| dc.description.abstract | River and tidal networks form important routes for the transportation of goods to and from sea
ports. These networks are made up of different nodes connecting different sections, confluences
and bifurcations. While a lot of research has been done on confluences, focus on bifurcations is
relatively new. In tidal estuaries bifurcations occur in the form of tidal bars and rivers experience
avulsions and can start to bifurcate in deltas. Bifurcation is an important mechanism in the
distribution and partitioning of flow and sediment of two downstream channels and can thus
influence the entire network. When this distribution leads to a dominance of one channel over
the other, resulting in aggradation in one channel and erosion in the other we call this bifurcation
instable. Stable behavior is described when discharge and sediment transport is divided equally.
Modelling solutions have been sought after and found to better understand discharge and
sediment transport division at bifurcations in rivers, and the instability or stability that it can
provide. The most recent theory by Bolla Pittaluga et al. (2015) describes a relationship between
the width to depth ratio and sediment mobility upstream of the bifurcation and the stability
configuration of the bifurcation. There is however still a lack of both experimental and field data
for intermediate and high mobility flow conditions in sandy rivers. For tidal channel systems there
is no evidence that the same models and theory apply due to a lack of research. The aim of this
thesis is to test bifurcation stability under changing width to depth ratios for both intermediate
and high sediment mobility. During Influences of changing bank morphology and channel
curvature are minimized. Experiments were also conducted to test whether the same stability
theory applies to tidal channels. The experiments were done in HR Wallingford’s Fast Flow
Facility, a 70m by 40m flume capable of producing flow discharges in two directions. The large
size of this flume made it possible for experiments on a 28 meter long sand bed, split into two
symmetrical channels by use of a splitter plate of 16m long, to be conducted at intermediate and
high Shields sediment mobilities. The morphological evolution of the sand bed after a
perturbation was placed in one of the channels was measured by use of laser scanners. For
intermediate mobility experiments with a high width to depth ratio of the upstream channel
resulted in an increase of the perturbation between the two channels, indicating instability, while
the bifurcations with a low width to depth ratio reduced the perturbation and were found to be
stable. The same results were observed for high mobilities. It was thus concluded that, in
absence of any curvature and morphology creating transverse flow differences, the width to
depth ratio of the upstream channel can predict the stability or instability of the bifurcation.
Additionally, experiments were conducted with symmetrical tides at intermediate Shields
stresses. The experiments resulted in the same trends, with low width to depth ratios resulting in
stability and high width to depth ratios of the upstream channels resulting in instability of the
bifurcation. | |
