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

乍一看，许多沙滩似乎都是由大小相同的沙子组成的。大多数海滩形状变化的数值模型也假定整个模型范围内的颗粒尺寸是一致的。然而，即使颗粒大小的微小变化也会对沙子是否会移动以及沙子可以被输送多少产生很大的影响。目前这项研究的假设是，细小的颗粒尺寸变化可以确定海滩是否以及在哪里更有可能受到侵蚀。作为这项研究的一部分，将测量沙粒大小的变异性，无论是在自然海滩上的空间上，还是在暴风雨和平静时期的时间上。将研究沙粒大小的空间和时间变异性与不断变化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淤积模式的发展具有重要意义。例如，在裂谷河道中经常观察到较粗的沉积物，而在河道之间的浅滩上发现较细的沉积物。还观察到，潮间带海滩和近海沙洲中发现了较细的沉积物，而冲浪带的颗粒尺寸要粗得多。此外，这一粗泥沙地区随着潮汐的起伏在海滩上上下移动。为了测量沙粒粒度，将使用数字成像系统(DIS)，该系统由水下外壳中带有微距镜头的数码相机组成。对沙子的图像进行处理，以给出平均粒度的估计。使用DIS，可以获得比传统沉积物采样技术在逻辑上可行的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的粒度测量。这些观察结果将与最先进的波浪、水流和海滩变化模型对不同沙粒大小类别运动的预测进行比较。结果将被用来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迁移方面的重要性，并在广泛的环境条件下改进海滩地貌动力学的预测。本研究将有助于增进对沿海变化的认识和预测。这种类型的预报目前被联邦、州和地方政府用来提供海岸规划和管理，用于军事行动，并被科学家用来更好地了解负责沿海地区沙子和其他物质移动的物理过程。此外，由此改进的模型将导致更好地预测受到极端海浪和海平面上升威胁的海岸的沉积物迁移和海岸线变化。世界上有很大比例的人口生活在沿海地区，更好地了解和预测影响海岸的物理过程将有助于规划、管理和缓解世界各地更安全的海岸。

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.