A coastal erosion risk assessment was framed as the basis for the intervention of coastal adaptation strategies under time-variant scenarios. The framework was devised to assess the influence of coastal erosion on coastal defense, the coastal inundation induced by the erosion-induced malfunction of defense, and risks using a downscaling analysis and the mechanism of the compound hazard interaction, which are innovative and practical for the application of coastal management in Taiwan. In addition, the vulnerable socio-economy was also taken into consideration in risk assessment. The adaptive strategy is proposed in terms of the risk origins and time-variance of risk forecasts, and the risk origins were assessed based on the Fuzzy Delphi Method and the analytic hierarchy process instead of subjective consideration. Within the erosion-induced hazard and risk assessments, this study considered erosion rates, decreases in defense elevation due to land subsidence, and population development in time-variant scenario analyses to estimate risk forecasts. Furthermore, a case study of the Yunlin coastal area was undertaken to demonstrate the feasibility of the proposed method. The presented results are informative for coastal hazard reduction and the promotion of the sustainable development of coastal zones.

In this study, a Bayesian network (BN) module is proposed for coastal hazard risk assessment and forecasting on the basis of the statistical prior and conditional probabilities, which indicates the mechanism of transitivity and causality. The BN module is applicable for quantifying the probabilities of various risk possibilities, which materialize and interpret the phenomenon of natural science. BN-based risk assessment is quantified by the causality of factors in this study instead of individual concerns. Using long-term coastal, environmental, and humanities data from Taitung city, which is situated on along the coast of Taiwan, causality and conditional probability are proposed to forecast hazard risk in the 2°C global warming scenario. The causality verification indicates a strong correlation (R-squared ≈ 0.81) between the wave energy and shoreline change rate, which reflects the performance of the BN module in this study. In the 2°C global warming scenario, the hazard risk degree of the wave energy-propagated shoreline change rate factor will increase in the estuary sector and the sediment supply termination sector; therefore, the maximum probability of a high hazard risk degree will increase to 60% in the estuary sector of the Bainen River. The degree of risk to the shoreline change rate has been revealed to be more polarized under climate change. The estuary and sediment supply termination sectors are identified as erosion hotspots under the climate change scenario. Hazard forecast analysis via a BN is an informative and well-founded process for sustainable coastal management.

The construction of coastal infrastructure alters the natural hydrodynamics, leading to irreversible changes in coastal morphology. Furthermore, there has been an increasing concern about global climate change in recent years, which requires examining how climatic shifts impact the mechanisms that govern oceanic processes, the trends in morphological changes, the extent of the impacts, and the corresponding weightings assigned to coastal infrastructure. This study aims to assess the impacts of climate change on the wave distribution and coastal morphology around the two breakwaters perpendicular to the shoreline of Hsinchu Fishery Harbor in Taiwan at the end of the 21st century. The findings reveal that, by the end of the century, during extreme climatic events, such as typhoons (increasing the maximum wind speed and extreme sea levels), the wave heights around the Hsinchu Fishery Harbor, compared to the present day, may increase by 5.94% to 81.25%. Regarding the potential coastal morphological changes, erosion trends are evident around the harbor, but there is a pronounced tendency toward accretion in the sheltered area. The impact range and weightings of the Hsinchu Fishery Harbor breakwaters under climate change on coastal morphology were then determined based on historical bathymetric data and simulated bathymetric changes using the empirical orthogonal function (EOF) method. Under different-intensity climate change scenarios, the EOF analysis indicates no significant differences in the impact range and weightings on coastal morphological changes. In light of the analysis results, it is evident that structures have a much more substantial impact on coastal morphological change than climate change does.