Hyperpycnal and buoyant plumes from a sediment‐laden river

The interaction of river plumes and sediment transport is examined using a three‐dimensional numerical model which couples the hydrodynamics and sediment transport. Only large rivers in midlatitudes are considered so that the effect of Coriolis deflection becomes important. Winds and tides are excluded. In an initially quiescent coastal sea, sediment‐freshwater mixture is released from the inner reaches of a river. A high ratio of sediment to buoyancy input is chosen so that the initial sediment‐freshwater mixture is denser than the receiving seawater. As the dense underflow enters the adjacent sea, release of sediment to the sea floor changes the plume from hyperpycnal to buoyant. The forced convection of freshwater from depths triggers an expansive buoyant plume expanding in the direction of Kelvin wave propagation. The resulting buoyant plume is far more expansive than a typical buoyant plume in clear seawater produced by a comparable buoyancy input. Circulation associated with the plume is anticyclonic throughout the water column, lacking a cyclonic gyre underneath the anticyclone as one would expect from a typical buoyant plume in clear seawater. A heuristic argument follows to explain the lack of baroclinic flow structure in plumes forced by the sediment‐freshwater mixture.