5% - eTheses Repository - University of Birmingham

than the more manageable video camera. Still photography has the
major disadvantage in that it is difficult to capture vortex
events clearly since the motion data is lost due to slow frame
rates. What can be captured are events like that shown in figure
4 where a region of obvious vorticity is swept from one region
to another.
Visual observation of the flow shows that at all conditions
vortices are responsible for lifting sediment from the crest and
lee slope regions. Vortices are clearly seen, and the eye is
sensitive enough to follow their progress both from formation at
or near the crest or from upstream bedforms. Regularly the flow
reverses direction at a fixed point say 0.01m above the lee slope
as the size of the separation region changes, sweeping away the
vorticity from the crest.
Also visually observed was the effect that these vortices have
on sediment at the crest and on the lee slope. At the crest the
finer particles (say less than 200 microns diameter) are captured
by the vorticity and moved away from the crest to fall out either
on the lee slope or on the next bedform. Larger particles are
seen to jet out into the flow before being captured by the return
flow and brought to rest on the lee slope. Regular lift-up of
sand from the lee slope by impinging vortices is also visible,
this being confined to the smaller particles depending upon the
intensity of the incident vortex.
A8-5