Field observations of sediment transport across the rocky coast of east Taiwan: Impacts of extreme waves on the coastal morphology by Typhoon Soudelor

After the 2015 Typhoon Soudelor, large boulders and mixed sediments deposited extensively across the beach along the east coast of Taiwan. Field measurements of these storm-related sediments provide the first-hand evidence to document the impacts of extreme waves induced by Typhoon Soudelor. Although the maximum inundation distance is only 170 m due to the lack of beach buffer zone, the weight of the largest boulder was estimated over 1 t and the maximum run-up was estimated up to 11.9 m. The clast distribution of the washover and the beach face sediments exhibited reverse directions of coarsening, reflecting different stages of wave propagation. Our field records show that a super typhoon like Soudelor can potentially leave footprints of extreme values in the geologic records. Therefore, a comprehensive understanding on the occurrence of the sediments and the depositional environment should be carefully considered to pin down the origin of the archived extreme events. In addition, the formation of the higher extreme wave berm and the destruction of the lower swell berm at the Yen-liao Beach displayed the power of the storm surges, suggesting the important role of the run-up on the evolution of the berm.