Human and physical factors causing river flooding

Physical factors that influence flooding

River discharge is the volume of water that flows in a river channel and is measured in cubic metres per second. An increase in discharge causes river levels to increase. Flooding occurs when the bankfull capacity of a river is exceeded (water spills over the banks of the river). There are a range of physical factors that can lead to flooding.

Prolonged rainfall

Soil becomes saturated after prolonged rainfall. This leads to an increase in surface run-off as rainfall can no longer infiltrate the soil. This leads to more water entering the river channel increasing the likelihood of flooding.

Heavy rainfall

heavy rainfall can result in water arriving too quickly to infiltrate the soil. This increases surface run-off, leading water to reach the river channel quicker, resulting in a greater risk of flooding.

Geology

Impermeable surfaces such as clay and granite do not allow infiltration leading to greater surface run-off. The risk of flooding increases as water reaches the river channel quickly, increasing discharge and the risk of flooding.

Relief

The steeper the slope the more rapid the flow of water into a river channel, increasing the risk of flooding.

Human causes of flooding

Building

Constructing buildings and surrounding roads and paths creates impermeable surfaces which increase surface run-off. Drains also increase the flow of water into surrounding rivers, increasing the flood risk.

Deforestation

Vegetation, including trees, intercept rainfall, slowing its movement. Some of this water is stored before evaporation returns moisture to the atmosphere. Trees also absorb water from the soil allowing greater infiltration into the soil, reducing surface run-off. When vegetation is removed, infiltration and interception reduce and surface run-off increases. This leads to a greater risk of flooding as more water reaches the river channel.