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复合体。这些洪泛平原和组成它们的沉积物具有相当重要的环境意义。例如，洪泛平原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的干扰。此外，泥沙淤积和河流迁移改造过程之间的平衡决定了泥沙在河漫滩的停留时间。这对地球化学循环至关重要（例如。以及与沉积物有关的营养物和污染物的迁移和归宿。数值模型是重要的工具，需要预测的方式，洪泛区建立随着时间的推移和再循环沉积物，以提供定量的了解洪泛区功能的背景下，上述各种环境应用。然而，尽管需要这样的模型，目前没有模型存在，能够代表在这些应用程序相关的时间尺度（几十年到几千年）的建设和演变的洪泛区所涉及的过程。其原因是，河漫滩演变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，有必要解决流体运动方程，这是耗时的（在计算方面）。出于这个原因，现有的模型的洪泛区建设忽视这些水力和水文控制，因此，无法预测洪泛区如何演变的方式，是物理现实的。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，以解决这一基本问题。这里开发的模型将适用于长达10万年的时期，但其核心是一个基于物理的流体动力学模型，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）使用高性能计算来简化代码的实现;以及（2）开发一系列新的方法来参数化精细尺度洪泛区地形的影响，以允许模型以降低的网格分辨率来实现，从而大大提高模型效率。这种建模方法的关键优势在于，它将允许使用以健全的流体动力学原理为基础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实地和遥感数据进行评估，该系统不受人类活动或全新世海平面变化的影响。模型评价将在过去的世纪和全新世进行。在此之后，该模型将用于进行一系列数值实验，旨在研究洪泛平原演变，沉积物和环境条件（气候，洪水状况，沉积物供应，洪泛盆地几何形状和构造环境）之间的关系。相结合，现场证据和模型模拟将提供新的定量洞察有关洪泛区功能的问题，从来没有得到解决，由于目前缺乏一个物理现实的水力驱动的模型长期洪泛区演变。

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.