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    Turbulent mixing in a strongly forced salt wedge estuary

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    2009JC006061.pdf (2.403Mb)
    Date
    2010-12-09
    Author
    Ralston, David K.  Concept link
    Geyer, W. Rockwell  Concept link
    Lerczak, James A.  Concept link
    Scully, Malcolm E.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/4304
    As published
    https://doi.org/10.1029/2009JC006061
    DOI
    10.1029/2009JC006061
    Keyword
     Mixing; Turbulence; Salt wedge estuary 
    Abstract
    Turbulent mixing of salt is examined in a shallow salt wedge estuary with strong fluvial and tidal forcing. A numerical model of the Merrimack River estuary is used to quantify turbulent stress, shear production, and buoyancy flux. Little mixing occurs during flood tides despite strong velocities because bottom boundary layer turbulence is dislocated from stratification elevated in the water column. During ebbs, bottom salinity fronts form at a series of bathymetric transitions. At the fronts, near-bottom velocity and shear stress are low, but shear, stress, and buoyancy flux are elevated at the pycnocline. Internal shear layers provide the dominant source of mixing during the early ebb. Later in the ebb, the pycnocline broadens and moves down such that boundary layer turbulence dominates mixing. Mixing occurs primarily during ebbs, with internal shear mixing accounting for about 50% of the total buoyancy flux. Both the relative contribution of internal shear mixing and the mixing efficiency increase with discharge, with bulk mixing efficiencies between 0.02 and 0.07. Buoyancy fluxes in the estuary increase with discharge up to about 400 m3 s−1 above which a majority of the mixing occurs offshore. Observed buoyancy fluxes were more consistent with the k-ɛ turbulence closure than the Mellor-Yamada closure, and more total mixing occurred in the estuary with k-ɛ. Calculated buoyancy fluxes were sensitive to horizontal grid resolution, as a lower resolution grid yielded less integrated buoyancy flux in the estuary and exported lower salinity water but likely had greater numerical mixing.
    Description
    Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C12024, doi:10.1029/2009JC006061.
    Collections
    • Applied Ocean Physics and Engineering (AOP&E)
    Suggested Citation
    Journal of Geophysical Research 115 (2010): C12024
     

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