*2.3. On-Site Bathymetric and Topographic Surveys*

Multiple methods can be used for bathymetric surveys, e.g., multibeam and singlebeam surveys. An accurate bathymetric survey allows the researcher to measure the depth of a water body and map the underwater features of a water body. To analyze the longterm erosion and accretion in the study area, topographic and bathymetric surveys were conducted through the measurement of 43 cross sections along the coastline 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. Figure 4 demonstrates the spatial distribution of the cross sections for sampling bathymetry and topography. As shown in Figure 3, the planned survey track lines for the sonar collection were spaced approximately 500 m apart in the alongshore direction and were of varying length to allow the survey to be completed in 2 days. The bathymetric survey was performed offshore from north of the Wushi Fishery Port to the south of the Lanyang River estuary, utilizing a boat-mounted Global Positioning System (GPS) device, with a

single-beam echosounder. The boat bathymetric survey was carried out until the measured water depth was approximately 10–15 m. This is based on the empirical formula proposed by Houston [36], i.e., *hc =* 6.75 *Hsm*, where *hc* is the closure depth (a water depth of the survey endpoint in nearshore waters) and *Hsm* is the long-term averaged significant wave height, which represents the wave climate of the study area. An *Hsm* value of 1.14 m was adopted in the present study, according to the statistical data for waves issued by the CWB of Taiwan. The *hc* value is estimated to be approximately 7.7 m for the nearshore waters of Yilan using Houston's equation. In order to avoid the inaccuracy of the depth sounder, the present study extended *hc* to 10–15 m to ensure the minimal *hc* (i.e., 7.7 m) is within the bathymetric surveys. A surveying-quality GPS unit and an electronic distance measurement (EDM) total station theodolite (TST) were used to determine the locations of features shown in the present study, to conduct a topographic survey on the beach.

**Figure 4.** Spatial distribution of the cross sections for sampling bathymetry.
