4 SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes transformation can lead to the development of different flow types within one current, both vertically and from front to back This co-occurrence of different flow types is especially common in flows with a dense basal layer and more dilute upper part Thus, classification schemes which subdivide flows on the basis of discrete flow types not recognise the diversity of natural flows, in which different types of flow may occur simultaneously and vary in relative importance in time and space as the flows evolve Internal and External Influences on Flow Behaviour Flow behaviour is influenced both by internal factors such as concentration and grain size distribution and external factors such as input conditions and topography Flow Velocity The driving force, and hence velocity of subaqueous gravity currents increases with both concentration and flow size However, resistance to internal shear will increase with increasing viscosity due to increasing particle concentrations, and with increasing yield strength caused by cohesive particles This will inhibit the increase of flow velocities However, because concentration-induced resistance to shear does not scale with flow size, it can more readily be overcome by the higher gravitational driving forces of larger flows, which are, therefore, faster than smaller flows Flow Duration and Run-Out Length Slope failure-induced slumps and slides that not transform into debris flows and/or turbidity currents will generally be of short duration and have run-out lengths on the order of the initial failure size If the failed sediment mass does transform into a debris flow, the duration and run-out length depend on the mobility as described above, with larger flows travelling further However, because debris flows stretch out as they are flowing and because they may incorporate material by erosion, their run-out length may not be directly related to the initial failure size The duration and run-out length of turbidity currents depend on their size and sediment content, and hence also on their formation mechanism Sustained input from rivers or glacial plumes can result in long duration flows, even if the input concentration is low Turbidity currents that are generated from slope failures can have a short duration input, but tend to stretch considerably due to turbulent mixing and will thus increase in flow duration provided the transported sediment is kept in suspension The ability of a flow to keep sediment in suspension, known as the flow ‘efficiency’, directly affects flow run-out lengths Flow efficiency depends on flow magnitude, with larger flows being more efficient, and on grain size, as finer grains settle out more slowly than coarser grains The presence of fine sediment in the flow also increases the ability to carry coarse sediment so both types of sediment will be carried further and both flow duration and run-out length will be increased Spatial and Temporal Changes to Flow Flows are influenced both by the input conditions and by the terrain over which flow takes place Flow behaviour therefore varies both temporally and spatially, causing local areas of erosion and deposition that lead to a deviation from a simple decelerating depositing flow and complicate the depositional pattern Both spatial and temporal changes in flow behaviour can be caused by changes in sediment content of the flow: erosion adds driving force to the flow and increases velocity, while deposition slows flows down Temporal changes to flow can also be caused by changing input conditions River input from floods leads to flows that initially have a progressive increase in velocity followed by a long period of decreasing velocity In retrogressive failure ongoing detachment of discrete sediment masses will result in pulsed sediment input; the rate of input generally tends to peak rapidly, and then diminish as successive slope failures reduce in size Local spatial changes in flow are caused by changes in the topography (Figure 4) The angle of the slope on which flow takes place is obviously important for gravity driven flows; when slope angle increases, the flow will go faster although the velocity increase will be diminished by the increase of friction with the ambient water Nevertheless, small changes in slope angle can change flow behaviour If the slope angle decreases, very dense flows can be stopped as the basal friction becomes too high More dilute flows may undergo hydraulic jumps, in which they abruptly thicken and decelerate This deceleration can cause coarser sediment to be deposited Local changes to flow can also be caused by changes in the constriction of the flow path When a flow goes into a constriction, velocity will increase Where a flow can expand, as at the end of submarine canyons, velocity will decrease Momentum Loss The evolution of flow behaviour can be different along flow-parallel and flow-transverse directions Momentum will be greater in the direction of flow