NATURAL DISASTERS - EXPLOSIVE VOLCANIC-ERUPTIONS AND GIGANTIC LANDSLIDES

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.