Do floods matter? Bridging the gap between fluvial morphodynamics and alluvial architecture

大多数低地河流流经由洪水期间沉积的沉积物组成的大型洪泛平原。这些洪泛平原及其组成的沉积沉积物具有重要的环境意义。例如，洪泛区沉积物经常被用来推断过去气候变化的性质和时间，或评估人类活动对上游景观干扰的影响。此外，泥沙淤积过程与河流迁移过程之间的平衡决定了泥沙在漫滩的停留时间。这对生物地球化学循环至关重要。碳)以及沉积物相关营养物质和污染物的运输和归宿。数值模型是预测洪泛区随着时间的推移形成和沉积物循环的重要工具，以便在上述各种环境应用的背景下定量了解洪泛区的功能。然而，尽管需要这样的模型，但目前还没有模型能够在与这些应用相关的时间尺度上（几十年到几千年）表示洪泛平原的建设和演变过程。这样做的原因是，河漫滩演化的现实模型需要代表控制沉积物运输和沉积的洪水的复杂行为。然而，要做到这一点，必须求解流体运动方程，这是耗时的（在计算方面）。由于这个原因，现有的洪泛区建设模型忽略了这些水力和水文控制，因此，无法以物理上真实的方式预测洪泛区如何演变。该项目旨在通过开发和评估新一代的水力驱动方法来模拟洪泛区的建设和演变，从而解决这一基本问题。这里开发的模型将适用于长达10万年的时间，但其核心是一个基于物理的水动力学模型，通常仅限于涉及个别洪水的应用。这将通过使用两种方法来减少模型运行时间来实现：(1)使用高性能计算并行化实现代码；(2)开发一系列参数化小尺度洪泛平原地形影响的新方法，使模型能够在较低的网格分辨率下实现，从而大大提高模型效率。这种建模方法的关键优势在于，它将允许使用基于健全流体动力学原理的方法来模拟洪泛区的长期演变。该模型将使用从广泛的自然洪泛区系统收集的野外和遥感数据进行评估，该系统既不受人类活动影响，也不受全新世海平面变化的影响。模式评估将在过去一个世纪和全新世进行。在此之后，该模型将用于开展一系列数值实验，旨在研究洪泛区演化、沉积沉积和环境条件（气候、洪水状况、沉积物供应、洪水盆地几何形状和构造背景）之间的关系。现场证据和模型模拟相结合，将为有关洪泛区功能的问题提供新的定量见解，这些问题由于目前缺乏物理上真实的洪泛区长期演变的水力驱动模型而从未得到解决。

Most lowland rivers flow across large floodplain complexes composed of sediment deposited during floods. These floodplains, and the sedimentary deposits of which they are composed, are of considerable environmental significance. For example, floodplain deposits are regularly used to infer the nature and timing of past climate change, or to assess the impact of upstream landscape disturbance by human activity. Furthermore, the balance between processes of sediment deposition and reworking, due to river migration, determines the residence time of sediment in the floodplain. This is critically important for biogeochemical cycling (eg. of Carbon) and for the transport and fate of sediment associated nutrients and contaminants. Numerical models are important tools that are needed to predict the way that floodplains build up over time and recycle sediment, in order to provide quantitative understanding of floodplain functioning in the context of the diverse environmental applications outlined above. However, despite the need for such models, no model currently exists that is capable of representing the processes involved in the construction and evolution of floodplains over the timescales relevant to these applications (decades to millennia). The reason for this is that realistic models of floodplain evolution need to represent the complex behaviour of the floodwaters that control sediment transport and deposition. However, to do this it is necessary to solve the equations of fluid motion, which is time consuming (in computational terms). For this reason, existing models of floodplain construction neglect these hydraulic and hydrologic controls and, consequently, are unable to predict how floodplains evolve in a way that is physically realistic. This project aims to address this fundamental problem by developing & evaluating a new generation of hydraulically-driven approaches to modelling floodplain construction and evolution. The model developed here will be applicable over periods of up to 100,000 years, yet will have at its core a physically-based hydrodynamic model more usually restricted to applications involving individual floods. This will be achieved by using two approaches to reduce model run times: (1) Parallelising the code for implementation using High Performance Computing; and (2) Developing a series of novel methods of parameterising the effects of fine scale floodplain topography to allow the model to be implemented at reduced grid resolutions, thus substantially increasing model efficiency. The key strength of this modelling approach is that it will allow long-term floodplain evolution to be simulated using an approach underpinned by sound fluid dynamics principles. The model will be evaluated using field and remote sensing data collected from an extensive, natural floodplain system, that is unaffected by either human activity or the effects of Holocene sea level change. Model evaluation will be carried out over the past century and over the Holocene. Following this, the model will be used to conduct a series of numerical experiments designed to investigate the relationships between floodplain evolution, sedimentary deposits and environmental conditions (climate, flood regime, sediment supply, flood basin geometry, & tectonic setting). In combination, field evidence and model simulations will provide new quantitative insight into questions concerning floodplain functioning that have never been addressed due to the current lack of a physically-realistic hydraulically-driven model of long-term floodplain evolution.