Riverbed roughness and interlayer structures

Abstract

This research aims to enhance the scientific insight into dynamic processes between sediment and flow occurring in the near-bed region of rivers. Initially, this work focuses on strengthening the understanding of grain-scale roughness parameterisation. To achieve this, close-range photogrammetry is employed to obtain accurate Digital Elevation Models (DEM) of field and laboratory gravel patches. An evaluation of analytical procedures including DEM size, grid spacing and detrending method is undertaken on an extensive suite of roughness parameters including standard deviation of elevations, skewness, inclination index, and horizontal roughness lengths from second-order structure functions. A minimum DEM size of 16 × D50A in both directions (where D50A is the median grain size of the surface), grid spacing of 1 mm or below and a moving-window detrending method provide optimal grain-roughness parameterisation. The use of a single roughness parameter is deemed inappropriate due to spatial variability across surfaces. Subsequently, variations in roughness statistics across a gravel bar in the field are quantified and used to infer sedimentation patterns. Further, the importance of contextualising patch-scale field research is demonstrated with an example of the effect of vegetation on DEM quality. Secondly, spatial grain-scale interactions are quantified over fixed beds, obtained through the successful replication of natural surfaces using a casting technique, which provides a bridge between field experiments and laboratory analysis. Dense Acoustic Doppler Velocimeter (ADV) measurements along a pseudo-plane parallel to the bed are used to investigate the effect of submergence and quantify the spatial structure of near-bed flow properties. Flow properties including mean velocities, turbulence intensity, turbulent kinetic energy, Reynolds stress and quadrant events are calculated. Submergence exerts an influence on flow structures in the near-bed, as under high submergence, systematic imprints of bed topography on flow properties are observed, compared to a lack of coherence under low submergence. The novel presentation of isopleth maps of flow properties indicates uniformity in the scale of flow properties structure across four beds. Further, a decrease in surface roughness displays an increase in average near-bed velocity, however to the contrary, increased turbulence characteristics were observed across beds with increased roughness.