泥沙物理模型翻译（养殖浮筏对海州湾水交换能力影响的研究）

摘 要：

海州湾是我国最大的紫菜筏式养殖基地。采用数值模拟方法研究了养殖浮筏对海州湾水动力条件和水交换能力的影响。首先，利用基于有限体积的海洋数值模式(FVCOM)建立了海州湾水动力模型，并在模型中引入考虑浮筏影响的双阻力模型，对海州湾海域的水动力场、纳潮量和水交换能力进行了模拟。结果表明，养殖浮筏严重削弱了海湾内水动力场，使养殖区域的海表流速降低10%～50%;纳潮量减少，最大减少了0.21×108 m3;海水半交换周期延长了16个潮周期，平均半交换时间延长了20%,表明海湾内养殖浮筏的存在降低了水交换能力。最后，通过数值实验，探讨了在养殖浮筏区设置水流通道对提高海湾水交换能力的贡献。

关键词:

海州湾; 养殖浮筏; FVCOM; 双阻力模型; 水交换能力; 水动力效应; 水力特性；

作者简介：

王松(1998—),男,硕士研究生,主要从事物理海洋学研究。E-mail:13998464727@163.com;

*张瑞瑾(1975—),女,副教授,博士,主要从事物理海洋和流体力学教学与科研工作。E-mail:ruijinz@dlou.edu.cn;

基金：

国家重点研发计划项目(2019YFC1407700);

国家自然科学基金项目(31302232);

设施渔业教育部重点实验室项目(2021-MOEKLECA-KF-06);

山东省重点研发计划项目(2019JZZY020713);

引用：

WANG Song, ZHANG Peng, XI Yanbin, et al. Study on the effect of culturing floating raft on water exchange capacity in Haizhou Bay [J]. Water Resources and Hydropower Engineering, 2021, 52(10): 109- 120.

王松, 张鹏, 席彦彬, 等. 养殖浮筏对海州湾水交换能力影响的研究(英文)[J]. 水利水电技术(中英文), 2021, 52(10): 109- 120.

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0 Introduction

Haizhou Bay is located in the western semi-open sea of the South Yellow Sea in China, with an area of approximately 820 km2 (see Fig.1). The water quality in the bay is fertile, bait organisms are rich, which provides a good growth environment for cultivating laver in Haizhou Bay. Haizhou Bay is the largest laver culture base in Asia, where the laver cultured area accounts for approximately one-third of the cultured area of China. In the laver cultivation, a culturing floating raft is used and the raft significantly influences the hydrodynamic current and water exchange capacity of the cultured area. The hydrodynamic environment, as an important natural condition in the cultured area, is related to the transport and distribution of nutrients, particulate matter and other substances. The existence of culturing floating rafts has hampered the hydrodynamic environment, which is the basic and key condition for laver culture. Therefore, it is of great significance to study the influence of culturing floating rafts on hydrodynamic current and water exchange capacity.

图1 计算区域的地理位置,浮筏区域,潮汐及潮流验证点

Research has been conducted to investigate the effect of laver culture facilities on the hydrodynamic field and water exchange capacity. First, the change in flow with and without culturing facilities has been observed. Waite et al. found that water movement through a mussel farm in New Zealand was attenuated to approximately 30% of the flow around the farm. In one of the main Chinese cultured sites (Sungo Bay), Zhao et al. suggested that there has been a 50% decrease in the flow velocity over 11 years of increasing mariculture development. Baba et al. measured the velocity around a laver culture facility and found that the velocity was reduced in the compartment of the facility. Lin et al. studied the hydrodynamic effect of a mussel suspended culture farm on tidal currents off Gouqi Island, outside the Changjiang Estuary, China, by using observational data. They founded that tidal current velocity was reduced by 75%-90% in the top layer and by approximately 45% at the bottom. According to Newell and Richardson, drag caused by culturing ropes and supporting infra-structure accounted for 75%-80% of flow attenuations. The decrease in the flow velocity will also have a certain impact on the organisms in the farming area. Navarro et al. found that the position of a floating raft in the estuary of Ria de Arosa caused significant differences in the feeding behavior of mussels, and the growth rate of mussels in the front end of the floating raft was always higher than that in the back end. Pilditch et al. observed that suspended scallop culturing currents were reduced by 40% of the ambient flow. Strohmeier et al. found that the average velocity of 30 m in a cultured area was 30% lower than that of the outside. The study showed that lower current speeds depleted food (Chl a) and lower meat content in a longline mussel farm. Seston depletion had been recorded in the water overlaying natural beds of filter-feeding bivalves and mussel rafts.

Second, the drag caused by culturing facilities was introduced to a numerical model to study the influence of culturing facilities on the hydrodynamic field and water exchange capacity. Duarte et al. conducted a three-dimensional (3 D) hydrodynamic numerical simulation of a raft mussel cultured at the mouth of the Ria de Ares-Betanzos estuary in Iberia and found that the substantial reduction of the subtidal circulation of the Ria de Ares-Betanzos was due to the presence of culturing facilities. A reduction in flow would negatively affect the carrying capacity of mussel farms and would favor the occurrence of harmful microalgae that proliferates in stratified slowly moving environments. Modeling results by Boyd and Heasman showed that the flow velocity in the cultured area decreased by 28% in Saldanha Bay, South Africa. Plew used a numerical model to consider the effects of longline mussel farms on tidal currents within two embayments in Pelorus Sound, New Zealand. The longline farms were parameterized in the model by modifying the bed friction coefficient to account for drag from the farm structures. Using Sungo Bay in China as an example, Grant and Bacher constructed a finite element circulation model to assess the magnitude of friction due to the cultured structure and found that suspended cultures result in a 54% reduction in the flow rate in the middle of the cultured area and 41% reduction in the water exchange rate. Shi et al. extended the work of Grant and Bacher in Sungo Bay to three dimensions and adopted a physical-biological coupled model to study the growth rates of cultured kelp farms. The simulation results showed that the culturing floating raft reduced the average surface current speed by 40% and reduced the water exchange capacity with the open sea.

Then, through field investigation and model simulation, the influence of culturing facilities of different cultured areas on tidal current fields was studied. On the basis of field measurements and simulation results, Jackson and Winant measured the obstructing effect of a kelp cultured area on tidal current and found that the current speed in the cultured area was approximately one-third of the outside current; the modeling results confirmed that the internal current decreased by 42%-68%. According to Jackson′s research, Grant et al. estimated that the drag generated by a raft cultured in mussel in Saldanha Bay of Benguela was approximately 30 times that without raft farming using the square of velocity damping. O′Donncha et al. used numerical modeling to investigate the viability of suspended cultured farms. On this basis, they also conducted field observation, and the observations demonstrated that flow speeds within the longline were reduced by 25%-30% from ambient, and material transport to downstream droppers was significantly reduced. These results suggest that neglecting the physical barrier imposed by culturing installations would result in a considerable overestimation of nutrient supply to the bivalve and thus an overestimation of carrying capacity. Currents measured in the vicinity of longlines iNew n Zealand showed a reduction in an ambient flow of 30% in . H. Yagi et al. performed numerical simulation of the hydrodynamic environment and suspended particulate matter dynamics in the local area around culturing facilities and bay scale area and conducted field experiments to measure the hydraulic resistance and velocity distribution in the compartment of the culturing facilities. Newell and Richardson combined field measurements and three-dimensional numerical simulations to develop guidelines for minimum ambient flows to provide optimal clearance rates for mussel growth. Fan et al. combined measured and numerical simulation methods to discuss the vertical structure of tidal current in a typical coastal raft cultured area. They observed that the vertical structure of seawater had changed significantly. The surface drag and bottom drag of culturing floating rafts were much higher than those of non-raft-cultured areas, and they changed significantly with the tidal current. Modeling results indicated that the surface current velocity was greatly damped because of the drag of culturing floating raft.

So far, researchers have studied the influence of culturing facilities on hydrodynamics through field observation and numerical simulation methods, but the influence of culturing facilities on water exchange capacity in culturing areas has not been carried out. And the water exchange capacity is of great importance to the breeding biology at the culturing floating raft area. Therefore this paper takes Haizhou Bay as an example to discuss the influence of culturing floating raft on water exchange capacity. This study developed a dual-drag model of culturing floating rafts on the basis of the hydrodynamic model and discussed the influence of culturing floating rafts on the hydrodynamic conditions and water exchange capacity of Haizhou Bay.

1 Materials and Methods

1.1 Construction and verification of the hydrodynamic model

The hydrodynamic model of the calculation area was established and verified to provide the hydrodynamic conditions for culturing floating rafts. The hydrodynamic component was based on the Finite Volume Coastal Ocean Model (FVCOM). The σ-coordinate transformation was used on the vertical axis to obtain a smooth representation of irregular bottom topography. The σ-coordinate transformation is defined as

where σ varies from -1 at the bottom to 0 at the surface. In this coordinate, the governing equations consist of the following continuity, momentum, temperature, salinity, and state equations (see Eq. (2)-(11)). The definite conditions consist of the free surface boundary condition, the seabed boundary condition, and the lateral solid boundary condition (see Eq. (12)-(15)). Each variable is described in Table 1.

We set calculation domain for Haizhou Bay in the South Yellow Sea of China (119.18°—120.27° E; 34.4°—35.78° N) according to the cultured floating raft area. The calculation domain was discretized by unstructured triangular meshes and the finite volume method based on space. We set the spatial resolution of the shoreline as 0.005° and that of the open boundary as 0.03°. The entire computing region consists of 28,755 grids and 14,843 nodes (see Fig.2). Since the water depth in the study area was relatively shallow, with an average water depth of less than 20 m, 18 layers were vertically divided to effectively describe the variation of velocity between layers. The water depth data were collected from the ETOPO1 Global Relief Model data, and the water depth at the nearshore area was adjusted using the chart published by the ministry of maritime assurance of China. We imposed the open boundary based on water level, considering eight foremost astronomical tides. The horizontal eddy viscosity was calculated using the Smagorinsky parameterized turbulent closure model, which was set as a constant value of 0.2. The vertical turbulent vortex viscosity coefficient was calculated using the Mellor-Yamada turbulence closed model of order 2.5, which was set as a constant value of 0.000 1 in this model. We adopted an internal and external mode splitting algorithm to improve computational efficiency. According to the stability standard of gravity wave Courant-Friedrich-Levy conditions (CFL), we set the time step of the external model as 1 s and the time step of the internal model as 10 s to ensure the stability of numerical calculation results. We verified the model by the measured data, which was showed in .

图2 计算区域的地形和网格

1.2 Dual-drag model

Because of the existence of culturing floating rafts in Haizhou Bay, the hydrodynamic structure of the sea area was different from that of other sea areas, and related studies showed that the drag formed by culturing facilities was greater than the effect of bottom friction, which severely changes the hydrodynamic structure of the cultured area, especially that the influence on the surface velocity was particularly significant. Given the differences in the hydrodynamic characteristics of other sea areas, we decided to investigate the influence of cultured areas on the hydrodynamic field using a hydrodynamic model with two types of drags. One drag was the surface stress due to culturing floating rafts. It depends not only on the current speed but also on the size of culturing facilities. The other was the bottom friction of the sea bed, which depends on the seabed topography. The drag coefficients CDS and CDB were, respectively, used to represent the drag of the surface boundary layer and the bottom boundary layer. Research has shown that the current speed distributions of the surface boundary layer and bottom boundary layer were similar. The logarithmic law of the wall approximately fitted the flow structures in both the surface boundary layer and the bottom boundary layer in.

Bottom stress τB in coastal ocean models is typically parameterized with a quadratic relationship as

where ρ represents the seawater density, CDB represents the bottom drag coefficient which is [κ/ln(z/z0B)]2, and UB represents the bottom horizontal current velocity and is defined by

where z represents the vertical length of the water column from the seabed, z0B represents the bottom rough length, κ=0.4 is the Von Karman constant, and u*B represents the bottom friction velocity, which is

The surface drag of culturing floating rafts is similar to that at the bottom. This has been confirmed in previous studies. Therefore, the surface stress τS in the ocean can be parameterized as

where CDS= [κ/ln(z/z0S)]2 is the surface drag coefficient and US represents the surface horizontal current velocity. Since the surface friction velocity isu*S=|τS|/ρ−−−−−√, we can apply the logarithmic law to the surface water to obtain.

where z0S represents the surface hydraulic roughness, and u*S represents the surface friction velocity.

The floating raft laver cultivation in Haizhou Bay is mainly composed of two parts: raft frame and net curtain. The raft frame is fixed by cable and insert rod. The cable is about 20 m long, the insert rod is 7～15 m long and the diameter is 6～9 cm. The net curtain is hung horizontally in the middle of two raft racks and fixed on the sea surface. The length of the net curtain is about 9 m and the width is about 1.6 m. One hectare of sea surface set up 180 floating rafts.

The surface drag coefficient is mainly based to the research results of H Yagi et al. They pointed out that the current speed of the research area was approximately 0.32 m/s, and the value of the drag coefficient was 0.02. In this study, according to the velocity of the floating raft of the cultured area in Haizhou Bay, the drag coefficient was selected as 0.01.

2 Results and Discussion

2.1 Effect of culturing floating raft on hydrodynamic condition

On the basis of the verification of tidal level and current, the tidal current field in Haizhou Bay was simulated. The tidal current in Haizhou Bay was dominated by reciprocating currents. At flooding tide, the tide flow from northeast and east into Haizhou Bay. At ebb tide, the current direction is opposite.

Fig.3 shows the velocity distributions of the surface layer, middle layer, and bottom layer without or with floating raft drag during the spring tide in Haizhou Bay. Without the influence of the culturing floating raft, the average velocity of surface water was approximately 0.43 m/s, that of middle water was approximately 0.34 m/s, and that of bottom water was approximately 0.21 m/s. Because of the existence of floating rafts, the flow velocity from the surface to the bottom of the cultured area decreased to varying degrees, and the surface velocity decreased significantly. From the outside culture area to the cultured area, the surface velocity was reduced by 10%-50%, with an average flow velocity of approximately 0.30 m/s. The attenuation of the flow velocity in the middle and bottom layers was not obvious, and the flow velocitywas reduced by 5%-20%. On the contrary, affected by the resistance of the culturing breeding raft, the velocity outside the cultured area increased from 10% to 35%.

图 3 海州湾有无浮筏时表层、 中层和底层流速(左: 无浮筏; 右: 有浮筏)

2.2 Influence of culturing floating raft on tide volume

Tide volume is the amount of tidal water that the bay may accept under the condition of average tidal range, and the calculation formula of the tide volume is

where P represents the tide volume, Si represents the area of the i grid, H1i represents the high water level, H2i represents the low water level, and n represents the total number of grids for selected sea areas. Fig.4 shows the area for the tide volume calculation.

图4 海州湾纳潮量计算区域

In this region, the variation of tide volume of spring, middle, and neap tides in Haizhou Bay without and with culturing floating rafts was calculated, respectively. Table 2 shows the calculation results. Because of the obstructing effect of floating rafts, the tide volume decreased, with the most at spring tide, which was 0.21×108 m3.

2.3 Effect of culturing floating raft on water exchange

On the basis of the hydrodynamic model, the DYE tracer model was coupled to study the water exchange capacity. Fig.5 shows that the tracer concentration at 25, 50, 75, and 100 days corresponded to the change in the conditions without or with a culturing floating raft. In the absence of a culturing floating raft, the decline rate of concentration in Haizhou Bay was significantly different for varying distances from the bay mouth section; even at the same distance, the decline rate of concentration in different regions was also different.

图5 示踪剂浓度水平分布的演化(左：无浮筏；右：有浮筏)

图5 示踪剂浓度水平分布的演化(左：无浮筏；右：有浮筏)

In the absence of a floating raft, the low-concentration seawater from the open sea diffused from the bay mouth to the bay at a faster speed from 0 to 25 days, and the diffusion rate in the center of the bay was faster than that on both sides. From 25 to 50 days, the low-concentration seawater in the northwest part of Haizhou Bay diffused rapidly into the bay, but the diffusion in the southwest direction was slower, and the diffusion range was smaller than that in the northwest part. From 50 to 75 days, the low-concentration seawater continued to diffuse into the bay, but the concentration of seawater in the southwest was still high. The high concentration in this area was mainly caused by the existence of a counterclockwise residual eddy, where many conservative tracer materials were restricted, resulting in a slow diffusion rate and a high concentration. From 75 to 100 days, except for a high concentration area in the southwest of the gulf, most of the sea areas were involved in water exchange. In the case of floating rafts, the concentration distributions of 0-25 days were the same as those without floating rafts. From 25 to 50 days, the concentration distributions in the middle part of the bay accelerated rapidly, but the diffusion of the two sides of the bay was relatively slow, and the concentration of water at the top of the bay was higher than that without a floating raft. From 50 to 100 days, the concentration distributions were the same as those without floating rafts, but the concentration in the southwest of the bay was higher than that without a floating raft.

Fig.6 shows the semi-exchange time distribution of seawater in Haizhou Bay without or with floating rafts. In the absence of floating rafts, the exchange time at the bay mouth was the shortest, and the exchange time in the northern part of the bay was less than that in the southern part of the bay. In the waters near the bay mouth, the semi-exchange time in most areas was less than 30 days, and the semi-exchange time in the northwestern part of the bay was 50 to 70 days, whereas the semi-exchange times at the top of the bay and in the southern part of the bay were more than 110 days. The shorter the distance to the bay mouth, the shorter the residence time, and at the same distance from the bay mouth, the northern part of the bay was shorter than the southern part of the Bay. In the case in which there are floating rafts, the distribution of semi-exchange time was changed. The area at the bay mouth was still the shortest semi-exchange time area, but near the top of the bay, the semi-exchange time became longer, from the original 40～70 days to 50～80 days, and the semi-exchange time in the lower part of the bay was significantly increased, the average semi-exchange time of the bay was increased by approximately 20%, and the water exchange capacity was weakened.

图6 半交换时间分布

2.4 Effect of changing the cultured layout on water exchange

To promote the water exchange capacity, numerical experiments have been conducted, where three schemes were arranged in the cultured floating raft area. In these schemes, the waterway was made by moving the floating raft (see Fig.7), and the water exchange capacity was studied, which is shown in Table 3. This area is a tidal current dominating area and the tide and the tidal current is reciprocating flow. Therefore, the direction consistent with the flow direction of the main tide was selected as the main direction of waterway. The culturing floating raft in the sea area is composed of two parts: the north and the south. In the design of the waterway, factors such as setting it in different areas and the distance between waterways and the outer sea were considered. Meanwhile, due to the weak water exchange time in the southern floating raft area, two waterways were set up.In Scheme 1, the semi-exchange time decreased from 45 to 44 days, shortening 2 tidal cycles. In Scheme 2, the semi-exchange time decreased from 45 to 43 days, shortening 4 tidal cycles. In Scheme 3, the semi-exchange time was reduced from 45 to 41 days, shortening 8 tidal cycles. Under the three schemes, semi-exchange time was shortened, and that of Scheme 3 was more obvious. Therefore, along with the actual situation, a reasonable layout of culturing floating rafts could make the water exchange with the sea faster, thereby promoting the exchange capacity of the bay.

图7 水流通道布置方案

3 Conclusions

A hydrodynamic numerical model with floating raft drag based on the FVCOM was developed to study the influence of floating rafts on tidal current fields, tide volume, and water exchange in Haizhou Bay. The existence of culturing floating rafts in the bay decreased the tidal current velocity and water exchange rate. The floating raft drag had a wide influence on the entire sea area. In the cultured area, the surface velocity was reduced by 10%-50%, and the flow velocities in the middle and bottom layers were reduced by 5%-20%. The flow velocity outside the cultured area increased by 10%-35%. The presence of culturing floating rafts reduced the tide volume, and the reductions were 0.21×108, 0.13×108, and 0.04×108 m3 during spring, middle, and neap tides, respectively. Compared with before large-scale culturing, the semi-exchange period of seawater was extended by 16 tidal cycles, and the average semi-exchange time was extended by 20%.

Finally, numerical experiments were conducted to discuss the contribution of setting up waterways in the cultured floating raft area to improve the water exchange capacity of the bay, which showed that a reasonable layout of culturing floating rafts could improve the water exchange capacity.

Raft culture is one of the three main forms of mariculture. The layout of floating raft has a great influence on the hydrodynamic conditions and water exchange capacity of the culture area. The results of this paper can provide reference for studying the flow blocking effect of culture facilities in other areas, and the waterway scheme proposed in this paper can also provide ideas for improving the water exchange capacity of existing culture floating areas.

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水利水电技术（中英文）

水利部《水利水电技术（中英文）》杂志是中国水利水电行业的综合性技术期刊（月刊），为全国中文核心期刊，面向国内外公开发行。本刊以介绍我国水资源的开发、利用、治理、配置、节约和保护，以及水利水电工程的勘测、设计、施工、运行管理和科学研究等方面的技术经验为主，同时也报道国外的先进技术。期刊主要栏目有:水文水资源、水工建筑、工程施工、工程基础、水力学、机电技术、泥沙研究、水环境与水生态、运行管理、试验研究、工程地质、金属结构、水利经济、水利规划、防汛抗旱、建设管理、新能源、城市水利、农村水利、水土保持、水库移民、水利现代化、国际水利等。

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