The 1990s have been designated the International Decade of Natural Dis
aster Reduction; and this paper discusses two natural disasters. In th
e first we show how to model massive, ground-hugging ash flows, known
as pyroclastic flows by geologists, in terms of particle-driven, turbu
lent gravity currents. A framework for solving forward problems is set
up so that, for different geometries, all the flow and deposit proper
ties can be predicted given the initial conditions of the flow followi
ng a volcanic eruption. This is then used to discuss inverse problems,
for which only the details of the deposit are provided and the initia
l conditions of the flow are to be calculated. This method of approach
is applied to analysing the eruption of Taupo, New Zealand, about 180
0 years ago. We demonstrate that the ash-laden flow travelled over the
ground in a current of order 1 km high travelling at a typical speed
of 200 ms(-1). The second study concerns the runout of massive landsli
des. A model employing the concepts of the flow of granular materials
is presented in which the interior of the rockfall propagates uniforml
y above a thin shear layer of rocks through which all the rocks eventu
ally fall, to leave the flow and add to its deposit. Quantitative pred
ictions from this theoretical model are shown to agree well with obser
vations from about 50 rockslides on Earth, the Moon and Mars. Our mode
l suggests that simulation of such granular flows would be extremely d
ifficult, if not impossible, to achieve in the laboratory.