PSI - Issue 84

Michele D’Amato et al. / Procedia Structural Integrity 84 (2026) 536–543

542

contribution to the scoured sample despite intermediate f c values. This behavior is consistent with the morphological instability of alluvial channels, which often develop non-ephemeral incisions causing flow misalignment during flood events and frequent exposure of foundation elements (Oliveto et al., 2004; Oliveto et al., 2006). The combined effect of riverbed typology and flow misalignment therefore represents a key control on scour occurrence. Another crucial factor in the study of erosion phenomenon is the reduction of the effective flow width beneath crossing structures. In this study, the contraction factor C a is defined according to MIT (2020) as the ratio between the overall width of the incised channel occupied by the footprint of piers and abutments, W al , and the overall width of the incised channel upstream of the bridge, W a : = , ∗100 (4) This parameter plays a key role in erosion and scours processes because it quantifies the reduction in effective channel width, which leads to increased flow velocity and turbulence. Fig. 6a illustrates the distribution of analyzed structures by contraction factor. Moderate constrictions (10-20% and 20-30%) dominate scour occurrences. These two classes together account for approximately 56% of the scoured bridges ( f s ) and nearly 47% of the total sample ( f t ) (Fig. 6a). This suggests that such reductions accelerate local flow and turbulence, enhancing scour potential. For factors exceeding 30%, smaller sample sizes limit representativeness. These findings align with established hydraulics, where contraction quantifies channel narrowing's role in erosive processes (MIT, 2020). Basin size is the last factor considered (Fig. 6b), with the sample classified into small (0–30 km²), medium (30– 200 km²), and large (>200 km²) catchments following Evangelista et al. (2024). Scour was observed in all classes, but medium-sized basins show the highest susceptibility, with the largest within-sub-sample scour rate and contribution to the scoured sample ( f c =76.7%, f s =47.9%), likely reflecting the higher occurrence of morphologically active alluvial channels within this basin class. Small and large basins exhibit lower scour incidence. This trend is consistent with the hydrological response of the analysed catchments, for which medium-sized basins more frequently generate flow conditions critical for scour at bridge crossings. 5. Conclusion This study presents a statistical analysis of scour-affected bridges along the provincial road network of Matera, aimed at preliminarily identifying the geometric and hydraulic factors most influencing scour occurrence and severity. The results show that rectangular piers with rounded corners are the most frequently affected by scour, while hexagonal and square shapes exhibit lower susceptibility, although based on a limited number of bridges. Among hydraulic variables, moderate contraction ratios (10–30%), non-zero flow attack angles, and alluvial or embanked riverbeds are consistently associated with higher scour frequencies. These findings confirm that scour vulnerability arises from the combined effects of pier geometry and hydraulic forcing, rather than from a single dominant parameter. As a result, hydraulic conditions play a key role in driving the Overall CoA of bridges, often corresponding to the highest vulnerability levels identified at the network scale. The study highlights the importance of integrated inspection and monitoring strategies that jointly address hydraulic and structural aspects, particular for bridges located in morphologically sensitive river reaches. The main findings of this work may support the improvement of the multi-level approach applied within the Italian bridge Guidelines for risk assessment of existing bridges. Acknowledgements This study was supported by FABRE – “Research consortium for the evaluation and monitoring of bridges, viaducts and other structures” (https://www.consorziofabre.it/). Any opinion expressed in the paper does not necessarily reflect the view of the funder.