Influence of bed morphology on double-averaged turbulent quantities in low submergence gravel-bed flows.

In river rehabilitation studies it is necessary to assess the impacts of the modified sand content, namely on near-bed flow hydrodynamics. This laboratory study addresses the changes that different bed morphology configurations and sediment transport impose on hydrodynamic variables relevant for the momentum and turbulent kinetic energy (TKE) budgets. Bed morphology was characterized by the void function, sand content, basal porosity and bed thickness. Special emphasis was given to the characterization of mean Reynolds and form-induced stresses, near-bed pressure, mean longitudinal velocity, momentum diffusivity and TKE production. Five laboratory tests simulated different stream conditions and sand contents in a framework-supported gravel bed. Instantaneous velocity maps were obtained with Particle Image Velocimetry (PIV). The collected data was analysed and theoretically framed with double-averaged methods (DAM). The impacts of the variations in the bed morphology over the near-bed flow hydrodynamics are discussed by direct comparison of the normalised DA flow variables. It was found that the effect of near-bed sediment movement is felt primarily in the wake production terms, in the shear rate below the plane of the crests and, to a lesser extent, in the form-induced shear stresses.