The Living World
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Rivers and Flooding

A river flood occurs when a river overflows its banks and water spreads onto nearby land areas. This happens when the volume of water in the river exceeds its capacity.

The factors influencing flood risk are varied and include precipitation, geology, relief and land use. Understanding the impact of each factor is essential to understand how external processes lead to potentially catastrophic flooding.

Precipitation

Precipitation refers to any form of water (liquid or solid) falling from clouds and reaching the ground. This includes various forms like rain, snow, sleet, and hail.

The release of water in various forms can impact a river in different ways.

Rainfall Intensity: Heavy or prolonged rainfall can overwhelm rivers, causing them to exceed capacity and flood nearby areas.

Rainfall Duration: Persistent or near constant rainfall over an extended period saturates the ground, increasing river runoff and elevating flood risk.

Seasonal Patterns: Seasonal variations in rainfall, such as monsoons or wet seasons, can significantly impact river levels and the likelihood of floods.

Geology

The geology of an area can significantly influence the likelihood of flooding. Depending on certain factors, geology can help an area resist flooding.

Permeability: The ability of rocks and soil to absorb water affects how quickly precipitation infiltrates the ground or runs into rivers. If the ground is permeable, that means it allows water to infiltrate, reducing surface runoff. The more permeable the ground is, the more it will resist heavy levels of precipitation to prevent flooding.

Impermeable Layers: Impermeable rock layers or dense clay soils slow water infiltration. This increases surface runoff and the likelihood of floods. When rocks and soil are impermeable then the precipitation will follow gravity to its lowest point, which can be a river. If it’s heavy rain, then this can lead to rapid flooding.

Aquifers and Springs: Aquifers are permeable rocks that can contain or transmit water, a spring serves as an exit point where groundwater emerges to become surface water. Underground water sources can contribute to flooding when they become overwhelmed and discharge into rivers.

Relief

Relief refers to the variations in elevation of the ground surface. This includes the hills, mountains, cliffs and slopes that make up the varied elevation of an area. The relief of an area can have a significant effect on whether a river floods or not. 

  • Steep Slopes: Mountainous or hilly areas experience faster runoff as water accelerates down slopes, increasing the risk of flash floods.
  • Narrow Valleys: Rivers flowing through narrow valleys or gorges can increase flood risk as water accumulates and rises rapidly in narrow spaces.

Land Use

How humans use the land can also have an impact on whether a river floods. Human activity often means that an area’s geography changes dramatically, which leads to natural processes being disrupted. A large consequence of this is an increased risk of flooding.

  • Urbanisation: The expansion of cities and towns often involves replacing natural, absorbent surfaces with impermeable materials like concrete and asphalt. This leads to an increase in surface runoff and flash flooding.
  • Deforestation: The removal of forests reduces the interception of rainfall, increases runoff, and can contribute to soil erosion. This can affect river channels and increase flood risk.
  • Agricultural Practices: Poor land management techniques, such as excessive irrigation or inadequate soil conservation, can increase runoff and sedimentation in rivers. This increases the potential of floods.
  • Channel Modifications: Changes to river channels, such as straightening or narrowing, can impact water flow dynamics and potentially increase flood risk.

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