High-resolution observations of lateral circulation at an inflection bend of the Oujiang River Estuary, China

Field observations were conducted at an inflection point of two bends of the Oujiang River Estuary, China, to investigate lateral circulation and its driving mechanisms, and the effect of stratification in a macrotidal estuary. Repeated cross-channel transect measurements of velocities were obtained by vessel-mounted acoustic Doppler current profiler at the inflection point during neap and spring tides. Conductivity, temperature, and depth were collected at both ends of the transects to estimate the lateral moment budget. We observed asymmetry of lateral circulation in intra-tidal and inter-tidal regions, with strong single-cell lateral circulation during the neap ebb tide. In contrast, during the flood, lateral circulation was weak but structurally complex. During the spring tide, lateral circulation only appeared at peak ebb and peak flood. This asymmetric variation in lateral circulation is induced by an asymmetric driving mechanism, investigated thoroughly and connected to water stratification. The significant effect of nonlinear advective acceleration in both highly stratified water and well-mixed water indicates the important influence of an inflection point of two bends on lateral circulation. Besides, the strong stratification plays a role to suppress the lateral circulation due to the tilted pycnocline spin opposite to lateral circulation. The Oujiang River Estuary is an important shipping area, and the results have practical significance for the development and utilisation of port and channel resources.