Composite Beaches - Can we protect our coastlines by mimicking nature?

项目背景复合海滩由较低的前滨沙子和自然形成的永久或半永久的后滨鹅卵石组成，在英国、美国和新西兰是普遍存在的海滩环境。众所周知，这些海滩是高度稳定的，因为它们结合了耗散沙坡和反射性鹅卵石护堤，这代表了海滩地貌动力学分类中最稳定的两个末端成员。此外，当存在永久的鹅卵石山脊时，它可以起到显著的天然海岸保护作用，稳定上层海滩，为腹地提供溢流保护。因此，建造的鹅卵石护道(或称动态护岸)正在试验中，旨在模仿合成的海滩鹅卵石山脊，为美国西海岸脆弱的海滩提供海岸防御，并取得了一些成功。尽管合成海滩普遍存在，而且作为海岸防御的一种自然形式，它们在学术文献中很少受到关注，迄今几乎没有开展专门针对合成海滩的研究。因此，对合成海滩的过程，如跨岸和沿岸的鹅卵石运输、波浪爬高和鹅卵石脊溢顶以及对海平面上升的响应的了解很少。项目目标和方法本项目旨在填补这一已确定的研究空白，并对合成海滩行为进行首批详细研究之一。这项工作将加强对这种有价值的海滩类型的了解，并获得洞察力，从而能够设计模仿合成海滩行为的基于自然的海岸防御结构，以提供可持续和低成本的海岸保护。1.分析现有的历史地形数据，以阐明综合海滩的中长期变化，以及它们对极端风暴和风暴序列的适应能力。这将包括通过新的“CoastSat”卫星沿海监测工具，利用卫星图像对综合海滩稳定性进行全球评估。对博斯海滩最近安装的(2021年6月)永久激光雷达站的数据进行了分析，该站以1赫兹的频率连续提供海滩剖面和海浪上升数据。这一分析将集中在风暴侵蚀和风暴后的响应。在威尔士和/或美国的2个或更多个合成海滩上，使用新开发的逐浪鹅卵石跟踪方法进行现场实验，以更好地了解合成海滩脊的短期变异性和单个鹅卵石的移动性。测试并有可能开发现有的先进的XBeach-G海岸模型，以预测复合海滩上的地形变化。如果成功，这将提高我们评估合成海滩对日益增长的暴风雨的脆弱性的能力，并为设计动态护岸海岸保护结构提供一个工具。

Project BackgroundComposite beaches consist of a lower foreshore of sand and a naturally occurring permanent or semi- permanent backshore ridge of cobbles, and are ubiquitous beach environments in the UK, USA and NZ. These beaches are known to be highly stable because they combine a dissipative sand slope and a reflective cobble berm, which represent the two most stable end members in the classification of beach morphodynamics. Furthermore, when a permanent cobble ridge exists it can play a significant natural coastal protection role, stabilising the upper beach and providing overtopping protection to the hinterland. As a result, constructed cobble berms (or dynamic revetments) designed to mimic composite beach cobble ridges are being trialled to provide coastal defence at vulnerable beaches on the US West Coast with some success. Despite the common occurrence of composite beaches and their value as a natural form of coastal defence they have received very little attention in academic literature with almost no composite beach- specific studies being undertaken to date. As a result, understanding of composite beach processes such as cross- and longshore cobble transport, wave runup and overtopping of cobble ridges and response to sea level rise is minimal.Project Aims and MethodsThis project aims to fill this identified research gap and undertake one of the first detailed studies into composite beach behaviour. This work will enhance understanding of this valuable beach type and gain insight which will enable the design nature-based coastal defence structures which mimic composite beach behaviour to provide sustainable and low-cost coastal protection. This will be achieved through the following activities:1. Analysis of existing historical topographic data to elucidate the medium to long-term variability of composite beaches and their resilience to extreme storms and storm sequences. This will include a global assessment of composite beach stability using satellite imagery via the new "CoastSat" satellite coastal monitoring tool.2. Analysis of data from the recently installed (June 2021) permanent Lidar station at Borth beach which provides beach profiles and wave runup data continuously at 1 Hz. This analysis will focus on storm erosion and post-storm response.3. Field experiments using a newly-developed, wave-by-wave cobble tracking methodology at 2 or more composite beaches in Wales and/or USA to better understand the short term variability of composite beach ridges and the mobilisation of individual cobbles.4. Test and potentially develop the existing state of the art XBeach-G coastal model to predict morphology changes on composite beaches. If successful, this will improve our ability to assess the vulnerability of composite beaches to increasing storminess and provide a tool for the design of dynamic revetment coastal protection structures.