NSFGEO-NERC: The Origin of Aeolian Dunes (TOAD)

风沙沙丘占据了地球表面的10%，既包括广阔的沙漠沙海，也是抵御沿海洪水的重要天然屏障。虽然影响成年沙丘形状、移动和模式的环境条件已经被广泛研究，但在我们对这些地貌的理解中，最持久的缺陷也是最深刻的：风吹沙丘是如何启动的？启动是理解沙丘作为主要地质单元的核心，包括这些景观对气候驱动因素、环境变化和社会影响的反应。沙丘启动对于更广泛地了解风沙系统及其背景具有重要意义，而外星沙丘场的发现为沙丘启动提供了最近的动力，这确保了启动问题在整个沙漠研究历史上一直是突出的。尽管如此，现有的解释沙丘起源过程的想法在很大程度上仍然是描述性的和未经证实的。关于沙丘形成的问题之所以持续存在，并不是因为没有认识到它的重要性，而是因为缺乏解决这一根本问题的手段。充分了解沙丘形成的关键障碍是，到目前为止，还不可能以超高的空间和时间分辨率测量必要的变量，以探测地表条件和风沙输送的小范围变化。地球科学的最新技术进步激发并支持了这一提议，因为它们现在提供了满足过程测量的苛刻要求的机会。克服沙丘启动这一长期存在的问题需要在研究设计中采用新的方法，我们的建议通过在实地工作和物理模型之间建立互补联系来解决小规模测量问题，以及通过计算机建模扩大详细过程发现的应用范围的能力。具体地说，这项提议将首次研究气流、表面特性、沙粒输送的变化和河床形状之间的关键相互关系，这些关系有助于对新生沙丘如何形成有意义的理解。通过现场监测极高分辨率的表面特性和床形变化，将实现对控制过程和床形发育的全面测量。实地考察工作的一个重要创新之处在于，它将在已知沙丘发育的三个精心挑选的地点进行，每个地点代表沙丘形成的三个不同驱动因素：表面粗糙度、表面水分和沙床不稳定。现场工作将为在定制的实验室水槽中进行的实验提供信息，该水槽旨在使用最先进的成像技术准确地描述非常接近模拟沙丘表面的流动。我们的野外和实验室数据集将被用来驱动一个计算机模型，然后我们将运行该模型来测试沙丘的形成和增长对沙漠、海岸和其他行星上一系列环境条件下不同控制措施的敏感性。我们的建议建立在以必要的特殊细节水平进行现场观测的新能力的基础上，并通过紧密耦合的最先进的实验室流动模拟，以及基于证据的建模的开发和应用来检查沙丘启动的驱动因素。总而言之，这种方法代表着一个前所未有的机会，可以克服真正持久的高原，了解一个主要陆地地貌系统的起源。

Aeolian (wind-blown) sand dunes occupy 10% of the Earth's surface, both in vast desert sand seas and as important natural defences against flooding along coasts. While the environmental conditions that influence the shape, movement and patterns of fully grown dunes have been extensively studied, arguably the most enduring deficiency in our understanding of these landforms is also the most profound: how do wind-blown dunes initiate?Initiation is central to understanding dunes as major geological units, including the response of these landscapes to climatic drivers, environmental change and societal impact. The significance of dune initiation for the wider understanding of wind-blown sandy systems and their contexts, for which the discovery of extra-terrestrial dune fields has added a recent impetus, ensures that the question of initiation has remained prominent throughout the history of desert research. Despite this, existing ideas proposed to explain processes of dune origin have remained largely descriptive and uncorroborated. The persistence of the question regarding dune initiation is not due to an absence of appreciation of its importance but, rather, a lack of the means to tackle this fundamental issue. The critical obstacle to a fully developed understanding of dune initiation is that, until now, measurement of the necessary variables, at the ultra-high spatial and temporal resolutions required to detect small-scale variations in surface conditions and wind-blown sand transport, has been impossible. Recent technological advances in the geosciences both inspire and underpin this proposal, as they now provide the opportunity to meet the demanding requirements of process measurement. Surmounting the abiding problem of dune initiation requires novel approaches in research design and our proposal tackles the issues of measurement at small scales by forging complementary links between fieldwork and physical modelling, as well as an ability to widen the application of detailed process findings through computer modelling. Specifically, this proposal will for the first time examine the key inter-relationships between airflow, surface properties, changes in sand transport and bedform shape that lie behind a meaningful understanding of how nascent dunes emerge. Full measurement of controlling processes and bedform development will be achieved through field monitoring of surface properties and bedform change at extremely high resolution. A key novelty of the fieldwork is that it will be carried out at three carefully chosen locations of known dune development, with each location representing the 'type site' for three different drivers of dune initiation; surface roughness, surface moisture and sand bed instability. The fieldwork will inform experiments undertaken in a bespoke laboratory flume that is designed to enable accurate characterisation of flow very close to the 3D surface of modelled dunes using state-of-the-art imaging techniques. Our field and laboratory dataset will be used to drive a computer model that we will then run to test the sensitivity of dune initiation and growth to different controls in a range of environmental conditions in deserts, coasts and on other planets. Our proposal is built on a new capability to make field observations at the requisite exceptional levels of detail, augmented by closely coupled state-of-the-art laboratory flow simulations, plus the development and application of evidence-based modelling to examine drivers of dune initiation. In concert, this approach represents an unprecedented opportunity to overcome a truly enduring plateau for understanding the origins of one of the major terrestrial landform systems.