Freshwater inputs originating from terrestrial streams and gullies that discharge into quiescent, semi-enclosed coastal regions (such as estuaries, tidal inlets or lagoons), typically provide point sources of nutrients (e.g. nitrates, phosphates) and/or contaminants (e.g. pesticides, pathogens) that may have a deleterious impact on water quality. Many of these sheltered coastal regions also increasingly support aquaculture operations (e.g. finfish, shellfish, or seaweed farms), which can therefore be directly impacted by nutrient and contaminant inputs. Dynamically, these terrestrial freshwater inflows behave as surface buoyant jets or plumes within the coastal saline or brackish receiving waters, due to the salinity-induced density gradients. As such, the presence of infrastructure associated with aquaculture operations in sheltered coastal waters can provide obstruction to the propagation characteristics and residence times for these surface freshwater flows. Consequently, an improved physical understanding of the flow-structure interaction is clearly crucial to assessing the potential contamination risk of aquaculture products. The aim of the current study is therefore to explore, through scaled laboratory experiments within a channel-basin facility, the impact of physical obstruction induced by a vertical grid structure on the flow evolution of a 2D – 3D expanding, surface buoyant jet. Two grid obstructions with different solidity ratios are tested, along with surface gravity currents of different density excesses and freshwater inflows to infer the influence of different parametric conditions on the propagation, blockage and mixing characteristics of the surface current in the vicinity of the grid obstruction. Measurements of the velocity structure and thickness of the expanding surface plume are obtained by ultrasonic velocity profilers, while the density excess in the evolving plume is measured by micro-conductivity probes. Dye visualization results also show that, in the presence of the grid obstruction, the generation of shear-induced billows at the lower interface of the expanding surface current is largely blocked and a local deepening of the fresh-salt water interface in the immediate vicinity of the grid obstruction is observed. In this sense, the obstruction imposed by aquaculture infrastructure in coastal domains can have a considerable influence of the local turbulent mixing and vertical transfer of substances (e.g. nutrients and contaminants), but is likely to have relatively minimal impact in the final dispersion of the surface plume.

中文翻译：

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表面浮力射流与网格障碍物相互作用的实验研究：对水产养殖的影响


源自陆地溪流和沟壑的淡水输入，排入静止的半封闭沿海地区（如河口、潮汐入口或泻湖），通常提供可能对水质产生有害影响的营养物质（如硝酸盐、磷酸盐）和/或污染物（如杀虫剂、病原体）的点源。许多这些避风港沿海地区也越来越多地支持水产养殖作业（例如有鳍鱼、贝类或海藻养殖场），因此这些养殖活动可能会受到营养物质和污染物输入的直接影响。从动态上讲，由于盐度引起的密度梯度，这些陆地淡水流入在沿海咸水或咸水接收水域表现为表面浮力射流或羽流。因此，在避风沿海水域存在与水产养殖作业相关的基础设施，可能会阻碍这些表层淡水流的传播特性和停留时间。因此，提高对流动-结构相互作用的物理理解对于评估水产养殖产品的潜在污染风险显然至关重要。因此，本研究的目的是通过在河道盆地设施内进行大规模实验室实验，探索垂直网格结构引起的物理阻塞对 2D – 3D 膨胀、表面浮力射流演变的影响。测试了两种不同固性比的网格阻塞，以及不同密度超额的表面重力流和淡水流入，以推断不同参数条件对网格阻塞附近表面流的传播、阻塞和混合特性的影响。 通过超声速度剖面仪测量膨胀表面羽流的速度结构和厚度，而演变的羽流中的密度过剩则通过微电导探针测量。染料可视化结果还表明，在网格阻塞存在的情况下，在膨胀的表面流的下界面处产生剪切诱导的波浪在很大程度上被阻止，并且在网格阻塞附近观察到淡水-盐水界面的局部加深。从这个意义上说，沿海地区水产养殖基础设施施加的阻碍可以产生当地湍流混合和物质（如营养物质和污染物）垂直转移的相当大的影响，但对表面羽流的最终扩散的影响可能相对较小。