A transferable multicriteria methodology for flood risk assessment: two case studies in Greece and Cyprus

Floods are the most frequent disasters, affecting the largest number of people. In 2023, 164 floods were recorded worldwide, killing 7763 people, affecting 32.4 million people, and resulting in 20.4 billion USD losses (CRED 2023 Disasters in Number report). To mitigate flood risk, decision makers and civil protection authorities need reliable information on flood risk assessment, covering all disaster management stages.

In the framework of a Programming Agreement with the Prefecture of Attica, NOA/IAASARS/BEYOND, in cooperation with NTUA/ITIA, developed a holistic multiparameter methodology that was implemented in five flood-stricken river basins at high spatial resolution. The research teams collected all available Earth Observation data, spatial data and technical studies; conducted detailed field visits; and modified the DEM and land cover accordingly. Following rainfall-runoff modeling and hydraulic modeling, the flood hazard was assessed for different scenarios. Vulnerability was considered a weighted estimation of population density, population age, and building characteristics on the basis of the population-housing census at the building block level. Exposure was based on the land value. Flood risk was eventually assessed on the basis of the combination of flood hazard, vulnerability, and exposure. Moreover, critical points, which were identified from the field visits, were also crosschecked with the flood inundation maps. Finally, refuge areas and escape routes were proposed for the worst-case flood scenario. This innovative methodology was applied, among other methods, in the Mandra river basin and was validated with the results of the urban flash flood, which took place in 2017, the deadliest flood in Greece in the last 40 years. BEYOND developed a user-friendly web GIS platform in which all the collected and produced data, including flood risk maps, critical points, refuge areas and escape routes, are made available.

This flood risk assessment methodology was applied, following adaptation, in the Garyllis river basin in Cyprus, within the framework of the EXCELSIOR project, as part of the collaborative activities between ECoE and BEYOND. Data were collected from multiple sources, including satellite missions, governmental portals, in situ measurements, and historical records, at different resolutions. The collected data were calibrated via onsite visits and discussions with relevant actors, harmonized in terms of spatial and temporal resolution and used as inputs to estimate the flood hazard for different return periods. The vulnerability levels of the study area were quantified via the weighted linear combination of population density, population age, and building characteristics at the road level. The exposure levels were quantified in terms of the land value. Flood risk levels were estimated as a product of hazard, vulnerability and exposure levels. The validity of the proposed methodology was evaluated by comparing the critical points identified during the field visits with the estimates of the flood risk levels. Consequently, escape routes and refuge regions are recommended for the most extreme scenario.

This work supports relevant authorities in improving disaster resilience and in implementing the EU Floods Directive 2007/60/EC, the Sendai Framework for Disaster Risk Reduction, the UN SDGs, and the UN Early Warnings for All initiative.