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控制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波纹都是河床地形、湍流和泥沙输送复杂相互作用的明显特征。波纹间距通常被用作古代波浪条件和水深的指标，而现代波纹影响河床粗糙度。然而，相对于快速变化的波浪条件，涟漪往往是不平衡的，古代和现代的涟漪通常都包含复杂的缺陷--偏离直线、平行的波峰--这些缺陷看起来是不平衡的特征，但人们对这些缺陷知之甚少。因此，我们解释二维波纹模式的能力，或者对这些模式如何应对变化的波浪条件进行建模的能力，是有缺陷的。该项目将通过实验室波浪水槽实验、河床演变的数值模拟以及暴露在露头岩石中的古代涟漪和暴露在海岸线上的现代涟漪的实地研究相结合，来研究波浪涟漪对波浪条件变化的响应机制。首先，在实验室的一系列波浪水槽实验中，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的阶跃变化的响应，并最终生成不同类型的波纹缺陷的相图。其次，我们将开发一种新的数值方法来模拟河床地形和振荡水流的共同演变，并将使用该模型来更好地理解在波浪水槽中观察到的瞬时波纹演变。第三，我们将把实验室和数值实验的结果与露头岩石中的古代涟漪和海岸线上的现代涟漪进行比较。主要成果将是对广泛的波浪波纹模式的新解释，以及对瞬时河床演变进行模拟的新框架。由流动的沙子产生的模式，如世界各地海岸线上常见的涟漪，是关于古代和现代水流条件的丰富信息来源。这些河床形态也可以影响其他地质流动：现代波纹使沙层变得粗糙，减缓了海岸流动，而沉积岩中的波纹可以影响渗透率，后者控制着陆地表面下的水、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波纹中常见不规则现象的能力，并将产生一个新的计算框架来模拟它们的形成。除了对海岸环境中波纹模式的惊人变化提供更好的解释外，我们的结果还将为地质学家、沉积学家和海岸工程师提供预测河床形成和演化的新工具，并将帮助地球物理学家和水文学家了解对储层特征的控制。

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.