Classification of Buoyant River Plumes from Large Aspect Ratio Channels

Abstract

The dynamics of the Grand River plume, a major tributary of Lake Michigan, was studied in the vicinity of the Grand Haven coast on the east side of the lake. The river spread laterally from the mouth and formed a thin surface buoyant plume with various shapes that have not been previously reported. To understand the influence of different driving forces, primarily wind, buoyancy, and ambient currents, extensive field experiments were carried out over four periods in August and June 2006, and June and July 2007. They included aerial photography over the plume, acoustic Doppler current profiler (ADCP) deployments, meteorological buoys, drifters, SF6 and Rhodamine WT tracer releases, three-dimensional (3D) CTD profiling over the plume, and CTD casts at the river mouth. The results showed more flow classes than included in previous studies. A modified classification scheme based on the relative magnitude of the plume-crossflow length scale and a Richardson number were devised that includes longshore current components and onshore-offshore wind effects. A ratio of length scales and the Richardson number could predict whether the plume was shore attached or unattached, and how the onshore wind can spread the unattached plume offshore, deflect it back to shore, or diffuse it.