Human-induced sediment supply altered natural shoreline evolution of the Nandu River estuary of Hainan Island, South China

Estuarine shorelines are subjected to significant human stress, which gradually alters their natural evolution. However, the impact of artificial variations in sediment transport on shoreline evolution remains poorly understood. Hainan Island, one of the most promising regions in South China, along with its largest estuary, the Nandu River estuary (NRE), is experiencing considerable anthropogenic pressure. Based on five field observations of 12 profiles during 2020–2023, we examined the spatio-temporal patterns of shoreline evolution in the NRE. Results show that most profiles displayed significant retreat, except for some profiles distant away from the river mouth, corresponding to the 1^st EOF mode driven by river sediment discharge. This mode accounted for 70.1% of the total variation, which is regarded as the most important spatio-temporal evolution pattern. Other minor patterns included shoreline retreat dominated by sediment transport under wave actions and shoreline progradation, resulting from artificial sand nourishment, taking 18.6% and 7.4% of the total variation, respectively. Recent sediment discharge from the Nandu River has decreased by more than 58% due to human activities, resulting in abnormal changes in the shoreline shape. Our conceptual model suggested that the dynamic mechanism was the change in mode of transition from unsaturated sediment transport to saturated sediment transport alongshore, which was caused by insufficient river sediment supply. In the short term, artificial sand nourishment could mitigate shoreline retreat; however, its positive effects are limited in both time and space. Therefore, additional soft measures must be implemented, such as ensuring the sediment transport function of the river through reservoir dispatch system, and improving the flow field structure of the shoreline through vegetation stabilization, to ensure long-term shoreline stability. These findings underscore the critical role of human-driven sediment transport/supply in estuarine shoreline evolution, offering new insights and recommendations for coastal management and restoration engineering.