人工坝体对长江上游鱼类栖息地流域水动力学特性的影响

长江上游鱼类栖息地的水动力学研究对于栖息地生态治理与恢复、评估水生物的生境条件有重要科学意义。该研究以长江上游四川省泸州市江段人工修复鱼类栖息地为研究对象,根据现场施工图、水下地形测绘图和航拍图,等比例建立了三维流场模型。基于CFD数值模拟结合现场试验勘测,研究了6种不同流速下的栖息地水动力学特性,分析了多个人工坝体对该江段水流环境的影响机制、湍流/涡结构的演化与发展规律,利用了水流均匀性指数、傅汝德数、局部断面流速等水力指标对修复后的栖息地水环境进行评估,并与现场地形和流速测绘、鱼群调查等试验数据对比分析。研究表明:在该江段试验区形成多个大漩涡区,涡流形态基本不随来流速度的变化而改变,逆时针漩涡区的上升流现象明显,且流速稍高于顺时针漩涡区;试验区的水流速度偏低,流动均匀性差,流态紊乱复杂,而对照区的流速相对较快,流动均匀性好,水流方向变化不明显;随来流速度增加,傅汝德数不完全呈线性增加趋势,不同水深截面上的水流均匀性指数的变化趋势相近;现场鱼群调查相同测绘距离内,对照区域为0.714尾/1000呯,试验区域为3.157尾/1000呯,说明流动不均匀的低速涡流区有利于聚集鱼群,经修复后的栖息地能够为水生物提供良好的栖息、繁衍、庇护等生存条件;当来流速度为2 m/s时,试验区域两条测试路径的平均流速分别为0.53和0.79 m/s,在同一鱼群探测试验测绘路径上,数值模拟结果与现场试验数据相差约10%,验证了数值计算方法的可靠性。研究结果对于大水面渔业生态工程的水动力学研究有重要的参考意义。

Effects of artificial dams on hydrodynamic characteristics of fish habitats in upper reaches of Yangtze River

Habitat of fish species has been an essential component of riverine ecosystems in natural rivers. However, a series of ecological problems have posed a practical challenge on fish habitats and blocks migration routes, particularly in the upstream reach of the Yangtze River, mostly due to the construction of hydroelectric dams rapidly increased in recent years in China. Therefore, it is necessary to explore the hydrodynamic characteristics of fish habitats for the ecological management and restoration, especially for the rare and unique fish, and thereby to evaluate the habitat conditions of aquatic organisms in the upper reaches of the Yangtze River. In this study, a three-dimensional model of flow field was established, corresponding to the artificial restoration of rare fish habitats in the river section of Yangtze River upstream, in Luzhou city, Sichuan province, China. The hydrodynamic characteristics of different flow velocities were investigated, based on the Computational Fluid Dynamics (CFD) numerical simulation combined with an experimental survey. The evolution and development of the turbulence/vortex structure were analyzed, in order to obtain the influence mechanisms of several artificial dams on the abdominal flow law of the basin, compared with the experimental data, including the site topography, flow mapping of the habitats, and the survey of fish shoal. The results showed that the large vortex areas were formed in the test area of the habitat basin, where the shape of vortex was almost invariant with the change of the stream speed. The upwelling occurred obviously in the anticlockwise vortex region, of which the velocity was slightly higher than that in the clockwise vortex region. The flow velocity of the habitat basin in the test area is lower than the control area, but the uniformity of water flow is baddish. The Froude number does not increase linearly with the inflow velocity, and the variation trends of flow uniformity index on different water depth sections of the habitat basin are similar. By the field survey of fish shoal in the same mapping distance, the area outside the dam was 0.714 in 1 000 scanning times, while, the area inside the dam was 3.157 in 1 000 scanning times, revealing that the low-velocity turbulence/vortex structure in the test area of the habitat basin can gather fish better, and thereby the artificial restoration of fish habitat can provide suitable living conditions for aquatic organisms to roost, breed, and shelter. Specifically, the average flow velocity in the abdomen of the basin was about 0.5-0.8 m/s, when the flow velocity was 2 m/s. The difference between the numerical simulation results and the test data is about 10% on the same mapping path, which verify the reliability of the numerical calculation method. The findings can offer a sound theoretical reference for the hydrodynamics of large-water fishery ecological projects.