Preliminary analysis of scour due to unsteady plunging jets

Scour process due to plunging jets is one of the most relevant topics for hydraulic engineers, especially because of the consequences of an excessive erosion. Generally, a scour process takes place downstream of hydraulic structures and its mechanism and evolution can be extremely complex, depending on both structure typology and geometric configuration. Therefore, a deep understanding of the scour behaviour assumes a fundamental importance. Scuor mechanism has been extensively studied in the last decades. In particular, relevant studies have been conducted focusing on both main scour hole length prediction and on the evolution kinetics. The erosive phenomenon can be either bi-dimensional (2D) or three-dimensional (3D), according to both the equilibrium morphologies and jet diffusion dynamics. Pagliara et al. (2006) analysed the main parameters governing the 2D scour process, concluding that it depends on several non-dimensional groups. A similar analysis was conducted by Pagliara et al. (2008a) for 3D scour, revealing that its mechanism is governed by the same parameters of 2D. Finally, scour depth evolution represents another important issue, and it is characterized by two different phases (developing and developed). In particular, Pagliara et al. (2008b) furnished useful equations to estimate the evolution trend, concluding that it depends on jet inclination and the developed phase is characterized by an asymptotic behaviour. Nevertheless, all the mentioned studies analysed the scour mechanism under steady flow conditions, i.e., for a constant jet discharge. To the authors’ knowledge, there are no studies present in the literature analysing the main scour features due to unsteady plunging jets. It is worth noting that such type of scour mechanism generally occurs downstream of transversal hydraulic structures (e.g., weirs, check dams, etc.) during flood events or as a consequence of multiple-floods. Therefore, it assumes a relevant practical importance. This paper aims to furnish a very preliminary analysis of the scour process, by analysing longitudinal profiles due to different inflow hydrographs. The preliminary analysis focuses on the comparison of dynamic equilibrium profiles under unsteady conditions with those due to a steady discharge equal to the peak hydrograph discharge.