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

海岸沙丘发展的标准之一是海岸风。因此，很难解释在主要风吹向近海的海岸上存在大型沙丘系统，通常将其归因于过去发生的向岸风，并暗示随后的气候变化。最近的研究表明，离岸风可以被现有的沙丘偏转或“转向”，从而改变方向。这可能发生到这样的程度，即可能出现称为“逆流”的过程，由此最初的离岸风实际上在海滩流向岸上。这一过程很重要，因为它会导致沙子从海滩吹到沙丘上，导致沙丘生长。这可能是解释沿海地区存在大面积沙丘的核心原因，因为那里的主导风是离岸风，但在风暴期间，沙丘在波浪侵蚀后如何恢复也很重要。传统上，近海风被排除在海岸沙丘沉积物预算计算之外，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，导致低估了风输送的沙子量。申请人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转向相关的向陆地的风成（风吹）沉积物输运。在这个建议中，我们打算调查这个过程中的控制（例如，沙丘的形状和风速下，流逆转发生）和流动变形和由此产生的泥沙输运所涉及的机制。最终目标是量化离岸风对沙丘发育和行为的作用。我们将使用风和泥沙输运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，离岸风大大有助于背风海岸的沙丘行为。我们建议在一个理想的现场，Magilligan Strand，北方爱尔兰，这是一个海岸为主的离岸风和各种foredune地形，良好的排序，均匀的沙子和利益是在一个安全的军事区，以测试这一假设。它的另一个好处是拥有一个已建立的气象站，并且容易到达，以便在出现合适的风力条件时迅速部署设备。为了与NERC战略保持一致，该提案汇集了工程和环境科学的新颖方法的独特组合（为雪漂移开发的建模方法），以解决背风海岸风成沙丘的起源和形态动力学的重要问题。我们的研究方法涉及应用程序，第一次，3-D计算流体动力学（CFD）建模自然海岸沙丘，再加上一系列的高频泥沙输运和气流测量使用的设备由研究人员开发。该研究的一个特别新颖的组成部分是，该模型将用于指导现场仪器的部署，以实现最佳数据收集，然后将使用这些现场数据来修改和改进模型模拟。该研究小组结合了建模、高分辨率实地测量和风成沉积物运输物理学方面的必要专业知识，以便能够采用综合方法进行研究。由于所研究的自然环境的复杂性，调查必然需要大量设备。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前所未有的现场数据集，该数据集具有足够高的分辨率，能够分析湍流区和内部边界层动力学特性，这对于了解近海风下的沉积物输运至关重要。

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.