Part size, shape and distance moved, particle size

Part
1 – From Figure 1, comparing tracer size, shape and distance moved,
particle size appears to be the most significant factor in controlling
transport distance. The particles have been classified using Zingg’s (1935)
classification of particle shape as spheroids, rods, discoids or blades, determined
by whether the axial ratios, a:b and c:b, equal 0.67 (Benn and Ballantyne,
1993). Particle size was partitioned using Warburton and Demir’s (2000) classification
of small (32 to < 64 mm), medium (64 to < 128 mm) or large (> 128 mm)
clasts. The boxplots show that there is a general decrease in distance
transported with tracer size which prevails in all four shape classes. Discs are consistently transported the greatest
distance in all three size classes showing a somewhat lesser dependence on
particle shape in determining transport distance. The size-selective nature of
particle movement is reinforced in Figure 2, which displays a negative linear
correlation between the b-axis of particles and distance transported. The
relationship shows a decrease in particle mobility with increasing size
fractions, with smaller particles moving furthest downstream. The mean size of particles
which were transported > 25 m is approximately half of those which travelled
smaller distances; 73 mm and 141 mm respectively. The size-selective transport
demonstrated is consistent with larger particles possessing a greater weight
and therefore a greater inertia, which is the principal force resisting motion
(Gintz et al. 1996). Ferguson and Wathen (1998) propose that size-selective
transport dominates in gravel bed rivers, rather than equal mobility. Size
selective transport governs that grains have differential transport rates based
on their b-axis; whereby small particles are more mobile and able to travel further
than their larger counterparts (Gomez et al. 2001).

The hypothesis
of size selective transport in Troutbeck is consistent with the downstream
fining of sediment determined by Wolman Plate analysis, displayed in Figure 3. If
equal mobility was present at all times, downstream fining could only occur through
abrasion and weathering (Lisle, 1995). However, the maximum diameters of the
particles surveyed have greatly reduced within the 25 m reach; decreasing from
256 mm in Pod 2 to 90 mm in Pod 5. Therefore, it can be inferred the rapid
downstream fining is the result of different fractional transport rates for the
varying size classes (Rice and Church, 1998).

However, the
scatter in Figure 2, implies a large amount of variability which is unexplained
by the simple relationship. The scatter suggests that, as well as the individual
characteristics of the grains, the sedimentological characteristics of the bed
and local flow conditions exert a control on the transport of bedload (Pryce,
2001). The interaction between these factors leads to a wide range of
displacements under the same bed and flow conditions, which means even particles
which begin in identical positions, such as the tracers studied, show variable
transport distances (Brayshaw, 1985).

One factor,
which may have produced the observed differences between initial tracer
distribution and those transported, is the arrangement of particles on the
channel bed. On a bed of mixed grain sizes, distinctive bed micro-topography
can exist due to the complicated packing of sediment (Ferguson et al. 2002).
The arrangement of particles can thus enhance the stability of individual
grains or increase their liability to movement, as clasts can become hidden,
interlocked with neighbouring grains or left in isolation (Bridge and Bennett,
1992). The packing of particles is an important control in gravel bed rivers as
cluster bedforms are common (Mao and Surian, 2009). Clusters augment the
stability of the grains in a structure as an obstacle particle acts as a
nucleus for collections to build in its sheltered stoss and lee side (Billi,
1988). The concentration of particles projects up from the bed surface creating
a physical barrier to flow which produces strong relief (Laronne and Carson,
1976). Therefore, bed topography creates spatial variability in the magnitude
of the hydraulic forces that influence sediment transport (Goode, 2009). Schmidt
and Ergenzinger (1992) propose the packing of particles on the bed also
enforces weak shape selective transport, as rods become easily trapped in clusters,
whereas spherical particles do not. The trapping of elongate particles corresponds
with the data collected at Troutbeck as prolate tracers showed minimal movement
in all three size classes.

 Another
factor which may have produced the observed differences between initial tracer
distribution and those transported is the grain size distribution of the bed
(Wilcock and Crowe, 2003). The composition of grain sizes on the bed impacts
the movement of individual particles as entrainment depends on a particles’
relative as well as absolute size (Ashworth and Ferguson, 1989). If a clast is
coarser than the average particle on the bed, it protrudes into the fluid flow
and is superiorly exposed to motion rather than resisting forces (Fenton and
Abbott, 1977). In contrast, finer than average particles are more difficult to
move as they become hidden within the bed layer (Church et al. 1991). The grain
size of the bed similarly influences a particle’s probability of movement as it
determines the pivoting angle a particle must overcome in order to initiate
motion (Johnson, 2014). On a bed of comparable grain size, pivoting angle can
be greatly increased as particle movement is inhibited by its connection with
neighbouring grains (Hassan et al. 1991). In contrast, if a clast is superior
in size to its neighbours it will protrude into the flow, thus reducing the
pivot angle it must overcome (Wilcock and McArdell, 1993). Although the tracer
particles were placed in comparable locations on the surface, following initial
displacement, the clasts will have been deposited in contrasting areas, thus
affecting their resultant transport (Church and Hassan, 1992). Moreover,
angular grains have a higher pivot angle required for transport than spherical
clasts as the