Influence of wind stress and ambient flow on a high discharge river plume

Abstract

The response of a high discharge river plume to an alongshore ambient flow and wind forcing is studied with a three-dimensional numerical model. The study extends prior model studies of plumes by including (1) a very large volume discharge (14,000 m3 s-1, about twice the maximum used in other models), (2) ambient flow in a direction opposite to that of the propagation of coastally trapped waves, and (3) a sequence of wind direction reversals. The magnitude of the ambient flow, wind stress, estuary width, and river outflow are based on typical values for the Columbia River on the Washington coast. The model results challenge two longstanding notions about the Columbia plume; first, that plume orientation is in a relatively stable southwest position in summer. For example, with average discharge conditions (7000 m3 s-1) a summertime downwelling event can erode the southwestward plume and advect it to the north of the river mouth over several days. Second, the plume is not always unidirectional; branches can occur both upstream and downstream of the river mouth simultaneously. The model also provides an explanation for the observation that the plume rarely tends southward during the winter season; in contrast to summer conditions the rotational tendency of the plume and the ambient flow are in the same direction, so that wind stress must be be significant (>1.4 dynes cm-2 for at least 2 days) to reverse the plume direction. Distinct anticyclonic freshwater pools form in modeled plumes both north and south of the river mouth under steady forcing conditions when ambient flow is present. The scale of modeled pools is consistent with features observed in the Columbia plume.