Abstract: The rapid evolution of beach morphodynamic under the impact of extreme weather events causes beach erosion and shoreline retreat, which seriously endangers coastal engineering protection, as well as the sustainable development and utilization of beach tourism resources. Based on the measured data of profile elevation, surficial sediments and offshore waves and tides at Wanwei Jintan Beach, Guangxi Province, before and after the 2024 4th Typhoon "Prapiroon", this study explores the beach morphodynamic process in response to typhoon impact. The results show that: (1) After the encounters with typhoon event, the sand dune-type profile and flat -type profile, which were respectively located in the western and central parts of Jintan Beach, correspondingly exhibited weak erosion (mean erosion height of 0.50 cm) and strong erosion (mean erosion height of 2.50 cm).The sand dune-type profile was as followed: beachface erosion in backshore dunes and lower foreshore, and accretion in the upper and middle parts of the foreshore, while that of flat -type profile was manifested as a continuous decrease in erosion height from the backshore to the middle foreshore, with a transition to accretion in the lower foreshore. In contrast, the sandbar-type Profile in the eastern part of Jintan Beach showed weak accretion (mean accretion height of 0.64 cm). Moreover, the sand dune-type profile presented an intermittent distribution pattern in erosion-accretion variation, which was featured by of backshore erosion-upper foreshore accretion-middle foreshore erosion-lower foreshore accretion. (2) Sand dune and ridge-trough system were the beach geomorphic units where the surficial sediments varied significantly in response to typhoon event. In specific, for the eroded sand dunes where strong reflectivity and sediment transport dynamics prevailed in their windward slopes, the coarser bed loads were removed, resulting in surficial sediments becoming finer and worse sorting after typhoon. For the trough of the sandbar topography where a complex bidirectional current and sediment transport environments existed, its sedimentary response to typhoon event was characterized by "erosion at the sandbar crest with increase in bed load components-accretion in the trough with increase in saltation load components". In comparison, for the flat zone where beach was highly dissipative and sediment-carried forces remained relatively stable, the ratios of sediment components hardly changed after the typhoon impact; (3) Large-scale backshore sand dunes and ridge-trough sandbar topography played a positive role on shaping the spatial erosion/accretion pattern of beach profile, as well as alleviating local beach erosion due to their functions of dissipating wave energy and lowering current velocity. On the contrary, the coastal engineering constructed in the backshore could intensify the erosion degree of the upper beach during the spring high tide coupled with storm surges.

Key words: Morphodynamic process, Typhoon "Prapiroon", Beachface erosion/accretion, Sandbars, Surficial sediment, Wanwei Jintan Beach