5% - eTheses Repository - University of Birmingham

4.1 Interactive Fraction (FI)
Our jetting studies identified seven main modes of suspension
ranging from short paths onto the upper lee slope to long
trajectories over the recirculation zone. From figure 14 we see
that increasing U/V is associated with increasing proportion of
longer (higher M value) trajectories.
For bedforms with a wide
particle size range our data suggests the bedload will experience
a broad dispersive sorting process during ejecting over the
crest. The largest particle grades (U/V about 4) will follow
trajectories onto the lee slope; whereas the finest grades (U/V
about 10) will be suspended by the shear layer and deposited
further downstream. Particles for which U/V is between 4 and 10
will deposit in more or less rank order between these limits and
this represents an 'interactive fraction'. Here the forces
acting on the particles are sensitively balanced such that slight
adjustments in U/V determines whether gravity or the flow forces
are dominant.
We may also define FI 0 as the grain size for which U/V=7: ie the
midpoint of the range within FI. In accelerating flows, such as
tidal estuaries, the FI concept may be helpful as an indication
of effective mobility. Knowing the stream speed and sediment
size distribution we can extract the proportion that travels with
each bedform as opposed to moving between bedforms. This
distinction has direct implications for the total sediment
transport rate and the bedform migration velocity both of which
are of concern in civil engineering hydraulics.
3-17