The spatial characterization of turbulence around large roughness elements in a gravel-bed river

This study characterizes the flow field above and around multiple instream submerged cobbles, boulders, and pebble clusters in order to obtain a better understanding of the hydrodynamics associated with large roughness elements (LREs) in gravel-bed rivers. Spatially distributed high frequency, three-dimensional velocity measurements were recorded in situ using acoustic Doppler velocimeters at different flow stages. The spatial distributions of turbulent kinetic energy, ke, longitudinal component integral timescales, ITSu, and Reynolds shear stresses were characterized and are presented for selected sites. The longitudinal-vertical Reynolds shear stress increased with flow stage more strongly than the longitudinal-lateral or lateral-vertical Reynolds shear stresses and dominate at the highest measured flows. Canonical redundancy analysis was used to relate LRE morphometrics and mean flow conditions to the turbulence parameters estimated in the LRE wakes (i.e., ke, ITSu, and Reynolds shear stresses). LRE size and mean unobstructed velocity explained the highest proportion of the variance in the turbulent wake statistics. Multivariate regression models based on LRE width, mean unobstructed longitudinal velocity and flow depth are presented offering a tool to predict LRE wake turbulence.