The Characteristics of Turbulent Flows on Forested Floodplains

这项研究是基于高地水研究集水区（HWRC），一个国家参考网站的低地洪泛平原森林溪流。森林洪泛区代表了大多数温带和热带河流系统的未受干扰的土地覆盖，但它们正受到人力资源管理的威胁。原则上，模拟森林洪泛区流量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;以及物理栖息地的演变。然而，在实践中，森林漫滩流相对较浅，强烈修改的复杂漫滩地形和林地地面上的植被和有机碎片的存在。在这种情况下，流动阻塞和改道是常见的，并且有可能经常从次临界流动切换到局部超临界流动。这种条件也可能有利于通过尾流和剪切产生强烈的湍流。不幸的是，这种复杂性意味着，目前的技术水平是这样的，它根本不清楚什么项支配着控制流动动量方程，或者可能需要什么形式的湍流模型来封闭它们。即使可以进行数值模拟，经验数据的缺乏（森林漫滩流量的复杂性提供了高度的空间变异性，抑制了现场数据收集）否定了模型验证的可能性。为了解决这些问题，该项目将获得高质量的三维流速数据，以进一步了解森林漫滩的流动机制。如前所述，高度的空间变异性阻碍了实地数据的收集。因此，唯一可行的方法是在实验室水槽的受控条件下复制自然洪泛区水流。然而，这提出了新的挑战，复制洪泛区地形和相关的水力学，使水槽实验是一个精确的自然条件模型。我们将通过采用南安普顿的大规模奇尔沃思水槽设施，以实现1：1的液压缩放与地面激光扫描的应用程序，以获得一个自然的洪泛区数字地形模型（DTM）在非常高的分辨率。激光扫描的使用是一项关键的创新，因为产生的DTM数据将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），以生产高精度（<1 mm误差）复制洪泛区，也是1：1比例。然后，PTM可以设置在水槽中，并经受与HWRC研究现场观察到的代表性越岸事件相同的稳定流量排放。水槽内的自然洪泛区形态和洪水水文学的精确复制将使我们能够使用声学多普勒测速仪在密集的3D测量网格上的每个节点处获取空间分布的3D流速数据。然后将分析网格化的流速数据，以便在所有情况下在不同的流动区域内区分动量平衡中的主导项，以及识别湍流产生和消散的机制。这项研究代表了第一次研究湍流的性质在地形复杂的洪泛区。该项目将提供洪泛区流动力学的原始经验模型，该模型将用于为森林洪泛区流动的稳健计算模拟设计提供信息，并提供将向社区提供的基准模型验证数据集。作为一个辅助产品，该项目还将“概念验证”一个新的和可转移的地形建模技术，使复杂的地形表面的精确复制。

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.