Bedforms and Gravity Underflows in Marine Environments

201 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2003.This investigation was motivated principally by the increasing necessity of reliable scientific evidence that could explain the presence of (1) vast fields of long, upslope-migrating sediment waves that appear covering extensive areas in continental rises or submarine-channel levees, and (2) systems of subparallel, longitudinal gullies that are commonly observed in continental slopes. Combined analytical and experimental evidence supporting the argument that supercritical density and turbidity currents can develop long-wavelength, upstream-migrating bedforms is presented for the first time. Additional experimental evidence of the coexistence of upstream-migrating antidunes and small-scale downstream-migrating ripples created by both supercritical saline density currents and turbidity currents is also presented. Linear, multiple-scale-based stability analysis was developed using a one-dimensional, layer-averaged model for conservative density currents (Ellison and Turner, 1959) that includes an additional equation for the layer-averaged turbulent kinetic energy (Parker et al, 1986). Similar methodology was also extended to the case of two-component, dilute turbidity currents, in an attempt to explore the effects of flow disequilibrium on bedform formation and development. Based on results of stability analyses presented here (considered valid approximations in the range of current bulk Richardson number of 0.5 < Rio < 1), it is suggested that dynamic differences between non-uniform supercritical density and dilute turbidity currents are imprinted in bedforms they can create: characteristic wavenumbers of bedwaves are given, together with dimensionless plots including rates of change of bedform geometric parameters, for either of those gravity underflows, as functions of bulk Richardson number. Theoretically-derived bedwave properties presented are compared, when possible, with experimental observations, and are proposed to be used in combination with field observations, for better interpretations of geologic records and inferences on the nature and properties of flows that created those sediment waves. Finally, experimental evidence on the formation of longitudinal channels, similar to slope gullies, is presented. Longitudinal features resembling slope gullies were observed after running successive sustained turbidity currents confined in a wide channel in which a slope break, similar to that observed in continental margins at the transition shelf-slope, was introduced.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD