Floods are amongst the most frequent disasters in terms of human and economic impacts. This study provides new insights into the frequency of loss of life at the global scale, mortality fractions of the population exposed to floods, and underlying trends. A dataset is compiled based on the EM-DAT disaster database covering the period 1975 until 2022, extending previous studies on this topic. Flood impact data are analysed over spatial, temporal and economic scales, decomposed in various flood types and compared with other natural disasters. Floods are the most frequent natural disasters up to 1000 fatalities, and flash floods lead to the highest mortality fractions per event, i.e. the number of deaths in an event relative to the exposed population. Despite population growth and increasing flood hazards, the average number of fatalities per event has declined over time. Mortality fractions per event have decreased over time for middle- and high-middle-income countries, but increased for low-income countries. This highlights the importance of continuing and expanding risk reduction and adaptation efforts.

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Reliable hazard analysis is crucial in the flood risk management of river basins. For the floodplains of large, developed rivers, flood hazard analysis often needs to account for the complex hydrology of multiple tributaries and the potential failure of dikes. Estimating this hazard using deterministic methods ignores two major aspects of large-scale risk analysis: the spatial–temporal variability of extreme events caused by tributaries, and the uncertainty of dike breach development. Innovative stochastic methods are here developed to account for these uncertainties and are applied to the Po River in Italy. The effects of using these stochastic methods are compared against deterministic equivalents, and the methods are combined to demonstrate applications for an overall stochastic hazard analysis. The results show these uncertainties can impact extreme event water levels by more than 2 m at certain channel locations, and also affect inundation and breaching patterns. The combined hazard analysis allows for probability distributions of flood hazard and dike failure to be developed, which can be used to assess future flood risk management measures.

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Flood risk analysis focused on small sections or regions of embanked systems ignores the dy-namics of large-scale floods through time and space. Resulting measures are oftentimes inef-ficient due to an over estimation of risk or be-cause they simply transfer risk downstream due to increased localised protection upstream. This present work addresses this problem through a ‘system behaviour’ analysis, in which the com-plete river-dike floodplain system is assessed using stochastic simulations of flood events and defence failures. Such analysis can provide decision-makers with accurate estimates of local and system-wide risk from which efficient and effective FRM strate-gies can be developed. Furthermore, it creates a repository of realistic flood event simulations that inform emergency responders and provides data for future analyses. System behaviour analyses were implemented on two of the most developed floodplain regions of Europe; the Po River in Italy and the Dutch Rhine-Meuse delta. In both cases, the analysis underlined the importance of assessing risk of complete systems, as it provided more accurate estimates of current flood hazard, defence fail-ure probabilities and risk. The analysis can also be used in the evaluation of new measures such as dike strengthening and detention areas, both in the case studies and in other protected river systems. Finally, the development of map-based tools allows for a clear interpretation of the data for decision makers and researchers that wish to further investigate aspects of the system. @en

Increasing amounts of data, together with more computing power and better machine learning algorithms to analyse the data, are causing changes in almost every aspect of our lives. This trend is expected to continue as more data keep becoming available, computing power keeps improving and machine learning algorithms keep improving as well. Flood risk and impact assessments are also being influenced by this trend, particularly in areas such as the development of mitigation measures, emergency response preparation and flood recovery planning. Machine learning methods have the potential to improve accuracy as well as reduce calculating time and model development cost. It is expected that in the future more applications will become feasible and many process models and traditional observation methods will be replaced by machine learning. Examples of this include the use of machine learning on remote sensing data to estimate exposure and on social media data to improve flood response. Some improvements may require new data collection efforts, such as for the modelling of flood damages or defence failures. In other components, machine learning may not always be suitable or should be applied complementary to process models, for example in hydrodynamic applications. Overall, machine learning is likely to drastically improve future flood risk and impact assessments, but issues such as applicability, bias and ethics must be considered carefully to avoid misuse. This paper presents some of the current developments on the application of machine learning in this field and highlights some key needs and challenges.

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To make informed flood risk management (FRM) decisions in large protected river systems, flood risk and hazard analyses should include the potential for dike breaching. 'Load interdependency' analyses attempt to include the system-wide effects of dike breaching while accounting for the uncertainty of both river loads and dike fragility. The intensive stochastic computation required for these analyses often precludes the use of complex hydraulic models, but simpler models may miss spatial inundation interactions such as flows that 'cascade' between compartmentalised regions and overland flows that 'shortcut' between river branches. The potential for these interactions in the Netherlands has previously been identified, and so a schematisation of the Dutch floodplain and protection system is here developed for use in a load interdependency analysis. The approach allows for the spatial distribution of hazard to be quantified under various scenarios and return periods. The results demonstrate the importance of including spatial inundation interactions on hazard estimation at three specific locations, and for the system in general. The general modelling approach can be used at a local scale to focus flood-risk analysis and management on the relevant causes of inundation, and at a system-wide scale to estimate the overall impact of large-scale measures.@en

Hazard analysis is a crucial step in flood risk management, and for large rivers, the effects of breaches need to be taken into account. Hazard analyses that incorporate this overall “system behaviour” have become increasingly popular in flood risk assessment. Methods to perform such analyses often focus on high water levels as a trigger for dike breaching. However, the duration of high water levels is known to be another important failure criterion. This study aims to investigate the effect of including this duration dependency in system behaviour analyses, using a computational framework in which two dike breach triggering methods are compared. The first triggers dike breaches based on water levels, and the second one based on both water-level and duration. The comparison is made for the Dutch Rhine system, where the dike failure probabilities are assumed to conform to the new Dutch standards of protection. The results show that including the duration as a breach triggering variable has an effect on the hydraulic loads and overall behaviour in the system, therefore influencing the risk. Although further work is required to fully understand the potential impact, the study suggests that including this duration dependency is important for future hazard risk analyses.

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Reliable hazard analysis is a crucial step in flood risk management, and for large river systems, the effects of breaches on downstream regions should be taken into account. Accounting for these breaches in hazard analyses is often termed ‘hydrodynamic system behaviour’ analyses, and has become increasingly popular in flood risk assessment. Methods to perform such analyses usually focus on high water levels as a trigger for dike breaching. However, the duration of high water levels is known to be an important criterion in the mechanisms that cause dike breaching, for example piping. This study aims to demonstrate the effect of the duration on hydraulic system behaviour analyses, using a computational framework in which two dike breach triggering method are compared in a large river system. The Dutch Rhine is used as a case-study. The first method triggers dike breaches based on water levels, and the second method is dependent on both water-level and duration, with the relationship of the two variables inferred via expert opinion. This comparison is made for dike failure probabilities based on the proposed future standards of protection. @en