No abstract

Coastal erosion is a major natural hazard along the northeastern shoreline (i.e., Yilan County) of Taiwan. Analyses of the evolution of the 0 m isobath of the Yilan County coastline indicate that erosion and accretion are occurring north and south of Wushi Fishery Port, respectively, because of jetty and groin construction. Topographic and bathymetric surveys involving the measurement of 43 cross sections were conducted in 2006, 2012, 2013, and 2019. The cross-shore profile comparisons reveal that the erosion of onshore dunes is significant in the northern Jhuan River estuary. Due to the establishment of a nature reserve in the southern Lanyang River estuary, the sediments are carried northward by tidal currents, and accretion is inevitable in the northern Lanyang River estuary. The results of the bathymetric surveys also suggest that the shoreline of Yilan County tends to accrete in summer because of abundant sediment from the rivers; however, it is eroded in winter, owing to the large waves induced by the northeast monsoon. Additionally, the calculated net volume of erosion and accretion between each pair of cross sections shows that the length of coastline impacted by estuarine sediment transport is approximately 2 km long from north to south along the coastline of the Lanyang River estuary.

While land subsidence (LS) caused by groundwater overuse is a significant contributor to inundation in coastal areas, rising sea levels remain the critical factor affecting coastal flooding globally. In this work, we projected potential inundation areas on the midwestern coast of Taiwan in 2100 using a high-resolution numerical model with light detection and ranging (LiDAR)-based digital elevation data. Two simulation approaches, namely, constant spring tide (CST) and dynamic astronomical tide (DAT) approaches, were employed to pair various sea-level rise (SLR) magnitudes from the 6th Assessment Report of IPCC with LS magnitudes reported by the Water Resource Agency of Taiwan. The CST method always projected larger maximum inundation areas (MIAs) than the DAT method. However, the discrepancies in the MIA projections between the CST and DAT methods showed exponential decay as the SLR magnitude increased. Through 1320 scenario simulations, the projected worst-case median MIAs derived using the CST and DAT methods were approximately 1100 and 1000 km2 in the year 2100, respectively, and the synergistic effect of SLR and LS significantly exacerbated the coastal inundation. This study indicates that the DAT approach is more realistic than the CST approach for modelling actual tidal variations and is suitable for assessing coastal inundation due to rising sea levels and subsiding lands worldwide.