2 SEDIMENTARY PROCESSES/Particle-Driven Subaqueous Gravity Processes lakes or oceans and continue flowing underwater if the rates of mass flux are sufficiently high Grain Transport Mechanisms Matrix Strength and Particle-Particle Interactions Within dense flows, grains can be prevented from settling as a result of matrix strength (Figure 1) This strength may arise if some or all of the particles are cohesive The resulting cohesive matrix prevents both cohesive and non-cohesive particles from settling out In addition, particles can be supported by matrix strength within flows of non-cohesive grains if the particles are in semi-permanent contact, as is the case for flows whose densities are close to that of static, loose-packed sediment For slightly lower concentrations, inter-particle collisions will help keep particles in suspension Hindered Settling and Buoyancy Settling of particles can be slowed down by water displaced upwards by other settling particles (Figure 1) Such hindered settling is especially effective in dense mixtures with a range of grain sizes so that the smaller particles are slowed down by settling of the larger particles The presence of smaller particles also increases the effective density of the fluid that the particles are settling in and thus enhances the buoyancy of the suspended particles and reduces settling rates Turbulence The motion of sediment-laden flows can generate turbulence through shear at the bed, internally in the flow or at the top of a dense layer The turbulent bursts generated at the bed tend to have an asymmetrical vertical velocity structure, with slower downward sweeps and more rapid upward bursts This turbulence pattern counteracts the downwards settling of particles, moving them higher up in the flow (Figure 1) Turbulence generation is hindered and dissipation increased, however, if the particle concentration is high, or if the flow is very cohesive or highly stratified Flow Types Broadly speaking, flows can be divided into three main types, depending on density: Dense, Relatively Undeformed Flows, Creeps, Slides and Slumps Flows of this type essentially have the same density as the pre-failure material In each case the sediment moves as one large coherent mass, but with varying amounts of internal deformation Grains remain in contact during flow and thus matrix strength is the main sediment transport mechanism Such flows will stop moving or shear stress becomes too low to overcome friction, at which point the entire mass comes to rest Flow thickness and deposit thickness are essentially the same, although flows may thicken via internal thrusting or ductile deformation as they decelerate prior to arrest Slope creep caused by gravity moves beds slowly downslope with gentle internal deformation of the original depositional structure Slides undergo little or no pervasive internal deformation, while slumps undergo partial deformation but the original internal structure is still recognisable in separate blocks Thicknesses of slides and slumps range from several tens of metres to 1–2 km and travel distances can be up to about 100 km, with displaced volumes of up to 1012 m3, although most flows are considerably smaller Dense, Deformed Flows: Rockfalls, Grain flows, Debris Flows and Mudflows In flows of this type, sediment still moves as one coherent mass, but concentrations can be lower and the mass is generally well mixed, with little or no preservation of remnant structure from the original failed material Sediment support mechanisms are matrix strength, buoyancy, hindered settling, and grain-grain collisions Rheologically such flows are plastic (i.e., they have a yield strength) Clast types generally range from purely cohesive in mudflows, to cohesive and/or non-cohesive in debris flows (Figure 2) and purely non-cohesive for grain flows and rockfalls (where movement is by freefall on very steep slopes) These types of flow are formed as a Figure Schematic illustration of the principal grain transport mechanisms, shown in decreasing order of concentration from left to right