Abstract
Over the past few decades, the topography and river-tide-salt dynamic characteristics of the Pearl River Delta (PRD) have undergone a myriad of changes due to the unnatural evolution process induced by diverse human activities, such as dam construction, land reclamation, sand excavation, and dredging for navigation. To investigate the impact of human activities on hydrodynamic structures in the PRD of the Modaomen Estuary (ME) during the dry season, a three-dimensional river-tide-salt dynamic model was used to simulate the hydrodynamic characteristics of the ME for different historical periods. The model results indicate that large-scale land reclamation weakened the tidal dynamics and mixing effects in the ME, promoting gravity circulation with opposite velocity directions at the surface and bottom within 15 km downstream of the estuary. Additionally, riverbed downcutting enhanced the tidal dynamics, which intensified saltwater intrusion, leading to the spatial scale-of-gravity circulation expanding 1–2 times. The enhancement of riverbed downcutting on the tidal dynamics in the ME was significantly greater than the weakening effect of land reclamation. Hence, due to the comprehensive influence of human activities between the 1970s and 2010, the hydrodynamic structures in the ME changed from a state of atypical gravity circulation, with nonobvious stratification, to a state of highly stratified and large-scale gravity circulation. The pollutant diffusion in the ME under different scenarios is also discussed in this paper. The results show that reclamation results in weakening of tidal dynamics, which is not conducive to the mixing and diffusion of pollutants in the estuary. However, the narrowed estuary due to land reclamation is conducive to the rapid entry of pollutants into the open sea.
Highlights
Estuaries are the primary transitional zones between land and sea [1,2]
The changes in the coastline caused by land reclamation and riverbed downcutting, which are induced by human activities such as sand excavation and dredging for navigation, have different effects on the hydrodynamic process of the Pearl River Delta (PRD) estuary
The results obtained from this study provide new insights into the evolutionary process of hydrodynamic characteristics driven by human activities in the PRD
Summary
Estuaries are the primary transitional zones between land and sea [1,2]. There is much research on tidal rivers all over the world, such as the Amazon in Brazil, the Columbia, Fraser, and Saint Lawrence in North America, as well as the Yangtze and Pearl in China [3]. Large-scale anthropogenic activities (e.g., dam construction, land reclamation, sand excavation, and waterway dredging) significantly change the riverbed topography and estuarine coastlines [6,7]. These changes, which are different from those of natural evolution, cause corresponding adjustments in the hydrodynamic characteristics of the estuary [8,9]. The changes in the coastline caused by land reclamation and riverbed downcutting, which are induced by human activities such as sand excavation and dredging for navigation, have different effects on the hydrodynamic process of the PRD estuary. This is expected to provide scientific guidelines for water resource utilization, flood prevention and control, saltwater intrusion prevention, and other general water management methods in the PRD
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