River plume
River plumes generally occupy wide-but-shallow sea surface layers bounded by sharp density gradients.
[3] At the edges of this water mass mixing takes place, creating a region adjacent to the river plume which is diluted and fresher compared to the open ocean, but does not have a clear boundary.
[3] Due to the indirect influence of freshwater discharge, ROFIs incorporate the dynamics and spatial extent of the river plumes but are typically assessed on seasonal, annual, and decadal timescales.
River discharges provide large fluxes of buoyancy, heat, terrigenous sediments, nutrients, and anthropogenic pollutants to the ocean.
Due to this complexity there is not (yet) a general, simple theory that offers quantitative predictability for the motion of particles and the structure of river plumes;[1] however, some theories incorporating simplified assumptions have helped in understanding the important aspects of buoyancy-influenced coastal flows.
[4] As is commonly used in fluid dynamics, the description of these complex flows is aided by scaling analysis to determine the relevant processes.
The primary parameters which define the structure and scale of an individual river plume are freshwater discharge, tidal energy, coastline bathymetry/geometry, ambient ocean currents, wind, and the rotation of the Earth.
[1] In the source or estuarine region, the buoyancy and momentum of the freshwater inflow from the estuary are the dominant properties that determine the initiation of the river plume.
The competition between river-induced stratification and tidal mixing sets the river plume's characteristic properties.
) indicates that freshwater processes are dominant compared to the tidal influence, and one can expect development of a river plume.
[1] In case of strong riverine forcing, often with a large estuarine Richardson number, the front of the plume separates from the bottom.
The position at which this flow separation occurs is called the liftoff point and sets the landward edge of the near-field.
Both the liftoff point and the outer boundary of the near-field, the plume front, are characterized by critical flow conditions (
[8] The momentum balance is dominated by barotropic and baroclinic pressure gradients, turbulent shear stresses, and flow acceleration.
This is for example the case if the width of the river mouth is large relative to the Rossby radius of deformation,
[9] The area at which the near-field inertial jet transfers into a flow in which geostrophic or wind-driven processes are dominant is the midfield-area.
The momentum balance of the mid-field is dominated by the rotation of the Earth (Coriolis effect), cross-stream internal pressure gradients, and sometimes centripetal acceleration.
When the influence of wind forcing is small, outflows can sometimes form a recirculating bulge;[1][6] however, evidence of such a feature in field observations is scant.
The momentum balance of the far-field is dominated by the rotation of the Earth (Coriolis effect), buoyancy, wind forcing, and bottom stress.
Bottom-advected plumes are often characterized by large discharge conditions and are generally less sensitive to wind forcing and corresponding advection and mixing.
[6] This type of advection is driven by bottom Ekman transport, which drives the fresh or brackish river outflow with density
When the frontal zone is far enough from the shore, thermal wind dynamics can transport the complete volume flux away from the estuary.
the bottom Ekman layer cannot transport the river outflow offshore and another process governs the propagation.
In the region near the mouth the initial momentum of the river outflow is the dominant mechanism, after which other processes such as wind forcing and the Coriolis effect take over.
Non-dimensional parameters have the benefit of simplifying the dynamics of the relevant processes by evaluating the magnitude of different terms.
[14] This barotropic variation in tidal velocity magnitude and direction gives rise to variability in the strength and stability of the river plume.
[7] This is already clear from the competition between river discharge and tidal mixing, captured in the (dimensionless) estuarine Richardson number
[5] Therefore, these baroclinic upwelling effects can cause ebb flows to transport nutrients and sediment towards the coast.
The initial jet-like structure gradually transfers into a far-field plume further offshore, which is deflected to the right as would be expected on the Northern Hemisphere due to the Coriolis effect.
Due to the high discharge, the corresponding momentum of the outflow, and the equatorial latitude, the dynamics of the plume are mainly characterized by the internal Froude number.
