Studies on boundary conditions for fine-sediment transport

Boundary conditions for sediment-transport models must reflect bed properties and particulate mass-transport functions at the seafloor. My dissertation research focuses on four topics regarding near-bed particle motion and material properties of cohesive suspensions contributing to boundary conditions for fine-sediment transport. This program of analysis and experimentation yields the following observations:; Particle fall velocity in channel flow is reduced from still-water values due to nonlinear drag associated with fluid turbulence and gravitational drift of particles. A mechanics-based model describing discrete-particle motion indicates that, for quartz-density particles, this effect is most severe at diameters between 20 {dollar}mu{dollar}m and 200 {dollar}mu{dollar}m. A 50-{dollar}mu{dollar}m diameter, quartz-density particle, for example, is predicted to settle through near-bed turbulent boundary layers at about 85, 70, and 33% of its still-water velocity when boundary shear stresses are 0.1, 0.4, and 1.6 Pa, respectively. In the case of 700-{dollar}mu{dollar}m diameter, low-density polystyrene particles, model simulations indicate and experimental observations confirm a 60-65% reduction in particle average fall velocity in near-bed channel flow as boundary shear stress increases from nil to 0.1 Pa.; The same mechanics-based model also describes saltation motion of quartz-density particles with known impact response in low transport-stage channel flows. At a given flow intensity, newly-observed relative heights of individual, smooth, glass spheres in saltation trajectories over a stationary bed of identical grains confirm model predictions and are greater by a factor of two than relative heights observed in similar earlier experiments.; Rigidity and yield stress of abiotic, clay-silt-seawater mixtures exhibit power-law dependence on solids concentration and primary-grain size; exponent values indicate scaling behavior of cohesive-network structures analogous to that of percolating systems. Scaling relationships between rheological and sedimentological properties contribute to improved understanding of factors controlling suspension properties and can be used in a newly-developed model that predicts erosion resistance of cohesive muds under steady flows.; Cohesive structures also impart power-law scaling behavior to properties of muds from the Amazon shelf. Stress-relaxation measurements additionally reveal that particle interactions in these natural muds give rise to solid-like behavior and broad spectra of viscoelastic response times with discrete values of up to 300 s. Interactions of oscillatory strain and response-time distributions result in measurably increasing mud elasticity with increasing frequency of deformation. Viscoelasticity of non-rigid but otherwise stationary mud layers on the inner Amazon shelf may give rise to thicknesses of consolidating deposits most resistant to wave-induced transport.