PSI - Issue 62

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

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dangerous in practical applications, since the scour depth may be larger than that characterizing the corresponding configuration without sill. Furthermore, the lateral expansion of the scour hole at the sill toe may endanger bank stability; d) the surface roughness slightly delays the process but does not significantly affect the scour mechanism.

Fig. 3. Diagram sketches illustrating the different phases of the scour process in the presence of a sill located at L / D =0: Phase (a) 1; (b) 2; (c) 3; (d) 4. 3. Macro-roughness elements Pagliara et al. (2015) tested an alternative methodology to control scour evolution at bridge piers. Namely, the authors conducted an experimental campaign to assess scour evolution characteristics at bridge pier with and without debris accumulation (3D and 5D) and with different macro-roughness arrangements downstream of the pier. More specifically, the concentration of macro-roughness elements  =N(  D 50 2 /4)/(bL x ) ranged from 0 (absence of macro-roughness elements) to 0.032, with N indicating the number of elements and D 50 their average diameter. Such concentrations were obtained by locating macro-roughness elements in rows, characterized by a longitudinal and transversal distance d x and d y , respectively, with L x indicating the longitudinal distance between the first and last row (Fig. 4). Based on dimensional arguments, Pagliara et al. (2015) found the z max /D can be expressed as follows, where F d =U/{g[(  s -  )/  ]d 50  0.5 denotes the densimetric Froude number and g is the gravitational acceleration: � ��� � ���∆ , � , ℎ � , � � � , � � � ,  , ∗ � (2) The authors analyzed the scour mechanisms focusing on the effect of macro-roughness elements on equilibrium geometry and scour kinetics, under different configurations. Experimental evidenced allowed to conclude that: a) the protection effect of macro-roughness elements mainly depends on their concentration and slightly on dimensions of single elements. Nonetheless, smaller elements may perform slightly better than larger counterpart for high values of  A; b) the kinetics of the scour process is strongly affected by debris accumulation and macro-roughness elements. Namely, three phases can be distinguished. Phase 1 starts from the beginning of the test up to when the macro roughness elements roll into the scour hole. Such phase is very rapid and generally ends at T * 1  2000. Phase 2 is characterized by a removal of sediment from the scour pothole. The dune starts forming and migrating downstream.