PSI - Issue 84

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

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foundations are often exposed above the bed due to construction constraints and the progression of general and contraction scours, which tends to increase the maximum scour depth (Breusers and Raudkivi, 1991). For rectangular pier-foundation systems, Jones et al. (1992) observed a reduction in scour depth for foundations located either above or below the bed surface, though their experiments were limited to piers aligned with the flow direction. Parola et al. (1996) reported similar conclusions but did not examine foundation effects at skewed piers. Castellarin (2004) further investigated the influence of foundation size and exposure relative to the riverbed. For a circular pier on a rectangular foundation aligned with the flow and with its top at the bed level, scour was reduced compared to the case without a foundation. Increasing the plan dimensions of the foundation led to additional scour reduction, and beyond a certain threshold the foundation fully protected the bed, preventing scour altogether. Despite these insights, the influence of foundations beneath skewed elongated pier-foundation systems remains largely untested, and available literature is insufficient to draw general conclusions. It is important to note that perfectly aligned pier-foundation systems are rarely encountered in real-world conditions, as even small angular deviations commonly arise due to ongoing morphodynamic adjustments of the river.

Fig. 1. Scheme of the laboratory flume used to develop the localized scour experiments

In the present work, experiments from physical models focus on elongated piers with rounded noses and tails, set on footings of varying planimetric dimensions. Firstly, scour at an elongated pier with varying skew angle is compared with that at a circular pier with diameter equal to the width of the elongated pier. These experiments maintain consistent hydraulic conditions and are conducted without foundations. Then, the influence of the skew angle of the pier foundation system and the foundation protrusion is evaluated by comparing results with scour at a pier alone under the same experimental conditions. Finally, the study’s results are compared with the CSU formula (HEC-18) as a predictive method, also considering the influence of experiment duration. This comprehensive approach aims to provide new insights into the complex interactions between pier geometry, foundation characteristics, and scour development in skewed configurations. 2. Experimental set-up Experiments were conducted at the Laboratory of Hydraulics and Hydraulic Constructions of the Department of Civil, Environmental and Architectural Engineering at the University of Padova, within a flume 1 m wide, 1 m deep, and 16 m long (Fig. 1). The elongated pier measured 50 mm × 200 mm with rounded noses, while two rectangular foundations of 90 mm × 240 mm and 130 mm × 280 mm were tested. The pier-foundation systems were examined with the top of the foundation aligned with the initial riverbed level, and the largest foundation has also been tested with a 20 mm exposure. All configurations, including the pier alone, were tested at skew angles of 0°, 15° and 30°. Additionally, a circular pier of 50 mm diameter without the foundation was tested for comparison. In all the experiments, the flow depth was set 10 cm, with flow velocity maintained at 90% of the critical value to approach the maximum scour depth (e.g. Melville and Coleman, 2000). The critical velocity, calculated using the Laursen (1963) formula, was experimentally verified in the flume. Each experiment lasted 48 hours to grant proper scour evolution. It is to note that for wide circular piers, scour depth at 48 hours is approximately 80% of the maximum (Giaretta, 2025). Water supply to the flume was provided by a circulation system powered by an electric pump (Flygt model CS 3102 LT) with a 45 L/s maximum capacity, equipped with an inverter regulated by a PID controller (Automation Progetti model AP222). Flow rate was controlled using a flow meter (Endress-Hauser model Promag W) on the 150