Turbulence at a roughness transition in a depth limited flow over a gravel bed

Turbulence statistics derived from velocity measurements along a longitudinal transect characterized by a roughness transition are presented. The bed morphology along the transect is dominated by composite scales of roughness ranging from an armoured bed in the upstream section to the presence of large, protuberant clasts superimposed on the bed in the downstream section. An array of four electromagnetic current meters operating at a frequency of 20 Hz was used to measure the longitudinal and vertical velocities at various heights above the bed surface and at different positions along the transect. Emphasis is put on the longitudinal and vertical distributions of turbulent flow statistics such as Reynolds shear stress, uv cross-correlation, and quadrant analysis of velocity fluctuations. The results indicate that the presence of protuberant clasts modifies the turbulence structure, increases turbulence intensity and appears to dominate turbulence generation. Distinct flow zones are observed. In particular, a clear distinction is made between (i) a sublayer below the top of the major roughness elements identified by fluid motions predominantly towards the bed surface and characterized by intense eddying, with some vortices or rollers possibly being trapped in this near-bed flow region and (ii) an inner layer, located immediately above the major protuberant clasts, identified by positive vertical velocities and hence upward-directed fluid motion. The dominant generative mechanism of flow structure appears to be the shedding of vortices from the lee of obstacles. Interaction of vortices stretched by shear into the main body of flow downstream from major obstacles dominates the turbulence structure in depth-limited flows over coarse and irregular bed surfaces.