Laboratory investigation on the plume-to-plume interaction on a rotating table

When freshwater from rivers flows into the coastal ocean, it forms a buoyant plume near the river mouth and serves as an important source of terrestrial materials in the coastal ecosystem. While most of previous researches have been focusing on an isolated river plume formed by a single large river, fewer investigations have considered on overlapped plumes developed by multiple rivers which deliver freshwater together and form a coastal current that can propagate far away from river mouths. This phenomenon includes a large river estuary with multiple channel outlets or multiple small rivers in fairly close distance that may interact and coalesce as freshwater from different drainage area encounters in the coastal ocean. Considering only the buoyancy and the earth rotation, river outflow leaving the estuary forms a two-part structure, a recirculating bulge extending offshore near river mouth and a coastal current propagating in the downstream direction. Following the classical bulge ballooning theory, the bulge grows forever in size to balance the momentum flux of the alongshore coastal current. Under plume-to-plume interaction condition, however, we hypothesize that the upstream plume will serve as an extra source to close the alongshore momentum budget of the downstream bulge. To test this hypothesis, we conducted laboratory experiment to simulate two adjacent river outflows on a 3-m rotating table. Horizontal velocity fields of each layers were measured using PIV technique. Our results show that the presence of upstream plume inhibits the growth of the downstream bulge and its offshore migration. As such, more incoming freshwater goes into the alongshore coastal current rather than into the growing bulge near the mouth. Results from idealized laboratory simulations may provide new insights into understanding the interaction between multiple plumes and estimating total freshwater transport along the coast, such as Chile’s central coast, Ob-Yenisei buoyant plume, western Iberian buoyant plume, etc.