Mean flow, turbulence structure, and bed form superimposition across the ripple-dune transition

[ 1] This paper describes a laboratory study of the dynamics of flow associated with three different stages of bed form amalgamation across the ripple-dune transition. Measurements of flow velocity were obtained over simplified fixed bed forms, designed to simulate conditions at the ripple: dune transition, using a 2D laser Doppler anemometer. This yielded information detailing the mean velocity field, turbulence statistics, local turbulence production, local deviations from the mean pressure caused by dynamic effects, turbulent kinetic energy and the contributions to the total Reynolds stresses from different coherent turbulent events. The results broadly confirm previous hypotheses that as bed form amalgamation proceeds across the ripple-dune transition, a superimposed bedstate can induce a series of critical changes to the flow structure, with higher Reynolds stresses being produced near and downstream of flow reattachment. In particular, quadrant 4 events ( turbulent flow structures with a downstream velocity greater than average, and directed towards the bed) dominate the vertical turbulent diffusion of longitudinal momentum in near-bed regions close to the crest of the next downstream ripple, thus providing the potential for increased erosion and sediment transport. These experiments using simple fixed beds also provide support for recent measurements that document increased suspended sediment concentrations across the ripple-dune transition (Schindler and Robert, 2004, 2005), and that at the transition the largest bed forms are not necessarily those associated with the most intense shear layer activity ( Schindler and Robert, 2005). Bed form superimposition and amalgamation may thus significantly alter the mean and turbulent flow field as compared to bed forms without superimposition.