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Extreme floods: What turns a rainstorm into a catastrophe?

Floods can devastate communities. Homes are lost, lives are changed, and the land is forever altered. We know rising waters cause extreme floods, but there’s a lot more to the story.

What lurks beneath the surface that turns a rainstorm into a catastrophe? A new study from the Helmholtz Centre for Environmental Research (UFZ) reveals the complex interactions that drive extreme flooding.

What makes a flood?

Think about the conditions leading up to a flood. Yes, there’s heavy rainfall, but there’s often more at work:

Air temperature

Warmer air can hold more moisture than cooler air. This is a basic principle of atmospheric science. When warm, moist air cools down, it can no longer hold as much water vapor, leading to condensation and precipitation.

In the context of flooding, this means that periods of warm air can lead to increased evaporation and, subsequently, more significant rainfall. Additionally, warmer temperatures can exacerbate snowmelt, contributing further to potential flooding.

Soil moisture

The soil’s moisture content plays a critical role in flood development. If the ground is already saturated from previous rainfall or snowmelt, it cannot absorb much more water. This condition is similar to a saturated sponge that can’t hold any more liquid.

When additional rain falls on saturated ground, the water runs off the surface instead of being absorbed, leading to increased runoff that can contribute to flooding.

The capacity of the soil to absorb water is influenced by its composition, vegetation cover, and the presence of urban surfaces, which are less permeable than natural landscapes.

Snow depth

In regions with significant snowfall, the depth of snow accumulated over the winter can be a pivotal factor in spring flooding.

Rapid snowmelt, driven by sudden warm temperatures or rain, can release large volumes of water into rivers and streams in a short period. This sudden influx of water can exceed the carrying capacity of the waterways, leading to overflow and flooding in surrounding areas.

Snowmelt contributes to some of the most significant flooding events in mountainous and northern regions.


The intensity and duration of rainfall are critical in flood formation. A short, intense downpour may lead to flash flooding, especially in urban areas where the ground surface is largely impermeable.

Conversely, prolonged rain over several days can saturate the ground and fill rivers, leading to more extensive flooding.

The timing and rate of precipitation, combined with the other factors mentioned, determine the immediate impact on the landscape and the potential for flooding.

Factors that cause extreme flood

The UFZ team analyzed over 125,000 flood events across the globe and found that in most cases, there’s not just one culprit. Over half of all floods involved precipitation teaming up with at least one more factor.

Additionally, the most extreme floods often had multiple drivers working together. Think of it like a supervillain team-up – the more villains, the more trouble.

Not all regions are affected by extreme floods

It turns out, where you live matters regarding flood risk. The UFZ team coined the term “flood complexity,” which means how many different factors usually cause floods in the area. Here’s the general idea:

  • Northern areas and the Alps tend to primarily have floods driven by snowmelt, regardless of how bad the flood gets.
  • Places with frequent rainy seasons, like the Amazon Basin, tend to have floods driven mostly by oversaturated soil.
  • Regions with a mix of weather conditions, like Eastern Europe and the US West Coast, tend to have more complicated floods with a mix of causes.

River basin

How several factors come together depends on the unique characteristics of your area. Things like soil type, whether it’s usually wet or arid, and even how steep the land is all play a role.

 “In drier regions, the mechanisms that lead to flooding tend to be more heterogeneous. For moderate floods, just a few days of heavy rainfall is usually enough. For extreme floods, it needs to rain longer on already moist soils”, says lead author Dr Shijie Jiang, who now works at the Max Planck Institute for Biogeochemistry in Jena.

Understanding extreme flooding

The UFZ scientists didn’t just analyze data; they developed new analysis methods using something called explainable machine learning. They can be used to:

Improve extreme flood predictions

The ability to accurately predict floods is paramount in safeguarding human lives, properties, and ecosystems.

With advancements in technology and research, flood prediction models have become more sophisticated, incorporating a variety of data sources and methodologies:

Hydrological and meteorological data

Modern prediction models use real-time data on rainfall, river levels, soil moisture, and other relevant factors. By analyzing this data, scientists can forecast the likelihood of flooding with greater accuracy.

Remote sensing and satellites

The use of satellite imagery allows for the monitoring of snowpack, the extent of floodwaters, and changes in land use that may affect flooding.

This broad perspective is crucial for assessing risk and predicting floods over large areas.

Machine learning and AI

Artificial intelligence and machine learning algorithms can analyze vast datasets to identify patterns and predict flood events more accurately. These technologies can continually learn and improve, enhancing the precision of flood forecasts over time.

More accurate flood warnings enable governments, communities, and individuals to prepare more effectively. This can include evacuating vulnerable areas, reinforcing flood defenses, and securing property.

Protect communities

Knowledge of flood risks and the implementation of strategies to mitigate these risks are crucial for community resilience.

Protecting communities involves a combination of planning, infrastructure development, and public education:

Resilient infrastructure

Building flood defenses such as levees, floodwalls, and reservoirs can help protect communities from floodwaters.

Additionally, designing infrastructure to withstand flood conditions ensures that essential services can continue during and after a flood event.

Zoning and land use planning

By understanding the areas most at risk, planners can restrict development in high-risk zones and promote land use that reduces vulnerability to flooding.

This includes preserving wetlands that can absorb floodwaters and implementing green infrastructure like rain gardens to manage runoff.

Emergency response strategies

Effective emergency planning and communication are critical in protecting communities during flood events.

This includes having evacuation plans in place, ensuring that vulnerable populations receive assistance, and establishing clear communication channels for disseminating warnings and information.

Overall, floods are far more intricate than just “too much rain.” By understanding the interplay of temperature, soil, snow, and precipitation patterns, we gain critical insights that could save lives and property.

The study is published in Science Advances.


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