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.
