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 mechansims. This is a problem because many theories of large-scale, far-field plume structure are based on outflow properties of the estuary. Also modern numerical models of buoyant flow on the continental shelf are not able to adequately resolve processes that occur within the supercritical outflow region. Because the water leaving the estuary may be modified considerably within the near-field, both numerical and analytical predictions of far-field plume structure will be incorrect if they are based directly on estuary outflow properties.
The overall objective of this project is to understand and predict water mass changes that occur within the supercritical outflow region. We propose to accomplish this 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 our understanding of river plume dynamics, and improve predictions of coastal buoyant flows.
The image below (with a link to the associated movie) shows an ideal numerical simulation of a near-field plume. The upper panel shows a gray-scale of surface salinity, the lower panel shows a cross-section of salinity down the center of the plume. The fresh water flow is initiated at t=0 hours. Note the very small temporal and spacial scales.
Download movie [2.2 M].