Spain’s flash floods reveal a desperate need for improved mitigation efforts

In late October, flash floods devastated the Valencia region of eastern Spain. In some places, a year\’s worth of rain fell in just a few hours. At least 221 people have died and at least 5 are still missing.

This is Spain\’s biggest deluge since 1982, when the same region as well as the Pyrenees mountain range flooded. Those events inspired me to study floods: how they arise and how to reduce their devastating effects on human settlements.

Because flash floods are likely to become more frequent and intense due to climate change, it\’s high time that governments and communities strengthen their systems for forecasting floods, warning people, and responding to floods.

The first step is to recognize that these are not unexpected events. The Mediterranean basin is no stranger to heavy rainfall. Air masses carrying moisture from the Atlantic Ocean and subtropical regions converge there.

And water evaporates from the warm ocean. All this moist air, which is forced to rise, is trapped by the surrounding mountains – leading to thunderstorms and intense flooding in small areas. Over the centuries, frequent floods have scarred the landscape, creating many small, steep-banked, ephemeral rivers.

But now, the Mediterranean Sea is warming – its surface reached a record daily average temperature of 28.9°C in August. This creates additional moisture and intensifies rainfall. Climate change increased the rainfall recorded across Spain in the October floods (D. Faranda et al. preprint on Zenodo https://doi.org/nsp9; 2024).

Yet buildings – including many with basements that are likely to flood – are still being built close to floodwaters.

Preventive measures can help. For example, after a major flood in 1957, the Turia River that flows through Valencia was diverted to flow through a concrete channel on the outskirts of the city. This partially controlled the water and limited the amount of deaths and damage caused by the October floods. But such structural interventions are not enough. And they can put people at risk through the \’levee effect\’ – building flood-protection infrastructure gives residents a false sense of security and prompts them to move into flood-prone areas.

\’Nature-based solutions\’ are also important, and should be explored in the Valencia region. For example, allowing floods to flow freely in less populated areas can reduce pressure on urban areas. Floodwater can also be diverted into wetlands. The Brazilian city of Paraty reduced flood risk by improving connections between water bodies in urban parks, ponds, rivers, floodplains and storage areas to contain and disperse excessive flows.

Storms and floods are local meteorological phenomena – they can be predicted, although with many uncertainties. Forecasts must improve. Radar and satellite observations, aided by artificial-intelligence tools, can help meteorologists track the emergence of heavy rainfall.

To predict floods, weather forecasts must be integrated with hydrological forecasts. This requires close collaboration between meteorological and hydrological agencies. For small basins with fast-flowing currents like Valencia, closely monitoring rainfall and water levels is essential.

Alerts are crucial to reducing the number of deaths and the amount of damage. Flash floods are deadly because they develop very quickly: people have at most a few hours to reach safety. And this is where Valencia\’s system failed.