How Floods Happen

Flooded building of Snoqualmie-Fall City Road

A building swallowed almost entirely by water just off Snoqualmie-Fall City Road, in 2009

Floods are known for being both destructive and a benefit, as they can bring relief to people and ecosystems that have been suffering from prolonged drought. However, floods are considered to be, in some parts of the Planet, the most costly natural disasters.

According to scientists, a flood is “a temporary rise of the water level, in a river, lake or along a seacoast, resulting in its spilling over and out of its natural or artificial confines onto land that is normally dry”.

Floods usually appear after excessive runoff from precipitation, snow melts, coastal storms or other similar phenomena. Sometimes, they are described according to their statistical occurrence. For example, a fifty-year flood has a magnitude that is reached in a particular area, on average, once every fifty years, while in any other period, the statistical chances of its occurrence are of only two percent.

Moreover, some floods develop gradually, while others can take shape in just a few minutes, without even visible signs of rain. The latter are called flash floods.

Flood Types

Depending on the way they take shape, floods are divided into four main types.

1. Areal floods

Areal floods happen on flat or low-lying areas when the ground is saturated and water can’t run off quickly or can’t run off at all in order to stop the process. This can lead to a river flood as water moves away from the floodplain into local rivers and streams.

They can also occur if water falls on an impermeable surface (concrete, paving, frozen ground) and can’t rapidly dissipate into the ground. Moreover, they can also be caused by localized heavy rain from a series of storms moving over the same area. When the rate of rainfall exceeds the drainage capacity of the area leads to a muddy flood, where sediments are picked up by run off and carried as suspended matter or bed load.

2. Riverine floods

These types of floods are produced on rivers and they can occur at different rates. River flows may last from a few minutes to several weeks, depending on the type of river and the source of the increased flow. For example, in large rivers with large catchment areas, slow rising floods are likely to appear. In these cases, the increase in flow can result from sustained rainfall, tropical cyclones, rapid snow melt or monsoons.

On the other hand, rapid flooding events (flash floods) are more likely to occur on smaller rivers, with steep valleys, or rivers that flow for much of their length over impermeable terrains. These can be caused by convective precipitations or sudden release from an upstream reservoir created behind a dam, landslide, or glacier.

3. Coastal floods

They are frequently caused by a combination of sea tidal surges produced by winds and low barometric pressure. Coastal floods can be exacerbated by high upstream river flow, storm events at sea, tsunami or tropical cyclones – in most severe cases.

Moreover, when a storm surge is caused by an extratropical cyclone or by a tropical cyclone, it is included in this category.

4. Catastrophic floods

They are usually associated with major infrastructure failures, such as the collapse of a dam. Moreover,  catastrophic floods can also occur as a result of damage sustained in an earthquake or volcanic eruption.

Flood Damages

Damages caused by floods is, first and foremost, the loss of life. Additionally, floods can cause extensive damage to buildings, bridges, roadways, canals and sewerage systems, which can impair a city’s infrastructure. This can lead to the loss of power and the loss of sewage disposal facilities which can cause severe water contamination. The latter may bring the risk of waterborne diseases, such as typhoid, giardia, cryptosporidium, cholera and many other diseases.

If a flood produces damages to roads and transport infrastructure it also makes it harder for authorities to provide emergency rescuers or ambulances.

On the other hand, a flood continues to cause damages even after the phenomenon itself ends, as it produces economic hardship. This is caused by the costs needed to rebuild houses and buildings or because of the food shortages that lead to price increases.

When deaths, serious injuries and loss of property occur, people may also suffer traumatizing psychological damage.

The deadliest floods in the world include events with total death tolls at or above 100,000. Most of these have occurred in China on the Yellow River (Huang He) and on the Yangtze River. The Indian Ocean Tsumani, which took place in Indonesia in 2004, killing 230,000 people, is another example of how devastating a flood can be.