Spatial structure of near-bed flow properties at the grain scale

Abstract

Expanding the existing knowledge of the role of roughness on near-bed flow properties is vital to provide deeper insights into the interactions between the flow, sediment transport, morphology and ecology. This study uses four fixed beds, from both the laboratory and the field, to characterise the structure of near-bed flow properties. Dense measurements over a grid of 6 mm spacing were conducted across each cast, using an Acoustic Doppler Velocimeter. These measurements followed the line of the bed topography, with the sampling location being 2 mm away from the maximum bed elevations. Near-bed flow properties were calculated across the surface; including velocity, turbulence intensity, turbulent kinetic energy, Reynolds stress and quadrant analysis. Improved visualisation of flow properties involves contour plots of the spatial distribution of flow over the DEM of the gravel bed to facilitate attributions of flow patterns to roughness and topography. Using the standard deviation of elevations as a proxy for grain roughness of each surface, we observed that a decrease in roughness led to an increase in the average near-bed velocity. In contrast, an increase in roughness resulted in an increase in both turbulence intensity and turbulent production (TKE). Analysis of quadrant events dominating across the surface can provide indications into the potential for erosion and/or deposition across the natural mobile surfaces. Finally, second-order structure function analysis is used to quantify the spatial structure of flow properties, presenting novel isopleth maps of flow properties. For the majority of flow properties across all surfaces, the range in the spatial extent of flow structures is between 1 and 2.25 × D50A, except for transverse and vertical velocities, which have a wider range up to 3.5 × D50A.