**4. Discussion**

According to the evolution of the 0 m isobath and the cross-sectional bathymetric data surveyed in various years, the present study reveals that a drastic morphodynamic pattern is found around the Lanyang River estuary and is particularly obvious in the southern Lanyang River estuary. The topographic and bathymetric data of transects S30–S43 surveyed in June 2006, April 2013, and April 2019 were utilized to clarify the mechanism of estuarine sediment transport for the Lanyang River. The spatial distributions of transects S30–S43 and their corresponding serial numbers for the net volume variation (i.e., V29–S43) are delineated in Figure 12a for June 2006 to April 2013 and in Figure 12b for April 2013 to April 2019. A comparison of Figure 12a,b shows that the erosion and accretion phenomena in the northern Lanyang River estuary are contrary to those in the southern Lanyang River estuary for the two periods. This is because a nature reserve in the southern Lanyang River estuary cannot be developed from 2013. Although the 13-year (June 2006 to April 2019) variations still show a slight accretion in the south of the Lanyang River estuary (Figure 7), erosion began in 2013. The erosion and accretion along the shoreline near the Lanyang River estuary are highly dependent on the estuarine sediment discharge and transport of the Lanyang River. Therefore, the limit of estuarine sediment transport is considered to play an important role in the mechanism of coastal erosion and accretion and can be used to examine how far estuarine sediment can be supplied. The net volume variations along the coastline of the Lanyang River estuary for the periods from June 2006 to April 2013 (cyan bar) and from April 2013 to April 2019 (red bar) are presented in Figure 12. As shown in Figure 13, erosion and accretion began to reverse at V34 and V38 between the two periods. The accretion turned into erosion from V34 to V38 and then returned to accretion at V39 during the 7-year period (from June 2006 to April 2013, cyan bar in Figure 13). A contrary phenomenon appeared in the same interval (between V34 and V38) in the 6-year period (from April 2013 to April 2019, red bar in Figure 13); in other words, accretion occurred within V34 to V38, but erosion occurred outside this interval. Therefore, it is believed that the extent affected by estuarine sediment is approximately 2 km long from north to south along the coast of the Lanyang River estuary.

Infrastructure and human activity, such as establishing nature reserves and constructing jetties along the coast, modify many of the shorelines worldwide and drive change in coastal geomorphology. These artificial modifications resulted in impairment to the balance of the erosion and accretion in the estuarine and coastal environment. The on-site data surveyed in the present study provides insightful and valuable suggestions in scientific perspectives and coastal zone management.

The extensive set of observed data in the present study are undoubtedly helpful for future comparisons with the results from numerical models to determine the beach shoreline evolution [37,38]; in fact, monitoring activities significantly contribute to improving the robustness and reliability of numerical models [39,40].

**Figure 12.** Long-term erosion and accretion and spatial distribution of the cross sections for sampling bathymetry near the Lanyang River estuary during the period of (**a**) June 2006 to April 2013 and (**b**) April 2013 to April 2019.

**Figure 13.** Net volume variations between erosion and accretion around the Lanyang River estuary (corresponding to V29 to V41 in Figure 11) during the period of June 2006 to April 2013 (cyan bar) and April 2013 to April 2019 (red bar).
