Secondary airflow patterns under offshore winds over coastal foredunes: implications for aeolian sediment transport

沿海沙丘开发的一个被引用的标准是陆上风。因此，在以近海风为主的海岸上存在大型沙丘系统是很难解释的，通常情况下，它们被归因于过去发生的陆上风，从而暗示着随后的气候变化。最近的研究表明，近海的风能可以被现有的沙丘转向或“操纵”，从而改变它们的方向。这可能会发生到这样的程度，即可能会出现一种被称为流动反转的过程，即最初离岸的风实际上是在海滩上流动的。这个过程很重要，因为它会导致沙子从海滩吹到沙丘里，导致沙子生长。这可能是解释海岸上存在大片沙丘的核心原因，那里的主要风向是近海，但对于沙丘在风暴期间经历海浪侵蚀后如何恢复也很重要。传统上，沿海沙丘的沉积物平衡计算不包括近海风能，但如果它们确实将沙子输送到岸上，这可能是一个重要的疏忽，导致低估了风能输送的沙量。申请者最近的工作首次能够测量(而不是简单地推断)与离岸定向气流的局部地形操纵有关的向陆地的风沙(风吹)泥沙输送。在这项提案中，我们打算研究对这一过程的控制(例如，发生水流逆转的沙丘形状和风速)以及水流变形和由此产生的泥沙输送所涉及的机制。一个最终目标是量化离岸风对森林沙丘发育和行为的作用。我们将使用风和泥沙输送的现场测量与最先进的空气动力学模型相结合。我们的工作假设是，离岸风对背风海岸的沙丘行为有很大贡献。我们建议在一个理想的野外场地-北爱尔兰的Magilligan Strand-验证这一假设，这是一个以离岸风为主的海岸，具有各种沙丘地形、分选良好、均匀的沙子，以及处于安全的军事区的好处。它还有一个额外的好处，那就是有一个已建立的气象站，而且很容易到达，以便在出现合适的风力条件时快速部署设备。根据国家环境研究中心的战略，这项建议将工程学和环境科学的新方法(模拟方法是为雪堆而开发的)的独特组合结合在一起，以解决背风海岸风沙沙地的起源和地形动力学这一重要问题。我们的研究方法首次涉及将三维计算流体动力学(CFD)模型应用于天然沿海沙丘，并使用研究人员开发的设备进行了一系列高频泥沙输送和气流测量。这项研究的一个特别新颖的组成部分是，该模型将被用来指导部署外地仪器，以便最佳地收集数据，然后这些实地数据将被用来修改和改进模型模拟。研究小组结合了建模、高分辨率现场测量和风沙输运物理方面的必要专业知识，以实现对研究的综合方法。由于所研究的自然环境的复杂性，调查必然是设备密集型的。然而，设备阵列的高时间分辨率和紧密的空间部署将使我们能够收集具有足够高分辨率的史无前例的现场数据集，以便能够分析湍流区和描述内部边界层动力学，这对于了解近海风下的沉积物传输至关重要。

One of the cited criteria for development of coastal sand dunes is onshore wind. The presence of large sand dune systems on coasts where the predominant wind blows offshore is therefore difficult to explain and usually they are attributed to the past occurrence of onshore winds and, by implication, subsequent changes in climate. Recent studies have shown that offshore winds can be deflected or 'steered' by existing dunes so that their direction changes. This can occur to such an extent that a process known as 'flow reversal' can arise, whereby the initially offshore wind actually flows onshore at the beach. This process is important because it can cause sand to be blown from the beach and into the dunes, causing them to grow. This may be central in explaining the presence of extensive dunes on coasts where the dominant wind is offshore, but is also important in how dunes recover after periods of wave erosion during storms. Offshore winds have traditionally been excluded from sediment budget calculations for coastal dunes, but if they do transport sand onshore, this may have been an important oversight leading to underestimates of the volume of sand being transported by wind. Recent work by the applicants has for the first time been able to measure (rather than simply infer) landward aeolian (wind-blown) sediment transport associated with local topographic steering of offshore directed airflow. In this proposal we intend to investigate the controls on this process (for example, the dune shape and wind velocities under which flow reversal occurs) and the mechanisms involved in deformation of the flow and resulting sediment transport. An ultimate goal is to quantify the role of offshore winds on foredune development and behaviour. We will use a combination of field measurement of wind and sediment transport coupled with state-of-the-art aerodynamic modelling. Our working hypothesis is that offshore winds contribute substantially to foredune behaviour on leeside coasts. We propose to test this hypothesis on an ideal field site, Magilligan Strand, Northern Ireland which is a coast dominated by offshore winds and with a variety of foredune topography, well-sorted, uniform sand and the benefit of being in a secure military zone. It has the added benefit of having an established meteorological station and being easily accessible to facilitate rapid equipment deployment in response to the occurrence of suitable wind conditions. In keeping with the NERC strategy, this proposal brings together a unique combination of novel approaches from the engineering and environmental sciences (the modelling approach was developed for snow drift) to address the important question of the origin and morphodynamics of aeolian dunefields on leeside coasts. Our research methodology involves the application, for the first time, of 3-D computational fluid dynamics (CFD) modelling to natural coastal dunes, coupled with an array of high frequency sediment transport and airflow measurements using equipment developed by the researchers. A particularly novel component of the study is that the model will be used to steer the deployment of the field instruments for optimal data gathering and this field data will then be used to modify and improve the model simulations. The research team combines the necessary expertise in modelling, high-resolution field measurement, and the physics of aeolian sediment transport to enable an integrated approach to the research. The investigation is necessarily equipment-intensive due to the complexity of the natural environment being studied. However, the high temporal resolution and close spatial deployment of the equipment array will allow us to collect an unprecedented field dataset that is of sufficiently high resolution to enable analysis of turbulence zones and characterisation of the internal boundary layer dynamics that are essential to understanding sediment transport under offshore winds.