High-frequency in situ measurements of beach responses to Tropical Storm Bebinca at Qing’an Bay, Guangdong Province, China

Abstract Tropical cyclones are one of the primary driving forces that cause beach evolution in South China. Because of the harsh working conditions, few high-frequency in situ observations of beach responses to tropical storms have been reported. In this study, high-frequency in situ measurements were conducted over six and a half days at Qing’an Bay beach, Xuwen County, Guangdong Province, China, during the strong tropical storm Bebinca in 2018. The measured parameters included overall hydrodynamics, hourly variations of beach surface elevations, and daily sediment sampling. Four main analysis results were identified. (1) Dominated by the spatial distribution of Hainan Island and the Leizhou Peninsula and the long-lasting and changeable characteristics of Bebinca, the hydrodynamic processes of Qing’an Bay were complex, but the variation range was not large. (2) In each tidal cycle, both erosion and accretion occurred in different parts of the beach profile. (3) During Bebinca, there were two main erosion patterns: first vertical and then horizontal. First, the response processes of the spring tidal berm comprised three stages: rapid erosion, slow erosion, and oscillatory recovery. The oscillatory recovery rate during the storm could reach one-quarter of the maximum erosion depth. Thereafter, the storm berm gradually receded laterally after the rapid erosion of the spring tidal berm. A sandbar appeared at the breaking point of the high tide, formed by gradual sand transportation from the beach berm to the sea, and its position was relatively stable. (4) Changes in grain size parameters of beach surface sediments were complex and were significantly influenced by the vertical distribution of grain size components, sediment confluence, and wave sorting.