Density Currents and Upstream Intrusions

Density currents are buoyancy driven far-field flows that are defined by transverse horizontal spreading while being advected downstream by an ambient current.

As illustrated by the image on the right, these spreading processes can intrude into the ambient flow, forming a buoyant upstream wedge and stagnation point.

These flows are caused by the density difference of the mixed flow relative to the ambient density. Density currents are preceded by turbulent jet mixing in the near-field and are followed by passive diffusion in the far-field.

Density currents may or may not form upstream intrusions, depending upon the crossflow magnitude and internal buoyancy at boundary interaction.

Surface Buoyant Jet/Density Current Mixing

Positively buoyant jets discharged horizontally along the water surface from a laterally entering channel or pipe as shown in the right image bear some similarities to the more classical submerged buoyant jet. For a relatively short initial distance, the effluent behaves like a momentum jet spreading both laterally and vertically due to turbulent mixing.

After this stage, vertical entrainment becomes inhibited due to buoyant damping of the turbulent motions, and the jet experiences strong lateral spreading. During stagnant ambient conditions, ultimately a reasonably thin layer may be formed at the surface of the receiving water; that layer can undergo the transient density current buoyant spreading motions (Image a).

In the presence of ambient crossflow, buoyant surface jets may exhibit any one of following three types of flow features: They may form a weakly deflected jet that does not interact with the shoreline (Image b, above). When the crossflow is strong, they may attach to the downstream boundary forming a shore-hugging plume (Image c). When a high discharge buoyancy flux combines with a weak crossflow, the buoyant spreading effects can be so strong that an upstream intruding plume is formed that also stays close to the shoreline (Image d).

Buoyant Spreading Models in CORMIX

Density current are effective transport mechanisms that can quickly spread a mixed effluent laterally over large distances in the transverse direction, particularly in cases of strong ambient stratification. In this case, effluent of considerable vertical thickness at the terminal level can collapse into a thin but very wide layer unless this is prevented by lateral boundaries.

If the discharge is non-buoyant, or weakly buoyant there is no buoyant spreading region in the far-field, only a passive diffusion region.

Depending on the type of near-field flow, ambient density stratification, and boundary interaction process, several types of density current buoyant spreading may occur:
(i) spreading at the water surface,
(ii) spreading at the bottom,
(iii) spreading at a sharp internal interface (pycnocline) with a density jump, or
(iv) spreading at the terminal level in continuously (e.g. linearly) stratified ambient.

Additional Mechanisms in Density Current Mixing

Two additional density current mixing mechanisms may be considered within CORMIX. These mehanisms effect the density difference between the flow and the ambient environment. These mechanisms are considered for the following pollutant types:

1. Heated Discharge: A heat loss coefficient may be specified for additional buoyancy loss due to surface heat exchange .
2. Sediment Discharge: Particle settling will cause additional buoyancy loss due to particle settling. Sediment discharge can be simulated with CORMIX-GTS Advanced Tools Sediment Release.