*2.1. Description of Study Site*

Yilan County covers the entire shoreline of northeastern Taiwan, with a total length of 101 km. The whole coastline is categorized as a fault coast in northern and southern Yilan and an alluvial-plain coast in middle Yilan according to the geological features. The section of shoreline selected for topographic and bathymetric surveys extended from north of the Wushi Fishery Port to south of the Lanyang River estuary, with a length of nearly 20 km (red line in Figure 1a), and extended from the shoreline to approximately 800 to 1000 m offshore. Figure 1b shows an aerial image of the Wushi Fishery Port taken in the present study. As shown in Figure 1b, an offshore jetty and an extended jetty, with lengths of 500 m and 400 m, respectively, lie to the east and north of the Wushi Fishery Port, respectively, to stabilize the oscillation caused by waves. A wave buoy located west of Gueishan Island (the orange triangle in Figure 1a) is managed by the Central Weather Bureau (CWB) of Taiwan. The hourly measurements of significant wave height and current recorded at the buoy from 2006 to 2020 are graphed in Figure 1c,d, respectively. It is obvious that the predominant waves come from the northeast with a height of 1–2 m (Figure 1c); however, this coastal area is threatened with big waves (significant wave height > 3.0 m) during the passage of typhoons [34,35]. The principal currents range from 0.25 to 0.5 m/s, with a flow direction to the southeast (Figure 1d). Erosion and accretion phenomena are expected to be found in the

north and south of Wushi Fishery Port, respectively, due to the physical characteristics of the marine environment and jetty effect. According to the report from the Water Resource Agency (WRA) of Taiwan, the Lanyang River is the largest river (with a watershed area of 978 km2) in Yilan County and has an average annual runoff of 74.24 ms<sup>−</sup><sup>1</sup> and a sediment yield of 6 million m3. An aerial image (taken by the present study) of the Lanyang River estuary is shown in Figure 1e, and a sandbar in the south of the Lanyang River estuary is clearly shown. Additionally, the daily sediment emissions from the Lanyang River estuary were measured by the WRA on the specified days in 2020. Figure 2 illustrates the daily variations of estuarine sediment volume at the Lanyang River. It can be seen that the months with higher daily sediment volume in the Lanyang River estuary are the end of April to September each year, due to their being wet and typhoon seasons. The median particle diameter (D50) of the Lanyang River estuary is 5.949 mm, according to the report from the WRA.

**Figure 1.** (**a**) The location of the study area, (**b**) an aerial image of the Wushi Fishery Port, (**c**) directional distribution of significant wave height, (**d**) directional distribution of current measured at a wave buoy near Guishan Island from 2006 to 2020, and (**e**) an aerial image of the Lanyang River estuary.

**Figure 2.** Measurements of daily sediment discharged from the Lanyang River estuary in 2020.

### *2.2. Evolution of the 0 m Isobath*

To better understand the shoreline evolution in the coastal area with significant erosion and accretion phenomena, the 0 m isobaths for the northern and southern Wushi Fishery Port and for the Lanyang River estuary surveyed in 2006, 2013, 2013, and 2019 were collected and compared. Figure 3a,b illustrate the 0 m isobaths along the northern and southern Wushi Fishery Port, respectively, and the 0 m isobath for the Lanyang River estuary is depicted in Figure 3c. As shown in Figure 3a, the 0 m isobath extended offshore to the north of the Wushi Fishery Port by 2013 due to the construction of the jetty and groin. The 0 m isobath in the southern Wushi Fishery Port stretched offshore and reached its maximum in 2013, after which erosion occurred until 2015 (as shown in Figure 3b). The 0 m isobaths crossing the Lanyang River estuary in various years are shown in Figure 3c. The 0 m isobaths around the Lanyang River estuary trended offshore by 2012 because of abundant sediments supplied by the Lanyang River; however, they retreated landward until 2015 as a result of the river mouth moving northward and reduced sediment transport southward.

**Figure 3.** Evolution of the 0 m isobath along (**a**) the northern Wushi Fishery Port, (**b**) the southern Wushi Fishery Port, and (**c**) the Lanyang River estuary.
