Human and Natural Causes of Flooding

River flooding occurs when a river exceeds the capacity of its channel and overflows onto the floodplain. Flooding is usually the result of a combination of natural factors and human activities, which affect how quickly water reaches the river channel.

Natural Causes of River Flooding

Heavy or Prolonged Rainfall

• Long periods of rainfall saturate the ground.
• When soil becomes saturated, infiltration decreases.
• More water enters the river as surface runoff, thereby increasing discharge.
• If discharge exceeds channel capacity, flooding occurs.

Intense Rainfall

• Short periods of very heavy rainfall can overwhelm drainage systems.
• Rainfall intensity may exceed infiltration rates.
• This leads to rapid surface runoff and a sudden rise in river levels.

Snowmelt

• Rapid snowmelt during warmer weather releases large volumes of water.
• If the ground is frozen, infiltration is limited.
• Meltwater flows quickly into rivers, increasing discharge.

Geology

• Impermeable rock (such as clay) prevents infiltration.
• This increases surface runoff into rivers.
• Flood risk is higher in drainage basins with impermeable geology.

Relief

• Steep slopes cause water to flow quickly downhill.
• Faster runoff reduces the time water is stored in the drainage basin.
• This leads to a rapid rise in river discharge.

Drainage Basin Characteristics

• Small or circular drainage basins respond quickly to rainfall.
• Water reaches the main river channel at similar times, increasing flood risk.

Human Causes of River Flooding

Urbanisation

• Urban areas have large areas of impermeable surfaces, such as roads and buildings.
• Infiltration is reduced.
• Surface runoff increases and reaches rivers quickly via drains.
• This causes a rapid rise in discharge and increases flood risk.

Deforestation

• Removal of vegetation reduces interception.
• Plants absorb less water.
• More water reaches the ground surface and flows into rivers.
• Surface runoff increases, raising discharge.

Farming Practices

• Soil compaction from heavy machinery reduces infiltration.
• Overgrazing removes vegetation cover.
• This increases surface runoff into rivers.

River Management

• Channel straightening increases river velocity.
• Water moves downstream more quickly.
• This can increase flood risk further downstream.
• Embankments may prevent local flooding but increase flood risk elsewhere.

Climate Change

• Climate change is increasing the frequency of extreme rainfall events.
• Heavier rainfall leads to higher peak discharge.
• Flooding is becoming more frequent and severe.

How Natural and Human Factors Combine

Flooding often occurs when several factors act together, for example:

• Heavy rainfall on saturated ground
• Urban development on floodplains
• River management that increases the downstream flow
• These combined effects can significantly increase flood risk.