During the River Influences on Shelf Ecosystems (RISE) project, suspended particle concentration and size data were collected simultaneously with turbulence, velocity and density data in the near field of the Columbia River plume. This energetic region of the plume is characterized by rapid mixing, high primary productivity, significant release of particles from the plume and, ac-cording to the work described in this proposal, re-suspension and entrainment of particles from the seafloor. In this project, the corresponding sediment and turbulence data sets are used to determine whether predicted relationships between particle size and small-scale turbulent shear are observed in river plumes. These results are then extended to determine how turbulence generated by frontal processes, bottom shear and internal waves modifies the retention and removal of particles in different regions of the plume. A second goal of the proposed work is to investigate the process of sediment re-suspension and its subsequent entrainment into the plume. Observations during RISE and previous cruises suggest that a significant amount of sediment is re-suspended during energetic ebbs in the stratified region offshore of the plume lift-off point. Such processes may explain observations of increased micronutrient levels in the plume near-field relative to the ocean and estuary, and help to explain the high productivity in the plume system. The passage of the plume front, which penetrates well below the base of the plume, will be examined in detail to determine its role in re-suspending and re-entraining sediment into the plume. This work is being done in parallel with many other physical, biological and geochemical studies of the Columbia Plume that are included in the RISE project. In addition to addressing questions relating to the role of turbulent processes on particle dispersal, the proposed work will benefit from and contribute to the larger interdisciplinary study of the plume system. It will leverage work already in place to provide key information about the role of sediment in the system, which was not included in the original proposal.
Intellectual merit: Particle transport is determined in large part by particle size via settling velocity. The proposed work will test relationships between particle size and turbulence in a productive river plume and relate variability in the particle field to plume processes. It will also determine the relative contributions to the plume from vertical (turbulent) and horizontal (advective) particle fluxes. Knowledge of these processes will contribute to the understanding of river-influenced ecosystems, the ability to model sediment processes in plumes, and the capability to sense suspended sediment from space.
Broader impacts: The proposed research will help explain the dynamics of sediment particles in turbulent environments and thus the transport of pollution, pathogens or other particle-bound matter. The highly productive Columbia Plume also provides habitat and food to endangered salmonids. Resuspended particles enhance primary productivity by supplying micronutrients. Turbidity provides relief from predation for juvenile salmonids. The project will provide support for Emily Spahn, a graduate student in Civil and Environmental Engineering. In addition to training, it will provide support for Ms. Spahn to attend professional conferences. The project will also provide support for an undergraduate student, thus introducing a student to different aspects of physical oceanographic research. Efforts will be made to recruit a student from an underrepresented group with the assistance of the College of Engineering's Diversity and Student Services (DASS) office. Finally, the project will support an early career assistant professor.
