Understanding a “100-Year flood”: Paulo Diogo explains hydrological risk and flood management during extreme weather in Portugal
Following the recent storm events that placed several regions of Portugal under elevated flood risk, particularly in the Coimbra region, Paulo Diogo, MARE and ARNET researcher and Professor at NOVA FCT, was invited to provide scientific clarification on national television regarding the hydrological dynamics at play.
During the live broadcast at RTP Notícias, he addressed the concept of a “100-year flood,” explaining that it is a statistical return-period concept rather than a deterministic prediction. A 100-year flood, he clarified, “refers to an event with a statistical probability of occurring once every hundred years on average. Importantly, this does not mean such an event cannot occur in shorter intervals. Moreover, the relationship between return periods and flood magnitude is not linear, since the difference between a 50-year and a 100-year event represents a disproportionately larger hydrological impact”.
Beyond terminology, Paulo Diogo emphasised the central role of monitoring in flood risk management. Continuous data collection, not only during crisis moments but throughout the year, is essential to understand system behaviour and anticipate peak flows. Using the Mondego basin as an example, he explained how dam management is based on:
- Real-time hydrological monitoring
- Forecasts of precipitation and inflows
- Storage capacity margins
- Safety thresholds designed to prevent structural failure
The researcher noted that large dams are dimensioned for extremely rare return periods (e.g., 1000-year events) and must always maintain safety buffers to avoid overtopping. At the same time, he highlighted the limits of engineering solutions: “It is impossible to prevent a flood of this magnitude if rainfall continues. It is not possible to build a dam that prevents all floods.”, since prolonged rainfall leads to soil saturation, reducing infiltration and increasing surface runoff. When soils are saturated, nearly all precipitation flows directly into river systems, accelerating peak discharge and increasing flood severity. In the case of landslides, saturated soils also become mechanically unstable, increasing the destructive potential of mass movements.
Importantly, Paulo Diogo reinforced that flood risk mapping already exists, and areas with high probability of flooding are identified in official cartography, supporting preventive evacuation and civil protection measures.
Through his intervention, Paulo Diogo contributed to strengthening public understanding of hydrological risk, infrastructure management, and the importance of prevention in a context of increasingly intense extreme weather events. The final remark was sound and clear: flood risk tends to fade from public attention once waters recede. Monitoring, preparedness, and coordination between technical authorities and civil protection must remain continuous priorities.
Text: João Pequeno