🌊 Environmental Management Study Notes: Flooding (Topic 6.3 & 6.6) 🌊

Hello future Environmental Manager! This chapter is all about understanding one of the world's most common and destructive natural hazards: flooding. We will look at what causes rivers and coastlines to overflow, the devastating impacts these events have, and, most importantly, how we can manage and reduce the risk.

Don't worry if this seems tricky at first—we will break down the causes and management strategies into simple, easy-to-remember points!

1. Defining Flooding

A flood happens when an area of land that is usually dry becomes submerged under water. This usually occurs when a river overflows its banks or when coastal defenses are breached by the sea.

Key Term:
Floodplain: The low-lying land near a river that is naturally covered by water when the river level is high. While floods are natural, human activity often makes them more dangerous.

2. Causes of River and Inland Flooding (Physical Factors)

Flooding occurs when the rate of water supply (input) exceeds the rate at which water can be removed or absorbed (output).

A. Rainfall Characteristics
  • Heavy Rainfall: A large amount of rain falls very quickly. The ground and river systems cannot cope with the sheer volume of water entering the drainage basin in a short time. This leads to very fast surface run-off.
  • Prolonged Rainfall: Even if the rain is light, if it lasts for many days, the ground becomes completely saturated (full of water). Once the soil is saturated, any new rain immediately becomes surface run-off, increasing river levels rapidly.
  • Snowmelt: In colder regions, a rapid rise in temperature can cause large amounts of snow and ice to melt very quickly. This sudden influx of melted water acts like intense, prolonged rainfall, overloading the river system.
B. Land Characteristics
  • Land Relief (Topography): Steep slopes mean water travels downhill very quickly, increasing the speed of run-off and reducing the time for infiltration (water soaking into the ground). Flat, low-lying areas, however, are more likely to be submerged once the river overflows.
  • Saturated Soil: Think of soil like a sponge. Once the sponge is completely wet (saturated), it cannot absorb any more water. Any further rainfall will flow directly over the surface, straight into rivers.
  • Compacted Soil: When soil is heavily used (e.g., by machinery or livestock), the particles are pushed closer together. This reduces the gaps (pores) in the soil, preventing water from infiltrating. The result is increased surface run-off.

Quick Review: Floods are more likely when the input (rain/snowmelt) is high and the ground's ability to absorb water (soil saturation/compaction) is low.

3. Causes of Flooding (Human Activity)

Human actions often worsen flood risk because they increase the amount of rapid surface run-off.

  • Deforestation: Trees intercept rainfall, reducing the amount that hits the ground. Their roots also absorb water and hold the soil structure together, promoting infiltration. Cutting down trees removes these natural defenses, leading to faster run-off and greater flood peaks.
  • Cultivation (Farming Practices): Ploughing fields up and down steep slopes creates small channels that direct water rapidly downwards, increasing run-off. Over-cultivation can also compact the soil.
  • Urbanisation: This is a major factor. When we build cities, we replace natural surfaces (like soil and grass) with impermeable surfaces like concrete, tarmac, and buildings.
    Analogy: Imagine putting a plastic sheet over your garden. Rain can’t soak in!

    This prevents infiltration, meaning almost 100% of rainfall becomes surface run-off, reaching the river much faster and causing a quicker, higher flood peak.

4. Causes of Coastal Flooding and Climate Change

Floods aren't just caused by rivers; the sea can also overflow coastal areas:

  • Storm Surges: These are temporary rises in sea level caused by strong winds associated with intense storms (like tropical cyclones) pushing water toward the shore. Low air pressure also contributes by 'sucking' the water level upwards.
  • Tsunamis: These are huge waves caused by underwater earthquakes or volcanic eruptions. While rare, they cause catastrophic coastal flooding.
  • Rise in Sea Level through Climate Change: Global warming causes ice caps and glaciers to melt, and seawater expands as it warms. This permanent rise in average sea level means coastal areas and low-lying islands are increasingly vulnerable to flooding, especially during high tides or minor storms.

5. The Impacts of Flooding (Syllabus 6.5)

Floods cause devastating consequences for people, the economy, and the environment.

A. Social and Health Impacts
  • Loss of life: People may drown or be killed by collapsing buildings or debris.
  • Displacement and Trauma: People lose their homes, requiring emergency shelters. This creates immense psychological stress and trauma.
  • Water-Related Disease: Floods contaminate drinking water supplies when sewage systems overflow and mix with floodwaters. This increases the risk of serious infectious bacterial diseases like typhoid and cholera.
B. Economic Impacts
  • Damage to Buildings and Infrastructure: Homes, hospitals, roads, bridges, and power lines are damaged, costing huge amounts to repair.
  • Loss of Crops and Livestock: Farmland is submerged, destroying harvests and killing farm animals, leading to food shortages and large financial losses for farmers.
  • Contamination of Water Supplies: Even after the water recedes, the water treatment plants may be damaged, leaving communities without safe (potable) drinking water.
C. Environmental Impacts
  • Habitat Loss: Terrestrial (land) habitats are destroyed, and animals may drown or be displaced.
  • Sedimentation: Floodwaters dump mud and silt over large areas, which can sometimes ruin productive farmland or smother sensitive ecosystems.

Did you know? The financial impact of a single major flood in a developed city can easily reach billions of dollars, making disaster preparedness a huge economic priority.

6. Managing the Impacts of Flooding (Syllabus 6.6)

Flood management involves strategies used before, during, and after a flood event to minimise loss and maximize recovery.

A. Monitoring and Warning Systems (Before the Flood)
  • Hydrological Monitoring: Using automatic sensors placed in rivers to constantly measure water levels and rainfall.
  • Flood Warning: Issuing immediate warnings to threatened communities (e.g., via text message, sirens, or local media) to allow for evacuation and protection of property.
B. Flood Management Techniques (Hard and Soft Engineering)

These techniques aim to control the flow and volume of water:

  • Dams and Reservoirs: (Hard Engineering) Large structures built across rivers to store excess water, releasing it slowly and preventing flooding downstream.
  • Embankments / Levees: (Hard Engineering) Raised banks built along the river to increase its capacity and contain high flows.
  • Land Use Zoning: (Soft Engineering) Regulations that restrict building houses or important infrastructure on the high-risk floodplains. Instead, the land might be used for parks or grazing, which can tolerate occasional flooding.
  • Afforestation: (Soft Engineering) Planting trees in the river basin increases interception and infiltration, slowing down run-off.
C. Understanding Flood Risk with Storm Hydrographs

A storm hydrograph is a graph showing how the river's discharge (water volume) changes over time following a rainfall event.

  • Run-off: The water flowing over the surface directly into the river.
  • Through-flow: Water flowing sideways through the soil.
  • Groundwater Flow (Base Flow): Water moving very slowly through deeper rock layers.

If the hydrograph shows a very short lag time (the time between peak rainfall and peak river discharge) and a high peak discharge, the risk of rapid flooding is high. Urbanisation usually leads to a shorter lag time!

D. Immediate and Post-Flood Strategies (During and After the Flood)
  • Shelters and Rescue: Setting up temporary emergency shelters (like schools or community halls) and having trained emergency teams ready for rescue operations.
  • Rebuilding of Damaged Areas: Clearing debris, repairing infrastructure, and ensuring new buildings meet higher flood resilience standards.
  • International Aid: Providing financial, medical, and practical assistance (such as food and temporary housing) from other countries and organisations, especially to Less Economically Developed Countries (LEDCs).

Key Takeaway: Effective flood management requires a balance between engineering solutions (dams, embankments) and sustainable solutions (land use zoning, afforestation) to reduce both the speed and volume of water reaching the river.