PSI - Issue 84

Pietro Giaretta et al. / Procedia Structural Integrity 84 (2026) 449–456

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pier, while the 30° skewed elongated pier exhibits the deepest scour. At longer durations (48 hours), scour at the skewed elongated piers increases more rapidly than at the circular and parallel elongated piers, exceeding the latter maximum scour depths. These results indicate that short-duration laboratory tests may significantly underestimate the ultimate scour depth. From a practical perspective, this highlights the potential cumulative impact of multiple flood events in increasing scour depth at bridge piers. For the 15° skewed elongated pier, the presence of a foundation consistently increases scour depth relative to the pier-alone configuration. Moreover, increasing the foundation size (when the top is aligned with the riverbed) leads to greater maximum erosion. When the larger foundation protrudes above the riverbed, scour depth increases further, representing conditions in which foundations become exposed due to general or contraction scour in real rivers. In contrast, for the 30° skewed pier-foundation system, scour depth seems to be independent of foundation size and protrusion here investigated, remaining comparable to that observed for the pier alone. Although this does not imply a safer condition, it suggests that, at large skew angles, maximum scour depth may be relatively insensitive to foundation geometry. Different behaviour is observed for the pier aligned with the flow. The smallest foundation reduces the intensity of the downflow, resulting in lower scour depth compared to the pier-alone case. The largest foundation appears to fully protect the pier, producing negligible erosion under laboratory conditions. However, when this foundation is exposed by 2 cm, significant scour occurred, exceeding that observed at the pier alone. This indicates that the apparent protective effect of large foundations for parallel piers is limited to highly controlled conditions and may not be representative of real-world scenarios, where general and contraction scour, as well as channel curvature, prevent perfectly aligned flow conditions. Finally, comparison between the 48 hours experimental results and predictions from the CSU equation shows that the latter overestimates scour depth for pier-foundation systems, while providing good agreement for isolated piers. When the CSU predictions are adjusted by reducing scour depth through the sediment coarseness coefficient ( 4 she ) and the temporal evolution of scour in the laboratory experiments is accounted (with 48 hours corresponding to approximately 80% of equilibrium scour), the CSU equation yields reasonable estimates for most configurations of pier-foundation systems. Conversely, for the pier-alone cases, the CSU equation tends to underestimate the measured scour depth. Acknowledgements The authors thank Sara Salvalaio for conducting part of the experimental work as part of her master’s thesis in Civil Engineering at the University of Padova. References Arneson, L. A., Zevenbergen, L. W., Lagasse, P. F., Clopper, P. E., 2012, Evaluating scour at bridges, Hydraulic Engineering Circular No. 18, Fifth Edition, FHWA HIF 12-003, Federal Highway Administration, Washington, D.C. Breusers, H. N. C., Raudkivi, A. J., 1991. Scouring. Balkema, Rotterdam. Castellarin, M., 2004. Influenza delle fondazioni sullo scavo localizzato in corrispondenza di pile da ponte. L'ACQUA, (5/2004), 29-39. (in Italian). Chabert, J., 1956. Etude des affouillements autour des piles de ponts. Rep., Laboratorie National d’Hydraulique, Chatou. Chiew, Y. M., 1984. Local scour at bridge piers [Doctoral thesis, University of Auckland] Ettema, R., 1980. Scour at Bridge Piers. Department of Civil Engineering: Report No. 216. Department of Civil Engineering, University of Auckland. Ettema, R., Kirkil, G., Muste, M., 2006. Similitude of large-scale turbulence in experiments on local scour at cylinders. Journal of Hydraulic Engineering, 132(1), 33-40. Ettema, R., Constantinescu, G., and Melville, B. W., 2011. National Cooperative Highway Research Program (NCHRP Project 24-27(01)). In Evaluation of bridge scour research: Pier scour processes and predictions. Washington, DC: NCHRP. Giaretta, P., 2025. Countermeasures against scour around piers of existing bridges [Doctoral thesis, University of Padova] Jones, J. S., Kilgore, R. T., Mistichelli, M. P., 1992. Effects of footing location on bridge pier scour. Journal of Hydraulic Engineering, 118(2), 280 290. Lagasse, P. F., Clopper, P. E., Zevenbergen, L. W., and Girard, L. G., 2007. National Cooperative Highway Research Program (NCHRP Report

593). In: Countermeasures to protect bridge piers from scour. Washington, DC: NCHRP. Landers, M. N., 1992. Bridge scour data management. USGS Staff--Published Research, 141.