Quadrant Analysis of Shear Events in Open Channel Flows Over Mobile and Immobile Hydraulically Rough Beds

Sediment overfeeding may induce important changes in the structure of the near-bed region of gravel-bed river flows, mainly in what concerns exchange of momentum and mass between the flow within the riverbed roughness elements and the flow in the upper regions. Although the turbulent structure of flows over gravel- bed rivers is object of several previous studies, it is not well-known how statistics characterizing coherent turbulence events responsible by the generation of turbulent shear stresses are affected by bed load transport in flows over hydraulically rough beds with low relative submergence. This study is aimed at bridging this research gap. It is based on two-dimensional instantaneous velocity data, in the stream-wise and vertical directions, acquired with Particle Image Velocimetry in a laboratory flume. Two tests simulated framework gravel beds with sand matrixes, one of which fed with sand at near capacity conditions. The framework, immobile under the imposed flow conditions, consists of coarse gravel whose diameters range between 0.5 cm and 7 cm. Matrix and imposed sand feature a median diameter of 0.9 mm. For both tests, the quadrant threshold analysis technique was employed and transported momentum were analyzed and discussed in what concerns their intensity distribution for events in the four quadrants, and for several positions within the flow. It is shown that under mobile bed conditions, sweeps are dominant in the turbulence production in the pythmenic region of the flow. In the outer region of the flow, this is independent from the channel bed; in the overlapping intermediate layer, between the inner region and the pythmenic region, the flow characteristics depend on the position in relation to the crests and troughs of the bed.