Initial expansion of the Columbia River tidal plume: Theory and remote sensing observations

Analysis of the Columbia River tidal plume using Lagrangian frontal equations provides a concise description of the evolution of frontal depth H , velocity U , reduced gravity g ′, and frontal internal Froude number F R . Because the estuary mouth is narrow, the initial radial plume motion is supercritical ( F R > 1) for up to 12 hours. Understanding this supercritical phase is vital, because plume properties change rapidly, with strong ecosystem impacts. To analyze this expansion, analytical and numerical models (the latter with three mixing formulations) were tested. Model results are compared to synthetic aperture radar images to verify that the predicted frontal properties are realistic. Lagrangian theory provides especially simple constraints (independent of the mixing model) on spatial variations in F R and Ug ′ H . For parameters representative of the Columbia River plume, the plume spreads 10–35 km and thins to 25–60% of its initial depth before becoming subcritical. After liftoff, F R increases as the front accelerates and thins, it then decreases to unity; g ′ decreases, but more slowly than H and U . H , controlled by a balance between spreading and mixing, first decreases then increases. The strength of vertical mixing and the mixing efficiency E F (ratio of buoyancy flux to dissipation) both play a significant role in determining plume properties, and it is important to include both the horizontal gradient in g ′ and surface slope component of the pressure gradient. A model with an Ellison‐Turner type entrainment scheme predicts the plume trajectory better than one that assumes a constant interfacial gradient Richardson number.