The Characteristics of Turbulent Flows on Forested Floodplains

这项研究是基于高地水研究集水区（HWRC），这是一个关于低地洪泛区森林溪流的国家参考站点。森林泛滥平原代表了大多数温带和热带河流系统未受干扰的土地覆盖，但它们受到人类资源管理的威胁。原则上，模拟森林泛滥平原流动的能力将巩固我们对这些系统中一些关键环境问题的理解，包括：洪水动力学；沉积物、营养物和污染物通量；自然栖息地的进化。然而，在实践中，森林泛滥平原的水流相对较浅，并且受到复杂泛滥平原地形和林地地面植被和有机碎屑的强烈影响。在这种情况下，流动堵塞和改道是常见的，并且有可能频繁地从亚临界流切换到局部超临界流。这样的条件也可能有利于尾流和切变产生强烈的湍流。不幸的是，这种复杂性意味着，目前的技术水平是这样的，我们根本不清楚哪些项主导着控制流动动量方程，或者需要什么样的湍流模型来关闭它们。即使可以进行数值模拟，由于缺乏经验数据（森林泛滥平原流动的复杂性提供了高度的空间变异性，阻碍了实地数据的收集），也就否定了验证模型的可能性。为了解决这些问题，该项目将获取高质量的3D流速数据，以进一步了解森林泛滥平原的流动机制。如前所述，高度的空间变异性抑制了现场数据的收集。因此，唯一可行的方法是在实验室水槽的受控条件下复制洪泛区的自然流动。然而，这提出了新的挑战，即复制洪泛区地形和相关的水力学，以便水槽实验是自然条件的精确模型。我们将利用南安普顿的大型Chilworth水槽设施实现1:1的水力缩放，并结合地面激光扫描的应用，以获得高分辨率的自然洪泛区数字地形模型（DTM）。激光扫描的使用是一个关键的创新，因为所得的DTM数据将被输入到一个创新的制造过程中，该过程能够铣削高密度聚氨酯物理地形模型（PTM），以产生高精度（误差小于1毫米）的复制品洪泛区，也是1:1的比例。然后可以将PTM设置在水槽中，并进行与在HWRC研究现场观察到的具有代表性的河岸事件相同的稳定流量排放。利用声学多普勒测速仪，精确复制天然洪泛平原形态和水槽内的洪水水文，将使我们能够在密集的3D测量网格上获得每个节点的空间分布的3D流速数据。然后对网格化的流速数据进行分析，以便区分动量平衡中的主要项，以及识别湍流产生和消散的机制，在所有情况下，在流动的不同区域内。这项研究首次对地形复杂的洪泛平原上湍流的性质进行了研究。该项目将提供河漫滩流动力学的原始经验模型，该模型将用于为森林河漫滩流动的强大计算模拟设计提供信息，并提供基准模型验证数据集，该数据集将提供给社区。作为辅助产品，该项目还将“概念验证”一种新颖的、可转移的地形建模技术，能够精确复制复杂的地形表面。

This study is based on the Highland Water Research Catchment (HWRC), a national reference site for lowland floodplain forest streams. Forested floodplains represent the undisturbed land cover of most temperate and tropical river systems, but they are under threat from human resource management. In principle, the ability to simulate forested floodplain flows would underpin our understanding of a number of key environmental issues in these systems, including: flood dynamics; sediment, nutrient and pollutant fluxes, and; the evolution of physical habitat. However, in practice forested floodplain flows are relatively shallow and strongly modified by the complex floodplain topography and the presence of vegetation and organic debris on the woodland floor. In such instances flow blockage and diversions are common, and there is the possibility of frequent switches from sub-critical to locally super-critical flow. Such conditions may also favour intense turbulence generation, both by wakes and by shear. Unfortunately this complexity means that the present state of the art is such that it is not at all clear what terms dominate the governing flow momentum equations or what forms of turbulence model might be needed to close them. Even if numerical simulations could be undertaken, a lack of empirical data (the complexity of forested floodplain flows provides a high degree of spatial variability that inhibits field data collection) negates the possibility of model validation. To address these issues, this project will acquire the high-quality 3D flow velocity data needed to further our understanding of the mechanics of flow across forested floodplains. As noted previously, the high degree of spatial variability inhibits field data collection. The only feasible methodological approach, therefore, is to replicate natural floodplain flows within the controlled conditions of a laboratory flume. However, this raises the new challenge of replicating floodplain topography and associated hydraulics so that the flume experiment is a precise model of natural conditions. We will achieve this by employing Southampton's large-scale Chilworth flume facility to achieve 1:1 hydraulic scaling in tandem with the application of terrestrial laser scanning to obtain a natural floodplain Digital Terrain Model (DTM) at very high resolution. The use of laser-scanning is a key innovation as the resulting DTM data will be input to an innovative manufacturing process capable of milling a high-density polyurethane Physical Terrain Model (PTM) to produce a highly precise (<1mm error) replica floodplain, also at 1:1 scale. The PTM can then be set in the flume & subjected to a steady flow discharge identical to a representative over bank event observed at the HWRC study site. Precise replication of the natural floodplain morphology and flood hydrology within the flume will enable us to acquire spatially distributed 3D flow velocity data at each node on a dense 3D measurement grid, using Acoustic Doppler Velocimetery. The gridded flow velocity data will then be analysed to enable discrimination of the dominant terms in the momentum balance, as well as identification of mechanisms of turbulence production & dissipation, in all cases within different regions of the flow. This research represents the first study into the nature of turbulent flows over topographically complex floodplains. The project will deliver an original empirical model of floodplain flow mechanics that will be used to inform the design of robust computational simulations of forested floodplain flows, as well as to provide a benchmark model validation data set that will be made available to the community. As an ancillary product the project will also 'concept-proof' a novel and transferable terrain modelling technique that enables precise replication of complex topographic surfaces.