Author(s): Wei-Po Huang; Jui-Chan Hsu; Chun-Jhen Ye
Linked Author(s):
Keywords: Morphological change empirical orthogonal function estuarine sediment transport
Abstract: Taitung Coast, located in south-eastern Taiwan, is subjected to shoreline recessions. It was designated as the coastal erosion hot spot in the "Integrated Coastal Zone Management Plan" (CPAMI, 2017). According to the Plan, factors causing coastal erosion should be identified, and possible solutions should be proposed. The coast is famous for its natural landscape and only a few artificial developments. Natural factors that may affect the evolution of coastal morphology include estuarine sediment supplies and marine climate. In this context, the present case study aims to assess the mechanism of coast morphology through the empirical orthogonal function (EOF) method (Winant et al., 1975). The EOF approach is a mathematical tool to identify a set of orthogonal functions or eigenvectors and decompose the original bathymetry data set over the study period. It can provide a simple and effective way to determine the impact factor on the coastal morphology, including weighting and extent. The method employs spatial eigenfunctions (spatial components) and temporal eigenvectors (temporal components), which can be used to explain the variations of the bathymetric changes. Spatial values indicated the impact regions of the corresponding factor, while the temporal coefficients described the annual variation of the corresponding impact factor. The spatial eigenfunctions and the corresponding temporal coefficients of the primary and the secondary impact factors on morphological variability in the Beinan and the Lija estuaries are shown in Fig. 1 and Fig. 2, respectively. In Fig. 1 (a) and Fig. 2 (a), there are two color regions; one is orange, and the other is green. The intensity of the color contours sheds light on the correlation between the corresponding factor and the coastal bathymetric change, with orange representing a positive correlation and green representing an uncorrelated degree. The temporal coefficients represent the change in the temporal domain of the corresponding factors affecting coastal bathymetric change shown in Figures 1 (b) and 2 (b). A time series of the corresponding temporal coefficient shows the yearly changes of the dominant factor at different times. If the corresponding value suddenly increases or decreases at a particular time, an unusual event may occur compared to other years. The temporal coefficient distribution and the time series of the abovementioned mechanisms were compared. If the two-time series have the same change trend, the impact factor corresponding to this mode can be recognized. In Fig. 1 (a), the white belt paralleled to the coastline dividing the orange and green regions are within the isobaths from -8m to -12m, which is in the range of the so-called ‘closure depth’ (Hallermeier, 1983). Therefore, wave energy is considered the primary mode's governing factor. In Fig. 2 (a), two positive correlation areas are around two estuarine regions in the secondary mode. Estuarine sediment transport is the most likely primary cause of bathymetric change in these two areas. Wave energy and river discharge versus the temporal coefficient, plotted as time series in Figures 1 (b) and 2 (b), demonstrate the accuracy of the dominant factors assumed. The correlation of the former is 0.72, and the latter is 0.97. As a result of the high correlation, it is possible to determine what factors are responsible for the primary and secondary modes of operation. Furthermore, the corresponding weights affecting the primary and second modes' coastal morphology are 99.8% and 0.16%. That is to say, the effect of "wave energy" on bathymetric changes in the Taitung coast is much more significant than that of "river flow". It might be possible to consider artificial beach nourishment using dredged sand from the two rivers in the future coastal erosion control strategy since estuarine sediments that flow into the sea cannot be carried to the adjacent coasts on both sides by marine forces.
Year: 2025