Flood risk assessment

River floods are caused by the relatively slow overflow of waterways after heavy rainfall and/or snowmelt.

Flash floods mainly occur in small catchments as a result of heavy rainfall, and are characterised by a rapid rise in water levels and high flow velocities.

Heavy rainfall brings large amounts of water to the Earth's surface in a matter of minutes or hours. This can overload sewer systems and cause localised flooding, known as pluvial flooding, particularly in urban areas.

Climate change is altering the likelihood of extreme weather events occurring, including flooding. However, these changes vary regionally and depend on the flood type. In the case of localised floods caused by short, heavy rainfall, such as thunderstorms, there is a very direct link between climate change and floods. Global warming leads to an increase in localised heavy rainfall, and in Germany, we see a trend towards more convective weather conditions that favour the formation of thunderstorms and heavy rainfall.

The effects of climate change on river flooding are less clear. Here it depends on the interaction of heavy precipitation and water storage in the catchment area (e.g., in the soil and groundwater). In mountainous regions, the effects of climate change on snowfall and snowmelt also play a role. Overall, however, we will find ourselves in a situation more often than before in which the existing protective measures are overloaded.

One study has examined changes in flood runoff across Europe over the last few decades. The results show predominantly statistically significant increases in Central and North-Western Europe, indicating that flood peaks have risen across large areas in recent decades. The observational data also indicates an increase in extreme precipitation. For instance, one study evaluated the number of record precipitation events from thousands of climate stations. This shows that, in recent years, the number of record precipitation events (i.e. precipitation that has not yet been measured at these stations) has been 30 per cent higher worldwide than would be expected in a world without climate change.

The following overview provides an overview of significant flood events in Central Europe since the 1990s.

Overview of flood events (1993–2024)

• 1993 + 1995 | Rhine (Germany, Netherlands)
  Flooding affecting a wide area, within a few years.
• 1997 | Oder (Poland, Germany)
  The “Oder floods” were one of the worst floods in Central Europe, caused by a Vb weather pattern accompanied by prolonged heavy rain.
• 2002 | Elbe (Germany, Czech Republic)
  Extreme weather event with record rainfall; extensive damage was also caused by tributaries such as the Weißeritz and the Mulde. Dresden, Grimma, Pirna, Riesa, Bitterfeld and the Erzgebirge were particularly hard hit.
• 2013 | Elbe and Danube (Central Europe)
  Water levels in some places higher than in 2002; severe damage to infrastructure, particularly in Saxony-Anhalt, Saxony and Bavaria.
• 2021 | The Eifel region, particularly the Ahr, Rur and Erft rivers (Rhineland-Palatinate, North Rhine-Westphalia, Belgium, the Netherlands, Luxembourg)
  Extreme flash flooding following heavy rain, over 230 fatalities and record damage.
• 2023/24 | Winter flooding (Germany)
  Persistent rain and saturated ground led to prolonged river flooding.
• 2024 | Summer floods (southern Germany)
  Heavy rain with weather conditions similar to those seen in Vb.

From a hydrological point of view, we don't talk about floods of the century, but about 100-year floods. This term expresses the fact that we have to reckon with such an event at a particular location on average once every 100 years. However, this does not mean that such an event occurs every 100 years, as extreme floods do not occur regularly. It means that we have a 1% probability of such an event occurring in a given year. Several such events can also occur in a few years.

Another aspect is that the term 100-year flood assumes a constant environment. However, climate change and interventions in our landscapes and rivers are changing the probability of a particular flood occurring. In a “rapidly” changing climate, this statistical term thus loses its basis. What was a 100-year flood yesterday may be a 20-year event today, for example.

For these reasons, but also for psychological reasons, the term “flood of the century” is problematic. It can easily be interpreted as the worst flood that can happen in a century. And if such a flood of the century has occurred, it sounds as if the next comparably severe event can only happen far in the future. But neither is the case. Even if it is not as easy for citizens to understand as the term “flood of the century”, it would probably be better to operate with probabilities. And to communicate that these probabilities can change, especially due to climate change.

In addition to technical protection measures, such as embankments and retention basins, better preparedness to floods is required. One sensible measure would be to improve flood warning systems and how the population responds to extreme situations. Only when people are informed and know what to do can they respond effectively in an emergency. Another measure would be to subject our critical infrastructure (e.g. the energy and water supply and hospitals) and our sensitive infrastructure (e.g. nursing homes) to stress tests in order to identify critical points and ensure that damage and failures are reduced as much as possible in such cases.

Furthermore, it is sensible to develop and implement risk models that can simulate the occurrence and effects of flooding.

The accuracy with which floods can be predicted at a regional scale depends among others on the accuracy of precipitation forecasts. Although general weather conditions can be predicted relatively reliably several days in advance, thunderstorms make forecasting considerably more difficult. Therefore, combinations of steady rain and thunderstorms pose major problems for warning management.

Floods can cause long-lasting health problems. People's quality of life and daily functioning can be badly affected by the psychological impact. This is partly due to people often thinking about the flood. For many, these memories continue long after the water has receded.

How a person is evacuated can also affect their recovery. Timely and effective evacuation can save lives and reduce the risk of long-term mental health problems, but it can also have consequences. In the 2021 flood in Germany, for example, the duration of displacement had a measurable effect on the psychological burden. The longer people were displaced, the greater the burden.

In the aftermath of the 2021 flood, homeowners who carry more financial stress, face longer repair timelines, and have deeper emotional ties to their homes often report worse quality of life and functioning than tenants. Financial stability also played a major role in recovery. Households with higher incomes or those that received timely insurance payouts, private donations, or fair compensation claims reported better health and daily functioning. In contrast, delays or disputes in financial recovery was found to prolong stress and hinder overall wellbeing.

These findings highlight a critical point: Supporting the health and quality of life of flood-affected communities should be a priority alongside rebuilding infrastructure.