Page 99 - Caldera Volcanism Analysis, Modelling and Response

74 Roberto Sulpizio and Pierfrancesco Dellino

where U(1, 2) is the velocity of the interface that separates regions with different
densities r 1 and r 2 , and j(r 1 ) j(r 2 ) the difference in density flux between the two
regions. The evolution of the system is determined by the motion of the interfaces,
and the nature of their interactions leads to a final state in which large density
contrasts occur (Lee and Leibig, 1994).
The contrasting effects of shear stress and particle concentration control the
distribution of turbulence within each pulse and subsequently determine its
sedimentological character (Figure 7n). The organisation of the current into trains
of pulses occurs in both concentrated and dilute PDCs (Sulpizio et al., 2007).
This determines two important consequences: (i) PDCs are intrinsically unsteady
phenomena; and (ii) deposition occurs stepwise (Figure 7).
Field evidence that supports a wide spectra of PDC behaviour is illustrated in
Section 5.

4.3. Influence of topography on deposition
With the exception of completely flat topography, the landscape morphology exerts
a major influence on depositional mechanisms of gravity-driven currents (e.g.
Fisher, 1990; Woods et al., 1998; Kneller and Buckee, 2000; Branney and Kokelaar,
2002; Pittari et al., 2006). The mechanisms that can influence mobility and
deposition in PDCs are basically: (i) changes in turbulence within the current;
(ii) changes in particle concentration in the flow-boundary zone; (iii) blocking or
stripping of the current. The influence of topography on gravity-driven currents
has been demonstrated in many field-based studies (e.g. Cas and Wright, 1987;
Saucedo et al., 2004; Branney and Kokelaar, 2002; Sulpizio et al., 2007 among
many others), but the effective mechanisms have been investigated primarily
through small-scale laboratory experiments (e.g. Edwards et al., 1994; Kneller and
McCaffrey, 1999; Woods et al., 1998) and numerical modelling (e.g. Dobran et al.,
1994). However, both simple laboratory conditions (use of fluids with different
densities, limited grain sizes distributions) and very dilute flows used in numerical
simulations (particle sizes between 10 and 100 mm, concentrations below a few
volume percent) only partially resemble the complexities of natural cases. This
implies that field data, which represent natural cases, have to be accurately collected
and interpreted in order to better understand the influence of topography on the
physical behaviour of PDCs. Here, some real examples of interactions between
PDCs and common landscape morphologies will be illustrated and discussed,
although they do not exclusively represent possible natural conditions.

4.3.1. Break in slope (concave local curvature)
PDCs may flow along the ground for some distance without significant deposition,
due to the prevalence of driving forces over resisting ones. The simplest way to
change the partition between the two forces in gravity-driven currents is to reduce
the gravitational driving force by reduction in local curvature of the topographic
slope (Figure 8a). The response of the moving flow includes two components. One
component reflects a simple trade-off between reduced driving stress and increased