Impacts of physical alterations on salt transport during the dry season in the Modaomen Estuary, Pearl River Delta, China

Physical alterations (e.g., land reclamation and riverbed down-cutting) caused by human activity may significantly affect the process of salt transport in estuarine systems. Between the 1970s and 2010s, the Modaomen Estuary, an outlet of the Pearl River Delta (PRD), was subject to large-scale land reclamation and riverbed downcutting, which exerted dramatic changes in the transport of salt. In this study, a three-dimensional hydrodynamic model (EFDC) is used to quantitatively evaluate the impacts of different physical alterations in the estuary during the dry season. The modeled results indicate that large-scale land reclamation has decreased landward salt transport, which has weakened the tidal mixing and diffusion in the estuary and has resulted in decreased salinity and a shortened salt intrusion length. In addition, the significant degree of riverbed down-cutting has caused the import of more salt into the estuary, which has enhanced the estuarine circulation to cause further increases in salinity and an extended salt intrusion length. Further, the enhancement of landward salt transport owing to the riverbed down-cutting is much stronger than the weakening effect of land reclamation. All of these local physical alterations have significantly augmented the salt intrusion extent and has intensified the salinity stratification. Consequently, the Modaomen Estuary has changed from partial stratification to high stratification during the past four decades. Although both upstream riverbed down-cutting and sea-level rise facilitate salt intrusion, the impacts from these processes are one order of magnitude smaller than those from the physical alterations made in the estuary. The results obtained in this study have significant implications for the sustainable development of estuarine systems and provide scientific guidelines for general water management, particularly for the prevention of brine tides in the PRD.