Experiments on bidisperse, constant-volume gravity currents: propagation and sediment deposition

Laboratory experiments show that the propagation and sedimentation patterns of particle-laden gravity currents are strongly influenced by the size of suspended particles. The main series of experiments consisted of fixed-volu me releases of dilute mixtures containing two sizes of silicon carbide part icles (25 mu m and 69 mu m mean diameter) within a 6-m flume. Polydisperse experiments involved mixtures of five different particle sizes and variatio n of the amounts of the finest and coarsest particles. All variables apart from the initial relative proportions of particles were identical in the ex periments. The effects of mixing different proportions of fine and coarse p articles is markedly non-linear. Adding small amounts of fine sediment to a coarse-grained gravity current has a much larger influence on flow velocit y, run-out distance and sedimentation patterns than adding a small amount o f coarse sediment to a fine-grained gravity current. The experiments show t hat adding small amounts of fine particles to a coarse-grained current resu lts in enhanced flow velocities because the fine sediment remains suspended and maintains an excess current density for a much longer time. Thus, the distance to which coarse particles are transported increases substantially as the proportion of fines in the flow is increased. Our experiments sugges t that sandy turbidity currents containing suspended fines will be much mor e extensive than turbidity currents composed of clean sand.