Spatially-induced momentum transfer over water-worked gravel beds.

The spatial variability in turbulent flows over water-worked gravel beds has been
studied. Spatially distributed measurements of velocity were obtained using Particle
Image Velocimetry for a range of hydraulic conditions over two water-worked gravel
beds. A different approach to bed formation was achieved by feeding sediment into a
flume to form deposits that result from the, arrangement of the grains by the flow, as
would be observed in a river.
It was shown that, even over macroscopically flat deposits, time-averaged streamwise
velocities display considerable spatial variability. However, the level of variability was
a magnitude lower for time-averaged vertical velocities. The degree of spatial variability
in both streamwise and vertical velocities was shown to increase with relative
submergence. Spatial flow variability was present in the logarithmic and outer regions
of the flow, which contradicts previous thinking that spatial variability is only present in
the near bed form-induced sublayer. The time-averaged flow velocities displayed a
considerable degree of organisation over both beds, indicating the existence of spatially
coherent time-averaged flow structures.
Form-induced stresses caused by the spatial heterogeneity in the flow were estimated.
These were found to be significant for flows over both beds, and did not disappear
above the form-induced sub layer as previously thought. Measurements of Reynolds
stress alone, whether spatially-averaged or not, cannot be used to determine the mean
bed shear stress over water-worked gravel beds. Increases in relative submergence
resulted in changes in the level of momentum carried by spatial deviations in the flow,
and by inference, those carried by the turbulent fluctuations. This occurred even when
the average rate of momentum transfer at the bed was the same. It was concluded that
relative submergence could have a more important influence on the spatial variability of
fluid stresses than the bed surface topography of water-worked gravel deposits.