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.

While studies on vegetated channel flows have been developed in many research centers, studies on jets interacting with vegetation are still rare. This study presents and analyzes turbulent jets issued into an obstructed cross-flow, with emergent vegetation simulated with a regular array of cylinders. The paper presents estimates of the turbulence diffusion coefficients and the main turbulence variables of jets issued into a vegetated channel flow. The experimental results are compared with jets issued into unobstructed cross-flow. In the presence of the cylinder array, the turbulence length-scales in the streamwise and transverse directions were reduced, relative to the unobstructed crossflow. This contributed to a reduction in streamwise turbulent diffusion, relative to the unobstructed conditions. In contrast, the transverse turbulent diffusion was enhanced, despite the reduction in length-scale, due to enhanced turbulent intensity and the transverse deflection of flow around individual cylinders. Importantly, in the obstructed condition, the streamwise and transverse turbulent diffusion coefficients are of the same order of magnitude.

This paper describes a theoretical approach of turbulent flow in an infinite square array of emergent cylinders distributed uniformly along a channel bottom. Laboratory experiments were carried out to investigate the flow velocity characteristics in order to confirm this theoretical approach. A model array of vertical, rigid, circular, and threaded steel cylinders, of regular distribution, was mounted on the bed of a recirculating hydraulic channel. Measurements of the three-dimensional flow velocity components were carried out using a 3D acoustic Doppler velocimeter. At a certain distance downstream of a lateral row of cylinders, the transversal profiles of velocities and turbulence parameters are expected to be periodic functions in the lateral position whose period is the centre-to-centre spacing of two side-by-side cylinders. Some empirical expressions for predicting the turbulent flow features have been established. The formulation proposed is consistent with the experimental data over a certain range of the Reynolds number based on the cylinder diameter. In addition, a new formula to estimate the bulk drag coefficient $\overline{C_D}$ of the array of cylinders is proposed and validated.

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.

Despite the numerous studies on scouring processes, the prediction of scour-hole dimensions downstream of hydraulic structures remains challenging because of the complexity of the phenomenon and its dynamic sensitivity to structure and sediment properties. This study experimentally focuses on scour-hole development downstream of a sloped grade-control structure (GCS) in alluvial channels. A large series of laboratory experiments were carried out in a rectangular channel with a noncohesive sediment bed. Based on the data from this study and data collected from previous studies, the effect of the downstream face slope of a GCS on scour morphology was analyzed. In this regard, it was found that the face slope has an effect only if it is smaller than the slope of the upstream equilibrium scour side obtained with a GCS of a vertical downstream face. Before reaching an equilibrium state, the scour process evolves into three distinct phases, a very rapid initial phase, an intermediate gradual phase, and an equilibrium phase. A general empirical expression for predicting temporal scour evolution is proposed and extended to different types of GCS. Moreover, a new scaling approach is proposed that leads to the derivation of new equations predicting equilibrium scour profiles with different entering jet-flow typologies. To make these equations operational, a series of estimating expressions for the characteristic lengths of equilibrium scour holes is also proposed.