Morphological consequences of upstream water and sediment changes and estuarine engineering activities in Pearl River Estuary channels over the last 50 years.

The relative contributions of decreased upstream sediment loads and local estuarine engineering activities to the estuarine channel geometry are poorly understood. In this study, we analyze the hydrological changes and identify the location, duration and intensity of the estuarine engineering activities based on the channel morphologic changes from 1965 to 2017 at the five stations in the Pearl River Estuary. Thereafter, the Mann-Kendall (M-K) statistical test, empirical orthogonal function (EOF) tests, and channel geometry reconstruction based on the hydrological coefficient were performed to quantitatively estimate the relative contributions from upstream dam construction and estuarine engineering activities. The results show that the geometric changes in the five transects over the last 50 years could be divided into three stages. Stage I extends over approximately 23-33 years at the different channel transects, during which the channel geometries were mainly influenced by natural factors, with a balance between erosion and deposition. Stage II occurred during the next 11-20 years and the changes in the cumulated water depth in comparison to the values in the previous adjacent years at this stage are approximately 5-25 times the values in stage I. The human activities (e.g., sand excavation) contribute to >70-90% of the extreme geometric changes. Stage III lasted for <3-11 years in the different transects with a slight depositional trend, and policies regulating sand excavation were implemented during this stage. The rapid increase in the channel area and water depth caused by sand excavation can cause the downcutting of the riverbed, a decrease in the water level, and redistribution of the water and sediment discharge. Therefore, the monitoring, simulation and analysis of the variation in the typical channel geometry over the long term provide important means to understand the human activities occurring and insights for future sustainable estuarine management.