The proposed research will extend a recently developed framework for analyzing flow/sediment bed interactions to bottom currents, debris flows and tsunamis. Differences with regard to wave orientation relative to flow direction and slope, up and downstream symmetry, direction of migration etc. will be compared in detail to corresponding field observations, in order to draw conclusions about the genesis of specific sediment wave fields. The framework for conducting both linear stability analysis and nonlinear simulations is based on the full three-dimensional Navier-Stokes equations, rather than the depth-averaged shallow water equations most commonly employed to date. A new Ph.D. post-doc will do most of the work in collaboration with the PI. The PI hopes that the proposed research will elevate the description of sediment wave formation from largely qualitative arguments to advanced, quantitative models on which we can base rigorous linear stability analyses as well as fully nonlinear simulations. In this way, it will enable us to distinguish between sediment waves generated by turbidity and bottom currents, debris flows and tsunamis, so that different sediment wave fields can be classified according to their mechanism of generation.
Broader impact: Stated broader impacts include support of quantitative, engineering-based modeling approaches within the earth science community. The work has relevance to the mission of the NSF-sponsored Community Surface Dynamics Modeling System (CSDMS). The project will also support a graduate student, as well as undergraduates and outreach to high school students.
Turbidity currents are gravity-driven, turbulent underwater sediment flows along the bottom of lakes or the ocean. By accounting for the majority of sediment transport from the continental shelves to the deep ocean, they play a crucial role in the global sediment cycle. Their transport distances range from hundreds of meters to thousands of kilometers, and over geological time scales they can form vast sediment accumulations up to millions of km3. Their interaction with the seafloor can result in the formation of large-scale geological features, such as submarine fans, channles and sediment waves. Ancient deposits of turbidite sand represent an important class of hydrocarbon reservoirs, adding an economic dimension to their exploration. In an environmental context, turbidity currents are responsible for much of the sedimentation in reservoirs, with consequent loss of water storage capacity. They furthermore pose a substantial danger to submarine equipment such as pipelines, resulting in significant environmental risks for the coastal ocean. By triggering submarine landslides, especially in conjunction with earthquakes, they can lead to the formation of near-shore tsunamis, thereby posing a threat to coastal population centers. Due to their infrequent and unpredictable occurrence in remote and hostile environments, field data for turbidity currents have been extremely difficult to obtain, so that much of what we know about them today is due to small-scale laboratory experiments and computer simulations. In the current project, we addressed several open questions regarding turbidity currents and their interactions with the seafloor, among them: 1) The initiation of turbidity currents from river outflows: Here we identified double-diffusive sedimentation as a very important mechanism governing the transport of sediment from buoyant river plumes to the seafloor. Specifically, this mechanism can drastically increase the effective settling velocity of sediment. 2) Their interaction with seafloor topography, such as an isolated seamount: Here we investigated the three-dimensional flow structure and mixing dynamics of such currents, along with their erosional and depositional behavior. We identified several surprising effects related to seafloor topography. 3) Their interaction with periodic obstacles, such as sediment waves and dunes: Here we paid particular attention to the flow structures and forces created in such periodic arrays, in order to understand the growth mechnism of sediment waves. 4) Their description via a new class of reduced semi-analytical models: We developed a new class of analytical models for the description of gravity-driven currents that, in contrast to earlier models, does not require any empirical energy conservation assumptions. 5) Improved numerical modeling approaches for turbidity currents: We employed state-of-the-art computational simulation techniques, in order to obtain turbidity current simulations of a very high degree of accuracy.
