**1. Introduction**

Coastal erosion can lead to coastal retreat, habitat destruction, and loss of land, which result in significant negative ecological and socioeconomic impacts on global coastal zones. Beach and dune systems are the first line for defending against the damaging impacts of water-related natural hazards, such as coastal storms, hurricanes, and typhoons; therefore, shoreline erosion (retreat) poses a significant threat to settled coastal areas worldwide [1,2]. Rapidly changing coastlines are a serious problem in many areas of the world, such as sections of the Nile and the Yellow River Delta, which have stirred the interest of many researchers in different fields [3–20].

The sediment budget and geology determine coastal morphology and dynamics, which influence the nature and health of coastal ecosystems. Human activities affecting sediment dynamics, both on the coast and on land, modify the naturally occurring patterns of erosion and accretion. Additionally, human interventions have frequently altered the delivery of riverine sediments to coastal areas [21–24]. For example, reservoir/dam construction has trapped over 50% of the world's sediment flux [25], and most of the world's deltas have now been significantly dammed in their upper and central reaches [21,25].

**Citation:** Liang, T.-Y.; Chang, C.-H.; Hsiao, S.-C.; Huang, W.-P.; Chang, T.-Y.; Guo, W.-D.; Liu, C.-H.; Ho, J.-Y.;Chen, W.-B. On-Site Investigations of Coastal Erosion and Accretion for the Northeast of Taiwan. *J. Mar. Sci. Eng.* **2022**, *10*, 282. https://doi.org/ 10.3390/jmse10020282

Academic Editor: Rodger Tomlinson

Received: 7 February 2022 Accepted: 10 February 2022 Published: 18 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

According to the report from Warrick et al. [26], approximately 30 million tons (Mt) of sediment was deposited in the reservoirs of the river they studied before dam removal began in 2011.

Many areas of observed historical shoreline advances are related to reclamation and impoundment by coastal structures. These human activities modify coastal dynamics, typically resulting in downdrift erosion. Factors that influence coastal erosion and sedimentation encompass characteristics of coastal sediment; exchanges among the land, the coast, and the shelf; geomorphic responses to oceanic forcing. Human activities may both substantially influence and be affected by coastal erosion and sedimentation [2,27,28]. Currently, climate change impacts, including sea-level rise and potential increases in the frequency and intensity of severe tropical and extratropical storms, hurricanes, and typhoons, could accelerate coastal erosion or accretion, and recent observations have also indicated an acceleration in coastal cliff erosion [29].

Dadson et al. [30] reported erosion rates in the Taiwan Mountains estimated from modern river sediment loads. They suggested that Taiwan supplied 384 Mt yr<sup>−</sup><sup>1</sup> of suspended sediment to the ocean from 1970 to 1999, which represents 1.9% of the estimated global suspended sediment discharge but is derived from only 0.024% of Earth's subaerial surface. A better understanding of the nature and evolution of coastal (beach) erosion and accretion is necessary to inform and enact appropriate and timely disaster preparedness [31,32]. Moreover, to accurately assess coastal hazards in the face of future climate and land-use changes, it is necessary to understand the dynamics of shoreline erosion and accretion over the length and time scales relevant to the processes that drive change.

Direct measurements through traditional on-boat acoustic surveys have a high resolution, although they consume considerable manpower and material resources [33]. The present study investigated the timing of the transition from either stable or erosional conditions to accretional conditions in the study area by evaluating a time series of cross-shore positions. The aim and insights derived from the present study are expected to clarify coastal, beach, estuarine, and tidal flat managemen<sup>t</sup> strategies. The findings are also helpful for the development of similar coastal systems worldwide. The details of the study site information, the evolution of the 0 m isobath, on-site topography, and bathymetric survey are described in Section 2; the analyses of interannual and seasonal variability derived from the surveyed data are presented in Section 3. In Section 4, a discussion on the alongshore limit of the sediment delivered from the Lanyang River is given, and finally, the summary and conclusions are presented in Section 5.
