PSI - Issue 84

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

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mm diameter inflow pipe, with the signal adjusting the inverter frequency. The pump was installed in a tank beneath the flume, ensuring adequate recirculation volumes. A downstream flap gate allowed water level regulation. The localized erosion occurs in a fine sand layer 0.50 m thick, 1 m wide and 4 m long, bounded by coarser material to limit the bedforms development. The sand had a median grain size d 50 =0.4 mm, gradation coefficient = √ 84 / 16 =1.60 , and particle density ρ s =2650 kg/m³. Before each experiment, the canal bed was carefully leveled and slowly saturated. Inflow discharge was gradually increased to the predetermined value, after which the flow depth was set by adjusting the flap gate. Post-experiment, the flow was slowly decreased to prevent material deposition in the scour hole. Scour geometry was surveyed using photogrammetry (Agisoft Metashape software), with accuracy confirmed at 1 mm through comparison with a Vernier point gauge measurements in the current experimental setup. 3. Results and discussion Prior to assessing the foundation’s influence on skewed elongated piers, the elongated pier alone at skew angles of 0°, 15° and 30° was compared with a circular pier at two temporal scales: 4 hours and 48 hours. This comparison provides insight into how scour depth evolves over time as pier geometry transitions from simple to more complex configurations. It is important to note that the elongated pier measured 50 mm × 200 mm, while the circular pier had a diameter of 50 mm, maintaining a constant flow depth-to-pier width ratio (flow depth of 100 mm). Results are presented in Fig. 2, where the characteristic width D is fixed at 50 mm for all cases. As previously discussed, increasing the skew angle leads to larger scour depths at the elongated pier, a trend observed at both durations. After 4 hours, the circular pier produces greater scour than the parallel elongated pier and the pier skewed at 15°, while the 30° skew results in the largest scour depth. However, after 48 hours, the scour at the skewed elongated piers grows at a faster rate than that at both the parallel elongated pier and the circular pier. At this longer duration, the scour depth at the 15° and 30° piers becomes significantly larger, whereas the parallel elongated pier exhibits a scour depth smaller than that of the circular pier. These results highlight the critical role of time in both laboratory investigations and real-world conditions. Short-duration tests substantially underestimate scour depth, particularly for skewed elongated piers. In field settings, repeated flood events can therefore generate deep scour at elongated piers. Although aligning the pier parallel to the flow direction would minimize scour, such alignment is rarely feasible in practical applications.

Fig. 2. Comparison between the normalized scour depths (ds/D) at circular and elongated piers at two durations (4 and 48 hours)

After that, the scour at the elongated pier alone was compared with the corresponding pier-foundation systems to assess how foundation geometry influences scour depth. Figure 3 shows the ratio between the scour depth at the pier foundation system and that at the isolated pier, plotted against skew angle. A ratio equal to one indicates that the foundation has no influence, while values greater or less than one indicate that the scour depth is, respectively, larger or smaller than that at the pier alone for the same skew angle. When the smaller foundation (90 mm × 240 mm) is added beneath the pier with the top at the riverbed level, it reduces scour depth for the pier aligned with the flow direction. This occurs because the downflow is partially intercepted by the foundation, dissipating part of its energy. At a skew angle of 15°, however, the behavior reverses and the foundation increases the maximum scour depth. At