An experimental study on long local scouring downstream of bed sills in a monogranular sand bed was carried out in the hydraulic laboratory flume at the Mediterranean Agronomic Institute of Bari (Italy). The main objectives of this study were to determine scour hole dimension, with its maximum scour depth as a function of time and at the equilibrium stage, the scour hole shapes and the investigation of the influence of sills on the distribution of the three-velocity components through the scour hole at the same stage. Four experimental configurations were tested, the main difference between them being the distance between sills. Based on experimental data, the classical dimensional analysis of the variables that influence the development of the scour hole has been carried out in order to obtain two empirical formulas predicting the maximum scour depth and the length of the scour hole at the equilibrium stage. Moreover, it was observed that the distance between sills influences the scour hole dimension and shape. The three-velocity components of the flow, measured with an acoustic Doppler velocimeter, show that in the scour hole, at the equilibrium stage, the three components of the flow turbulence intensities are very high. Near-bed flow vortexes in addition to secondary currents are also observed. RÉSUMÉUne étude expérimentale sur l'affouillement local à long terme à l'aval des seuils le long d'un lit à sable mono-granulaire a été élaborée dans un canal d'essai au laboratoire d'hydraulique de l'Institut Agronomique Méditerranéen de Bari (Italie). Les objectifs principaux de cette étude sont; la détermination des dimensions de l'affouillement, à savoir, l'évolution de sa profondeur maximale en fonction du temps et à la phase d'équilibre, sa forme et l'analyse de l'influence du seuil sur la distribution des trois composantes de la vitesse d'écoulement lors de la même phase. Quatre configurations expérimentales ont été réalisées, la différence entre ces dernières étant l'écartement entre les seuils. Sur la base des résultats expérimentaux, l'analyse dimensionnelle classique des variables influençant le développement de l'affouillement a induit à l'obtention de deux formules empiriques permettant la détermination de la profondeur maximale et de la longueur d'affouillement à l'équilibre. En outre, il a été observé que l'écartement entre les seuils présente une influence aussi bien sur les dimensions que sur la forme de l'affouillement. Les mesures de la vitesse d'écoulement tridimensionnelle par le biais d'un Acoustic Doppler Velocimeter (ADV) a montré que dans l'affouillement, durant la phase d'équilibre, les trois composantes de l'intensité de la turbulence de l'écoulement sont très élevées. Une circulation de fluide en vortex à proximité du lit ainsi qu'un développement d'écoulement secondaire ont été observés.

Despite the many studies on flow in partly obstructed open channels, this issue remains of fundamental importance in order to better understand the interaction between flow behavior and the canopy structure. In the first part of this study we suggest a new theoretical approach able to model the flow pattern within the shear layer in the unobstructed domain, adjacent to the canopy area. Differently from previous studies, the new analytical solution of flow momentum equations takes into account the transversal velocity component of the flow, which is modelled as a linear function of the streamwise velocity. The proposed theoretical model is validated by different experiments carried out on a physical model of a very large rectangular channel by the research group of the Department of Civil, Environmental, Building Engineering and Chemistry of the Technical University of Bari. An array of vertical, rigid, and circular steel cylinders was partially mounted on the bottom in the central part of the flume, leaving two lateral areas of free flow circulation near the walls. The three-dimensional flow velocity components were measured using a 3D Acoustic Doppler Velocimeter. A comparison of the measured and predicted data of the present study with those obtained in other previous studies, carried out with different canopy density, show a non-dependence of this analytical solution on the array density and the Reynolds number. In the second part of the paper, detailed observations of turbulent intensities and spanwise Reynolds stresses in the unobstructed flow are analyzed and discussed. Differently from some earlier studies, it was observed that the peak of the turbulence intensity and that of the spanwise Reynolds stress are significantly shifted toward the center of the shear layer.

No abstract