Page 90 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 65

Within the initial grain size population of a PDC, turbulence supports only the
particles with low terminal velocity (smaller and/or lighter). This means that, with
the exception of very energetic PDCs, the size of particles fully supported by
turbulence never exceeds fine ash. These particles constitute the suspension
population, and they are transported at any level within a PDC as fluid components
(Figure 3d).
Particles with higher terminal velocity (coarser and/or denser) are intermittently
supported by fluid turbulence. This is because turbulence produces fluctuations in
the near-bed velocity that give rise to fluctuations in the forces on, and resultant
motion of, particles in the flow-boundary zone (Schmeeckle and Nelson, 2003).
These velocity fluctuations act on particles that spend the majority of their time in
the lower part of the current, where they form the intermittent suspension
population (Figure 3c). The motion of particles only intermittently supported by
turbulence is named saltation, common in unsteady and turbulent PDCs. The term
saltation indicates any process of clast bouncing at an interface, with the interface
supplying intermittent support.
A typical dimensionless relation characterising the limiting conditions for the
entrainment of bed particles into bedload motion can be written as (Raudkivi,
1990):

f ðRe ; t Þ¼ 0 (3)
p

where Re ¼ u d p =v denotes a particle Reynolds number, with u * denoting the
p
flow shear velocity, d p denoting a representative mean diameter of the entrained

particles and v denoting the kinematic viscosity of the fluid. The term t ¼
2

u =ð gRd p Þ denotes a dimensionless bed shear stress (i.e. Shields’ stress), with g
denoting gravitational acceleration, R=(r s r)/r denoting the submerged specific
density and r s and r denoting the density of the entrained particle and the density
of the fluid, respectively. It can be argued that Equation (3) can also be used to
represent a dimensionless relation characterising the limit of entrainment into
suspension of particles lying over a granular bed (Nin ˜o et al., 2003). In fact, u * is
a measure of the turbulence intensity in the near-bed region, and t * can be
interpreted as a measure of the ratio of turbulent lift to gravitational forces acting on
the particle.
Particles with the highest terminal velocity are transported at the current-
deposit interface by drag forces, and constitute the traction population (Figure 3b).
This type of support occurs when the terminal velocity of particles is comparable to
the drag force exerted by the fluid, causing the sliding and rolling of particles at the
flow boundary (a process also known as traction; Middleton and Southard, 1984).
Sliding and rolling processes are very common at the base of fully diluted and
turbulent PDCs, but can also occur in more concentrated PDCs, where large
blocks can roll at the flow-boundary under the drag force exerted by overriding
flowing material. The clasts transported by intermittent or continuous support at an
interface (e.g. at the deposit surface) can experience different segregation processes.
As an example, clasts that roll under the effect of fluid drag in turbulent PDCs can
result in alignments of clasts and/or crudely stratified layers with imbrication of