River Management » River Flooding Causes

What you'll learn this session

Study time: 30 minutes

Understand the physical and human causes of river flooding
Explore how climate change affects flood risk
Examine real-world case studies of major flood events
Learn about the factors that increase flood vulnerability
Discover how human activities can worsen flooding

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Introduction to River Flooding Causes

River flooding is one of the most common and devastating natural hazards affecting millions of people worldwide. Understanding why rivers flood is crucial for managing flood risk and protecting communities. Flooding occurs when a river's discharge exceeds its channel capacity, causing water to spill onto the surrounding floodplain.

Key Definitions:

Discharge: The volume of water flowing through a river channel at a given point, measured in cubic metres per second (cumecs).
Floodplain: The flat area of land beside a river that naturally floods during high water levels.
Peak discharge: The maximum amount of water flowing through a river during a flood event.
Lag time: The time delay between peak rainfall and peak discharge in a river.

🌧 Physical Causes

Natural factors that increase flood risk include heavy rainfall, snowmelt and ground conditions that affect water absorption.

🏭 Human Causes

Human activities like urbanisation, deforestation and river modifications can significantly increase flood risk.

Physical Causes of River Flooding

Physical factors are natural processes that contribute to river flooding. These factors work together to determine how quickly water reaches rivers and how much water flows through them.

🌧 Precipitation Patterns

The amount, intensity and duration of rainfall are the primary triggers for river flooding. Heavy rainfall over a short period can overwhelm river channels, whilst prolonged rainfall can saturate the ground, reducing its ability to absorb more water.

☔ Intense Rainfall

Short bursts of heavy rain can cause flash floods, especially in urban areas where surfaces are impermeable.

🌧 Prolonged Rainfall

Extended periods of moderate rain saturate the soil, increasing surface runoff and river levels.

❄ Snowmelt

Rapid melting of snow and ice can add significant volumes of water to river systems, especially in spring.

🌏 Ground Conditions

The condition of the ground affects how much rainfall can be absorbed before surface runoff begins. Saturated, frozen, or impermeable ground increases flood risk by forcing more water into rivers.

Soil saturation: When soil is already full of water, additional rainfall runs off the surface
Frozen ground: Ice prevents water absorption, increasing surface runoff
Rock type: Impermeable rocks like clay force water to flow over the surface
Vegetation cover: Plants intercept rainfall and reduce runoff through their root systems

Case Study Focus: Boscastle Flash Flood, Cornwall (2004)

On 16th August 2004, Boscastle experienced a devastating flash flood when 75mm of rain fell in just 2 hours. The steep valley sides, impermeable slate bedrock and saturated ground from previous rainfall created perfect conditions for rapid surface runoff. The narrow river channel couldn't cope with the sudden influx of water, causing severe flooding that destroyed buildings and vehicles.

Human Causes of River Flooding

Human activities have significantly altered natural drainage systems, often increasing flood risk. Understanding these human impacts is essential for effective flood management.

🏭 Urbanisation

The growth of towns and cities creates impermeable surfaces that prevent rainfall from soaking into the ground. This increases surface runoff and reduces lag time, making flash floods more likely.

🏠 Impermeable Surfaces

Concrete, tarmac and buildings prevent water absorption, forcing rainfall to flow directly into drainage systems and rivers.

🚧 Drainage Systems

Storm drains and sewers quickly channel water to rivers, reducing lag time and increasing peak discharge.

🌲 Deforestation

Removing trees and vegetation reduces interception and increases surface runoff. Trees normally absorb large amounts of water through their roots and slow down the flow of water to rivers.

Reduced interception: Fewer leaves to catch and slow rainfall
Soil erosion: Exposed soil is easily washed away, reducing water absorption
Increased runoff: Water flows more quickly across cleared land
Loss of root systems: Tree roots normally absorb groundwater and stabilise soil

🚧 River Channel Modifications

Human changes to river channels can increase flood risk downstream. Whilst these modifications might solve local problems, they often create issues elsewhere.

🚧 Channelisation

Straightening rivers increases water velocity, moving flood risk downstream to areas with natural meanders.

🏭 Embankments

Building walls along rivers can increase water levels and create catastrophic flooding if they fail.

🌊 Dams

Dam failure can release enormous volumes of water, whilst dam construction changes natural flow patterns.

Case Study Focus: River Severn Floods (2007)

The summer 2007 floods affected large areas of England, with the River Severn particularly badly hit. Gloucestershire received 120mm of rain in 24 hours - twice the normal July rainfall. Human factors worsened the flooding: urban development on floodplains, removal of natural flood storage areas and intensive farming practices that increased surface runoff. The floods caused £3 billion in damage and highlighted the need for better flood management.

Climate Change and Flooding

Climate change is altering precipitation patterns and increasing flood risk in many areas. Understanding these changes is crucial for future flood management strategies.

🌡 Changing Weather Patterns

Climate change is making extreme weather events more frequent and intense. This includes heavier rainfall events that can overwhelm drainage systems and river channels.

Increased rainfall intensity: More extreme downpours in shorter time periods
Seasonal changes: Wetter winters and more variable summer rainfall
Storm frequency: More frequent severe weather events
Temperature effects: Warmer air holds more moisture, leading to heavier rainfall

Factors Affecting Flood Risk

Several factors combine to determine how likely an area is to flood and how severe that flooding might be. Understanding these factors helps in flood risk assessment and management.

🌏 Drainage Basin Characteristics

The shape, size and slope of a drainage basin affect how quickly water reaches the main river. Steep, circular basins have shorter lag times and higher flood risk.

🌊 River Channel Features

Channel width, depth and roughness determine how much water a river can carry. Narrow, shallow channels are more likely to overflow.

📍 Location Factors

Where you are in a drainage basin affects flood risk. Areas near river confluences, at the bottom of valleys, or on floodplains face higher risks.

🌊 Confluences

Where rivers meet, combined discharge can cause flooding if channels can't cope with the increased water volume.

🌏 Valley Bottoms

Low-lying areas naturally collect water and are most vulnerable to flooding during high river levels.

🌧 Floodplains

These flat areas beside rivers are designed by nature to flood and provide natural water storage.

Case Study Focus: Cockermouth Floods, Cumbria (2009)

In November 2009, Cockermouth experienced severe flooding when the Rivers Cocker and Derwent burst their banks. The town sits at the confluence of these rivers, making it particularly vulnerable. Extreme rainfall (316mm in 24 hours) combined with steep valley sides, saturated ground and urban development on the floodplain created perfect conditions for devastating floods. The event highlighted how multiple factors combine to create flood disasters.

Managing Flood Causes

Understanding flood causes is the first step in developing effective management strategies. By addressing both physical and human factors, we can reduce flood risk and protect communities.

🌱 Natural Flood Management

Working with natural processes can help reduce flood risk whilst providing other environmental benefits. These approaches focus on slowing water flow and increasing natural storage.

Reforestation: Planting trees to increase interception and reduce runoff
Wetland restoration: Creating natural water storage areas
Floodplain reconnection: Allowing rivers to use their natural flood storage areas
Sustainable drainage: Using permeable surfaces and green infrastructure in urban areas

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