Sand-mud transition dynamics at embayed beaches during a typhoon season in eastern China

Sand-mud transition (SMT) is an important boundary of the beach system, and its location and evolution have been increasingly studied in recent years. This study focuses on the location and migration of SMTs and their influencing factors on three representative embayed beaches on the east coast of Zhujiajian Island, Zhejiang Province, China, characterized by different levels of human interventions. Beach topographies, surficial sediment characteristics and nearshore hydrodynamic data were obtained through three field campaigns carried out during the early, middle and late stages of the 2019 typhoon season. This typhoon season included four typhoons with nearshore significant wave height Hs exceeding 3.71 m, and maximum Hs of 6.77 m (Super Typhoon Lekima). Results show that the three beaches were all impacted by the high-energy wave conditions, although with some different behaviors. Sediments of the three beaches all coarsened with worse sorting during the typhoon season, with the nearshore surficial sediments showing similar patterns. The SMTs of the three beaches were stable or migrated seaward during the typhoon season. During the typhoon season, offshore SMT migration was positively correlated with the beach profile volumetric loss at three embayed beaches in this study. The SMTs of beaches with less human intervention are more stable during typhoon season. By including 12 additional embayed beaches of eastern China in our analysis, we found that the SMT offshore distance increases with increasing nearshore average significant wave height, increasing headland offshore extent and decreasing tidal range. Our study suggests that SMT is a relevant indicator of beach sediment stability, which can help to increase the understanding of embayed beach dynamics and to guide coastal management and planning during typhoon seasons at such embayed beaches.