Observations and Predictions of Sand Grain Size Variability and Morphodynamics on Beaches

乍一看，许多桑迪似乎由大小相同的沙子组成。大多数数值模型的变化形状的海滩也假定均匀的粒度在整个模型域。然而，即使是颗粒大小的微小变化也会对沙子是否移动以及可以运输多少沙子产生很大影响。本研究的假设是，微小的粒度变化可以决定海滩是否以及在哪里更有可能被侵蚀。作为这项研究的一部分，将测量沙粒大小的变化，无论是在自然海滩的空间，还是在风暴和平静时期的时间。将研究沙粒大小的空间和时间变异性与变化中的海滩形态之间的相关性，目的是确定颗粒大小的变化是否对侵蚀和加积模式的发展很重要。例如，粗糙的沉积物经常出现在离岸水道中，而较细的沉积物则出现在水道之间的浅滩上。还观察到，在潮间带海滩和离岸沙洲中发现了较细的沉积物，而碎波带中的粒度要粗得多。此外，该粗沉积物区域随着潮汐的涨落而在海滩上上下移动。为了测量砂粒尺寸，将使用数字成像系统（DIS），该系统由水下外壳中的带微距透镜的数码相机组成。对沙子的图像进行处理，以估计平均粒度。与传统的沉积物取样技术相比，使用DIS可以获得更多的粒度测量结果。作为这项研究的一部分，将定期进行高分辨率粒度调查。这些观测结果将与最先进的波浪、海流和海滩变化模型对不同沙粒级运动的预测进行比较。研究结果将用于探索移动不同大小类别的物理过程，评估空间和时间粒度变化在准确预测沉积物输运方面的重要性，并改进对各种环境条件下海滩形态动力学的预测。目前的研究将有助于改进对海岸变化的认识和预测。这种类型的预测目前被联邦、州和地方政府用于提供海岸规划和管理，用于军事行动，并且被科学家用于更好地理解负责移动沙子和其他材料的物理过程。此外，由此产生的改进模型将导致更好地预测受极端波浪和海平面上升威胁的海岸的沉积物迁移和海岸线变化。世界上很大一部分人口生活在沿海地区，更好地了解和预测影响沿海地区的物理过程将有助于规划、管理和减缓全球沿海地区的安全。

At first glance many sandy beaches appear to consist of sand that is all the same size. Most numerical models of the changing shape of the beach also assume uniform grain size across the model domain. However, even small changes in the size of the grains can have a large effect on whether or not the sand will move and how much sand can be transported. The hypothesis for the present study is that small grain size variations can determine whether and where a beach is more likely to erode. As part of this research, the variability of sand grain size will be measured, both in space on a natural beach and in time through storms and calm periods. Correlations between the spatial and temporal variability of sand size and changing beach morphology will be examined, with the goal of determining whether variations in grain size are important in the development of patterns of erosion and accretion. For example, coarser sediments are often observed in rip channels while finer sediments are found on the shoals between channels. It has also been observed that finer sediments are found high on the intertidal beach and offshore in sand bars, whereas grain sizes in the surf zone are much coarser. In addition, this coarse-sediment region moves up and down the beach with the rise and fall of the tides. To make measurements of sand grain size, a digital imaging system (DIS) will be used that consists of a digital camera with a macro lens in an underwater housing. Images of sand are processed to give estimates of mean grain size. With the DIS, many more measurements of grain size can be obtained than is logistically feasible with traditional sediment sampling techniques. As part of this study, high-resolution grain size surveys will be conducted regularly. These observations will be compared with predictions of the motion of different sand size classes from state-of-the-artmodels of waves, currents and beach changes. The results will be used to explore the physical processes moving different size classes, to assess the importance of spatial and temporal grain size variability in making accurate predictions of sediment transport and to improve prediction of beach morphodynamics for a wide range of environmental conditions. The present research will contribute to improved knowledge and prediction of coastal changes. Predictions of this type are presently used by federal, state, and local governments to provide coastal planning and management, for military operations, and by scientists to better understand the physical processes responsible for moving sand and other materials in the coastal region. In addition, the resulting improved model will lead to better predictions for sediment transport and shoreline changes on coasts threatened by extreme waves and sea level rise. A large percentage of the world's population lives within the coastal region and a better understanding and predictive capability of the physical processes affecting the coasts will allow planning, management and mitigation for safer coasts worldwide.