Long-Term Geomorphological Evolution of the Mouth Bar in the Modaomen Estuary of the Pearl River over the Last 55 Years (1964–2019)

Based on mass bathymetric data and remote sensing data in the Modaomen Estuary, this study explored the long-term evolutionary characteristics of the mouth bar in the Modaomen Estuary of the Pearl River from 1964 to 2019. In the past 55 years, due to the impact of human activities, such as shoal reclamation and estuarine regulation in the Modaomen Estuary, the river mouth moved out of the shallow sea covered by several islands and faced the South China Sea directly. Therefore, the mouth bay became a siltation center in the estuarine region and expanded outwards, gradually evolving a geomorphic pattern with three shallow shoals and two distributary branches; a west branch as the main branch accompanied by a small east branch. Over the past decade, high-intensity sand dredging activities in the mouth bar have led to a considerable deepening of the water depth and a significant refinement of bed sediments, forming a discharge pattern of a wide and shallow channel flowing into the sea. Therefore, the evolutionary characteristics of the mouth bar have become abnormal in recent years, so additional field bathymetric data and hydrological data are required for further research regarding the subsequent evolution of the mouth bar, against the background of a significant reduction of sediment discharge and high-intensity human activities.

Impacts of Human Activities on Hydrodynamic Structures during the Dry Season in the Modaomen Estuary

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.

Impact of upstream runoff and tidal level on the chlorinity of an estuary in a river network: a case study of Modaomen estuary in the Pearl River Delta, China

Saltwater intrusion exerts great impact on water supply and water withdrawal from estuarine areas. A chlorinity prediction model based on backpropagation neural network was constructed, calibrated, and validated, considering phase lags, with the Modaomen estuary in the Pearl River Delta (PRD), China as case study. This study aimed to investigate impacts of upstream runoff and tidal level on the changing properties of estuarine chlorinity. Nine boundary conditions – low tide and tidal range both with three different frequencies – were designed to explore the changing process of estuarine chlorinity and obtain the critical upstream runoff for saltwater suppression. Results indicated the model performed efficiently; Nash–Sutcliffe efficiency coefficient and R2 were both 0.91 in training period, 0.76 and 0.82 in testing period, and 0.64 and 0.77 in validation period, respectively, and estuarine chlorinity shows slightly different changing processes of decline rate under the nine boundary conditions when the upstream runoff increases. The higher the designed tidal range and lower daily tides together with the smaller the amount of upstream runoff, the higher the estuarine chlorinity. The critical upstream runoff of the Pinggang pumping station is 2,500 m3/s. These findings provide a foundation for water supply security and upstream reservoir dispatching in estuarine areas in dry periods.