The role of turbulence stress divergence in decelerating a river plume

Abstract

[1] Turbulence controls the composition of river plumes through mixing and alters the plume's trajectory by diffusing its momentum. While believed to play a crucial role in decelerating river-source waters, the turbulence stress in a near-field river plume has not previously been observationally quantified. In this study, finely resolved density, velocity, and turbulence observations are combined with a control-volume technique to describe the momentum balance in the Columbia River's near-field plume during 10 tidal cycles that encompass both large and small river flow. Turbulence stress varies by 2-3 orders of magnitude, both within a given ebb and between ebbs with different tidal or river forcing; its magnitude scales with the strength of the instantaneous ebb outflow, i.e., high stresses occur during peak flow of strong ebbs. During these periods, the momentum equation is represented by a balance between stress divergence and plume deceleration. As the flow relaxes, the stress divergence weakens and other terms (pressure gradient and Coriolis) may become appreciable and influence plume deceleration. While the momentum balance could not be closed during these weaker flow periods, during strong tidal pulses the time scale for decay based on observed stress is significantly less than a tidal half-period, indicating that stress divergence plays a fundamental role in the initial deceleration of the plume.

Transient river flow into a fjord and its control of plume energy partitioning

“…For example, in Kilcher et al . [], the buoyancy frequency squared was scaled by four (4 N 2 ) to given a maximum near‐surface ∼0.3 s −2 . Thus, the equivalent N 2 is ∼0.075 s −2 .…”

“…Spreading of the large‐scale plume is regulated by boundaries, rotation, wind stirring, internal wave generation, or upwelling as the bulges propagates and dilutes in the ambient flow structure and stratification [ Garvine , ; Horner‐Devine et al ., ]. Small‐scale plume evolution is controlled by geometry at the inflow location, shear‐driven turbulence, and horizontal advection [ Garvine , ; Kilcher et al ., ]. Large‐scale plumes with narrow river mouths, e.g., Columbia River, exhibit similar dynamics in the near field as small‐scale plumes [ Hetland , ].…”

“…Modulation of the near‐field plume structure during the initial phase of the ebb tide results in strong shear [ Nash et al ., ]. Associated turbulent momentum fluxes modify plume thickness over subsequent tidal cycles [ Kilcher et al ., ]. Variability of the freshwater inflows (akin to the spring freshet) was explored numerically by Yankovsky et al .…”

Evaluating the Surface Response of Discharge Events in a New Zealand Gulf-ROFI

“…Spreading of the large-scale plume is regulated at boundaries by rotation, wind stirring, internal wave generation or upwelling as the bulges propagates and dilutes in the ambient flow structure and stratification (Garvine, 1987;Horner-Devine et al, 2008). Small-scale plume evolution is controlled by geometry at the inflow location, shear-driven turbulence and horizontal advection (Garvine, 1995;Kilcher et al, 2012). Large-scale plumes with narrow river mouths, e.g., Columbia River, exhibit similar dynamics in the near-field as small-scale plumes (Hetland, 2005).…”

Effects of bottom topography on dynamics of river discharges in tidal regions: case study of twin plumes in Taiwan Strait

“…Previous studies of the topic have produced an extensive body of literature encompassing analyses of observational data (e.g. Boicourt, 1981;Münchow and Garvine, 1993;Simpson and Souza, 1995a, b;Zavialov et al, 2003;Guo and Valle-Levinson, 2007;O'Donnell et al, 1998;MacCready et al, 2009;Jay et al, 2009;Warrick and Stevens, 2011;Kilcher et al, 2012), theoretical and numerical approaches (e.g. Chao, 1988;Oey and Mellor, 1993;Garvine, 1995Garvine, , 2001Ruddick et al, 1995;Kourafalou et al, 1996;Yankovsky and Chapman, 1997;Kasai et al, 2000;Korotenko, 2000;Valle-Levinson et al, 2003;Cushman-Roisin et al, 2007;Schiller and Kourafalou, 2010;Hwang et al, 2011), as well as laboratory experiments (Whitehead and Chapman, 1986;Avicola and Huq, 2002;Lentz and Helfrich, 2002;Horner-Devine et al, 2006;Yeping and Horner-Devine, 2013).…”