6.3 Flooding

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe and explain the causes of flooding
  2. Apply knowledge of specific causes including: heavy rainfall, prolonged rainfall, snowmelt; land relief; saturated soil, compacted soil; deforestation, cultivation and urbanisation; storm surges, tsunamis; rise in sea level through climate change

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What is Flooding?

A flood happens when water covers land that is normally dry. This occurs when more water enters an area than can drain away naturally. Floods can happen near rivers, along coastlines, or even in cities. Understanding why floods happen is the first step to managing them.

Causes of Flooding

The causes of flooding can be grouped into two main categories: rainfall and water input causes and land-based causes. There are also special causes related to the sea and climate.

1. Heavy Rainfall

Heavy rainfall means a very large amount of rain falling in a short period of time — for example, during a thunderstorm or a tropical cyclone (a powerful spinning storm).

  • When rain falls faster than the ground can absorb it, the water cannot soak in.
  • Instead, it flows across the surface of the land — this is called surface run-off.
  • All this water rushes into rivers very quickly, causing river levels to rise rapidly and overflow their banks.
  • Urban areas (towns and cities) are especially at risk because roads and pavements are hard surfaces — water cannot soak through them at all.

Simple example: Imagine pouring a full glass of water onto a tile floor versus a patch of grass. On the tile, the water spreads everywhere immediately. On the grass, some soaks in. Heavy rain on a city behaves like water on a tile floor.

2. Prolonged Rainfall

Prolonged rainfall means rain that falls steadily over a long period of time — days or even weeks — even if it is not extremely heavy at any one moment.

  • At first, the soil soaks up the rain like a sponge.
  • But after a long time, the soil becomes saturated (completely full of water — it cannot hold any more).
  • Once the soil is saturated, any further rain has nowhere to go. It all runs off the surface into rivers.
  • Rivers keep receiving more and more water until they burst their banks and flood the surrounding land.

Key idea: Even gentle, steady rain can cause serious flooding if it goes on for long enough.

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