PSI - Issue 84

Maria Giovanna Tanda et al. / Procedia Structural Integrity 84 (2026) 272–279

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interest in literature for a long time. Starting from the experimental work of Laursen and Toch (1956) in the search for identifying the parameters that influence erosion, many researchers have attempted to deal with this subject. A valuable review of the existing literature can be found in Ettema et al. (2011, 2017). The complex flow pattern and scour mechanism around a pier immersed into the flow is well known and has been reported by various investigators (Ettema et al., 2011; Melville and Coleman, 2000). The presence of the pier modifies the flow field on the upstream side generating a downward flow, which then changes into an upward flow along the riverbed. This process leads to the formation of the so-called horseshoe vortex system around the pier's base. These vortices create a scour hole when the shear stress on the stream bed is greater than the critical shear stress, transporting sediment from the upstream to the downstream side. The greatest depth occurs on the upstream face in case of circular thin piers (Raudkivi and Ettema, 1983). Due to the importance of the problem for pier stability, a lot of efforts has been devoted to investigate not only the maximum depth of erosion but the countermeasures able to limit the groove and delay of scour in time. Due to the nature of the flow field, two categories of methods are commonly used to reduce scour depth (Melville and Coleman, 2000; Singh et al., 2020; Tafarojnoruz et al., 2010; Tafarojnoruz et al., 2012). The first one consists in increasing stream bed resistance to withstand erosion and is generally unfolded by placing armoring devices at the bed level: riprap protections or bed sills are two examples as they provide a physical barrier to resist against the erosive power of the flow. The second category consists in adopting flow altering countermeasures: among those, collars placed around the pier body are one of the mostly tested. A collar, hooked or unhooked (see Figure 1 for nomenclature), modifies the flow pattern around the pier by intercepting the downflow and hence decreasing the horseshoe vortex strength around the pier. Studies on circular piers have shown the effectiveness of collars in reducing the scour depth (Chiew, 1992; Ettema, 1980; Tanaka and Yano, 1967). Many authors, working independently, concluded that the efficacy of a collar in reducing the scour depends upon its width and elevation relative to the bed surface (Ettema, 1980; Kumar et al., 1999; Singh et al., 2001; Zarrati et al., 2004). The literature review shows that numerous studies have examined the effectiveness of collars in reducing local scour. However, few of them have considered piers with cross-sectional shapes other than circular together with collars installed below the bed of the watercourse, yet still significantly above the maximum scour depth that would occur without them. Hence, this work focuses on reduction of scour through collars applied around circular and round-nosed rectangular piers at a height close or well under the riverbed. In the case of circular piers, both simple collars and hooked collars were tested. The paper summarizes the results of the experimental tests conducted in the laboratory of Hydraulics and Hydraulic Structures of the Department of Engineering and Architecture of the University of Parma.

Figure 1-Definition sketch and nomenclature of the experimental setup: circular pier with simple/ hooked collar (left); round-nosed rectangular pier with simple collar (right).