PSI - Issue 62

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Michele Palermo et al. / Procedia Structural Integrity 62 (2024) 593–600 Author name / Structural Integrity Procedia 00 (2019) 000–000

Fig. 5. Comparison of the temporal evolution of maximum scour depth for selected tests conducted by Pagliara et al (2010a) and (2015). Tests conducted for Q=33.5 l/s, h/D=5.67, debris 5D, sills S1, S2, G, D 50 /D=0.5 and D 50 /D=0.83, and L/D = (a) 0 and (c) 0.5; tests conducted for Q=17.0 l/s, h/D=2.67, debris 5D, sills S1, S2, G, D 50 /D=0.5 and D 50 /D=0.83, and L/D = (b) 0 and (d) 0.5. The analysis of plots reported in Fig. 5 reveals that the presence of macro-roughness elements generally reduces the scour depth in the absence of debris accumulation. Such behavior can be explained considering that the development of the dune is delayed by the presence of macro-roughness that alters the shear stress distribution acting on the bed material. Consequently, the scour hole extension is inhibited, and the scour depth does not increase. However, hydraulic conditions play an important role, since the effectiveness of macro-roughness elements tends to vanish for lower h/D values in the absence of debris. Conversely, in the presence of debris accumulation, macro-roughness elements always perform better than the sills for L/D=0, whereas the effectiveness of the two countermeasures are comparable for L/D=0.5, regardless of the inflow conditions. It is worth remarking that such behavior resembles that caused by plunging jets in the presence of protection screens (Pagliara and Palermo, 2008, Pagliara et al. 2010b). Namely, by increasing the distance of the protection structure, the jet does not impinge directly on it. Thus, a macro eddy in correspondence of the scour does not occur. In other words, the confinement effect due to the presence of the structure is reduced and the flow power is dissipated by eroding a larger scour surface (Pagliara and Palermo, 2020, Palermo and Pagliara, 2020). Nonetheless, results present in the open literature do not allow to establish general criteria for optimal location of the protection sills. This issue still represents a significant limitation for practitioners. In particular, the tested range of L/B seems to be rather narrow to derive general tools for the design of sills in the presence of debris accumulation. 4. Conclusions This paper discusses different types of countermeasures generally adopted to control erosion at bridge piers. In particular, we focused on two of them, i.e., sills/gabions and macro-roughness elements. In general, in the absence of debris accumulation, sills and gabions are effective in reducing the scour geometry, limiting maximum depth, scour area and volume. However, with debris accumulation, the presence of a sill/gabion alters the kinetics of the scour process. Namely, the evolution is characterized by four phases and the last phase exhibits peculiar features, resulting in a significant increase of the scour depth and its velocity. Furthermore, the scour hole may also reach lateral walls, thus endangering bank stability. As for macro-roughness elements, they were found to be always