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When buoyant coastal currents interact

Despite being an important phenomenon, the dynamics and impacts of interacting buoyant coastal currents are poorly understood. Researchers joined forces to clear the waters in this respect by investigating the dynamics of these interactions in a series of studies and laboratory experiments.
When buoyant coastal currents interact
Buoyant coastal currents strongly influence coastal circulation and ecosystems as well as the redistribution of fresh water on a global scale. One example of a common interaction of buoyant currents from different river sources is the western Adriatic Sea coastal current. The way in which buoyant currents interact influences the downstream coastal distribution of waterborne materials contained in individual currents. This has significance in both ecological and societal terms, and calls for an understanding of how coastal waters can transport pollution, harmful algal blooms (HABs) and sediments away from a populated coastline.

Against this backdrop, the 'Interacting buoyant coastal currents' (INBUCOC) project set out to investigate the effects of multiple river plumes on the formation and dispersal of coastal waters. Researchers focused on the importance of the boundaries and relative location of these plumes, given the possibility of waters from one river being polluted or carrying harmful algae. To achieve their goals, they employed a simplified scenario involving two hypothetical rivers, the North River and the South River.

To determine various scenario outcomes whereby one river contained pollution or harmful algae, INBUCOC set a series of questions. These covered the possibility of effects to the populated coastline, whether one river acts as a barrier to such threats reaching the coast, and if the two water masses align relative to each other in the vertical and horizontal.

Primarily, the study aimed to quantify the possible horizontal and vertical alignment scenarios as a function of dynamically relevant non-dimensional numbers, using an analytical model. Laboratory results and analytical calculations showed that the frontal position, the depth profile, and the horizontal and vertical alignments of two buoyant fluids with different densities can be characterised by an important length scale, the Rossby radius of deformation.

Laboratory rotating experiments revealed that, after reaching equilibrium, the two buoyant currents align mainly horizontally. Alternatively, if the extent of the fronts between three different fluids is similar, the buoyant currents align mainly vertically.

INBUCOC provided insights on how the water masses of two individual buoyant coastal currents align relative to each other vertically and horizontally. While this can enhance our understanding of coastal current interactions, researchers cautioned that project results should be interpreted with care, due to their modelled, speculative, laboratory-based nature.

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