Abstract
Abstract. Floods can arise from a variety of physical processes. Although numerous risk assessment approaches stress the importance of taking into account the possible combinations of flood types (i.e. compound floods), this awareness has so far not been reflected in the development of early warning systems: existing methods for forecasting flood hazards or the corresponding socio-economic impacts are generally designed for only one type of flooding. During compound flood events, these flood type-specific approaches are unable to identify overall hazards or impacts. Moreover, from the perspective of end-users (e.g. civil protection authorities), the monitoring of separate flood forecasts – with potentially contradictory outputs – can be confusing and time-consuming, and ultimately impede an effective emergency response. To enhance decision support, this paper proposes the integration of different flood type-specific approaches into one compound flood impact forecast. This possibility has been explored through the development of a unified system combining the simulations of two impact forecasting methods: the Rapid Risk Assessment of the European Flood Awareness System (EFAS RRA; representing fluvial floods) and the radar-based ReAFFIRM method (representing flash floods). The unified system has been tested for a recent catastrophic episode of compound flooding: the DANA event of September 2019 in south-east Spain (Depresión Aislada en Niveles Altos, meaning cut-off low). The combination of the two methods identified well the overall compound flood extents and impacts reported by various information sources. For instance, the simulated economic losses amounted to about EUR 670 million against EUR 425 million of reported insured losses. Although the compound impact estimates were less accurate at municipal level, they corresponded much better to the observed impacts than those generated by the two methods applied separately. This demonstrates the potential of such integrated approaches for improving decision support services.
Highlights
Around the globe, floods regularly result in devastating impacts on human society
Numerous risk assessment approaches stress the importance of taking into account the possible combinations of flood types, this awareness has so far not been reflected in the development of early warning systems: existing methods for forecasting flood hazards or the corresponding socio-economic impacts are generally designed for only one type of flooding
This possibility has been explored through the development of a unified system combining the simulations of two impact forecasting methods: the Rapid Risk Assessment of the European Flood Awareness System (EFAS RRA; representing fluvial floods) and the radar-based ReAFFIRM method
Summary
Floods regularly result in devastating impacts on human society. With more than one trillion USD over the last four decades, floods accounted for about 40 % of all natural hazard-related economic losses (Munich Re, 2020). Climate change projections suggest that the frequency and magnitude of floods will increase in many parts of the world over the decades to come (IPCC, 2018). In combination with rising trends in urbanisation and population growth, the impacts of floods on society are expected to increase significantly if no further adaptation measures are adopted (Dottori et al, 2018). The development of early warning systems (EWSs) is a highly cost-effective way to reduce flood impacts as they support the coordination of emergency response measures, such as warnings to the population, evacuations, and the in-
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