Recent research has examined the factors controlling the geometrical
configuration of bifurcations, determined the range of stability
conditions for a number of bifurcation types and assessed the impact of
perturbations on bifurcation evolution. However, the flow division
process and the parameters that influence flow and sediment partitioning
are still poorly characterized. To identify and isolate these
parameters, three-dimensional velocities were measured at 11
cross-sections in a fixed-walled experimental bifurcation. Water surface
gradients were controlled, and systematically varied, using a weir in
each distributary.
As may be expected, the steepest distributary conveyed the most
discharge ( was dominant) while the mildest distributary conveyed the
least discharge ( was subordinate). A zone of water surface
super-elevation was co-located with the bifurcation in symmetric cases
or displaced into the subordinate branch in asymmetric cases. Downstream
of a relatively acute-angled bifurcation, primary velocity cores were
near to the water surface and against the inner banks, with near-bed
zones of lower primary velocity at the outer banks. Downstream of an
obtuse-angled bifurcation, velocity cores were initially at the outer
banks, with near-bed zones of lower velocities at the inner banks, but
patterns soon reverted to match the acute-angled case. A single
secondary flow cell was generated in each distributary, with water
flowing inwards at the water surface and outwards at the bed.
Circulation was relatively enhanced within the subordinate branch, which
may help explain why subordinate distributaries remain open, may play a
role in determining the size of commonly-observed topographic features,
and may thus exert some control on the stability of asymmetric
bifurcations. Further, because larger values of circulation result from
larger gradient disadvantages, the length of confluence-diffluence units
in braided rivers or between diffluences within delta distributary
networks may vary depending upon flow structures inherited from upstream
and whether, and how, they are fed by dominant or subordinate
distributaries. Copyright (C) 2011 John Wiley & Sons, Ltd.