Flood Risk Factors (Physical & Human)

Physical Flood Risk Factors

Physical factors are natural features that increase the risk of flooding in a river landscape. Understanding these helps explain why some areas flood more often or more severely than others.

Heavy Rainfall

Intense or prolonged rainfall increases flood risk because the river and surrounding land cannot absorb or carry away all the water quickly enough. This causes surface runoff to increase, raising river levels rapidly and potentially causing flooding.

For example, during storms or in autumn and winter, heavy rainfall is common in the UK uplands, increasing flood risk downstream.

River Channel Characteristics

The shape, size, and roughness of a river channel affect how quickly water flows:

• Narrow, steep channels speed up water flow, increasing flood risk downstream.
• Wide, shallow channels slow water flow, reducing flood risk.
• Channel roughness (e.g., presence of vegetation or debris) slows water, reducing flood risk.

Soil Type and Saturation

Soils vary in permeability:

• Sandy or gravelly soils are permeable, allowing water to soak in, reducing runoff and flood risk.
• Clay soils are impermeable, so water runs off the surface, increasing flood risk.
• Saturated soils (already holding maximum water) cannot absorb more, so excess water flows into rivers quickly, increasing flood risk.

Relief and Topography

The shape and steepness of the land affect flood risk:

• Steep slopes cause water to run off quickly into rivers, increasing flood risk.
• Flat land slows runoff, reducing flood risk but may cause water to pool and flood locally.
• Upland areas with steep relief, such as the Pennines, often experience rapid runoff and flash flooding.

For instance, if an area with steep slopes and clay soil receives heavy rain, the water will run off quickly into rivers, increasing flood risk downstream.

Human Flood Risk Factors

Human activities can increase flood risk by changing how water moves through the landscape or by increasing the number of people and properties in flood-prone areas.

Urbanisation and Impermeable Surfaces

Building towns and cities creates many impermeable surfaces like roads, pavements, and roofs. These surfaces prevent water soaking into the ground, increasing surface runoff and causing rivers to rise faster after rain.

Urban drainage systems often channel water quickly into rivers, increasing flood peaks.

Deforestation and Land Use Change

Removing trees reduces interception and evapotranspiration, so more rain reaches the ground and runs off into rivers. This increases flood risk.

Changing land use to agriculture or building on natural floodplains also reduces the lands ability to absorb water.

Floodplain Development

Building on floodplains increases flood risk because:

• Natural flood storage areas are reduced.
• More people and property are at risk of flooding.
• Floodwaters may be forced into smaller channels or diverted, increasing flood severity elsewhere.

Drainage and River Modification

Humans often modify rivers by straightening channels or building embankments to protect land or speed up drainage. While this can reduce flooding locally, it often increases flood risk downstream by moving water faster.

Drainage of wetlands or marshes for farming removes natural water storage, increasing flood risk.

Interactions of Physical and Human Factors

Physical and human factors often interact, affecting flood risk and human settlement patterns.

How Physical Factors Influence Human Settlement

People often settle near rivers for water supply, fertile land, and transport. However, physical flood risk factors like flat floodplains and heavy rainfall areas mean these settlements face flood risks.

For example, many UK towns are on floodplains because of fertile soil and easy access, despite flood risk.

Impact of Human Activities on Flood Severity

Human activities can worsen natural flood risks:

• Urbanisation increases runoff and flood peaks.
• Deforestation reduces interception, increasing runoff.
• Building on floodplains reduces natural flood storage.
• River modifications can speed water flow downstream, increasing flood severity.

Examples of Combined Effects in UK Rivers

The River Severn often floods due to heavy rainfall in upland areas (physical factor) combined with urban development in floodplains (human factor). This leads to rapid runoff and increased flood risk in towns like Shrewsbury.

Similarly, the River Thames floods more easily where urban areas like London have many impermeable surfaces, increasing flood peaks after rainfall.