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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1. Introduction Since bridges are crucial infrastructures, their design requires particular attention. Consequently, the analysis of effective countermeasures is of fundamental importance. Scour evolution and its equilibrium configuration depend on several factors, ranging from hydraulic characteristics of the flow/fluid (i.e., among others, the discharge Q, the approaching flow depth h and the density  ), the width of the channel b and pier diameter D, the properties of sediment (i.e., density  s and average diameter d 50 ) to the time t (Chiew, 1995, Lança et al., 2013, Melville and Chiew, 1999, Melville and Sutherland, 1988). The complexity of the scour mechanism is further exacerbated by the presence of debris accumulation (Melville and Dongol, 1992, Pagliara et al., 2010a, Palermo et al., 2021), that modifies both the kinetics of the scour and its equilibrium morphology. Therefore, countermeasures are usually adopted in order to control/limit local erosion. Among others, collars and submerged vanes are widely used in practical applications (see for instance Kumar et al.,1999, Ghorbani and Kells, 2008, Moncada et al., 2009, Odgaard and Wang, 1987, Zarrati, et al., 2006). However, more recently several studies have been presented focusing on the effect of sills presence on geometric features (e.g., Grimaldi et al., 2009a and 2009b, Pagliara et al., 2010a). Such studies revealed that the presence of a sill should be taken into careful consideration since it may also cause an increase of scour depth after a certain time depending on its location. Likewise, a novel approach to control scour was introduced by Pagliara et al. (2015), who tested macro-roughness elements located downstream of the pier. The authors analyzed the effect of such elements in delaying the dune and scour hole expansion. This study aims at comparing the scour mechanisms in the presence of sills/gabions and macro-roughness, highlighting similitudes and differences in the evolution process. Furthermore, the physics of the phenomenon is also discussed, evidencing interesting insights that may be helpful in practical applications and for future research.

Nomenclature A acc

total cross-sectional area blocked by both pier and debris accumulation

b

channel width pier diameter debris width

D d d d x d y

longitudinal distance between two macro-roughness elements transversal distance between two macro-roughness elements

d 50 D 50

average diameter of bed material

average diameter of macro-roughness elements

F d

densimetric Froude number approaching flow depth sill/gabion distance from the pier

h L

L x l d n s Q

longitudinal distance between the first and last row of macro-roughness elements

debris length

geometric surface roughness of the sill or gabion

discharge

t

time

t d

debris thickness

T * U U c

non-dimensional time average velocity threshold velocity maximum scour depth blockage ratio percentage

z max  A

non-dimensional concentration of macroroughness elements

 ξ i 

temporal evolution coefficient of the i-phase

water density sediment density

 s