Turbidity currents at continental margins are subaqueous density flows, in which the suspension of sediments in seawater produces a water-sediment mixture that is denser than the ambient seawater and hence flows downslope due to gravity along the seafloor. These currents carve submarine canyons in the continental slopes and deliver sediments to the abyssal plains. Deposits from turbidity currents are notably known for being significant hydrocarbon-rich and other minerals reservoirs. Turbidity currents also represent a natural hazard for the growing offshore industry as even small currents can damage the oil and gas pipelines and undersea telecommunications cables scattered on the seafloor (see the review of Meiburg and Kneller, 2010). Understanding the dynamics of such gravity currents is thus crucially needed. Until now the different scenarios that have been envisaged to explain the geological and geophysical observations on turbidity currents rely on qualitative models. There is no physical model for turbidity currents flowing down complex and erodible topographies. We know that particulate gravity currents flowing horizontally in a fixed V-shaped valley are progressively thinner and deposit predominantly in the centre of the valley compared to its flanks (Monaghan, Mériaux and others, 2009a & 2009b). However we do not know what is the influence of slopes on the deposition, and under which conditions erosion competes with deposition in a V-shaped valley. We know that any current consisting of multiple particles of the same density results in deposits that are vertically stratified with the coarser particles more frequent at the base, and whose grain size thins with distance from the source (Garcia, 1994), but we still do not really understand the particle-particle interactions within those currents (Mériaux and Kurz-Besson, 2012). Furthermore, few turbidity currents in the ocean have provided evidence of their capacity to damage structures laid on the seafloor but there has been no quantification of the forces exerted onto fixed or movable bodies by particulate gravity currents flowing along complex geometries such as a sloping V-shaped valley. Such questions require to be tested by physical and numerical models. ToSCAN will develop laboratory and numerical models of turbidity currents. In particular we will perform experiments in sloping V-shaped valleys, evaluate the forces on fixed/movable objects exerted by the impact of turbidity currents, and assess the erosional power of currents on pre-existent deposits. The laboratory experiments will be run in parallel with three-dimensional numerical simulations based on the Smooth Particle Hydrodynamics (SPH) method. The experiments and simulations, which we propose to carry out in this application, are all original and innovative. The findings will be compared with modern and ancient natural analogues including canyons of the Portuguese Margin (São Vicente, Lagos, Portimão), and the ancient outcropping turbidity deposits of the Southwest Iberia (e.g. Costa Vicentina, Portugal). Our analogue and numerical modelling will provide a framework to advance our understanding of 1) the characteristics of the deposits produced by turbidity currents flowing down an inclined V-shaped valley, 2) the conditions for erosion and deposition during a turbidity current event in a V-shaped canyon, and 3) the extent to which turbidity currents can damage or move any solid objects within their path. This study will be a key to the understanding of the complex geomorphology of the Portuguese canyon system as well as ancient turbidity deposits as our results will provide tools to interpret deposits resulting from turbidity currents. This study will also be beneficial to the offshore industry as we provide a reliable evaluation of the risks for pipelines and telecommunications cables when run over by turbidity currents. The research team has been framed to meet the challenges set for this project with a unique group of Portuguese and international experts in analogue and numerical modelling, and in geology and marine geology disciplines.

ToSCAN -Turbidity currents in Submarine CANyons

DI CELMA, Claudio Nicola
2012

Abstract

Turbidity currents at continental margins are subaqueous density flows, in which the suspension of sediments in seawater produces a water-sediment mixture that is denser than the ambient seawater and hence flows downslope due to gravity along the seafloor. These currents carve submarine canyons in the continental slopes and deliver sediments to the abyssal plains. Deposits from turbidity currents are notably known for being significant hydrocarbon-rich and other minerals reservoirs. Turbidity currents also represent a natural hazard for the growing offshore industry as even small currents can damage the oil and gas pipelines and undersea telecommunications cables scattered on the seafloor (see the review of Meiburg and Kneller, 2010). Understanding the dynamics of such gravity currents is thus crucially needed. Until now the different scenarios that have been envisaged to explain the geological and geophysical observations on turbidity currents rely on qualitative models. There is no physical model for turbidity currents flowing down complex and erodible topographies. We know that particulate gravity currents flowing horizontally in a fixed V-shaped valley are progressively thinner and deposit predominantly in the centre of the valley compared to its flanks (Monaghan, Mériaux and others, 2009a & 2009b). However we do not know what is the influence of slopes on the deposition, and under which conditions erosion competes with deposition in a V-shaped valley. We know that any current consisting of multiple particles of the same density results in deposits that are vertically stratified with the coarser particles more frequent at the base, and whose grain size thins with distance from the source (Garcia, 1994), but we still do not really understand the particle-particle interactions within those currents (Mériaux and Kurz-Besson, 2012). Furthermore, few turbidity currents in the ocean have provided evidence of their capacity to damage structures laid on the seafloor but there has been no quantification of the forces exerted onto fixed or movable bodies by particulate gravity currents flowing along complex geometries such as a sloping V-shaped valley. Such questions require to be tested by physical and numerical models. ToSCAN will develop laboratory and numerical models of turbidity currents. In particular we will perform experiments in sloping V-shaped valleys, evaluate the forces on fixed/movable objects exerted by the impact of turbidity currents, and assess the erosional power of currents on pre-existent deposits. The laboratory experiments will be run in parallel with three-dimensional numerical simulations based on the Smooth Particle Hydrodynamics (SPH) method. The experiments and simulations, which we propose to carry out in this application, are all original and innovative. The findings will be compared with modern and ancient natural analogues including canyons of the Portuguese Margin (São Vicente, Lagos, Portimão), and the ancient outcropping turbidity deposits of the Southwest Iberia (e.g. Costa Vicentina, Portugal). Our analogue and numerical modelling will provide a framework to advance our understanding of 1) the characteristics of the deposits produced by turbidity currents flowing down an inclined V-shaped valley, 2) the conditions for erosion and deposition during a turbidity current event in a V-shaped canyon, and 3) the extent to which turbidity currents can damage or move any solid objects within their path. This study will be a key to the understanding of the complex geomorphology of the Portuguese canyon system as well as ancient turbidity deposits as our results will provide tools to interpret deposits resulting from turbidity currents. This study will also be beneficial to the offshore industry as we provide a reliable evaluation of the risks for pipelines and telecommunications cables when run over by turbidity currents. The research team has been framed to meet the challenges set for this project with a unique group of Portuguese and international experts in analogue and numerical modelling, and in geology and marine geology disciplines.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11581/249365
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