Collaborative Research: Spreading, Entrainment, and Water Mass Modification in Near-Field River Plumes

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Intellectual merit: A near-field river plume can be characterized by supercritical Froude numbers, enhanced mixing, and rapid water mass modification. This supercritical outflow region separates the estuary from the subcritical 'far-field' plume beyond. The supercritical flow is initiated by topographic control at the estuary mouth and results in intense mixing and high flow speeds that are typically not present in the coastal ocean. Whereas the far-field plume is influenced strongly by the earth's rotation and local wind stress, the supercritical outflow region is dominated by local advective processes and internal shear instabilities. The area over which the estuary outflow is supercritical is only a small fraction of the entire river plume area, but salinity changes occurring within the region of supercritical flow may be similar in magnitude to salinity changes that occur within the estuary or the far-field plume. However, despite the importance of these salinity changes in determining the water mass characteristics and structure of the river plume as a whole, there is no theory that describes the nature of these transformations, or their dependence on varying forcing mechanisms. The overall objective of this project is to understand and predict water mass changes that occur within the supercritical outflow region. This will be accomplished through a combination of observational and numerical modeling techniques. The central hypothesis of this proposal is that the outflow properties of the near-field may be predicted given the local geometry, flow parameters at the estuary mouth, and tidal amplitude. The rationale for the proposed research is that better understanding of the near-field plume will broaden understanding of river plume dynamics, and improve predictions of coastal buoyant flows. The project will accomplish the overall objective by pursuing the following two specific objectives: 1) Relate mixing and spreading within the supercritical outflow region of the plume, and 2) Quantify dependence of near-field outflow properties to estuarine discharge characteristics. It is expected that this project will produce a quantitative understanding of the processes affecting water mass modification, most notably changes in salinity structure, in the near-field plume region. This understanding will fill in an important gap in relating estuarine outflow to large scale river plume properties. Broader impact: This study will improve analytical and numerical studies of shelf circulation through a better understanding of how estuarine outflow ultimately enters the broader scale shelf circulation. It will also improve understanding of near-field river plume dynamics and the role of the near-field plume in the context of the river plume as a whole. In that respect, many ongoing studies of river plumes will directly benefit. Another impact of the proposed work is the significance of measuring the turbulent field associated with the near field plume using three distinct techniques. Comparison of point measurements (microstructure) with the mean values provided by a control volume method will provide important context to both types of measurements in terms of understanding the intermittency of turbulence and its integrated effects. Cross comparison of the various techniques will also provide a critical test environment for the turbulence Autonomous Underwater Vehicle, which is a new and emerging technology. Furthermore, such a complete and multifaceted data set of turbulence from a stratified shear flow will be a boon to numerical modelers seeking opportunities to evaluate and test new turbulence closure techniques. Finally, in addition to writing peer-reviewed, scientific papers, interactive course materials appropriate for upper-level undergraduate and introductory graduate classes in engineering andphysical oceanography will be developed.