**1. Introduction**

Changhai County, Dalian, is located on the eastern side of the Liaodong Peninsula in the northern waters of the Yellow Sea in Liaoning Province, China(as shown in Figure 1), and its geographic coordinates are 122◦17 E–123◦13 E, 38◦55 N–39◦35 N. As the only county completely located on islands in Northeast China, Changhai County has an area of water covering 10,324 square kilometers, which is an ideal habitat for temperate marine organisms, such as fish, shrimp, shellfish, and algae. In recent years, with the rapid development of the marine aquaculture industry, floating raft and cage aquaculture industries have emerged in this open sea area, which has brought not only a great deal of economic benefits to residents, but also huge challenges to marine hydrodynamics and ecological environment protection. Some aquaculture farmers excessively pursue high yields with a lack of scientific and reasonable justification, so they tend to increase the scale and density of aquaculture in a disorderly manner. Due to the over-crowded raft areas, the rafts and facilities have

**Citation:** Wang, K.; Li, N.; Wang, Z.; Song, G.; Du, J.; Song, L.; Jiang, H.; Wu, J. The Impact of Floating Raft Aquaculture on the Hydrodynamic Environment of an Open Sea Area in Liaoning Province, China. *Water* **2022**, *14*, 3125. https://doi.org/10.3390/ w14193125

Academic Editors: Xiangli Tian and Li Li

Received: 21 July 2022 Accepted: 26 September 2022 Published: 4 October 2022

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**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/).

had a hindering effect on the hydrodynamic environment, inhibiting water exchange and weakening the substance transport and diffusion capacity of the water body. As a result, it is impossible for algae and bait to be evenly distributed with the hydrodynamic force, which would support the growth of marine organisms. This results in a phenomenon where the cultured organisms that were longitudinally arranged in a raft area appear to grow well, while those in the middle grow slowly or even die due to a lack of bait. Some aquaculture operators who have been cultivating scallops, oysters, or sea cucumbers in floating rafts and cages have gradually realized the severity of the problem. Thanks to such changes in their awareness, they are looking for a scientific and reasonable solution to the problem, with the ultimate goal of determining the degree of impact of the overall structure for aquaculture, including rafts, floaters, ropes and cages, and even the cultured organisms, on the hydrodynamic environment of an open sea area. The solution of this problem could provide technical support for the scientific formulation of a sowing density plan for cultured organisms, the rational selection of the location of an aquaculture area, and the precise placement of bait casting devices in bait-deficient zones.

In recent years, some researchers have carried out relevant studies on the mechanisms of interactions between raft placement and hydrodynamic environments in raft aquaculture areas; however, most of the studies have focused on the changes in water quality and the sediment environment or used field observations and model tests. For example, Zhao et al. [1] conducted a simulation-based assessment of the impact of the deep-sea cage aquaculture of *Lateolabraxjaponicus* on water quality and the sediment environment in the Yellow Sea of China based on a three-dimensional Lagrangian particle tracking model. Water quality simulations indicated that deep sea cages account for 26% of the total dissolved inorganic nitrogen and 19% of the active phosphorus content. The model results indicated that the installation of all deep-sea cages will lead to acceptable levels of water quality, but that sediments may become polluted. The coupled model can be used to predict the environmental impacts of deep-sea cage farming and provide a useful tool for

designing the layout of the integrated multi-trophic aquaculture of organic extractive or inorganic extractive species. Klebertet et al. [2] carried out field monitoring and modeling for the three-dimensional deformation of a large circular flexible sea cage in high currents using an acoustic Doppler current profiler (ADCP) and an acoustic Doppler velocimeter (ADV). The results showed a reduction of 30% in the cage volume for a current velocity above 0.6 m/s. The measured current reduction in the cage was 21.5%. Moreover, a simulation model based on super elements describing the cage shape was applied, and the results showed good agreement with the cage deformations. Dong et al. [3] conducted an experimental study involving an internationally advanced experimental model of fluid– structure interactions, which described the fluid–structure interactions of flexible structures, in a study on the cage aquaculture of Thunnusorientalis. They measured the drag force, cage deformation, and flow field inside and around a scaled net cage model composed of different bottom weights under various incoming current speeds in a flume tank. Results indicated that the drag force and cage volume increased and decreased, respectively, with the bottom weight. Owing to the significant deformation of the flexible net cage, a complex fluid–structure interaction occurred and a strong negative correlation between the drag force and cage volume was obtained. Furthermore, an area where the current speed was often reduced was identified. The intensity of this reduction depended on the incoming current speed. The results of this study can be used to understand and design optimal flexible sea cage structures that can be used in modern aquaculture. In addition, a team led by Dong used model-scale test and full-scale sea test techniques [4] to determine the hydrodynamic characteristics of a sea area near a cage aquaculture area for silver salmon. In that study, the results of model-scale and full-scale tests were compared, showing that under the impact of lower currents, only bottom mesh deformation was found. As for the observed trends, the resistance, cage deformation, and cross-sectional area estimated based on the depth data from the full-scale test were generally consistent with the results converted from the model-scale test using the law of similarity. However, the resistance value of a full-sized cage converted from the model-scale test was larger than the depth estimated based on the depth data from the full-scale test. Conversely, the result from the model-scale test was smaller than the estimate from the full-scale test. In the future, cage deformation should be investigated at higher flow rates, and resistance should be measured at full scale to verify the results of model-scale tests and hydrodynamic model tests. Sintef et al. [5] also observed and investigated the turbulence and flow field changes in sea cages for commercial salmon aquaculture and their wakes in their study, where an acoustic Doppler current profiler (ADCP) installed on the seabed was used to measure the flow rate and turbulence on a layered basis, and an acoustic Doppler velocimeter(ADV) was used to measure the velocity inside the sea cages; dissolved oxygen sensors and echo sounders were also arranged in the sea cages to measure fish distribution, in order to facilitate the acquisition of data. The final results showed that a reduction in strong currents in the wakes near the cages and the existence of high-turbulence columns in the upper part of the water were both caused by the cages. Measurements performed in the cages indicated that although fish aggregation reduced water flow, there was no evidence that fish generated secondary radial and vertical flows within the cages.Ji et al. [6] observed, in a study on a gulf ecosystem for shellfish aquaculture, that in a crowded area with suspended shellfish, the sedimentation effect of organisms was very obvious, and the hydrodynamic effect was obviously insufficient. Hatcher et al. [7] conducted a measurement in the mussel aquaculture area located in the Upper South Cove, Canada, and found that the settlement of the raft aquaculture area was more than twice that of the control area without aquaculture. Bouchet and Sauriau [8] found, in an ecological quality assessment on a shellfish aquaculture area in the Pacific Ocean, that the suspended aquaculture system resulted in higher organic matter enrichment compared with a bottom sowing culture.

With the rapid development of computer technology, mathematical models have been widely adopted in numerical-simulation-based studies on marine aquaculture. Panchanget et al. [9] stated that the mathematical modeling of hydrodynamic force and particle

tracking can be an effective method with which to study the laws of diffusion and transport of pollutants in aquaculture areas, and the fate and traceability of materials. Xing et al. [10] studied the impact of an aquaculture area on the distribution of the vertical structure of the water flow with a hydrodynamic model and found that the distribution of the vertical structure was mainly controlled by the bottom friction of the aquaculture area. Durateet et al. [11] calculated the hydrodynamic characteristics of the estuary in Galicia based on a three-dimensional numerical model. It was found, through the analysis of the residual current field, that raft aquaculture can reduce the flow rate of the residual current by at least 40%, which facilitates the development of harmful algal blooms, posing a serious threat to cultured organisms and the aquatic environment. Shiand Wei [12] simulated an aquaculture area in Sanggou Bay with an optimized POM and found that the high-density aquaculture and related facilities in Sanggou Bay reduced the flow rate by nearly 40% on average and increased the average half-exchange time by 71%. In summary, the valuable technical studies conducted by these researchers will greatly inspire our later studies.

The original intention of this work was to solve some problems with floating raft aquaculture areas. In this study, a typical floating raft aquaculture area located in Changhai County, Liaoning Province, was chosen as the research area on the basis of the successful establishment of the hydrodynamic model and tracer model in these area of Liaodong Bay, in order to quantitatively explain the impact of floating raft aquaculture on the hydrodynamic environment of an open sea area. Compared with the sea area of Liaodong Bay, the study area features a higher degree of openness. Aiming to comprehensively understand the temporal and spatial distribution and variation characteristics of hydrodynamic force in the waters near the floating raft aquaculture area located in Changhai County, Dalian, the project team simulated and analyzed the hydrodynamic field and water exchange rate in the sea area near the floating raft aquaculture area. In this study, depth-averaged two-dimensional shallow-water equations and three-dimensional incompressible Reynoldsaveraged Navier–Stokes equations were established for the open sea area. We described the impact of rafts (floaters, ropes, cages, cultured organisms, etc.) on hydrodynamic force in the aquaculture area by changing the Manning number of the seabed. Finally, the model was verified with the observed hydrodynamic data, and the results show that the model has great accuracy, stability, and universality, and it can provide an accurate prediction of the hydrodynamic environment of aquaculture in the raft area.

#### **2. Materials and Methods**

#### *2.1. Observational Data*

The project team set up a temporary tide-level observation station, T1, in the coastal waters of Dalian, and conducted tide-level observations for three months, from 00:00, 1 August 2021 to 23:00, 31 October 2021. Two continuous observation stations, P1 and P2, were set up for ocean current observation, where a total of 25 h of layered and synchronous continuous ocean current observations were carried out, from 11:00, 13 September 2021 to 12:00, 14 September 2021. The specific coordinates of the stations are shown in Table 1, and their locations are shown in Figure 2. Refer to Section 3.1 for the specific observation values below.

**Table 1.** Coordinates of the hydrometric stations for hydrological tide tests.


**Figure 2.** Locations of the observation stations.

## *2.2. Model and Methods*

The model is based on the solution of the three-dimensional incompressible Reynoldsaveraged Navier–Stokes equations. First, the integration of the horizontal momentum equations and the continuity equation over depth for the following two-dimensional shallow water equations was carried out [13–16]. Based on the aforesaid principle, the commercial model encapsulation platforms used in this study mainly included Hydro info, a water conservancy information system developed by Dalian University of Technology, China, and Mike, a commercial water simulation computing system developed by the Danish Hydraulic Institute (DHI).

Based on the above model and methods, the specific implementation process was completed, as follows. First, in order to accurately analyze the hydrodynamic conditions of the water area near Changhai County, two-dimensional models of the Yellow Sea and the Bohai Sea and the waters near Changhai County were created, where the open boundary of an open water area was driven by the time series file of the tidal level. Then, in order to reflect the hydrodynamic conditions of the sea area near the aquaculture area located in Changhai County in more detail, a three-dimensional model of a small area of interest in Changhai County was created with a nesting method [17,18] based on the tidal-level drive after the calibration of the two-dimensional model of Changhai County, where the calculation range mainly covered the area contained by the four control points C, D, E, and F shown in Table 2, and the locations of the control points are shown in Figure 3a.

**Table 2.** Coordinates of points in the range of the calculation domain.


**Figure 3.** Calculation ranges and grid distribution maps of the models.

#### 2.2.1. Hydrodynamic Model

The study area is located along the northern coast of the Yellow Sea (see Figure 1), where tidal currents play a dominant role in various flow components. The three-dimensional Navier–Stokes equations for the free-surface flow of incompressible fluid in the Cartesian coordinate system were used for description; on this basis, the horizontal momentum equations and the continuity equation for the three-dimensional shallow water form were integrated in the range *H = η + h* to obtain the following depth-averaged two-dimensional shallow water continuity equation:

$$\frac{\partial \eta}{\partial t} + \frac{\partial}{\partial x}(h\iota) + \frac{\partial}{\partial y}(h\upsilon) = 0\tag{1}$$

Momentum equations

$$\frac{du}{dt} - \frac{\partial}{\partial \mathbf{x}} \left( \gamma^h \frac{\partial u}{\partial \mathbf{x}} \right) - \frac{\partial}{\partial \mathbf{y}} \left( \gamma^h \frac{\partial u}{\partial \mathbf{y}} \right) - fv + \frac{gu\sqrt{u^2 + v^2}}{\mathbb{C}\boldsymbol{z}^2 H} = -g\frac{\partial \eta}{\partial \mathbf{x}} \tag{2}$$

$$\frac{dv}{dt} - \frac{\partial}{\partial \mathbf{x}} \left( \gamma^\mu \frac{\partial v}{\partial \mathbf{x}} \right) - \frac{\partial}{\partial \mathbf{y}} \left( \gamma^h \frac{\partial v}{\partial y} \right) + fu + \frac{gv\sqrt{\mu^2 + v^2}}{\mathbb{C}\_Z{Z}^2 H} = -g\frac{\partial \eta}{\partial y} \tag{3}$$

where *η* represents the sea surface fluctuation (tidal level) relative to the still sea surface; *h* represents the still water depth (the distance from the seabed to the still sea surface); *H = η + h* represents the total water depth; *C*<sup>z</sup> = *n*. *H*(1/6) is the Chezy coefficient; *n* = 1/*M* is Manning's roughness coefficient, and M represents the Manning number.

Equations (1)–(3) are the basic governing equations for solving the hydrodynamic elements. In order to comply with the uniqueness of solutions, the definite conditions must be given.

(1) Initial Conditions

The cold-start mode was used, meaning that the initial conditions were considered irrelevant to the final result of the calculation. In this study, the initial flow rate and tidal level were both determined as 0.

(2) Boundary Conditions

For the numerical model used in this study, two boundary conditions need to be given, including open boundary and closed boundary conditions. For a tidal flat near the island coastline, the position of the land–water interface changed with the fluctuation of the tide

level, and the dry–wet variation of the grid nodes in the moving boundary was taken into consideration in this work [19–21].
