Physical and numerical model of a new sediment trap

Several researches have been recently carried out using sediment traps, in order to study many biogeochemical processes in natural waters. Traditionally, the sedimentation process of these traps are approximately based on a vertical fall of particles through the water column. In highly turbulent environments, strong current flows generally oppose to the sedimentation, causing a horizontal outflow of particles. In these conditions, the trapping efficiency should be checked and improved. Consequently, the analysis of the flow field, near the mouth of the trap and inside it, is necessary also considering that internal eddies, due to turbulence, interact with the sedimentation process. The present study analyzes the behaviour of a new sediment trap design (Orsi et al., 1998), which differs from the previous ones in the geometry of both adduction channel and interface between the channel and the trapping area. This new trap, which is completely passive, can align to the stream and generate a consistent velocity reduction inside its trapping part. It was tested both by a physical and a numerical model. Firstly, the physical model was investigated at the Coastal Engineering Laboratory of the Technical University of Bari (Italy). The profiles of the longitudinal velocities were measured at different sections, by means of a 2D ADV. Successively, the trap was numerically simulated by means of the ANSYS software. The laboratory test was validated by the numerical results. In fact, the comparison between the physical and the numerical model show a good agreement, highlighting the presence of a dominant clockwise current in the trapping room.