Using unmanned aerial vehicle (UAV) photogrammetry for monitoring seasonal changes of barrier island in the southwestern coast of Taiwan

The change of barrier islands could be a precursor of coastal landscape evolution. The barrier islands on the coast of southwest Taiwan are continuing narrowing and landward moving in the past decades. The government has tried to install eight detached embankments to protect Dingtoue barrier island in 2001. In this study, we try to monitor the landform change by using UAV photogrammetry. Dingtoue barrier island is 1.3 km in length and with area of 30.5 ha. We have already conducted 4 campaigns of UAV photogrammetry between March 2018 and September 2019, and they can reveal the landscape of the end of summer and winter monsoon. We use Agisoft Metashape to process the aerial photos for acquiring the DEM and ortho-rectified image with the spatial resolution of 0.5 m and precision level of 0.04 m in both horizontal and vertical direction. We sub-divide Dingtoue barrier island into beach and sand dune zones for further analysis by using Arc GIS. The DEM of difference and areas will be obtained in beach and sand dune as well.

The results show that area of Dingtoue barrier island is increasing 5101.2 sq.m, while volume of Dingtoue barrier island is decreasing 26722.1 cu.m at the end of the 2018 summer monsoon. The beach part is increasing in both area and volume, while the sand dune part is decreasing in both area and volume. The northern part of the beach is extending to east and the sand dune zone is retreating to further east. The southern part of the beach is extending to west part, which is the sea in the past. Area of Dingtoue barrier island is increasing 719.4 sq.m, while volume of Dingtoue barrier island is increasing 36705.7 cu.m at the end of the 2018 winter monsoon. Area of the beach part is relative the same as the previous period be with some minor changes in the northern and southern part. The sand dune part is increasing in both area and volume. Area of Dingtoue barrier island is increasing 14616.2 sq.m while volume of Dingtoue barrier island is decreasing 23894.1 cu.m at the end of the 2019 summer monsoon. Areas of beach and sand dune are both increasing while volume of the sand dune is decreasing. The mid-part of the beach is occupied by sand dune and the beach is recovering to previous shape.

In general, Dingtoue barrier island is increasing 7% in area and is decreasing 13910.5 cu.m in volume between March 2018 and September 2019. The average surface lowering is 0.05 m in this period. The trend shows that typhoons will increase area of Dingtoue barrier island, but decrease volume. The winter will decrease area of Dingtoue barrier island but increase volume. So the main change of area is at the beach part and the main change of volume is at the sand dune part. From the installation of the eight detached embankments can stabilize Dingtoue barrier island by increasing area, and volume is decreasing during the summer period and increasing in the winter period.