Page 108 - Caldera Volcanism Analysis, Modelling and Response

Pyroclastic Density Currents 83

qualitatively in terms of the internal structure of the currents (the vertical gradients
of velocity and density) with respect to the scale of the local substrate roughness.
The anisotropy of magnetic susceptibility (AMS) shows both how building-induced
roughness was able to strongly influence flow directions and that the more
concentrated underflows in several streams followed external walls of the city and
filled internal roads (Figure 14; Gurioli et al., 2005). The roughness induced by
the close assemblage of buildings and roads locally increased turbulence, and the
vortex production was particularly enhanced when the space between edifices
equalled their heights (Oke, 1987). The development of strong vortices can
cause a decrease in temperature of hot PDCs in the order of 150 200 1C(Gurioli
et al., 2005).
The interaction with urban structures probably limited the runout of PDCs
by loss of kinetic energy on impacts against obstacles, deflection, bore formation
and increasing turbulence. As an example, a decrease in runout of 15–25%
was observed for volcaniclastic flows that interacted with urban settlements in the
Sarno area in comparison to others that flowed over undeveloped areas (Zanchetta
et al., 2004a).

Figure 14 In£uence of city buildings and roads on £ow directions of PDCs from the AD 79
eruption of Vesuvius. For the archaeological excavations of Pompeii the assessed equilibrium
temperatures are also reported for the town and in its surroundings (after Gurioli et al., 2005;
modi¢ed).