Direct measurement of secondary circulation in a meandering macrotidal estuary

Secondary circulation, which induces the typical helical flow motion along a curved channel, is responsible for the complex morphodynamic processes in fluvial streams and estuaries with meandering nature. Classical secondary circulation has been extensively documented through flume experiments and numerical simulations but field observations about this phenomenon are scarce and only limited to few channel cross-sections. In this study, intensive measurements of flow velocities were performed using a vessel-mounted ADCP in order to illustrate the spatial distribution of classical secondary circulation in a meandering macrotidal estuary for both flood and ebb phases. Negligible salinity is noted during field measurements and velocity profiles manifesting logarithmic patterns confirm open channel flow conditions. As the flow enters the bend, primary and secondary velocities are shifted towards the outer bank. At the bend apex, circulation is accelerated with near-surface primary and secondary velocities skewed to the outer bank while bottom currents are skewed to the inner bank. As the flow exits the bend, bottom secondary currents gain dominance over surface currents while the bottom velocity vectors are directed towards the inner bank. Both the primary and secondary currents cause the propagation of the three-dimensional helical flow alongside with the asymmetry in the bend cross-section and the formation of point bar. This study successfully pioneers in demonstrating the three-dimensional structure of classical secondary circulation in actual field conditions through intensive ADCP surveys.