The fate of discharges from small rivers and mountainous streams are little studied relative to the dynamics of large river plumes. Flows from small watersheds are episodic, forming transient low‐salinity surface plumes that vary tidally due to interaction of outflow inertia with buoyancy and ambient tidal currents. An implementation of the Regional Ocean Modeling System (ROMS v3.0), a three‐dimensional, free‐surface, terrain‐following numerical model, is applied to small river outflows (≤10 m3 s−1) entering a tidal ocean, where they form small plumes of scale ~103 m or smaller. Analysis of the momentum balance points to three distinct zones: (i) an inertia‐driven near field, where advection, pressure gradient, and vertical stress divergence control the plume dynamics, (ii) a buoyancy‐driven midfield, where pressure gradient, lateral stress divergence, and rotational accelerations are dominant, and (iii) an advective far field, where local accelerations induced by ambient tidal currents determine the fate of the river/estuarine discharge. The response of these small tidal plumes to different buoyancy forcing and outflow rate is explored. With increasing buoyancy, plumes change from narrow/elongated, bottom‐attached flows to radially expanding, surface‐layer flows. Weak outflow promotes stratification within the plume layer, and with stronger outflow, the plume layer becomes thinner and well‐mixed. When compared with prototypical large‐river plumes in which Coriolis effects are important, (i) in these small plumes, there is no bulge and no coastal buoyancy current, that is, shore contact is mostly absent, and (ii) the plume is strongly influenced by ambient tidal currents, forming a tidal plume that is deflected upcoast/downcoast from the river mouth.
In contrast to large river plumes, Coriolis effects are weak, and inertia is quickly depleted so that the fate and structure of small‐scale plumes are more sensitive to tide and wind. Advected alongshore by reversing tidal currents in absence of wind forcing, small buoyant plumes are persistently deflected downwind in presence of alongshore winds and exhibit little tidal variability. The effect of different upwelling/downwelling winds on buoyant outflows ∼10 m3 s−1is explored. With increasing wind, tidal variability decreases, as does asymmetry in plume characteristics—for strong winds upwelling/downwelling plume structure is similar as the plume is retained closer to the shore. Wind forcing is exerted directly by wind stress on the surface of the plume and indirectly by wind‐driven currents that deflect the upwind boundary of the plume. While inertia and buoyancy dominate the inner plume, and wind dominates the outer plume, the mid‐plume responds to an interaction of wind and buoyancy forcing that can be indexed by a Plume Wedderburn Number
- NSF-PAR ID:
- 10387921
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 127
- Issue:
- 12
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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