water Article Secondary Currents with Scour Hole at Grade Control Structures Mouldi Ben Meftah * , Diana De Padova , Francesca De Serio and Michele Mossa   Citation: Ben Meftah, M.; De Padova, D.; De Serio, F.; Mossa, M. Secondary Currents with Scour Hole at Grade Control Structures. Water 2021, 13, 319. https://doi.org/10.3390/ w13030319 Academic Editor: Roberto Gaudio Received: 25 December 2020 Accepted: 23 January 2021 Published: 28 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Civil, Environmental, Land, Building Engineering and Chemistry, Polytechnic University of Bari, Via E. Orabona 4, 70125 Bari, Italy; diana.depadova@poliba.it (D.D.P.); francesca.deserio@poliba.it (F.D.S.); michele.mossa@poliba.it (M.M.) * Correspondence: mouldi.benmeftah@poliba.it; Tel.: +39-080-5963508 Abstract: Most studies on local scouring at grade control structures have principally focused on the analysis of the primary flow field, predicting the equilibrium scour depth. Despite the numerous studies on scouring processes, secondary currents were not often considered. Based on comprehen- sive measurements of flow velocities in clear water scours downstream of a grade control structure in a channel with non-cohesive sediments, in this study, we attempted to investigate the generation and turbulence properties of secondary currents across a scour hole at equilibrium condition. The flow velocity distributions through the cross-sectional planes at the downstream location of the maximum equilibrium scour depth clearly show the development of secondary current cells. The secondary currents form a sort of helical-like motion, occurring in both halves of the cross-section in an ax- isymmetric fashion. A detailed analysis of the turbulence intensities and Reynolds shear stresses was carried out and compared with previous studies. The results highlight considerable spatial heterogeneities of flow turbulence. The anisotropy term of normal stresses dominates the secondary shear stress, giving the impression of its crucial role in generating secondary flow motion across the scour hole. The anisotropy term shows maximum values near both the scour mouth and the scour bed, caused, respectively, by the grade control structure and the sediment ridge formation, which play fundamental roles in maintaining and enhancing the secondary flow motion. Keywords: scour; equilibrium condition; velocity field; secondary currents; turbulence 1. Introduction The presence of natural or man-made structures on riverbeds plays an important role in the evolution of river morphology and sediment entrainment. Flow turbulence properties and secondary currents play a crucial role in sediment transport, and, in turn, suspended particle motion influences turbulence, such as Reynolds shear stress and veloc- ity [1]. In addition to the fluid–particle interactions, which definitely influence the flow velocity distribution, the fluid–structure interactions, i.e., with natural vegetations, riverbed debris, bridge piers and abutments, sills, sluice gates, spillways, weirs, spur dikes, off-shore platforms, wind turbines, energy converters, etc., cause additional complex effects on the flow hydrodynamic characteristics. Local scouring is produced due to these complex flow patterns occurring in the surroundings of such structures. The local scouring process has attracted the attention and interest of many scientists for decades [2–18]. Experimental studies on the scouring process at grade control structures (GCSs) in riverbeds [3,5,9,17,18] showed that the scour often developed downstream of the structure. The extension of the scour hole is strongly influenced by the properties of the incoming flow, which is usually a two-dimensional jet-like flow. Owing to the high velocity of the entering jet flow, a large amount of sediment erosion locally occurs downstream of the GCS, forming the scour hole. Because of the large velocity gradients among the jet flow and that in the scour pool, the jet diffuses near the bed, and is redirected at a reduced bed velocity. The equilibrium state occurs when the path of the impinging jet becomes long enough and its diffused velocity is reduced to values below the minimum value required to move Water 2021, 13, 319. https://doi.org/10.3390/w13030319 https://www.mdpi.com/journal/water