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Structure and dynamics of the Columbia River tidal plume front

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Title Structure and dynamics of the Columbia River tidal plume front
Names Kilcher, Levi F. (creator)
Nash, Jonathan D. (creator)
Date Issued 2010-05-29 (iso8601)
Abstract Time‐dependent buoyant plumes form at the outflow of tidally dominated estuaries.
When estuary discharge velocity exceeds plume internal wave speed c, a sharp front forms
at the plume’s leading edge that expands from the time‐dependent source. Using
observations of the Columbia River tidal plume from multiple tidal cycles we characterize
time‐evolving plume structure and quantify front speed Uf, plume internal wave speed c,
front curvature, and ultimate extent. We identify three distinct stages of propagation:
(1) Initially, the plume is strongly influenced by shallow bathymetry near the river mouth.
(2) As the front advances offshore the plume detaches from the bottom and expands as a
freely propagating gravity current with relatively constant U[subscript f], c and frontal Froude
number F = U[subscript f] /c. Ambient currents explain intracycle variability in U[subscript f] and winds alter
front shape. Variability in ambient stratification associated with previous cycles’ plume
remnants leads to complex fronts and internal waves. (3) Finally, the plume decelerates,
adjusts toward geostrophy, and may radiate additional internal waves. Using a simple
kinematic model, we suggest that constant frontal propagation speed, Uf = 0.9 ± 0.1 m/s,
during stage 2 is primarily controlled by linearly increasing volume flux from the Columbia
River mouth. As this discharge rate subsides, the plume expands as a fixed volume with
decreasing front speed (stage 3). The plume’s final extent is controlled by the Rossby radius,
which scales with a length based on the total volume discharged. This provides an integral
description of plume front evolution based on the time‐dependent estuary discharge.
Genre Article
Topic Columbia River tidal plume
Identifier Kilcher, L. F., and J. D. Nash (2010), Structure and dynamics of the Columbia River tidal plume front, J. Geophys. Res., 115, C05S90, doi:10.1029/2009JC006066.

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