Detection and Analysis of Coherent Flow Structures in a Depth-Limited Flow over a Gravel Surface

In gravel-bed rivers, the micro-topography of the bed is known to exert a significant influence on the generation of macro-scale turbulent flow structures that owe their origin to fluid shear generated near the bed. Although field and laboratory measurements have indicated that flows over gravel beds contain a range of coherent flow structures, the kinematic and dynamic properties of these structures are still poorly understood. Here, we demonstrate a new experimental methodology to quantify simultaneously both the kinematic and dynamic characteristics of coherent flow structures based upon combined planar laser-induced fluorescence and particle-imaging velocimetry (PLIFPIV). The results identify and demonstrate that the coherent flow structures generated over gravel surfaces are similar to both the model of Falco (1991) and previous field visualization results, where absolute and convective instabilities occur through bed-generated (i) pressure drag and (ii) Kelvin-Helmholtz roller-type instabilities generated along the separation zone shear layer formed around the larger anchor clasts in the bed.