Spatio-temporal evolutionary characteristics of sandy shorelines on a typical island under the influence of human activities

Long-term morphological changes in beaches are crucial for developing and managing coastal zones and have significant implications for coastal disaster prevention and mitigation, as well as sustainable development of sandy shorelines under global climate change conditions. Pingtan Island, located on the west coast of the Taiwan Strait, about 40 km south of the Minjiang River Estuary, features a 70 km -long sandy shoreline, and its developmental history exhibits distinct phases. This study analyzes the spatio-temporal evolutionary characteristics of sandy shorelines on Pingtan Island over the past 35 years by utilizing continuous satellite imagery from 1990 to 2024 and the CoastSat global shoreline mapping tool. The primary controlling factors influencing the evolution of sandy shorelines are also identified in this study. The results indicate that (1) driven by variations in coastal dynamic conditions, the evolution of the sandy shoreline along Pingtan Island exhibits notable spatial differences, with erosion intensities significantly greater in the northern and eastern regions of the island than in the southeastern and southern areas. (2) The sandy shoreline of Pingtan Island demonstrates clear phased evolutionary characteristics: from 1990 to 2010, the shoreline generally experienced erosion, with a higher erosion rate observed from 1990 to 1999, followed by a deceleration in erosion from 2000 to 2010. After 2010, the shoreline tended toward stability, which was related to changes in sediment discharge in the Minjiang River estuary. (3) Following an adaptation period of 5–15 years after sediment discharge from adjacent rivers stabilized, a new dynamic equilibrium was established. However, this equilibrium was subsequently disrupted by intense human activities, leading to renewed oscillations of erosion and accretion along the sandy shoreline.