The Impact of Floating Raft Aquaculture on the Hydrodynamic Environment of an Open Sea Area in Liaoning Province, China

Abstract

The sea area of Changhai County in Dalian City is a typical floating raft aquaculture area, located in Liaoning Province, China, where a key issue in determining the scale and spatial layout of the floating raft aquaculture is the assessment of the impact of aquaculture activities on the hydrodynamic environment. To address this issue, we established depth-averaged two-dimensional shallow water equations and three-dimensional incompressible Reynolds-averaged Navier–Stokes equations for the open sea area described in this paper. The impact of floating rafts for aquaculture on hydrodynamic force was reflected in the numerical model by changing the Manning number, where scenarios with different aquaculture densities were taken into account. Finally, the water exchange rate of the floating raft aquaculture area in the study area was calculated. It was found, through a comparison between the simulated value and the measured value obtained via layered observation, that the two values were in good agreement with each other, indicating that the model exhibits great accuracy. In addition, the calculation results for scenarios before and after aquaculture were compared and analyzed, showing that from low-density to high-density aquaculture zones, the variation in flow rate was greater than 80% at the peak of a flood tide. The water exchange rates of the water body after 1 day, 4 days, and 8 days of water exchange were also calculated, and the results show that they had been reduced by 17.92%, 13.59%, and 1.63%, respectively, indicating that the existence of floating rafts for aquaculture indeed reduced the water exchange capacity of the water body. The model described in this paper can serve as a foundation for other studies on aquaculture in open sea areas, and it provides a theoretical basis for the scientific formulation of marine aquaculture plans and the rational optimization of the spatial layout.

Application of a VOF Multiphase Flow Model for Issues concerning Floating Raft Aquaculture

“…Against the backdrop of the computed hydrodynamic force results and considering the resistance from the entire raft area, the water exchange performance of a nearby large-scale sea area was simulated. As for the simulation of the water exchange degree, the aforesaid transport equations, in combination with the tracer method [21], were used for computation, evaluation and description, and a comparative analysis of water exchange rates in the natural sea area and the sea areas near the floating raft aquaculture area was carried out. Figure 20 shows the distribution of substance transport and diffusion concentration fields at different times (2d, 4d, 6d and 8d) after the implementation of aquaculture activities.…”

“…Scott et al [20] calibrated the kinetic energy obtained in a laboratory flume experiment using horizontal velocity, Reynolds stress and vertical profiles of turbulent flows, wherein the change in the empirical correlation-based drag coefficient in the vertical direction was also taken into account, and quantitatively described the nutrient transport conditions affected by various aquaculture structures with calibrated turbulence parameters and drag coefficients. In order to comprehensively understand the flow of seawater through aquaculture facilities in an open-sea area of Changhai County, Dalian, Liaoning Province, Wang et al [21] introduced a dual-resistance model wherein the secondary drag force coefficient was considered to describe the impact of the floating raft facilities and organisms on the hydrodynamic environment. Zhang et al [22] estimated and evaluated the characteristics of hydrodynamic force and sediment transport under natural and aquaculture conditions based on observation data from a suspended kelp aquaculture area in Haini Bay.…”