Causes and effects of flooding (Edexcel GCSE Geography A): Revision Notes
Causes and effects of flooding
What causes river flooding?
River flooding happens when water levels in a river channel rise above the normal bank level, causing water to spill onto the surrounding floodplain. This occurs due to a combination of physical and human factors that increase the amount of water entering the river system.
Understanding flooding requires examining both natural processes and human activities. While physical factors like intense rainfall create the basic conditions for flooding, human activities can significantly worsen flood risk by altering natural water flow patterns.
Physical causes of flooding
Physical factors are natural processes that control how water moves through the landscape and into river systems. These factors determine the baseline flood risk for any given area.
Rainfall intensity
When there is heavy rainfall over a short period of time, the ground becomes saturated and cannot absorb any more water through infiltration. This means that excess water flows over the surface as overland flow (also called surface run-off) directly into rivers and streams. The more intense the rainfall, the greater the volume of water entering the river system rapidly.
Critical Concept: When rainfall intensity exceeds the soil's infiltration capacity, all additional water becomes surface run-off. This dramatically increases flood risk because water reaches rivers much faster than if it had soaked into the ground first.
Geology
The type of rock beneath the ground significantly affects flooding risk. Impermeable rocks such as granite prevent water from seeping through them. When rain falls on areas with impermeable geology, water cannot percolate downwards through the rock layers. Instead, it remains on or near the surface and flows quickly towards rivers as surface run-off, increasing flood risk.
Worked Example: How Geology Affects Flooding
Scenario A - Permeable rock (limestone):
- Rain falls on limestone area
- Water infiltrates into soil
- Water percolates through rock layers
- Slow groundwater flow to rivers
- Result: Lower flood risk
Scenario B - Impermeable rock (granite):
- Rain falls on granite area
- Limited infiltration occurs
- Water cannot percolate through rock
- Rapid surface run-off to rivers
- Result: Higher flood risk
Snowmelt
During spring, warmer temperatures cause accumulated snow and ice to melt rapidly. This sudden release of large volumes of water can overwhelm river systems, particularly when combined with spring rainfall. Areas that experience significant snowfall during winter are especially vulnerable to flooding during the melting season.
Drainage basin characteristics
The shape and features of a drainage basin influence how quickly water reaches the main river channel. Steep-sided valleys channel water rapidly downhill, reducing the time it takes for surface run-off to reach rivers. Similarly, areas with many tributaries (smaller streams that join the main river) can quickly funnel water from across the catchment area into the river system.
Drainage Basin Factors That Increase Flood Risk:
- Steep valley sides that speed up water flow
- High drainage density (many tributaries per unit area)
- Circular basin shape that concentrates water flow
- Impermeable valley floors that prevent water absorption
Human causes of flooding
Human activities can significantly increase flood risk by affecting how water moves through the landscape. Many human impacts result in more water flowing towards rivers as surface run-off rather than being absorbed naturally by the ground. This additional water reaches river channels more quickly, forcing water levels to rise above normal bank levels and potentially causing flooding.
Key Human Impact: Most human activities that increase flood risk work by reducing natural infiltration and increasing surface run-off. This creates a "double effect" - less water soaks into the ground AND more water flows directly to rivers.
Effects of flooding
Flooding creates both environmental and social impacts that can be devastating for affected areas. The consequences of flooding extend far beyond the immediate flood event, often causing long-term damage to both natural and human systems.
Physical effects
The environmental consequences of flooding include landslides on steep slopes where saturated soil becomes unstable and collapses. Soil erosion occurs as fast-flowing floodwater carries away fertile topsoil, damaging agricultural land and natural ecosystems. Natural habitats can be destroyed or severely damaged by the force of floodwater, affecting wildlife populations. Additionally, floodwater often becomes contaminated with sewage, chemicals, and debris, polluting water supplies and making them unsafe for consumption.
Environmental damage from flooding can take years or even decades to fully recover. Topsoil erosion is particularly problematic because it takes hundreds of years for natural processes to create new fertile soil layers.
Human effects
The social and economic impacts of flooding can be severe and long-lasting. Loss of life and the spread of waterborne diseases pose immediate threats to human health and safety. Property damage affects homes, businesses, and infrastructure, leading to costly insurance claims and reconstruction efforts. Many people lose their livelihoods when businesses are forced to close or when agricultural land and livestock are damaged. Transportation networks become disrupted when roads, railways, and bridges are damaged or blocked by floodwater, affecting emergency services and economic activity.
Cascading Effects: Flooding often creates a chain reaction of problems. Initial property damage leads to business closures, which cause job losses, which reduce local spending, which further impacts the local economy. This multiplier effect can make recovery much slower and more difficult.
Understanding flood hydrographs
A flood hydrograph (sometimes called a storm hydrograph) is a graph that shows how a river responds to a rainstorm over time. These graphs are essential tools for understanding flood risk and predicting when flooding might occur.
The graph displays two types of information: bar charts show the amount of rainfall measured in millimetres, while a line graph shows river discharge measured in cubic metres per second. The time scale along the bottom shows hours from the start of the rainstorm.
Reading Flood Hydrographs:
- The bars show rainfall input to the system
- The line shows the river's response (discharge output)
- The gap between peak rainfall and peak discharge reveals important information about flood risk
Lag time and flood risk
Lag time refers to the time difference between the peak of the rainstorm and the peak of the river discharge. This measurement is crucial for understanding flood risk because it indicates how quickly a river responds to rainfall.
Areas with shorter lag times face greater flood risk because water reaches the river channel rapidly, causing sudden increases in discharge. Steeper rising limbs on the hydrograph also indicate rapid increases in river levels. When lag time is short and the rising limb is steep, there is less time for flood warnings and emergency preparations, making flooding more dangerous.
Critical Warning Signs on Hydrographs:
- Lag time less than 6 hours = High flood risk
- Very steep rising limb = Rapid onset flooding
- High peak discharge = Potential for severe flooding
- These factors combined create extremely dangerous flood conditions
Key Points to Remember:
Physical causes include:
- Intense rainfall preventing infiltration
- Impermeable rocks stopping percolation
- Spring snowmelt releasing large water volumes
- Steep drainage basins with many tributaries
Human activities:
- Increase surface run-off
- Cause more water to reach rivers quickly
- Raise overall flood risk through landscape changes
Physical effects:
- Environmental damage through landslides and soil erosion
- Habitat destruction and ecosystem disruption
- Water contamination from pollutants and debris
Human effects:
- Immediate threats: death, disease, and injury
- Economic impacts: property damage and livelihood losses
- Infrastructure disruption affecting transport and services
Flood hydrographs:
- Shorter lag times indicate higher flood risk
- Steeper rising limbs show rapid river response to rainfall
- Both factors reduce warning time and increase danger