Why do floods happen and which areas are most susceptible?
Floods occur when water covers an area that is normally dry. We examine what the risk factors are and where floods are most frequent.
Flooding is a fairly common natural phenomenon that occurs when water overflows and runs across land that is generally dry. It involves an excess of water that neither the soil nor the drainage systems can absorb. According to the Organisation for Economic Co-operation and Development, floods cause more than 38 billion euros worth of damage worldwide each year.
Natural disasters such as floods have direct short-term effects on people's health, but they also have economic and environmental consequences. Experts estimate that, largely due to climate change, floods will become increasingly frequent and dangerous. Some floods take hours to develop while others occur quite suddenly. Flash floods tend to be much more dangerous and can, for example, burst the banks of a river and wash away everything in their path.
Common causes of floods
There are several causes of floods, such as heavy rainfall, snowmelt or rising sea levels. Those are all natural causes that are influenced by the type of rainfall, the terrain or plant cover of the location, soil moisture conditions or other issues. Depending on these factors, an area or region will be more or less prone to experiencing floods.
- The main cause of flooding is rainfall. Storms and heavy rainfall events cause surface runoff. Soil becomes saturated and can no longer store any more water.
- Another factor that can cause flooding is when a river breaks its banks. The flooding of a river can become catastrophic in the event of heavy and continuous rainfall. This happens, for example, in tropical climate zones or in the Mediterranean, where it is more common for a river to overflow due to heavy rainfall or snowmelt and meteorological phenomena such as the high-impact rainfall event known in Spain and France as a “cold drop”, or DANA (Spanish acronym for “isolated depression at high altitude”). This atmospheric phenomenon causes severe weather instability with lower temperatures and the arrival of storms and floods. Although DANAs can occur anywhere in Spain, they tend to be more dangerous when they originate over the Mediterranean, such as the one experienced in many areas of Valencia in October 2024.
- Another hazard directly related to flooding is dam failure, which is a major risk. Should this happen, the large amounts of water held in a reservoir will be released and flood nearby areas causing catastrophic damage.
Climate change and floods
More and more studies are linking flooding (and its increasing frequency and strength) to climate change. In fact, these effects are already very noticeable in some coastal areas and in the Mediterranean. Climate change and its effects increase the intensity and frequency of extreme rainfall events and, therefore, the risk of flooding.
It is therefore crucial that scientists and governments develop effective prevention and adaptation strategies, such as sustainable urban planning. Global warming is also linked to hurricanes moving more slowly, leading to greater rainfall. At the same time, melting glaciers are causing sea levels to rise, increasing the risk of chronic flooding.
Ultimately, climate change is a decisive factor that increases the frequency and intensity of these episodes, which tend to occur in regions such as East Africa, Pakistan and Brazil. These areas have suffered torrential rains and severe flooding in recent months, with injuries and thousands of incidents causing material damage. Hurricane-like storms and typhoons are also becoming more noticeable in Mediterranean regions such as Greece, Bulgaria and Spain. According to data from Oxfam Intermón, floods and droughts in the ten most affected countries - Somalia, China, the Philippines, Ethiopia, India, Brazil and Bangladesh - have increased sharply. In 2013 there were only 24. Last year there were 656.
Although these natural events cannot be totally avoided, there are certain measures that can help limit damage, such as proper waterway management or optimal urban planning. Understanding why and how floods occur is important to limit their impact on citizens and their environment.
