Modeling of the sediment transport and deposition in the Yellow River Estuary during the water-sediment regulation scheme

A large amount of terrigenous sediment is delivered to the Bohai Sea by the Yellow River annually. Approximately 30 % and 50 % of the annual water and sediment are pulsively delivered to the sea during the short period (15−20 days) of Water and Sediment Regulation Scheme (WSRS). Two distinct periods are identified with different characteristics of water and sediment discharge: (1) during the Water Regulation Period (WRP), sluice gate skim clear water; (2) during the Sediment Regulation Period (SRP), discharging the sediment silted in the reservoir. The observed differences in water discharge, concentration and composition of suspended sediment could lead to distinct sediment dynamics and geomorphological changes within the estuary, yet the full extent of these impacts remains not fully understood. In this study a calibrated coupled model (COAWST) was utilized to explore the processes of sediment transport and deposition during the two distinct periods of WSRS. Results showed that stratification of water column during the WRP was primarily driven by salinity, whereas during the SRP, it was induced by increasing suspended sediment concentration (SSC). The increase in SSC during the SRP also triggered the formation of hyperpycnal flow beneath the saline water, which replaced the hypopycnal flow during the WRP. As a result, high water discharge scoured the river mouth, depositing coarse sediments within 5 km of the mouth, while fine sediments were primarily deposited further offshore during the WRP. In contrast, during the SRP, sediments were mostly deposited in a fan-shaped pattern within 10 m of the former delta. Sensitivity diagnostic tests indicated that the variation in erosion and deposition patterns between these two periods are mainly due to variations in the incoming sediment coefficient (the ratio of SSC to water discharge) which was crucial in shaping the morphology of the Yellow River Estuary (YRE) and driving delta evolution. These findings provide an effective approach to preserve or even expand delta areas.