Risk Assessment, Analysis of Alternatives and Catchment Modeling for a Railroad Crossing Flood-prone Areas in south Italy

During the design of a railroad in south Italy a critical flood risk has been investigated. Several solutions have been studied to ensure the safety of the infrastructure, considering hydraulic, economic and alignment aspects. In particular, for each alternative, has been evaluated the effect of the structures’ layout in term of flooding hazard change, compared with the pre-construction scenario. Specific 2D models have been implemented for the design selected options using the software Infoworks ICM. A comparison with HEC-RAS has been also done to check the results. The outputs of the two models resulted to be very close in term of water depth, with a small over-estimation of the heights in the Infoworks model.

During the study, has been also performed a hydrological analysis to estimate the peak discharges and the input hydrographs of the river that causes the flooding. A relevant number of gauging stations has been considered and more than 70 years of observed rainfall data have been processed, using the best fitting statistical distribution. A rainfall-runoff model has been then applied by estimating the CN values of the basin with available soil use maps. The hydrographs shapes have been deduced using the Nash method. Thank to the availability of detailed digital ground elevation models, obtained by LIDAR data and other surveys, an accurate grid of the territory surface has been extracted. Once defined the flood modeling in the pre-construction condition (50, 100 and 300 year ARI), it was possible to update the hazard maps of the studied area, following the local laws criteria. The existing flooding maps, included in the regulation documents and draw many years ago, don’t highlight such a critical situation, probably because they were obtained with modeling tools now obsolete. The choice of the optimal alternative for flood risk mitigation structures came out from an analysis of alternatives in which the weights and scores of each criterion have been agreed with the local authorities, the infrastructure manager and the works financier. A multi criteria matrix has been implemented to identify the most suitable solution. The alternative that reached the highest score consisted in a stretch of floodwall, an alignment rising with a water-transparent concrete structure and an earth embankment with slopes lined with mattresses. This solution combined a low flood risk impact with reduced construction costs and maintenance issues, guaranteeing the rail passenger service even for the most extreme and rare events.