River plume
A river plume is a freshened water mass that is formed in the sea as a result of mixing of river discharge and saline seawater. River plumes are formed in coastal sea areas at many regions in the World. River plumes generally occupy wide-but-shallow sea surface layers bounded by sharp density gradients. The area of a river plume is 3-5 orders of magnitude greater than its depth; therefore, even small rivers with discharge rates ~1–10 m/s form river plumes with horizontal spatial extents ~10–100 m. Areas of river plumes formed by the largest rivers are ~100–1000 km2. Despite the relatively small volume of total freshwater runoff to the World Ocean, river plumes occupy up to 21% of shelf areas of the ocean, i.e., several million square kilometers.
In some occasions river plumes are spoken of as regions of fresh water influence, although it is preferred to use this term for regions in which multiple sources add to the fresh water input of the zone or for shallow, frictional shelves. ROFIs and river plumes differ in the variation at temporal and spatial scales. The river plume can be identified as a buoyant water mass that emerges due to river discharge into the coastal ocean and varies over diurnal to synoptic timescales. 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. This extended region is called the region of freshwater influence, ROFI. 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.
Processes
River plumes play an important role in global and regional land-ocean interactions. River discharges provide large fluxes of buoyancy, heat, terrigenous sediments, nutrients, and anthropogenic pollutants to the ocean. River plumes strongly influence many physical, biological, and geochemical processes in the coastal and shelf sea areas including stratification of seawater, coastal currents, carbon and biogeochemical cycles, primary production, and seabed morphology.A river plume is a dynamical system influenced by processes with a wide range of temporal and spatial scales, which depend on the size and shape of the estuary as well as on the type and variation of the forcing from the estuary and the ocean. Feedback mechanisms between sediment deposited by the plume at the submarine delta and the geometry of the delta make for a complex system. Due to this complexity there is not a general, simple theory that offers quantitative predictability for the motion of particles and the structure of river plumes; however, some theories incorporating simplified assumptions have helped in understanding the important aspects of buoyancy-influenced coastal flows. 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.
Structure
The balance between the important processes varies over the position in the plume. The following regions can be distinguished: the source region, the liftoff point, the front, and the near field region. Beyond the plume itself but within its area of influence are the mid-field region and the far field region.Liftoff point
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. This point is important in surface-advected river plumes.Near-field region
In the near-field the momentum of the plume is larger than its buoyancy. This balance is represented in the Froude number, and is larger than one in the near-field, indicating supercritical flow. Both the liftoff point and the outer boundary of the near-field, the plume front, are characterized by critical flow conditions and the flow in the near-field region shows features similar to a jet. The momentum balance is dominated by barotropic and baroclinic pressure gradients, turbulent shear stresses, and flow acceleration. Flow deceleration is mainly caused by the shear stresses on the interface of the plume with the ambient ocean. In some cases a near-field region will not exist. This is for example the case if the width of the river mouth is large relative to the Rossby radius of deformation,, and the fresh water inflow will leave the river mouth as a far-field plume. When tides are large, the near-field plume is also known as the tidal plume.Mid-field region
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, cross-stream internal pressure gradients, and sometimes centripetal acceleration. The initial momentum of the outflow from the source is lost and the wind forcing gradually takes over as the most important parameter. As a result, the flow changes its speed, direction, and spreading pattern. When the influence of wind forcing is small, outflows can sometimes form a recirculating bulge; however, evidence of such a feature in field observations is scant.Far-field region
Even further away from the source region is the far-field, where the plume has lost all memory of the outflow momentum. The momentum balance of the far-field is dominated by the rotation of the Earth, buoyancy, wind forcing, and bottom stress. The far-field can cover large areas, up to hundreds of kilometers from its source. Diurnal and semi-diurnal variability of the far-field region is generally governed by tides, synoptic variability by wind forcing, and seasonal variability by river discharge. In the absence of strong wind forcing and strong currents, the far-field plume can behave as a current of relatively fresh water in the direction of a propagating Kelvin wave. Examples of this can be observed in the Rhine ROFI, where the river plume can be traced all along the Dutch coast. The character of this coastal current is different in the case of shallow seas, when the current occupies the whole water column and its motion is affected by bottom friction, and in the case of a surface-advected plume whose vertical size is less than the water depth.Advection
At the most basic and idealized level, river plumes can be classified to be either surface-advected or bottom-advected. A plume is considered to be bottom-advected when it occupies the whole water column from the surface to the seabed. In this case its stratification is mainly horizontal as a result of strong advection over the whole water column, especially near the bed. A surface-advected plume does not interact with the bottom because its vertical size is less than its depth. In this case a plume is mainly vertically stratified. Differentiation between these two types of river plumes can be made by evaluating a set of parameters, as set up by Yankovsky and Chapman in their paper from 1997. The distance up to which the fresh water river plume is transported across-shelf by processes at the surface is given bywhere
- is the inflow velocity from the source region and the near-field jet,
- is the Coriolis force,
- is the buoyancy, and
- is the depth of the water column at the mouth of the river/estuary.
Bottom-advected plumes
Bottom-advected plumes are often characterized by large discharge conditions and are generally less sensitive to wind forcing and corresponding advection and mixing. This type of advection is driven by bottom Ekman transport, which drives the fresh or brackish river outflow with density and velocity from an estuary of width and depth to the frontal zone across the shelf. This is indicated in the figure to the right. When the frontal zone is far enough from the shore, thermal wind dynamics can transport the complete volume flux away from the estuary. The across-shore position, which denotes the width of the coastal current, and the equilibrium-depth at which the plume separates from the bottom can be calculated in equilibrium conditions with a certain bottom slope byNote that this is only valid when. When the bottom Ekman layer cannot transport the river outflow offshore and another process governs the propagation. In that case, only a surface-advected plume is found.