Developing an integrated methodology for flood-hazard assessment: application to the Pikrodafni River Basin (Attica, Greece)

Flood hazard assessment—together with vulnerability and risk analysis—is closely linked to flood resilience and has been extensively studied in densely populated areas, where the most catastrophic floods tend to occur. The need for a holistic and transferable methodology is critical considering that, different simulation approaches are often used, while key methodological phases are sometimes omitted. Within the framework of the Programming Agreement of the Prefecture of Attica, the BEYOND Centre (IAASARS/NOA), in cooperation with the NTUA research group have developed the methodology presented in this work. The methodology was implemented at high spatial resolution in five flood-affected river basins in Attica, with the Pikrodafni River basin being presented in detail in this study. Data acquisition constituted a core component of the methodology and involved targeted spatial datasets, Earth-observation imagery, time-series data, historical flood records, and relevant prior studies obtained from the competent authorities. Field visits were conducted to characterize site conditions and verify the collected datasets, identifying high-risk critical points, and measuring the dimensions of hydraulic structures (bridges, culverts) and channel properties. Regarding modeling, design-flood scenarios with typical return periods were analyzed in accordance with the Directive 2007/60/EC. HEC-HMS was used to generate hydrographs for each sub-basin, which were then imported into the quasi-2D LISFLOOD-FP model as a means to prepare and calibrate the HEC-RAS model, where a rain-on-grid methodology integrated the hydrologic and hydraulic flood processes at the area of interest. High spatial resolution was maintained throughout, with particular emphasis on uncertainty analysis and on the detailed representation of infrastructure and urban areas, given their strong influence on flood dynamics. Results indicate that overflow typically occurs in buried streams, along adjacent roads in the downstream reach of the river, at stream confluences, and at the upstream inlet where natural streams enter the drainage pipe network. Up to 200 critical points were identified, of which up to 35% were classified as first-priority sites for intervention.