Effect of sloping bottom on river plume dynamics on a laboratory-scale rotating table

In nature, plumes usually enter the coastal ocean after they leave the estuary, and most of them interact with the continental shelf slope. To understand plume dynamics, laboratory experiments were carried out on a rotating table to simulate the evolution of plumes over a sloping bottom. We modified reduced gravity g', Coriolis parameter f, and shelf slope alpha to study their impacts on plume characteristics and freshwater fate, and used the optical thickness method to obtain the depth field. We found that with the increasing g' or decreasing f, plume maximum depth h(max) decreases and plume maximum width W-max increases. We proposed a method to determine plume types based on their attachment to the shelf slope: when PCN (plume classification number) > 1.6, the plume is bottom-attached; when PCN< 1.6, the plume is surface-advected. In addition, we found the bulge will become unstable when BIN (bulge instability number)< 0.8. Our analysis shows that the sloping bottom is the most significant factor determining the ratio of freshwater accumulated in the bulge over transported with coastal currents. Generally, bottom-attached plume trapped near the coast inhibits offshore freshwater transport and promotes coastal current transport, while baroclinic instability tends to produce a large cyclonic vortex over a gentle slope which strongly enhances the offshore transport.

Automated summary

In nature, plumes entering the coastal ocean after leaving the estuary interact with the continental shelf slope. Laboratory experiments were conducted to simulate the evolution of plumes over a sloping bottom. The impacts of reduced gravity, Coriolis parameter, and shelf slope on plume characteristics and freshwater fate were studied, and the optical thickness method was used to obtain the depth field. It was found that increasing reduced gravity or decreasing Coriolis parameter led to a decrease in plume maximum depth and an increase in plume maximum width. A method was proposed to classify plumes based on their attachment to the shelf slope. Additionally, the study revealed the factors influencing the ratio of freshwater transport and accumulation in the bulge.