Flood risk is a key issue in IB Geography, particularly within Option A: Freshwater – Drainage Basins. Flood risk refers to the likelihood and potential impact of flooding within a drainage basin. It is influenced by a combination of physical characteristics and human activities, which together determine how quickly and how much water reaches river channels during rainfall events.

One of the most important physical factors influencing flood risk is rainfall. Intense rainfall over a short period is more likely to cause flooding than gentle, prolonged rain. Heavy rainfall can exceed the infiltration capacity of soils, leading to increased surface runoff. Antecedent rainfall is also important; if the ground is already saturated, additional rain will quickly run off into rivers, increasing flood risk.

Soil type and geology play a major role. Permeable soils and rocks allow water to infiltrate and percolate, reducing surface runoff. In contrast, impermeable soils such as clay limit infiltration, causing water to flow rapidly over the surface. Drainage basins with impermeable geology tend to experience faster river response and higher flood risk.

The shape and size of a drainage basin also influence flood risk. Circular basins tend to produce higher peak discharge because water from different parts of the basin reaches the river at similar times. Elongated basins spread runoff over a longer period, reducing peak discharge. Steep slopes increase the speed of runoff, while gentle slopes slow water movement and reduce flood risk.

Vegetation cover is another key physical factor. Dense vegetation intercepts rainfall, increases infiltration, and stabilises soil, reducing surface runoff. In contrast, sparse vegetation or bare ground increases runoff and erosion. Seasonal changes in vegetation can therefore influence flood risk.

Human activities often increase flood risk. Urbanisation is one of the most significant factors. Roads, buildings, and pavements create impermeable surfaces that prevent infiltration. Storm drains quickly transfer water into rivers, shortening lag time and increasing peak discharge. This creates flashy storm hydrographs and raises flood risk.

Deforestation also increases flood risk by reducing interception and soil stability. Without tree roots to bind the soil, runoff increases and river channels may become clogged with sediment, reducing channel capacity. Agricultural practices such as soil compaction and field drainage can further increase runoff.

River management can both increase and reduce flood risk. Embankments and channelisation may protect certain areas but can increase flood risk downstream by speeding up water flow. Poorly managed floodplains reduce natural storage and increase vulnerability.

In IB Geography, strong answers recognise that flood risk is the result of interacting physical and human factors. Understanding these interactions is essential for evaluating flood management strategies.

RevisionDojo helps IB Geography students analyse flood risk clearly by linking drainage basin characteristics to river behaviour and exam-style explanations.