Spatio-Temporal Morphodynamics of Huolongtan Sandbar, Penghu islands Taiwan - Using Short-term Monitoring from 2020 to 2024

Huolongtan is located in the east of Baisha Island, Penghu, Taiwan. It was formed by the 1986 typhoon and is the youngest sandbar island in Penghu. This study explores its changes at different time and space scales, focusing on the interaction between tropical typhoons and the northeast monsoon. The influence of its terrain.In this study, multi-period UAV photogrammetry was used to analyze the erosion and sedimentation status before and after typhoons and northeast monsoons using a DSM of difference (DoD). Grain size analysis and marine meteorological data were combined to explore natural variation factors. The USGU Digital Shoreline Analysis System (DSAS) was used to analyze shoreline changes.The research results show that the sandbar erosion and siltation responses showed different spatial trends during the three typhoons. The maximum wave height of Typhoon Koinu was 567 cm, and the volume of sedimentation was -7657.2m3, with a ratio of -0.03. During the passage of this typhoon, the wind was mainly from the north, causing accumulation on the south bank and erosion on the north bank, and its sand tail gradually swing south to change. During the northeast monsoon, the volume of sediments recovered, with a volume of 9048.5m3and an increase of 0.04, but the sandbar islands were eroded again in the late northeast monsoon. In addition, this paper found that the intertidal zone of several kilometers in the north would protect the terrain of the northern shore, and significant erosion would only occur with high wave heights or long-term monsoon waves. Comparison of the zero-meter contour lines shows that the main island of Huolongtan has a tendency to move southwest and sand tail has a tendency to move north. The maximum erosion of the coastline changes in the medium and short time intervals is 21.3m/y (end point rate (EPR)) and 8.2m/y (linear regression rate (LRR)); the maximum accumulation is 30.0m/y (EPR) and 16.4 m/y (LRR), and the Kalman filter was used to predict that the north bank would erode more landward and the south bank would advance toward the sea in 10 years.The research results can be applied to the sustainable development of recreational areas and seabird habitats on Huolongtan sandbar under climate change and frequent extreme events. This will also help management units to adapt to climate change in a changing environment.Keywords: UAV, Particle size analysis, DSAS, Coastal change