Author(s): M. J. Franca
Linked Author(s): Mário Franca
Keywords: Gravel riverbed; Log-profile; S-profile; Double boundary layer; Field measurements
Abstract: Field measurements in a shallow gravel-bed river are analyzed along a streamwise plane where a bed perturbation creates an inflection in the velocity distribution. Three types of velocity profiles may occur in these flows: log (undisturbed profile), s (caused by the perturbation) and double log (after the perturbation). The dynamics of the transition log-s2xlog is investigated. Three regions of the flow are identified, corresponding to the three mentioned types of profile: (1) convergence of the flow in the outer layers above the boulder; (2) separation of the flow due to the boulder creating a shielded inner zone where the mean velocity distribution is inflected, turbulence levels decrease and coherent structures reorganize; (3) momentum redistribution throughout the water depth. A parameterization of the s-profile with a hyperbolic tangent function is presented, where the main parameter is an energetic eddy size. An analysis of coherent structure scales indicates that this parameter corresponds to the mean integral scale in the roughness sublayer of the flow. Lateral low momentum convection occurs around the obstacle and is responsible for the formation of the inner boundary layer in region (3), whereas the boulder form-drag contributes to the shear in the outer boundary layer. Turbulence mean levels and coherent structures are investigated in the three regions of the flow. Turbulence intensity is modified due to shielding effects and due to the Kelvin-Helmholtz type instability in the inflection of s-profiles. Instantaneous shear stress events are conditionally sampled and the quadrant distribution is investigated.