Barrier islands are vital dynamic landforms that not only host ecological resources but often protect coastal ecosystems from storm damage. The Waisanding Barrier (WSDB) in Taiwan has suffered from continuous beach erosion in recent decades. In this study, we developed a SiamUnet network compared to three basic DeepUnet networks with different image sizes to effectively detect barrier waterlines from 207 high-resolution satellite images. The evolution of the barrier waterline shape is obtained to present two special morphologic changes at the southern end and the evolution of the entire waterline. The time periods of separation of the southern end from the main WSDB are determined and discussed. We also show that the southern L-shaped end has occurred recently from the end of 2017 until 2021. The length of the L-shaped end gradually decreases during the summer, but gradually increases during the winter. The L-shaped end obviously has a seasonal and jagged change. The attenuation rate of the land area is analyzed as −0.344 km²/year. We also explore two factors that affect the analysis results, which are the number of valid images selected and the deviation threshold from the mean sea level.

Chang, H.-K.; Chen, W.-W.; Cheng, C.-C.; Liou, J.-C., and Lin, S.-F., 2024. Assessment of tide model prediction and discrepancy in shallow waters of Taiwan to improve data fusion methods. Journal of Coastal Research, 40(1), 129–137. Charlotte (North Carolina), ISSN 0749-0208. Shallow bathymetry in the Taiwan Strait and corresponding complex hydrodynamics are key factors that make it difficult for hydrodynamic tide models to accurately calculate tide levels. Based on the Princeton Ocean Model with data assimilation from 2010 to 2014, the Ministry of Interior (MOI) developed a local tide model (MOI.18v1) for the waters around Taiwan that was imposed by harmonic analysis to have fast tide calculation at any time and at any location. This study first evaluated the prediction accuracy of the MOI.18v1 model using tide data at 22 gauge stations in Taiwan for 2016 and compared the results with those of NAO.99b. The MOI.18v1 root-mean-square error (RMSE) of residuals by subtraction with observations can reach about 13.49 to 18.26 cm, results lower than those of NAO.99b. A data fusion method is also proposed to further reduce the prediction discrepancy with RMSE of 3.67 to 6.35 cm, which means that the percentage of improvement increases the level of 25.85 to 47.08%. Data fusion can be used as a spatial correction for the predicted tide level using hydrodynamic tide models.

Barrier change is a complex process of evolution of coastal topography, which is related to the interaction of driving forces such as waves, tides and sea level rise (SLR) with beaches. The Waisanding Barrier (WSDB) in Taiwan has suffered from continuous beach erosion in recent decades. Some short-term studies have been carried out to understand the characteristics of the barrier change to provide a reference for future barrier protection. In this paper, the digital elevation model (DEM) measured by LiDAR (Light Detection and Ranging), over nearly two decades was used to analyze the morphological changes, the land area and volume. The changes in the morphology, including the whole shoreline, duneline height, width of forebeach and backbarrier, are investigated. The WSDB’s land area and land volume were analyzed to show a continuous decrease by a rate of −0.418 × 10⁶ m²/year and −3.96 × 10⁵ m³/year, respectively. The corresponding average land volume (LV) decrease in elevation can be estimated to be −0.0286 m/year. The changes in these features are discussed and relate to land subsidence, sea level rise and large waves induced by typhoons passing near WSDB.

Nodal tides, which follow an 18.6-year cycle, influence tidal variations at any given location in the ocean. Conventional nodal tide theory neglects land effects and topological change. Due to the complex seabed topography around Taiwan waters, the purpose of this paper is to use the long-term tidal data of six stations to discuss the effects of perigean and nodal tides on 20 constituents and to compare the results with previous theories. A modulation method is employed to fit the annual amplitude estimated by harmonic analysis (HA). The top four constituents of the fitted and theoretical values of nodal amplitude factor (AF) and phase factor (PF) are O₁, K₁, K₂, and Q₁. We find that perigean tides or second-order nodal tides considered in the fitting contribute to almost identical performance. The linear time change considered in the AF fitting has better fitting than the mean water level involved.

The Waisanding Barrier (WSDB) in Taiwan faces problems with regard to beach erosion and land area (LA) reduction. Due to the small amount of data generated and insufficient time periods used, the results of previous studies are only local rather than long term in nature. This study used 207 satellite images over 20 years to explore long-term and interannual changes in the LA of the WSDB. We developed both a surface fitting method (SFM) and a two-step interpolation method (TSIM) to reliably determine the LA of each image. When the tidal level of WSDB at the image acquisition is within ±0.25 m of the zero-meter shoreline, the LA obtained via the TSIM is similar to that of the SFM, while the other ones are quite different. The long-term decaying rate of the WSDB’s LA determined via both methods is about −0.40 × 10⁶ m²/year. The consistent differences in LA obtained from LiDAR and image data and the interannual variation in excess area (EA) are discussed, and the causes of these differences are land subsidence (LS) in the WSDB and excess wave energy (AEWE).