**3. Results**

### *3.1. Interannual Variability of Erosion and Accretion along the Surveyed Shoreline*

To analyze the long-term variation in erosion and accretion for the study coastline, bathymetric surveys were conducted in June 2006, April and September 2012, April and September 2013, and April and October 2019. A total of 43 transects (track lines) were distributed along the shoreline from north of the Wushi Fishery Port to the south of the Lanyang River estuary (as shown in Figure 4). Moreover, the net volume of erosion or accretion between two transects was estimated, which means that 41 net volumes were used to evaluate whether the sediment budget was increasing or decreasing along the studied coast. The updated bathymetric datasets were compared with the previous datasets to estimate depth changes during two periods. Figure 5a,b present the variations in bottom elevation over 7 and 6 years, respectively, with Figure 5a showing the changes between June 2006 and April 2013 and Figure 5b showing the changes between April 2013 and April 2019.

**Figure 5.** The spatial distribution of erosion and accretion along the shoreline during the periods of (**a**) June 2006 to April 2013 and (**b**) April 2013 to April 2019.

The comparisons indicate that erosion and accretion phenomena occurred alternately along the shoreline from north of the Wushi Fishery Port to the south of the Lanyang River estuary and from June 2006 to April 2013 and April 2013 to April 2019. This is particularly obvious in the waters near the Lanyang River estuary. As shown in Figure 5a, the bathymetries rose to a maximum of 3.0–4.0 m in the south of the Lanyang River estuary from June 2006 to April 2013; however, the water depths were reduced by 3.0–4.0 m in the same area from April 2013 to April 2019 (Figure 5b). Overall, the erosion and accretion trends for the waters close to the shoreline are found to be contrary to those of the water slightly farther from the shoreline for both periods. A similar phenomenon is discovered when Figure 5a is compared to Figure 5b. Slight erosion and accretion were distributed in the area somewhat far from the shoreline and the area close to the shoreline, respectively, along the coastline from north of the Wushi Fishery Port to the northern Lanyang River estuary during the period of June 2006 to April 2013 (i.e., a 7-year bottom elevation difference, as shown in Figure 5a). However, the opposite phenomenon of erosion and accretion occurred for the 6-year bottom elevation difference from April 2013 to April 2019 (Figure 5a). Figure 6a,b present the net erosion (positive quantity in Figure 6) or accretion (negative quantity in Figure 6) volume of each pair of transects in the 7-year and 6- year periods, respectively. Based on a comparison of Figure 5 with Figure 6, the distribution pattern of increases and decreases in the net volume is identical to that of the bathymetric changes alongshore. The maximal accretion volumes are approximately 2 × 10<sup>6</sup> m<sup>3</sup> at two intervals between transects S40 and S41 and between S41 and S42 (i.e., V39 and V40, as shown in Figure 6a), while the maximal erosion volume is nearly −2 × 10<sup>6</sup> m<sup>3</sup> at an interval between transects S41 and S42 (i.e., V40, as shown in Figure 6b). To evaluate the longer-term erosion and accretion along the studied shoreline, the differences between the bathymetric surveys in June 2006 and April 2019 were calculated. The variations in bottom elevation and net volume are depicted in Figure 6a,b, respectively. The 13-year alongshore erosion and accretion variations are minor relative to the 7-year and 6-year evolution. The changes in water depth are within ±2.0 m (as shown in Figure 7a), and the maximum accretion and erosion volumes are approximately 1.0 × 10<sup>6</sup> m<sup>3</sup> and −5.0 × 10<sup>5</sup> m<sup>3</sup> at V39 and V35, respectively (Figure 7b). This means that the sediment added to and removed from the studiedcoastalsystemisgraduallybalancedthroughlong-termsedimenttransport.

**Figure 6.** The erosion and accretion volume between each pair of cross sections during the period of (**a**) June 2006 to April 2013 and (**b**) April 2013 to April 2019.

**Figure 7.** The erosion and accretion volume between each pair of cross sections during the period of (**a**) June 2006 to April 2013 and (**b**) April 2013 to April 2019.

### *3.2. Seasonal Variability in Erosion and Accretion along the Surveyed Shoreline*

In addition to the interannual variability in erosion and accretion, estimating the seasonal variation in erosion and accretion is also important to coastal managemen<sup>t</sup> and development. The bathymetric data pairs surveyed in September 2012 and April 2013 and in April 2013 and October 2013 were adopted to analyze the variability in erosion and accretion along the studied coastline in winter and summer, respectively. The bathymetry differences between September 2012 and April 2013 are regarded as erosion and accretion variations in winter (as shown in Figure 8a), while the bathymetry differences between April 2013 and October 2013 are considered to be erosion and accretion variations in summer (as shown in Figure 8b). As shown in Figure 8a, erosion phenomena are obvious along the shoreline from north of the Wushi Fishery Port to the south of the Lanyang River estuary during winter, due to the sustained large waves caused by the northeast monsoon and the shortage of sediment discharged from the river. In contrast, in summer, accretion phenomena are found along the coastline from north of the Wushi Fishery Port to the Lanyang River estuary (Figure 8b) because of weaker waves and abundant sediment released from the river.

**Figure 8.** The spatial distribution of seasonal variability for erosion and accretion along the shoreline for (**a**) winter (from September 2012 to April 2013) and (**b**) summer (from April 2013 to September 2013).
