TURBULENCE STRUCTURE OVER 2-DIMENSIONAL BED FORMS - IMPLICATIONS FOR SEDIMENT TRANSPORT

Turbulence measurements from the flow over two-dimensional fixed dune shapes are presented along with analysis and discussion of the ramific ations of the observations for transport of sediment as bed load over bed forms. The spatial structure of the local transport rate determine s the shape and stability of bed forms such as ripples and dunes, and the transport of sediment is a highly nonlinear process that is profou ndly affected by the statistics of the temporal fluctuations in the ne ar-bed flow field. The measurements presented herein show strong spati al evolution of the joint probability distribution of the streamwise a nd bednormal fluctuating velocity components. Unlike measurements in u niform boundary layers, these distributions and the higher moments of the velocity do not scale with the local shear velocity, indicating th at it is probably inappropriate to use the shear stress to characteriz e the sediment flux. This conclusion is supported by observations of s ediment flux over a dune.