Salt transport during a dry season in the Modaomen Estuary, Pearl River Delta, China

Based on the salinity and current data from an intensive field survey carried out in the Modaomen Estuary, Pearl River Delta, China, this study reveals that the behaviors of the two major components of salt transport flux (i.e., the advective salt flux caused by mean advective flow and the tidal salt flux caused by tidal diffusive processes) are quite different at different stations along the estuary during a dry season. During the neap tide, the tidal salt flux and the advective flux are both important in driving a net landward salt transport at a downstream station, and the tidal salt flux is the primary component of landward transport at the station where the salt intrusion limit occurs, whereas the advective salt flux is more important at the station in the mesohaline region and at the estuary mouth. This situation changes during the following meso and spring tides, i.e., the advective salt flux becomes more important than the tidal salt flux. When an enhanced river flow and a strong down-estuary wind both occur at the spring tide, the surface seaward advective salt flux increases significantly, whereas the tidal salt flux is also enhanced, becoming the main contributor to the landward salt transport. The salt transport pattern is of great significance in the regional environmental management for salt intrusion prevention. An optimal freshwater releasing time from the upstream reservoir should be chosen at the spring tide. The implementation of an engineering project to prevent saltwater entering the Modaomen mainstem from the Hongwan Waterway (a branch of the estuary) should be based on a thorough understanding of the local salt transport process.