Morphodynamic processes downstream of man-made structural interventions: Experimental investigation of the role of turbulent flow structures in the prediction of scour downstream of a rigid bed

Experimental program was conducted to analyze scour – caused by a horizontal jet downstream of a rigid bed – and the flow velocity and turbulence characteristics within the scour hole. The experimental program was motivated by the fact that the knowledge of flow field, including turbulent characteristics, is fundamental to correctly simulate the bed-load transport and the temporal evolution of the scour hole and, thus, to define protective measures of scouring.In this paper some results of experiments – carried out to understand the role of flow turbulence in the prediction of scouring process and sediment movement – are reported. The analysis is conducted on the basis of detailed measurements of flow velocity at various sections in the scour hole region. Both a 2D LDA anemometer and an acoustic velocimeter profiler (DOP2000) were used to conduct the velocity measurements.The results show that the flow is characterized by alternating high-low speed fluid regions which lead to the formation of vertical turbulent coherent structures. The turbulent intensity assumes significant values along longitudinal planes (i.e. parallel to the flow direction) in the near-bank zones. Maps documenting the correlations between the vertical and the longitudinal turbulent intensities are shown. With the aid of the experimental data, a relationship between the bed shear stress and the volume of sand eroded is defined. The quadrant analysis is applied for the qualitative description of the turbulent events. Analysis indicates that the major turbulent activity occurs in the deepest region of the scour hole. It seems that sweep and ejection events, which correspond to positive production of the turbulent energy, play an important role in erosion and transport of the sediment particles.

► Alternating high-low speed fluid regions are found in the scour hole.
► The major turbulent activity occurs in the deepest region of the scour hole.
► Ejection events are found over the area of scour hole with negative bed slope.
► Sweep events are found over the area of scour hole with positive bed slope.
► A relationship between the bed shear stress and the volume of sand eroded is found.