Shoreline Behavior in Response to Coastal Structures: A Case Study in Haikou Bay, China

Abstract

The rapid development of coastal structures on sandy coastlines raises concerns about their impacts on the shoreline’s evolution and the sediment transport dynamics. This study utilized a numerical modeling approach to simulate the multi-year response of Haikou Bay’s coastline to various nearshore structures, including piers and a large artificial island. The LITLINE module of the MIKE21 (v2020) software was employed to analyze the sediment transport patterns across three distinct coastal segments. The simulation results indicated that the sediment transport directions varied significantly: from west to east in the western segment, from east to west in the middle segment, and convergence toward the center in the eastern segment, divided by a construction trestle. The net sediment transport rates were quantified as 2000 m³/year for the western segment, 8000 m³/year for the middle segment, and 13,000 m³/year (west) and 10,000 m³/year (east) for the eastern segment. Due to the conflicting sediment transport directions on each side of the breakwater, noticeable deposition occurred on both sides. The presence of the artificial island created notable deposition in its wave shadow area, while the overall impact on the shoreline changes diminished over time. These findings underscore the significant influence of human activities, particularly coastal structures, on the natural evolution of shorelines.

1. Introduction

Coastal regions in China are economically developed and densely populated, with nearly 42% of the population living in coastal areas, but these regions account for only 13% of the country’s land area. These regions contribute over 60% of the national GDP, and future trends indicate further urbanization and population growth along the coast. As geomorphic units of coastal areas, sandy coastlines are highly susceptible to changes in shape due to waves, wind, and human activities [1]. To meet the demands of economic development, maritime transportation, resource exploitation, and recreational activities, a large number of structures have been constructed near the coast, including breakwaters, piers, and artificial islands. Over 70% of China’s coastline is covered by seawalls, which are often collectively referred to as China’s “Great Wall on the Sea” [2]. In recent years, the construction of artificial islands through offshore reclamation has become increasingly common, driven by the need to alleviate the shortage of coastal land resources and address the growing demand for high-quality coastal leisure and recreational facilities. Haikou Bay in Hainan Province is a densely populated coastal tourism area with numerous coastal engineering projects. Evaluating the long-term impacts of these nearshore structures and offshore reclamation projects on shoreline evolution is essential in understanding how human interventions affect natural sedimentary processes. With its rapidly growing tourism industry and large number of coastal engineering projects, Haikou Bay provides a unique opportunity to study the specific effects of artificial islands and other coastal structures on sediment transport and shoreline behavior. The motivation for this study lies in the need to inform future coastal management and disaster mitigation strategies by analyzing these long-term impacts in a highly engineered bay system. High quality development of water conservancy need to be pay more attention and put forward new solutions in this new stage with full of opportunities and challenging [3,4].

Approximately 70% of the sandy coastlines worldwide are subject to erosion [5,6], and the causes of shoreline erosion arise from both the land and ocean [7,8,9]. In recent years, coastal tropical cyclones have caused an increase in coastal wave heights [10], and the frequency of extreme El Niño and La Niña climate events has caused the inundation and erosion of the coasts on both sides of the Pacific Basin [11]. The anticipated changes in climate are expected to significantly affect the beach dynamics through alterations of the mean sea level, wave climate, storm patterns, and river flow rates. For example, changes in the wave climate have reduced the sediment supply along the northwestern coast of Portugal [12] and variations in precipitation have altered river sediment transport, impacting the beach sediment supply [13]. Additionally, the large proportion of artificial coastlines is an important factor affecting the vulnerability of the coast to erosion, where hard coastal protection alters the dynamic equilibrium of the original sediment movement of the coast, bringing new coastal erosion or siltation and destroying the natural environment of the coast [14]. For example, after the construction of a harbor in Nouakchott, Mauritania, a beach erosion rate of 20 m/year was observed, which was 10 times higher than its natural state [15]. The disturbance of coastal sand transport patterns due to the construction of jetties and breakwaters led to the rapid erosion and recession of a pair of sand spits at the mouth of the Fudu River in Dalian [16]. Among the various coastal structures, the offshore construction of artificial islands is one of the main forms of reclamation. With their different layouts and relative positions, artificial islands can have varying impacts on wave fields, tidal currents, and shoreline erosion and accretion after their construction. Li et al. [17] used the LITLINE beach evolution model to simulate the effects of three artificial island layouts with different reclamation areas, offshore distances, and plane forms on the deformation of the nearshore coastline. The results showed that the artificial island layout with a large offshore distance and small area had the smallest adverse effect on the beach.

In recent years, with the increasing drive for ecological protection, coastal defense methods have gradually shifted from ‘hard’ to ‘soft’ approaches. For example, researchers have revealed the role of waves in mud deposition as well as the formation of the beach boundary [18], and efforts to mitigate beach erosion through coastal zone conservation and restoration projects, such as artificial beach nourishment, ecological restoration, and dynamic coast technologies, have been extensively implemented in European countries [19,20,21,22]. Submerged sandbars help to replenish beaches and cause waves to break and reflect earlier, reducing the wave energy reaching the shorelines [23,24,25]. Various nature-based shoreline protection technologies are being rapidly developed, although simple beach maintenance measures are insufficient to restore the dynamic sedimentary environments disrupted by human activity. While numerous studies have explored the shoreline erosion and sediment dynamics globally, the specific long-term effects of coastal structures, such as artificial islands, on shoreline evolution in regions such as Haikou Bay remain underexplored. This study uses the LITLINE model to simulate the multi-year shoreline evolution in Haikou Bay, providing valuable insights into the ways in which different coastal structures influence the sediment transport and accumulation patterns. This research is unique in its focus on a densely engineered coastal region, offering a comprehensive analysis that can inform both local- and broader-scale coastal management practices.

Numerical simulation is the most common method used in coastal protection and restoration for the design of solutions and prediction of outcomes [26,27,28]. The spatial and temporal scales of the shoreline evolution in Haikou Bay are extensive, and process-based numerical models such as Delft3D are not applicable at this scale. In contrast, one-line models, which can rapidly simulate large-scale coastal evolution, are preferred in practical engineering applications. One-line models assume that, during the processes of coastal erosion and accretion, the active beach face shifts in position without changing in shape under the influence of gradients in the alongshore sediment transport rates. They treat short-term dynamic processes as fluctuations within long-term shoreline evolution trends, thus significantly enhancing the computational efficiency. Classic one-line models, such as LITPACK, have been used to simulate the impacts of breakwater on shoreline morphology [29], and UNIBEST has been employed to assess the effects of artificial beach nourishment on the Dutch coast [30]. In recent years, the combination of one-line models and semi-empirical models has led to the development of coastline evolution models that consider multiple physical processes and are efficient in achieving complexity reduction. Hanson and Larson [31] introduced cross-shore sediment transport into one-line models through this approach. The COCOONED model [32], which is based on the coupling of first-principles models, Miller and Dean’s equilibrium model [33], and the Bruun rule, also incorporates source–sink terms and dune erosion terms, allowing it to consider the impact of dune evolution on coastline changes. However, this model also has certain limitations, such as its assumption of a constant beach profile; thus, it may not fully capture short-term, highly dynamic processes such as storm surges or extreme wave events. This study primarily focuses on long-term shoreline evolution; therefore, the aforementioned limitations in capturing short-term dynamic events do not significantly affect the overall conclusions drawn from the analysis.

This study focuses on the coastal areas surrounding Haikou Bay in Hainan Province, China—a region where the impacts of artificial islands and coastal structures on the shoreline have not been thoroughly investigated. We use the LITLINE module of the MIKE21 software to analyze the coastline evolution in this region. This study investigates the evolution of the coastline in three segments of Haikou Bay, determining the sediment transport directions and rates in different sections. A multi-year numerical simulation method is employed to examine the long-term effects of nearshore structures and the responses of coastlines to artificial island construction projects, providing valuable insights into the sustained impacts of human activities on natural sedimentary processes. This fills a gap in our understanding of the long-term effects of artificial islands and nearshore structures on the sediment dynamics. This study provides data support for major engineering projects and ecological environmental protection along the coast of Hainan Island.

2. Study Area and Research Methods

2.1. Study Area Description

2.1.1. Description of Topography and Landforms

Haikou Bay is located in the northern part of Hainan Island, south of the central section of the Qiongzhou Strait, stretching from Baisha Cape in the east to Houhai in the west. It is an open spiral bay that faces north (also known as a cape bay). The bay is bordered to the north by the Qiongzhou Strait, which is over 20 km wide. Most of Haikou Bay has a water depth of less than 5 m, with the underwater terrain sloping northward at approximately one thousandth. The bay is mostly characterized by shallow shoals.

To the northwest of the western end of Haikou Bay, there are sandy twin shoals rising up, with several shallow hills on top and water depths ranging from only 0.7 to 1.5 m. Southwest of the twin shoals, there is a deep trough with water depths ranging from 5 to 9 m. An analysis of the changes in the isobaths was conducted by the South China Sea Institute of Planning and Environmental Research of the State Oceanic Administration and the Hainan Institute of Marine Development Planning and Design Research based on hydrographic surveys conducted in 1963, 1996, and 2003 (as shown in Figure 1). It was observed that, between 1963 and 2003, the −20 m and −10 m isobaths in the twin shoal area remained relatively stable, but the −5 m and −2 m isobaths receded slightly inland. This indicates that the seabed terrain in the project area underwent mild erosion over this 40-year period. The changes in the seabed terrain are somewhat related to the continuous deepening of the Haikou Port channel, which intercepts sediment from the east to the west. Although sediment in the twin shoal area is continuously lost, this erosion trend will continue; however, it is still mild and will not cause the twin shoals to disappear.

The Mingzhu Artificial Island in Haikou Bay was constructed in two phases, with a straight-line distance of approximately 1.90 km from the New State Guesthouse and a reclamation area of approximately 2.50 km². As shown in the satellite images comparing the shorelines over the years (Figure 2), the construction of Hainan Mingzhu Eco-Island Phase I and Phase II in Haikou Bay had little impact on the extensive shoreline, with only some accretion occurring in the wave shadow area between the artificial island and the mainland. Figure 3 shows on-site photos of Haikou Bay. Here, (a) shows the beach on the west side of Xiuying Port, which is subject to strong erosion and is protected by artificial riprap. This section marks the apex of a cape bay coastline, where the net sediment transport occurs from east to west along the coast. However, due to the obstruction caused by the port structures and channels on the east side, only a small amount of sediment is able to replenish the shoreline, leading to erosion. Moreover, (b) to (d) show the beaches from westward to the wave shadow area of Mingzhu Island, exhibiting no significant erosion or accretion features and indicating relatively balanced sediment transport. Finally, (e) and (f) show sections of the shoreline to the west of Zhenhai Village Fishery Harbor, where human activities have caused the severe erosion of the beach.

The suspended sediment concentration in Haikou Bay is influenced by the strength of the tidal currents and wave action. When the wave intensity is weak (for example, northward wind waves of less than 3–4 on the Beaufort scale), the sediment concentration mainly depends on the strength of the tidal currents, typically resulting in low sediment concentrations, averaging between 0.015 kg/m³ and 0.06 kg/m³. The sediment concentration is higher at the bay’s mouth and gradually decreases toward the bay’s head. When the wave action reaches 3–4 or higher on the Beaufort scale, wave-induced sediment resuspension causes the entire bay to become turbid, with the sediment concentration evenly distributed throughout the bay. As the wave action intensifies, the sediment concentration increases rapidly.

The sediment in Haikou Bay is primarily composed of sand, silty sand, and sandy silt, accounting for 40.1%, 21.4%, and 17.2%, respectively, totaling 78.6%. The median particle sizes are 343.44 µm, 365.87 µm, and 80.82 µm, respectively. This is followed by clayey silt and sandy gravel, accounting for 17.2%, with median particle sizes of 16.68 µm and 2166 µm, respectively.

2.1.2. Hydrology and Meteorology

The eastern side of the Qiongzhou Strait is characterized by an irregular semidiurnal tidal regime from the South China Sea, while the western side is influenced by the regular diurnal tides from the Beibu Gulf. The strait is situated at the confluence of these two tidal regimes, resulting in exceptionally complex tidal dynamics. The tidal characteristics, such as the nature of the tides and the tidal range, change rapidly along the Qiongzhou Strait. The tidal pattern is classified as diurnal, semi-diurnal, or mixed using the tidal discrimination number (H_K1 + H_O1)/H_M2, where HK1, HO1, and HM2 represent the luni-solar declinational diurnal constituent, the lunar declinational diurnal constituent, and the principal lunar semi-diurnal constituent, respectively.

According to tidal data collected in 2002 at the Xiuying Oceanographic Station, the tidal discrimination number (H_K1 + H_O1)/H_M2 for Haikou Bay is 3.86, indicating an irregular diurnal tide. The average tidal range is 1.18 m, with a maximum tidal range of 3.60 m.

In June 2020, tide gauge stations were installed in Haikou Bay and Puqian Bay, including the Haikou West Coast and Jiangdong, for the simultaneous observation of the tide levels and currents. The locations of the tide gauge stations are shown in Figure 4a. The tidal time series for the West Coast station is shown in Figure 4b, while that for the Jiangdong station is shown in Figure 4c. During the observation period, the maximum tidal range at the West Coast station was 2.11 m and the minimum was 1.02 m. At the Jiangdong station, the maximum tidal range was 1.65 m and the minimum was 0.38 m.

Based on an analysis of the measured tidal current data, the main tidal current type in Haikou Bay is an irregular diurnal tidal current. According to the results obtained via the two tidal current stations, during spring tides, the CLHK01 station exhibits obvious reciprocating flow characteristics, with the flow direction mostly parallel to the shoreline of Haikou Bay, while the CLHK02 station shows some rotational flow characteristics. During neap tides, the CLHK02 station generally exhibits a reciprocating flow, while the CLHK01 station shows rotational flow characteristics. The average maximum flow velocity perpendicular to the coastline does not exceed 1.10 m/s, with the flow velocity during spring tides being significantly greater than that during neap tides.

Wind and waves dominate in Haikou Bay throughout the year, accounting for 76% to 85% of the total, while swells account for 14% to 23%. Wind waves are the most prevalent in winter, with fewer occurrences in other seasons. The prevailing wave direction is ENE, accounting for 30.1%, followed by NE, with a frequency of 22.9%. The least common wave direction is S to WSW. During the northeastern monsoon, the monthly average H1/10 wave height is higher than in other seasons, with the maximum wave height occurring from August to October.

2.2. Shoreline Evolution Model

To predict the shoreline changes in each working area, the LITLINE module of the LITPACK model, developed by the Danish Hydraulic Institute (DHI), was used to construct a shoreline evolution model. The LITLINE module can simulate shoreline evolution issues caused by changes in the coastal sediment transport capacity due to the construction of coastal structures. Similarly to other one-line models, the calculation of the shoreline evolution in LITLINE is based on the single-line theory, which assumes that the cross-sectional profile of the beach remains unchanged during deformation, the two boundary lines of sediment movement toward the shore and toward the sea remain unchanged, the isobaths are parallel to the shoreline, and the evolution of the beach can be simplified in terms of the advance or retreat of the profile. The module simulates shoreline evolution problems by solving the sediment mass conservation equation in the coastal zone, with the following governing equation:

$${\displaystyle \frac{\partial y_c(x,t)}{\partial t}}=-{\displaystyle \frac{1}{h_{act}(x)}}{\displaystyle \frac{\partial Q(x,t)}{\partial x}}+{\displaystyle \frac{Q_{sou}(x,t)}{h_{act}(x)\Delta x}}$$

(1)

In the above equation, $y_c(x,t)$ represents the offshore distance; t represents the time; h_act represents the actual cross-sectional elevation with erosion and deposition; Q(x,t) represents the alongshore volume sediment transport intensity; x represents the alongshore coordinate; Δx represents the alongshore spatial step length; and Q_sou represents the sediment source–sink intensity per unit spatial step length.

The LITLINE module’s governing equation is solved using the finite difference method, with the corresponding variables configured on a horizontally equidistant staggered grid. The grid spacing for shoreline calculation is 5 m. The model was used to simulate and calculate the shoreline deformation of Haikou Bay for 5, 10, and 20 years. The sediment transport rate input was obtained by interpolating sediment transport tables at spatial points calculated in advance using the LITTABL module. The transverse profiles of the shoreline and beach terrain used during model calculation were generated by interpolating data from naval charting department charts, C-map electronic charts, and surveyed terrain data near the project. The calculation area covered the shoreline from the new seaport terminal to the National Sailing and Windsurfing Base, bounded by the construction of the Meishu Bay near the Shoukai Bay area and the Zhenhai Village fishing port. The calculation area was divided into the western, middle, and eastern sections, as shown in Figure 5.

The model’s water level was determined based on the statistical characteristics of the measured tidal data from Xiuying Station, covering the period from 2000 to 2015. In the model’s water level, the average high water level of 0.56 m accounts for 25%, the mean sea level accounts for 50%, and the average low water level of −0.61 m accounts for 25%. The wave data for the Haikou Bay area were derived from the frequency and grade wave data from the Baishamen Station, collected from May 1984 to April 1985, calculated for the nearshore conditions as input. The sediment transport rates in each working area were obtained from the sediment transport tables provided by the LINTABL module. LINTABL calculates the sediment transport rates by utilizing the sediment transport rate calculation module LITDRIF to compute the coastal sediment transport under a range of specified conditions, which include the water level, flow velocity, wave height, wave period, and wave direction, thus obtaining the sediment transport tables. When simulating shoreline deformation using LITLINE, the sediment transport rates at the calculation grid points are obtained by interpolating the sediment transport tables provided by LINTABL. Sediment particle sizes were assigned based on the subdivision of the particle sizes in the nearshore area, as described in Section 2.1.1.

The sediment initiation velocity is the minimum flow rate required for sediment particles to begin moving in a stream when the dynamic forces of the fluid (e.g., drag and lift) exceed the stabilizing forces of the sediment particles (e.g., gravity). The initiation flow velocity under the action of tidal currents in the nearshore breaker zone was estimated using the sediment initiation formula from the Wuhan University of Hydraulic and Electric Engineering [34]:

$$U_c={({\displaystyle \frac{h}{D}})}^{0.14}{(17.6{\displaystyle \frac{{\rho }_s-\rho }{\rho }}\mathrm{D}+{6.0510}^{-7}{\displaystyle \frac{10+h}{D^{0.72}}})}^{0.5}$$

(2)

The terms are as follows: U_c is the initiation flow velocity, h is the water depth, D is the median sediment particle size, and ρ_s and ρ are the densities of sediment and water, respectively. In a nearshore area with a water depth of approximately 2 m, the sediment initiation flow velocity is approximately 0.53 m/s. Taking Haikou Bay as an example, in the nearshore area with a water depth of approximately 5 m, the sediment initiation flow velocity was approximately 0.62 m/s. In the nearshore area with a water depth of approximately 10 m, the sediment initiation flow velocity was approximately 0.68 m/s. According to the measured flow velocity data, the influence of the tidal currents on the sediment transport outside the breaker zone was significant. Inside the breaker zone, where the water depth was smaller, the sediment transport was mainly influenced by wave-driven alongshore transport, and the influence of the tidal currents was relatively small. Therefore, tidal current effects were not considered in this simulation.

3. Shoreline Evolution Simulation Results

3.1. Shoreline Evolution in the Western Section

Figure 6 shows the simulation results for the evolution of the shoreline in the western section. The simulation results for the western section, which was located between the Xinhaigang Terminal and the jetties near the Shoukai Meishuwu Bay, indicated that the direction of sand transport was mainly from west to east, with a net annual sand transport of approximately 0.2 million m³. This finding aligns with that of Li et al. [17], who reported similar sediment transport patterns in comparable geomorphic settings, highlighting the significant influence of the southeastern monsoon and tidal forces on the sediment dynamics in this region. In addition, the findings suggest that this section of the coastline is protected by surrounding structures such as piers and revetments, creating multiple, relatively independent units of sediment transport, resulting in minimal sediment accumulation in the vicinity of these structures. The presence of these artificial structures partly obstructs the natural sediment dynamics, leading to localized areas of sediment accumulation and erosion. Fu et al. [35] indicated that artificial structures can alter the local flow patterns, causing sediment to concentrate in certain areas or be washed away from others. In this study, the changes observed along the western coastline could be attributed to the interference of these structures with the sediment dynamics, further impacting the ecological balance of the area.

Moreover, the results revealed that, although the overall sediment transport direction was from west to east, there were significant variations in the sediment transport volume at different locations along the coastline due to the influence of local artificial structures. For instance, near the base of the revetments on the right side, there was relatively little accumulation of sediment, whereas, on the left side, altered water flows may have resulted in localized erosion. This uneven distribution of the sediment aligns with the findings of Huu et al. [36], who observed similar sediment dynamics affected by artificial islands.

These changes in the coastline have profound implications for both human and aquatic habitats. In terms of the human impact, coastline erosion can threaten infrastructure, reduce the amount of land available for development, and increase the vulnerability to coastal flooding, which could disrupt the local economy and affect tourism in Haikou Bay. While they are intended to protect certain areas, coastal structures may result in increased erosion in other areas, leading to uneven shoreline retreat and the loss of valuable land. For aquatic habitats, changes in the coastline can severely alter the nearshore environment, which serves as a critical habitat for various marine species. Changes in erosion and sedimentation patterns can modify the composition of the seabed, leading to habitat loss or changes in the distribution of benthic organisms. Increased sedimentation in certain areas may also affect the water quality, reducing light penetration and impacting seagrass beds, coral reefs, and other vital marine ecosystems. Additionally, the disruption of natural sediment flows can affect the spawning grounds and feeding areas of fish and other marine organisms, leading to long-term ecological consequences.

3.2. Shoreline Evolution in the Middle Section

Figure 7 presents the simulation results regarding the shoreline’s evolution in the middle section of Haikou Bay. This section lies between the protruding structures of Shoukai Meishuwang Bay and Zhenhai Village Fishing Port, with a net sediment transport direction from east to west, amounting to approximately 8000 m³ per year. Due to the sheltering effect of the protruding structures near Shoukai Meishuwang Bay, the sediment transport directions on the eastern and western sides of the protrusion are conflicting. This section is further divided into multiple sediment transport units by coastal buildings. In the left unit, significant accumulation occurs on both sides of the protruding structures, with the sediment primarily transported from east to west, creating zones of high deposition. Similar behavior has been observed in other coastal studies [37], where artificial structures led to localized patterns of sediment buildup and erosion.

In the central unit, rocks at the Hawaii Coast location contribute to partial sediment accumulation. The right section mainly experiences accumulation in the wave shadow zone of the offshore island. The interaction between the coastal morphology and artificial structures alters the natural sediment transport dynamics, which is consistent with findings obtained at the Bay of Santa Marta in Colombia [38]. The results for Haikou Bay reinforce the idea that, while coastal structures can protect specific sections of the coastline, they can also lead to unintended consequences in adjacent areas, including erosion and sediment deposition imbalances.

3.3. Shoreline Evolution in the Eastern Section

Figure 8 illustrates the simulation results regarding the shoreline’s evolution in the eastern section of Haikou Bay. The eastern shoreline is located between the Zhenhai Village Fishing Port and the National Sailing and Windsurfing Base, with the construction wharf of Mingzhu Island as the dividing point. The sediment transport direction on the western side of the construction wharf is from west to east, with an annual net sediment transport volume of approximately 13,000 m³. On the eastern side, the sediment transport direction is from east to west, with an annual net sediment transport volume of approximately 10,000 m³. The eastern shoreline shows significant accumulation in the wave shadow zone of the Phase 2 artificial island, with sediment transport along the coast from both the eastern and western sides of the wave shadow zone toward the center. The sediment accumulation in the wave shadow zone reflects a common phenomenon associated with artificial structures. Research on sandy beaches, such as those in the Caribbean and the Mediterranean, has shown that artificial islands and other coastal structures can significantly disrupt the natural sediment transport processes, leading to both erosion and deposition in unexpected areas [37].

Overall, the current impact of Mingzhu Island’s construction on the shoreline is gradually weakening, with only the shoreline in the wave shadow zone still undergoing adjustments after the island’s construction; it requires some time to reach sediment equilibrium.

4. Conclusions

This study analyzed the hydrological and sedimentological data, as well as the topography, of Haikou Bay, Hainan Province, employing the LITLINE module to investigate the sediment transport direction, sediment transport rate, and shoreline evolution, as well as the impacts of artificial island construction on the shoreline. The findings indicated that the sediment transport along Haikou Bay’s coast varied: it moved from west to east in the western section and from east to west in the central section, and it converged from both sides toward the center in the eastern section. The net sediment transport rates were 2000 m³/year, 8000 m³/year, and 13,000 m³/year on the western side of the construction wharf and 10,000 m³/year on the eastern side. Additionally, the western shoreline was segmented into multiple sediment transport units by revetments and reefs, leading to minimal accumulation near the revetment structures, while the central shoreline mainly exhibited accumulation at reef locations. In the eastern section, significant accumulation occurred within the wave shadow zone of the artificial island. The influence of the artificial island’s construction on the shoreline is gradually diminishing, with only the wave shadow zone still exhibiting post-construction adjustment, indicating that a certain amount of time is required to achieve sediment equilibrium.