The influence of different time varying antecedent flows on the stability of mixed grain size deposits.

The objective of this work was to examine the impact of unsteady flows on the erosion and
movement of mixed grain size sediment. Time varying flows were examined as flowrates in
natural rivers are rarely constant. There are very few reported studies on the movement of
sediment in unsteady open channel flow and most of those used single sized sediment. River
reach has its own sedimentological character and non-uniform beds exhibit very different
behaviour from that of single sized material. Therefore it was thought important to examine the
impact of time varying flow on the stability of water worked mixed grain size sediment beds.
The thesis reports on a series of laboratory experiments in which a bimodal sediment bed was
exposed to different flow hydrographs. The flow hydrographs consisted of constant flowrate
with different duration and time varying flows with different rising and falling limb but had the
same peak flowrate. Each experiment was followed by a stability test in which a standard
"triangular shaped hydrograph" was used to assess the stability of each water worked deposit.
The stability observation demonstrated that grain size fractions have different thresholds of
motion when beds are formed by different antecedent flow patterns. The bed stability increased
as the antecedent constant flow hydrograph progressed. The rising and falling limbs of the
flowrate hydrographs were found to have a significant effect on the bed stabilisation process. It
revealed that the shortest rising limb of flow hydrograph formed the weakest bed while the
longest recession limb of flow hydro graph formed the most stable bed. It is believed that the
short period of flowrate acceleration did not allow the coarse grains to stabilise with numerous
exposed large grains spread on the bed. In a longer duration of recession limb of hydrograph,
the coarse grains moved and eventually deposited over a length of time. As the flowrate
declined the finer grains also rolled and then deposited forming a strong bond with the coarse
grams.
These experiments also provided important information on the flow structures and the changes
in the bed topography as the tests progressed. There is strong evidence that only upward
interactions (ejections) with high momentum magnitude were able to transport coarser grains.
The lack of change in the distribution of downward looking-bed interactions (sweeps) in all
tests indicated that these features are not important in determining transport. Changes in bed
topography were also measured and characteristics of the distribution of bed surface elevation
were linked to the observed changes in bed stability.