Scour holes downstream of bed sills in low-gradient channels

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.