Disastrous floods have caused millions of fatalities in the twentieth century, tens of billions of dollars of direct economic loss each year and serious disruption to global trade. In this Review, we provide a synthesis of the atmospheric, land surface and socio-economic processes that produce river floods with disastrous consequences. Disastrous floods have often been caused by processes fundamentally different from those of non-disastrous floods, such as unusual but recurring atmospheric circulation patterns or failures of flood defences, which lead to high levels of damage because they are unexpected both by citizens and by flood managers. Past trends in economic flood impacts show widespread increases, mostly driven by economic and population growth. However, the number of fatalities and people affected has decreased since the mid-1990s because of risk reduction measures, such as improved risk awareness and structural flood defences. Disastrous flooding is projected to increase in many regions, particularly in Asia and Africa, owing to climate and socio-economic changes, although substantial uncertainties remain. Assessing the risk of disastrous river floods requires a deeper understanding of their distinct causes. Transdisciplinary research is needed to understand the potential for surprise in flood risk systems better and to operationalize risk management concepts that account for limited knowledge and unexpected developments.
The causative mechanisms of floods with disastrous consequences tend to be different from those of non-disastrous floods, and show anomalies in one or several flood- and loss-generating processes.
Past trends in flood hazard show both upward and downward changes. In some regions, anthropogenic warming is already strong enough to override other drivers of change.
Flood hazards and impacts are projected to increase for many regions around the globe. Future flooding hotspots are expected in Asia and Africa, owing to climate and socio-economic changes.
Reducing vulnerability is a particularly effective way of reducing flood impacts. Global decreases in flood-affected people and fatalities since the mid-1990s (despite a growing population) are signs of effective risk reduction.
Disastrous floods often come as a surprise. Effective risk reduction requires an understanding of the causative processes that make these events distinct and to address the sources of surprise, including cognitive biases.
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The authors declare that the data supporting the findings of this study are available within the article and its supplementary information files. Other data can be provided by the authors on request.
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This work was supported by the DFG projects ‘SPATE’ (FOR 2416) and ‘NatRiskChange’ (GRK 2043/1), the FWF ‘SPATE’ project (I 3174), the ERC Advanced Grant ‘FloodChange’ project (number 291152), the Horizon 2020 ETN ‘System Risk’ project (number 676027) and the Helmholtz Climate Initiative. P.B. was supported by a Royal Society Wolfson Research Merit award. J.C.J.H.A. was supported by an ERC Advanced Grant COASTMOVE (number 884442) and a NWO-VICI grant (number 453-13-006).
The authors declare no competing interests.
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- Annual average loss
(AAL). A widespread indicator for risk, it is the estimated average loss per year considering the full range of scenarios from frequent events (zero or small loss) to extreme events (large loss or worst-case scenario).
- Rain-on-snow events
Fall of rain onto existing snow, leading to flood runoff composed of snowmelt and rainfall.
- Atmospheric rivers
Long, narrow and transient corridors of strong horizontal water vapour, transporting on average more than double the flow of the Amazon river and delivering moisture as heavy precipitation.
- Runoff coefficient
The fraction of the event water input (precipitation or snowmelt within the catchment) that is not retained in the catchment and that directly contributes to discharge during the event.
- Direct impacts
Consequences occurring in the inundated region during a flooding event.
- Indirect impacts
Consequences occurring far away from the flooded region and/or after a flooding event.
- Intangible impacts
Consequences of a flooding event that are difficult or impossible to monetarize, such as loss of life or loss of memorabilia.
The ratio of the number of people who lose their lives in a flood to the number of people affected by the flooding event.
- Annual maximum flows
The highest streamflow peak in each year.
- Flood timing
The dates of the year when floods occur.
- Flood extent
The distance over which flooding occurs simultaneously.
- Major flood level
Level at which a flood causes extensive inundation, significant evacuations, or property transfer to higher ground.
- Action flood level
Level at which a flood does not cause damage but requires mitigation action in preparation for more substantial flooding.
- People displaced
According to the DFO, either the total number of people left homeless after the incident, or the number of people evacuated during the flood.
Coupled Model Intercomparison Project Phase 5; for coordinated climate change experiments for the Fifth Assessment Report AR5 of the Intergovernmental Panel on Climate Change and beyond.
- Return period
An indicator expressing the exceedance probability or rarity of an event. For instance, a 100-year flood discharge has a probability of 1/100 of being exceeded in a given year.
- Flood frequency curve
Relation between flood discharge and the associated return period.
- Risk-based decision-making
Optimizing risk reduction measures based on the best available knowledge.
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Merz, B., Blöschl, G., Vorogushyn, S. et al. Causes, impacts and patterns of disastrous river floods. Nat Rev Earth Environ 2, 592–609 (2021). https://doi.org/10.1038/s43017-021-00195-3