Development of Hydraulic Hazard Mapping within a GIS Environment: Interoperability Between HEC-RAS and QGIS

Biagio Saya 

Alluvial events represent some of the most destructive natural hazards threatening human
civilization. Over recent decades, the frequency of extreme rainfall has exposed the Italian
territory to periodic and highly damaging inundations, resulting in a dramatic escalation of
financial losses and mortality rates. In regions such as Sicily, several industrial and artisanal
clusters are strategically yet precariously situated adjacent to natural river embankments, continuously exposing them to high flood risks. Consequently, determining advance flood velocity thresholds across varying return periods, along with precise water depth metrics, is critical for localized risk mitigation. This paper outlines an integrated methodological framework that pairs advanced hydraulic numerical modelling with Geographical Information Systems (GIS) to execute robust hydraulic susceptibility analyses. Integrating hydraulic simulation data into QGIS yields high-resolution flood zoning maps that link physical hydraulic variables to regional vulnerability indices. Crucial spatial input layers, including topography (high-resolution LiDAR DTM/DSM), biology (riparian flood plain vegetation roughness), and bed-load granulometry, serve as fundamental parameters determining channel frictional coefficients. To demonstrate the interoperability of HEC-RAS and QGIS, a real-world application is presented along the Rosmarino River axis. The results illustrate how localized hydraulic outputs can be smoothly post-processed via Raster Calculators into statutory hazard zoning bands (P1 to P4), establishing a scientific tool for public territorial planning and Civil Protection execution.