Precipitation encompasses all forms of moisture reaching the Earth’s surface—rain, sleet, snow, and hail. The speed at which this precipitation enters a river channel determines flood risk. Surface runoff is the quickest pathway for water to reach the channel, so high runoff levels can rapidly raise the river’s discharge.
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 raises river levels. Other ways water enters a river include throughflow and groundwater flow. Flooding occurs when the bankfull capacity of a river is exceeded (water spills over the banks of the river). A range of physical and human factors affects flood risk.
What are the physical factors that affect flood risk?
Prolonged rainfall
After prolonged rainfall, the soil becomes saturated, causing an increase in surface runoff. As rainfall can no longer infiltrate the soil, more water enters the river channel, increasing the likelihood of flooding.
Heavy rainfall
Heavy rainfall can cause water to arrive too quickly for it to infiltrate the soil. This increases surface runoff, allowing water to reach the river channel more quickly and increasing the 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
Water can move swiftly downhill in regions with steep terrain, such as mountains. The steeper the slope, the more rapid the water flow into a river channel, increasing the risk of flooding.
What are the human factors that affect flood risk?
Landuse
Urban development also affects water movement. Hard, non-absorbent surfaces like concrete, asphalt, and drainage systems can speed up water flow to rivers. Plus, the scarcity of greenery means less rainwater is absorbed or evaporated. Drains also increase water flow into surrounding rivers, increasing the flood risk.
Agricultural practices can influence how water moves. For instance, fields without crops, especially during winter when soils are wet, can quickly channel water. Also, ploughing down slopes can form small channels that accelerate water flow directly to rivers.
Deforestation
Vegetation, including trees, intercepts 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 and reducing surface runoff. When vegetation is removed, infiltration and interception are reduced and surface run-off increases. Without trees, more water flows directly into rivers, increasing the risk of flooding. This increases the risk of flooding as more water reaches the river channel.
