5% - eTheses Repository - University of Birmingham

i——COMPARISON WITH PREVIOUS WORK
Soulsby (1989) performed velocity and suspended sediment
concentration measurements over a sandwave in the Taw estuary.
The observed velocity profiles at the bedform crests showed
similar jetting due to acceleration up the flank of the bedform
as our velocity profiles; see figures 1 and 2. The bedforms used
for our work were also similar in shape, with comparable
height:length ratio of about 1:15. We thus feel confident in
saying that the cloud of suspended sediment deduced by Soulsby
to move downstream from a sandwave, as shown in figure 3 is the
separated flow region being shed, as shown in plate 1.
The reasons for the unsteadiness in the local pressure gradient
around the crest may be due to surface waves excited by the
bedforms or by wind, the uneven flume walls, pressure
fluctuations caused by the passage of large scale structures shed
from upstream crests or possibly a combination of effects.
This shedding phenomenon highlights the complex nature of the
system. These complications hamper fundamental understanding of
particle-vortex interactions, which is surely the method by which
the sediment is moved from the bed into the separation cloud.
It points to the serious need to pinpoint the mechanism by using
single sharp crests in the absence of external forcing from the
eddies shed from upstream and by surface waves. When the
fundamental system is understood, its extension to the real
system may be trivial in the light of our current knowledge
A8-6