Structure and dynamics of stationary fronts in an angled river plume

In angled river plumes, the river flow is oriented obliquely to the coast, leading to a lateral expansion into the sea rather than a radial one. However, the frontal structure and dynamics in these systems are not straightforward, especially under the influence of coastal currents. Here, we present detailed observations of an angled river plume and its fronts in the Macao waters. During the ebb tide, the river plume transitions from subcritical to supercritical due to the acceleration caused by lateral contraction. Despite this acceleration, the plume front remains nearly stationary, meeting the critical conditions in a coordinate system normal to the front. This quasi-stationary pattern of the front results from the plume expansion, which is established as a steady gravity current. Furthermore, the adjustment of the cross-flow reduces the Froude angle, causing the frontal shape to a half parabola. These findings offer novel understanding of stationary surface fronts in an angled river plume, which may trap the river-borne materials and pose significant challenges to coastal environments and ecosystems.