*3.3. Analysis of Cross-Shore Profile Evolution*

Eight representative transects illustrated in Figure 9 were selected for cross-profile comparisons with the bathymetric surveys in June 2006, April 2012, April 2013, and April 2019. Cross-profile comparisons are beneficial for investigating nearshore morphological evolution and sediment transport mechanisms in coastal environments. Closure depth has been widely used within coastal engineering as an empirical measure of the seaward limit of significant cross-shore sediment transport on sandy beaches. Therefore, a closure depth of approximately 10 m was adopted as an offshore boundary for morphodynamics in the present study. The cross-profile surveys extended from dune ridges on the beach to the 10 m closure depth in the nearshore waters of the study site. Eight transects were measured in the northern and southern portions of the Wushi Fishery Port (transects S5 and S9 in Figure 9), in the northern and southern portions of the Jhuan River estuary (transects S10 and S11 in Figure 9), along the shoreline between the Jhuan River estuary and the Lanyang River estuary (transects S18 and S31 in Figure 9), and in the northern and southern portions of the Lanyang River estuary (transects S38 and S41 in Figure 9). Figure 10a–d represent the topographic and bathymetric data for transects S5 (Figure 10a), S9 (Figure 10b), S10

(Figure 10c), and S12 (Figure 10d). These transects were surveyed in June 2006, April 2012, April 2013, and April 2019. The cross-profile comparisons of transects S5 and S9 (surveyed in the northern and southern portions of the Wushi Fishery Port) in various years are shown in Figure 10a,c. The bathymetric surveys were all performed after the construction of the Wushi Fishery Port in 2002. The comparisons show that the accretion is sustainable in the northern portion of the Wushi Fishery Port (transect S5) and is up to 2 m at a distance of approximately 400 m from 2006 to 2019 (Figure 10a). Erosion phenomena were found in the southern portion of the Wushi Fishery Port (transect S9) at a distance of 100–400 m; however, weak accretion was detected at a distance of 600–800 m (Figure 10b). The construction of the Wushi Fishery Port, which created a jetty effect, is the major contributing factor to accretion and erosion on the northern and southern sides of the Wushi Fishery Port. The bathymetry changes are minor below a water depth below 0 m on the northern and southern sides of the Jhuan River estuary (transect S10 in Figure 10c and transect S12 in Figure 10d). However, a coastal dune with a height of approximately 7 m in transect S10 (northern side of the Jhuan River estuary) was eroded significantly within a distance of 150 m because of the strong northeast monsoon (Figure 10c). Transects S18 and S31 lay between the Jhuan River estuary and the Lanyang River estuary and feature a gently sloping seafloor (as shown in Figure 11a,b). The cross-profile comparisons are relatively stable because they are far from the estuaries and any artificial structures. Figure 11c,d illustrate the bathymetric variations along transects S38 and S41, respectively, in various years. As shown in Figure 11c (transect S38), erosion occurred in the north of the Lanyang River estuary with a distance between 700 m and 1300 m from 2006 to 2013, after which accretion was present in the same zone. The most significant erosion and accretion are observed in the southern Lanyang River estuary, i.e., transect S41 in Figure 11d. The accretion reached 2–4 m along all of transect S41 from 2006 to 2013, but the coastal and nearshore zones at distances beyond 200 m were eroded dramatically in 2019. These phenomena are identical to the evolution of the 0 m isobath in various years described in Section 2.2. The Lanyang River mouth moved northward gradually, and the supply of riverine sediment for the southern Lanyang River estuary has consequently decreased in the past two decades.

**Figure 9.** Spatial distribution of eight transects for the cross-shore profile comparison.

**Figure 10.** Cross-shore bathymetry profile comparisons for (**a**) S5, (**b**) S9, (**c**) S10, and (**d**) S12 crosssections measured in various years.

**Figure 11.** Cross-shore bathymetry profile comparisons for (**a**) S18, (**b**) S31, (**c**) S38, and (**d**) S41 cross sections measured in various years.
