6542 September 11~16, 2005, Seoul, Korea
FLOW DYNAMICS OVER A GRAVEL RIVERBED M.J. FRANCA Laboratory of Environmental Hydraulics - LHE École Polytechnique Fédérale de Lausanne, st 18, CH-1015 Lausanne, Switzerland (e-mail:
[email protected]) 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. Keywords: Gravel riverbed; Log-profile; S-profile; double boundary layer; Field measurements. 1. INTRODUCTION The present research on natural gravel-bed river flows concerns the flow around a bed protuberance. In shallow gravel-bed rivers, the mean velocity in the lower layers of the flow may show two kinds of profiles, logarithmic and s-shaped (Marchand et al., 1984 and Bathurst, 1988). Bathurst (1988) and Ferro and Baiamonte (1994) described the importance of relative roughness for the occurrence of s-profiles. Based on their measurements, Nelson et al. (1993), Baiamonte et al. (1995) and Buffin-Bélanger and Roy (1998) state that the wake effect formed by the bed forms and boulders is responsible for the deviations observed in the velocity profile of gravel-bed rivers. Nikora and Smart (1997) referred to a 3D inner layer of the flow called the roughness sublayer (z
