Determining the subtidal flow at a stratified tidal junction by a novel repeated measurements method

Abstract

The division of freshwater into separate channels at tidal junctions plays an important role in determining the hydrodynamics and material transport in multichannel estuaries. To obtain a better understanding of the net material transport, it is essential to precisely measure the residual flow near the tidal junction. In this study, the phase-averaging method (PAM) was applied to investigate the residual current and freshwater division at the lower reaches of the Modaomen Estuary (ME), China. This is the first PAM-based quantitative evaluation of the freshwater division at the tidal junction, which controls the freshwater division between the mainstem of the ME and the Hongwan Waterway (HW). The accuracy of the PAM results was examined by comparison with the subtidal flow obtained at a moored site near the estuary mouth. The results showed that the HW received freshwater discharge from the ME during both spring and neap tides. The stratification in the HW was determined by water masses advected from the mainstem of the ME. During the spring tide, the ME had a stronger longitudinal salinity gradient. Via interaction with the vertical shear of the along-channel velocity, the tidal straining tended to enhance the vertical salinity gradient, which generated more vigorous stratification in the channel during the spring tide than during the neap tide. In accordance with the stratified and longitudinal salinity gradient, the mainstem of the ME exhibited a stronger exchange flow during the spring tide. Harmonic analysis and error estimation showed that the PAM can effectively remove the tidal signals. The accuracy of our results suggests that the PAM can be applied to survey other tidal junctions.