacerbating' factors (human land use). A flood is rarely caused by one factor alone.
Scale of Impact: Remember that environmental impacts can be positive (nutrient cycling), whereas socio-economic impacts are almost exclusively negative in a human context.
