Wind Effects on Small-Scale River and Creek Plumes

Basdurak, N. B.; Largier, J. L.

Abstract. In contrast to large river plumes, Coriolis effects are weak, and inertia is quickly depleted so thatfate and structure of small-scale plumes are more sensitive to tidal and wind forcing. Advectedalongshore by reversing tidal currents in absence of wind forcing, small buoyant plumes arepersistently deflected downwind in presence of alongshore winds and exhibit little tidal variability.The effect of wind on buoyant outflows ~10 m3s-1 is explored for simulations with differentupwelling/downwelling winds. With increasing wind, tidal variability decreases, as doesasymmetry in plume characteristics – for strong winds upwelling/downwelling plume structure issimilar as plume is retained nearshore. Wind forcing is exerted directly by wind stress on thesurface of the plume and indirectly by wind-driven currents that deflect the upwind boundary ofthe plume. While inertia and buoyancy dominate the inner plume, and wind dominates the outer plume, the interaction of wind and buoyancy can be indexed by a Plume Wedderburn Number Wpl(wind stress versus density gradients): for weaker winds (Wpl < ~1) surface stress enhancesstratification through straining, lengthening low-salinity reach; for stronger winds (Wpl > 1) surfacestress mixes the plume vertically, shortening low-salinity reach. However, dilute plume watersextend furthest in strong winds, passively advected several kilometers downwind. Shorelineexposure to outflow transitions from a quasi-symmetrical tide-averaged zone of impact under zerowind to a heavily skewed zone with persistent weak wind forcing and a one-sided zone for strong wind forcing.

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