Sensitivity of interfacial hydraulics to the microtopographic roughness of water‐lain gravels

Abstract

Flow within the interfacial layer of gravel-bed rivers is poorly understood, but this zone is important because the hydraulics here transport sediment, generate flow structures and interact with benthic organisms. We hypothesized that different gravel-bed microtopographies generate measurable differences in hydraulic characteristics within the interfacial layer. This was tested using a high density of spatially and vertically distributed, velocity time series measured in the interfacial layers above three surfaces of contrasting microtopography. These surfaces had natural water-worked textures, captured in the field using a casting procedure. Analysis was repeated for three discharges, with Reynolds numbers between 165000 and 287000, to evaluate whether discharge affected the impact of microtopography on interfacial flows. Relative submergence varied over a small range (3.5 to 8.1) characteristic of upland gravel-bed rivers. Between-surface differences in the median and variance of several time-averaged and turbulent flow parameters were tested using non-parametric statistics. Across all discharges, microtopographic differences did not affect spatially averaged (median) values of streamwise velocity, but were associated with significant differences in its spatial variance, and did affect spatially averaged (median) turbulent kinetic energy. Sweep and ejection events dominated the interfacial region above all surfaces at all flows, but there was a microtopographic effect, with Q2 and Q4 events less dominant and structures less persistent above the surface with the widest relief distribution, especially at the highest Reynolds number flow. Results are broadly consistent with earlier work, although this analysis is unique because of the focus on interfacial hydraulics, spatially averaged 'patch scale' metrics and a statistical approach to data analysis. An important implication is that observable differences in microtopography do not necessarily produce differences in interfacial hydraulics. An important observation is that appropriate roughness parameterizations for gravel-bed rivers remain elusive, partly because the relative contributions to flow resistance of different aspects of bed microtopography are poorly constrained.

Sampling variability in estimates of flow characteristics in coarse‐bed channels: Effects of sample size

“…For example, the standard deviation of shear stress and roughness length derived from spatially averaged profiles was higher in the Rapid reach but, because of higher values of these parameters in comparison to Pool‐Riffle, the coefficients of variation were similar. The higher absolute variation in Rapid compared to Pool‐Riffle and in riffles relative to pools is in line with research indicating that small relative depth is associated with higher variability in flow [ Cooper et al ., ; Rice et al ., ]. The relative nature of the data can explain other, seemingly counterintuitive findings, for instance, higher coefficient of variation in riffles in terms of flow depth.…”

Bridging the lab‐field interface in fluvial morphology at patch‐scale: Using close‐range photogrammetry to assess surface replication and vegetation influence

“…We present the DoD between replica and natural surfaces, which previously have not been presented in those studies that have utilized the casting technique (Rice et al, ; Spiller et al, ), except in the initial outlines of the casting process (Buffin‐Bélanger et al, ; Chandler et al, ). No DoDs for casts made of silicone rubber have been presented previously.…”

“…Applications of the casting technique include multiple physical experiments undertaken to form a network of results (Table 3), and using fixed beds is advantageous for investigating flow structures and hydrodynamic forces (Cooper et al, 2017;Rice et al, 2014;Spiller et al, 2012). Although we currently recommend using the casting process for surface replication, there are opportunities in future research for the use of 3D printing (Viles, 2016) that could avoid the aforementioned issues encountered with the casting process (e.g., particle dislodgement understanding of the processes occurring at different scales (Yager et al, 2015), in order to progress scientific understanding and for river management applications.…”

“…Development of the casting technique and materials used produced replica surfaces that were visually deemed representative of the surface topography (Spiller, Rüther, & Baumann, 2012). Subsequently, casts are analysed in the laboratory to enhance the understanding of how topography influences flow properties (Rice, Buffin-Bélanger, & Reid, 2014), with recent quantification of flows over both permeable and impermeable beds (i.e., casts) providing novel insights into fundamental hydrodynamic differences in the near-bed region of gravel-bed rivers (Cooper, Ockleford, Rice, & Powell, 2017).…”

The Impact of Biological Bedforms on Near‐Bed and Subsurface Flow: A Laboratory‐Evaluated Numerical Study of Flow in the Vicinity of Pits and Mounds

“…River substrates are highly heterogeneous at a range of spatial scales, creating a topographically complex environment (Nikora & Rowinski, ; Rice et al, ; Sarkar et al, ). Topographic features range from individual grains or clusters of grains that protrude into the flow (Hassan & Reid, ; Heritage & Milan, ; Lacey & Roy, ) to bedforms incorporating many sediment grains, such as ripples and dunes (Elliott & Brooks, ; Keshavarzi et al, ; Vanoni, ), and width‐scale undulations in the bed surface (i.e., riffle‐pool units; Hein & Walker, ; Marion et al, ; Tonina & Buffington, ; Tubino et al, ).…”