Mass Gravity Flows Modelling

Abstract

In the last decade, offshore pipeline engineering extended its action field to very deep waters and continental slopes. This implies the necessity to deal with continental slopes instability and mass gravity flows. Even if a standard classification of mass gravity flows does not exist in literature, they can be divided in two main different classes having respectively laminar and turbulent regimes. The first class, namely debris flow, is a very dense laminar flow, up to 1800Kg/m ³, with Bingham fluid characteristics. A debris flow could occur on steep slopes with velocity estimated to reach 30m/s. The main cause of debris flows occurrence is the seismicity. The second class, namely turbidity current, is a turbulent Newtonian flux with density up to 1200Kg/m ³. They are usually associated to debris flows that generate a dense mixture of water and sediment that continues to flow down slope even after the debris flow has stopped. Maximum velocities reached by turbidity currents are of the order of 10 –15m/s; they generally last a long time and continue to flow down even on slopes of few degrees. Mass gravity flows are rare and have random occurrence and the direct measurement of the phenomena is practically impossible. This pushed toward the development of physical and numerical models apt to investigate the characteristics and intensity of the phenomena (Niedoroda et al., 2000a, Niedoroda et al., 2000b). In this paper two numerical models, one for debris flows and the other for turbidity currents, are presented. A further diffusion model has been implemented to couple these models, thus allowing the complete simulation of a mass gravity flow starting as a debris flow that flowing down generates a turbidity current.