Tidal effects on the dispersal and water age of the plumes from eight outlets of the Pearl river during the wet summer

Abstract

This study quantitatively evaluates the impact of tidal dynamics on dispersal and water age of the plumes from eight outlets of the Pearl River Estuary (PRE) using the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system. The results demonstrate that tidal forces play a crucial role in modulating estuarine transport and mixing processes. Regarding tidal effects on plume dispersal, tidal activity enhances the overlap of tracers released from the Yamen and Hutiaomen outlets in the Huangmaohai Estuary, extending their eastward and westward transport, respectively. However, for the Lingdingyang Estuary, tidal-induced convergence along the eastern side of the West Shoal significantly constrains the lateral spread of upstream plumes, leading to a predominant west-to-east distribution of high-concentration tracers from the Hengmen, Hongqili, Jiaomen, and Humen outlets. Regarding tidal effects on water age, tides generally increase surface water age by 1–4 days while reducing bottom water age by more than 5 days, resulting in a substantial decrease in the surface-to-bottom water age difference from 8.9 days in the no-tide scenario to 4.5 days in the realistic scenario. This reduction is primarily attributed to enhanced vertical mixing, which intensifies turbulence (by a factor of 5), increases downward buoyancy flux (by 1.4 times), and reduces stratification by 41 %. Additionally, the interaction between tidal forcing and seabed topography modulates bottom friction and vertical viscosity, weakening estuarine circulation and seaward tracer flux over shallow shoals while enhancing these processes in deep channels. Consequently, tides increase water age at shoals but decrease it in downstream channels and bulge regions. These findings highlight the pivotal role of tidal dynamics in modulating water age and freshwater fluxes during the wet summer season in the PRE. The study provides novel insights into estuarine transport processes, emphasizing the broader implications of tides on water quality and ecological dynamics. The findings offer science-based management solutions for the PRE, specifically: (1) tidal-phase-dependent pollutant load allocation, and (2) outlet-specific pollutant discharge regulation to maintain ecosystem health under dynamic hydrodynamic forcing.