Development of Kelvin Helmholtz coherent structures over a rough bed

The Kelvin-Helmholtz Coherent Structures (KHCSs) that can populate Shallow Mixing Layers (SMLs) over a hydraulically rough bed are investigated in an 18 m long and 1.98 m wide tilted open-channel flume. A comparison with SMLs developing over a smooth bed is also carried out. A total of eight transversely sheared flows are studied by varying: (1) the total discharge, i.e. the flow depth; and (2) the flow partition from either side of the splitter plate, i.e. the initial shear parameter (λ=(U₂-U₁)/ (U₂+U₁) (where U₂ and U₁ are streamwise time-averaged velocities characterizing the high-speed and low-speed streams, respectively). The potential impact of the bed roughness on the emergence and fate of the KHCSs is discussed. The emergence of KHCSs is found to be controlled by the shear parameter, with the same threshold- value (λ>0.3) required over rough and smooth beds. Over a smooth bed, it was observed that the longitudinal stretching and final decay of the KHCSs were controlled by the bed friction number, S= (C_fδ_v/4hλ) (with c_f = friction coefficient, δ_v = mixing layer width, and h = water depth). The longitudinal stretching starts S >~ 0.01 and the KHCSs vanish when S> ~. 0.1.Over the rough bed, two threshold values of S-parameter are also observed, which are of the same order of magnitude as the smooth-bed values.