河道弯曲度对改善城市中小河流生态环境的机理研究

River ecological restoration is one of hotspot issues in the research field of water environment and ecological water conservancy. Due to low flow, water pollution and channelization of small-medium city rivers, the health and ecological degradation problems of these rivers become much more prominent. Meandering of river shapes is a new restoration method of river topography and geomorphology. However, the water and sand movements, water quality improvement and river bed evolution with the actions of secondary flow and transverse surface slope in meandering flows are not yet fully known. Considering the meandering of river channel can not only restore the topographic features, but also improve the water quality and benthos habitat diversity in a river, in this research project, we plan to conduct following studies: (1) field investigation at river reaches with continuous meandering of a city river about its hydraulic conditions, water quality, macrozoobenthos and its habitat; (2) flow structure and river bed evolution in the meandering rivers; (3) influences of meandering flow on water quality and (4) the effectiveness of meandering degrees on the habitat quality of the macrozoobenthos. Through comprehensive analyses, we will reveal the mechanism of improving the water quality and habitat quality of zoobenthos in small-medium city rivers by different meandering degrees, and furthermore establish an assessment methodological system for improving water environment and ecology, which can provide theoretical and technical supports for improving water environment and water ecology of small-medium city rivers.

河流生态修复是水环境与生态水利研究中的热点问题之一。其中，城市中小河流因流量小、污染严重且大多渠化而面临的生态系统结构和功能退化问题尤为突出。河流形态弯曲化正逐渐成为河流地貌修复的一种新方法，然而，弯道水流在复杂二次环流及横比降共同作用下的水沙运动、水质改善、河床演变规律和机理却不成熟。鉴于此，本研究从弯曲河道既可实现河流地貌修复，又存在可利用其复杂水动力条件来改善河流水质和底栖生物生境条件的思路出发，开展如下研究：（1）城市连续河湾现场的水力学条件、水质、底栖动物及生境调查；(2）弯曲河道内水流结构与河床形态演变规律；（3）弯道水流对水质的影响机制；（4）不同弯曲度对底栖动物生境质量的改善机制。最终，揭示出河道不同弯曲度对改善城市中小河流的水质和水生生物生境条件的机理，并建立河道弯曲度对河流生态环境影响的评价体系，为改善城市中小河流的生态环境提供理论和技术支撑。

河流形态的弯曲化正成为河流生态修复的一种新方法。然而，弯道对水沙运动、水质改善、河床演变的机理却不成熟。为此，根据项目计划书，首先以岷江支流府河流经成都市华阳镇金子沱的弯曲河段为研究背景，开展了水动力、水质、底栖动物及生境调查，并构建了以底栖动物为指示物种的栖息地适宜性模型。. 其次，利用三维RNGκ～ε紊流模型和VOF自由面模型，系统研究了不同弯曲角对水流结构的影响，得了一些有价值的研究成果：（1）最大横比降出现在弯道中部，且会随着弯曲角的增加而增大；（2）弯道二次环流除形成一个顺时针方向的主涡之外，还会在靠近凹岸的水体上部形成一个逆时针方向的次涡，该次涡具有保护凹岸免受冲刷破坏的作用，且会随弯曲角的增大逐渐扩展并靠近凸岸；（3）弯道凹岸会形成一个低应力区，凸岸则会形成一个高应力区，且其尺寸均随弯曲角的增加而增大；（4）凸岸的最大剪切应力会随弯曲角的增大呈非线性增加，而在凹岸和河床底部的最大剪切应力则出现在120度弯道；（5）提出了凹岸次涡形成的判别条件。. 再构建了二维正交曲线坐标的水沙、河床演变、水质与栖息地模拟的数值模型，系统探讨了6种单一弯道和四种组合弯道对水沙、河床演变、水质与栖息地的改善效果，结果表明：（1）弯道的弯曲度越大，越能改善河道内的水质和增加栖息地加权可利用面积（WUA）；（2）顺直河道在高流量中几乎没有适合的底栖动物栖息地；弯道却能通过水动力、泥沙改善河床形态和水质，为底栖动物营造出适合其生存的避难生境，且弯曲度越大，栖息地的提升效果越好；（3）连续弯道对栖息地的改善效果比单一弯道更好，且随着流量的增高，连续弯道内的WUA相比顺直河道提高的倍数也越多。. 这些成果可为城市中小河流的水质和生境改善、水生态修复与评价提供一定的理论和技术支撑。.