Assessment of direct rainfall and flood-induced damage to land transport infrastructure using two-dimensional HEC-RAS 6.6 rain-on-grid simulations

This study explores the application of the Rain-on-Grid approach within the two-dimensional (2D) Hydrologic Engineering Center's River Analysis System (HEC-RAS, version 6.6) for a selected area (similar to 230 km(2)) of the Low Calore River catchment in Southern Italy, which was heavily hit by an extreme rainfall event on October 14-15, 2015. This event, lasting about 17 h, triggered a range of geo-hydraulic phenomena, including extensive flooding of the Calore River, with physical damage to the railway infrastructure. The hydrodynamic model was used to reconstruct the effects of the observed rainfall event by including relevant processes such as spatially distributed rainfall, upstream discharge input, infiltration losses, and flow propagation across hillslopes and within the Calore River. The results demonstrate that 2D Rain-on-Grid HEC-RAS simulations, which account for the minor tributary network, can reproduce realistic correlations with recorded damage to linear transport infrastructure. The comparison with a traditional fluvial flooding approach, where a given discharge hydrograph is used as the only input to the hydrodynamic model, shows that the traditional approach fails to evaluate the activation of the minor tributary network, leading to an underestimation of potential infrastructure damages, and the inability to explain observed damages. These results suggest that hazard maps should explicitly model pluvial and compound pluvial-fluvial flooding when assessing risks to transportation networks. However, the findings also highlight certain limitations, including the need for more detailed and spatially distributed input data and increased computational time.