TURBULENCE AT A ROUGHNESS TRANSITION IN A DEPTH LIMITED FLOW OVER A GRAVEL-BED

Turbulence statistics derived from velocity measurements along a longi tudinal transect characterized by a roughness transition are presented . The bed morphology along the transect is dominated by composite scal es of roughness ranging from an armoured bed in the upstream section t o the presence of large, protuberant clasts superimposed on the bed in the downstream section. An array of four electromagnetic current mete rs operating at a frequency of 20 Hz was used to measure the longitudi nal and vertical velocities at various heights above the bed surface a nd at different positions along the transect. Emphasis is pur on the l ongitudinal and vertical distributions of turbulent flow statistics su ch as Reynolds shear stress, uv cross-correlation, and quadrant analys is of velocity fluctuations. The results indicate that the presence of protuberant clasts modifies the turbulence structure, increases turbu lence intensity and appears to dominate turbulence generation. Distinc t flow zones are observed. In particular, a clear distinction is made between (i) a sublayer below the top of the major roughness elements i dentified by fluid motions predominantly towards the bed surface and c haracterized by intense eddying, with some vortices or rollers possibl y 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. T he dominant generative mechanism of flow structure appears to be the s hedding of vortices from the lee of obstacles. Interaction of vortices stretched by shear into the main body of flow downstream from major o bstacles dominates the turbulence structure in depth-limited flows ove r coarse and irregular bed surfaces.