The spatial organisation of time-averaged streamwise velocity and its correlation with the surface topography of water-worked gravel beds

An examination was made into the spatial pattern of time-averaged streamwise velocity in the near-bed region over two water-worked gravel beds. Laboratory observations revealed that there is considerable spatial variability in velocity. It was organised into streamwise streaks of high-speed fluid which were overlain by spots of low-speed fluid. This spatial pattern was found to be consistently and heavily dependent on relative submergence. The spatial pattern of velocity was shown to have little linear coherence with bed surface topography at the grain-scale. It suggested that for flows above the two beds studied here, bed surface topography at the grain-scale exerted less of an influence on the spatial organisation of time-averaged streamwise velocities than relative submergence.