Collaborative research: Laboratory and numerical experiments on the response of wave ripples to changes in oscillatory flow

对称沙纹是现代波浪环境和岩石记录中最常见的底形之一。无论是古代还是现代，视觉上引人注目的波浪涟漪图案都是河床地形、湍流和沉积物输运复杂相互作用的容易观察到的特征。涟漪间距通常用作古代波浪条件和水深的指标，现代波纹影响床面粗糙度。然而，波纹往往是不平衡的，相对于快速变化的波浪条件，和古代和现代的波纹往往包含复杂的缺陷-偏离直线，平行波峰-似乎是不平衡的功能，但知之甚少。因此，我们解释二维涟漪模式或模拟这些模式如何响应不断变化的波浪条件的能力是不足的。本项目将通过实验室波浪水槽实验、底形演变的数值模拟以及对岩石露头中暴露的古代波纹和海岸线上暴露的现代波纹的实地研究相结合，研究波浪波纹对波浪条件变化的响应机制。首先，在一系列实验室波浪水槽实验中，我们将使用延时摄影和图像分析来跟踪波纹床对波浪强迫阶跃变化的响应，并最终生成不同类型波浪涟漪缺陷的相图。其次，我们将开发一种新的数值方法来模拟床面地形和振荡流的共同演变，我们将使用该模型来更好地了解在波浪槽中观察到的瞬态涟漪演变。第三，我们将比较实验室和数值实验的结果与岩石露头的古代波纹和现代的海岸线上的波纹。主要成果将是对广泛分布的波浪涟漪模式的新解释，以及用于模拟瞬时底形演变的新框架。由移动沙子的流动产生的模式，例如世界各地沿着海岸线常见的波纹，是关于古代和现代流动条件的丰富信息来源。这些底形模式也会影响其他地质流动：现代的波纹使桑迪床变粗糙，减缓海岸流动，沉积岩中的波纹会影响渗透性，从而控制地表以下的水、石油和天然气的流动。这项研究将提高我们解释古代和现代波浪涟漪模式中常见不规则性的能力，并将产生一个新的计算框架来模拟它们的形成。除了提供了一个改进的解释，在沿海设置的惊人的各种涟漪模式，我们的研究结果将提供地质学家，沉积学家和海岸工程师与新的工具，用于预测的形成和演变的底形，并将帮助海洋学家和水文学家在理解水库特性的控制。

Symmetric sand ripples are among the most common bedforms in modern wave-dominated environments and in the rock record. Whether ancient or modern, visually striking wave ripple patterns are an easily observable signature of the complex interaction of bed topography, turbulent flow, and sediment transport. Ripple spacing is often used as an indicator of ancient wave conditions and water depth, and modern ripples influence bed roughness. However, ripples are often out of equilibrium with respect to rapidly changing wave conditions, and both ancient and modern ripples often contain complicated defects - deviations from straight, parallel crests - that appear to be disequilibrium features but are poorly understood. Our ability to interpret two-dimensional ripple patterns, or to model how those patterns respond to changing wave conditions, is therefore deficient. This project will investigate the mechanisms by which wave ripples respond to changes in wave conditions through a combination of laboratory wave tank experiments, numerical simulations of bedform evolution, and field studies of both ancient ripples exposed in rock outcrops and modern ripples exposed on shorelines. First, in a series of laboratory wave tank experiments, we will use time-lapse photography and image analysis to track the response of rippled beds to step changes in wave forcing, and ultimately produce a phase diagram for different types of wave ripple defects. Second, we will develop a new numerical method for modeling the co-evolution of bed topography and oscillatory flow, and we will use this model to better understand the transient ripple evolution observed in the wave tank. Third, we will compare the results of the laboratory and numerical experiments with ancient ripples in rock outcrops and modern ripples on shorelines. The main outcomes will be a new interpretation of widespread wave ripple patterns, and a new framework for modeling transient bedform evolution.Patterns generated by flows that move sand, such as the ripples that are a common sight along shorelines around the world, are a rich source of information about ancient and modern flow conditions. These bedform patterns can also influence other geologic flows: modern ripples roughen the sandy bed, slowing coastal flows, and ripples in sedimentary rocks can influence permeability, which controls the flow of water, oil and gas beneath the land surface. This research will improve our ability to interpret common irregularities in ancient and modern wave ripple patterns, and will also produce a new computational framework for modeling their formation. In addition to providing an improved explanation for the striking variety of ripple patterns in coastal settings, our results will provide geologists, sedimentologists, and coastal engineers with new tools for predicting the formation and evolution of bedforms, and will aid geophysicists and hydrologists in understanding the controls on reservoir characteristics.