Effects of a slotted bridge pier on the approach flow

Previous studies showed that a slot through a cylindrical bridge pier may reduce the scour depth around it. In fact, a part of the approach flow passes through the pier opening, influencing the downflow and, consequently, the horseshoe vortex. In the present study, a circular cylinder was placed in a sand-glued flume bed under steady and quasi-uniform flow conditions and two tests were performed. In Test 1, a four-beam side-looking acoustic Doppler Vectrino probe was used to measure the instantaneous velocity components upstream to the cylinder, in the plane of symmetry. In Test 2, a rectangular slot was drilled in the cylinder, near the bed, and the flow field measurements were repeated, in order to analyse and compare time-averaged velocity vectors and components, as well as turbulent characteristics, including turbulence intensity and turbulent kinetic energy. The flow depths upstream and downstream of the two cylinders were also measured. Results clarify that the presence of a slot through a cylinder alters and redistributes the profiles of velocity and turbulence characteristics close to the cylinder. An increase of flow velocity, streamwise and orthogonal turbulence intensities and turbulent kinetic energy and, on the contrary, a reduction of the spanwise component of turbulence intensity were observed in front of the slot at the locations nearest to the drilled cylinder.