**5. Summary and Conclusions**

A series of on-site topographic and bathymetric surveys were conducted along a 20 km long stretch of shoreline from north of the Wushi Fishery Port to the south of the Lanyang River estuary in June 2006, April and September 2012, April and September 2013, and April and October 2019. A total of 43 planned survey track lines designed for sonar collection were spaced approximately 500 m apart in the alongshore direction. Each track line was of varying length to allow the maximum water depth to reach 10–15 m (deeper than the closure depth), and topographic and bathymetric surveys were completed in 2 days. The bathymetric data obtained from several on-site surveys and the collected 0 m isobaths in various years were employed to investigate the long-term erosion and accretion along the studied shoreline. The 0 m isobath evolution reveals that the construction of a jetty and groin for the Wushi Fishery Port caused erosion and accretion to occur to the north and south, respectively. The interannual comparisons of the alongshore and cross-shore profiles indicate that dramatic erosion occurred in the southern Lanyang River estuary from 2013 to 2019 due to the establishment of a nature reserve. However, slight accretion occurred from 2013 to 2019 in the northern Lanyang River estuary because the riverine sediments were carried northward by tidal currents. In winter, the strong northeast monsoon induced sustained large waves, and the shortage of riverine sediment supply led to erosion of the coastal areas of northeastern Taiwan. In contrast to winter, weaker waves and large amounts of sediment released from the river (because of high river discharge) in summer were beneficial for accretion in the coastal areas of northeastern Taiwan. The cross-profile evolutions indicate that the supply of riverine sediment in the southern Lanyang River estuary was reduced in the past two decades as a result of the gradual northward movement of the Lanyang River mouth. The shoreline affected by estuarine sediment is approximately 2 km long from north to south of the Lanyang River alongshore. Long-term alongshore erosion and accretion are natural processes in coastal areas; however, the loss of coastal lands could be a result of overexploitation, e.g., the designs and constructions of the fishery port and backwater for the coastal areas. Although the present study provides insight into the problems of managemen<sup>t</sup> and development for the coastal environment, more frequent topographic and bathymetric surveys for the whole shoreline of Taiwan should be conducted in the future. Additionally, numerical simulations are necessary for future research to explain the seasonal mechanism of the simulations of the seasonal and annual change for wavefield, tidal current, and alongshore sediment transport.

**Author Contributions:** Conceptualization, T.-Y.L., S.-C.H., C.-H.C., T.-Y.C., W.-B.C., W.-P.H., W.-D.G., J.-Y.H. and C.-H.L.; on-site surveys, T.-Y.L., C.-H.C., T.-Y.C., W.-B.C., W.-P.H., W.-D.G., J.-Y.H. and C.-H.L.; data collection and analysis, T.-Y.L., C.-H.C., T.-Y.C., W.-P.H. and W.-B.C.; writing—original draft preparation, W.-B.C. and S.-C.H.; writing—review and editing, W.-B.C. and S.-C.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Science and Technology Center for Disaster (NCDR), Ministry of Science and Technology (MOST), Taiwan.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Publicly available datasets were analyzed in the present study. The data can be found here: https://ocean.cwb.gov.tw/V2/ (accessed on 5 January 2022).

**Acknowledgments:** The authors would like to thank the Central Weather Bureau and the Water Resource Agency, Taiwan, for providing the measurements of waves and currents.

**Conflicts of Interest:** The authors declare no conflict of interest.
