Observations of dune interactions from DEMs using through-water Structure from Motion
Observations of dune interactions from DEMs using through-water Structure from Motion
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DOI：
10.1016/j.geomorph.2020.107126
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发表时间：
2020-06
期刊：
影响因子：
3.9
通讯作者：
Renske C. Terwisscha van Scheltinga;G. Coco;M. Kleinhans;H. Friedrich
中科院分区：
文献类型：
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作者：
Renske C. Terwisscha van Scheltinga;G. Coco;M. Kleinhans;H. Friedrich
三维(3D)在调节水流和泥沙运动中的作用仍然知之甚少。由于不规则的沙丘形状和淹没，三维海床高程测量很难获得。在地表研究中，使用摄影测量工具重建地形已经变得很流行，但水的折射使通过水捕获淹没沙丘的图像仍然具有挑战性。我们表明，通过水图像和结构从运动(SfM)工具可以产生精确的数字高程模型(DEM)的测量沙丘领域，与高水平的细节。它是一种低成本、非侵入性的水下床高程测量替代方案。我们提供了详细的SfM工作流程，介绍和讨论了相机坐标和水下地面控制点的使用，以及获得高精度DEM所需的处理步骤。我们的方法在受控的实验室条件下得到了毫米分辨率的DEM。通过6次实验获得了23个沙地DEM的新数据，沙地DEM时间序列为研究沙丘相互作用过程提供了新的信息。在我们的实验中，合并和缺陷创建是最常见的。合并减少了床型的数量，而缺陷的产生导致了更多的床型。沙丘在达到平衡后继续相互作用和推挤。单峰比不规则峰、曲折峰或双峰更常见。观察到了二维(2D)的牙冠形状，尽管牙冠形状通常是3D性质的。观测到的沙丘迁移率并不稳定，在时间和空间上都是不同的。底形分裂加快了沙丘的运移，而邻近的沙丘则倾向于加速或减速。我们确定了一系列的过程，并表明需要高频的3D DEM来解决观测到的沙丘的快速变形和相互作用。已确定的沙丘相互作用过程是泥沙通量(变异性)所固有的，而与平均运移相关的基本泥沙运移过程的时间成分仍未得到很好的理解。
The role of three-dimensionality (3D) in modulating both flow and sediment transport remains poorly understood. 3D bed elevation measurements are difficult to obtain due to irregular dune shapes and submergence. Using photogrammetric tools for topographic reconstruction has become popular in surface studies, yet water refraction makes through-water image capturing of submerged dunes still challenging. We show that through-water images and Structure from Motion (SfM) tools can produce accurate Digital Elevation Models (DEMs) of a measured dune field, with a high level of detail. It is a low-cost and non-intrusive alternative for submerged bed elevation measurements. We provide a detailed SfM workflow, introduce and discuss the use of camera coordinates and underwater ground control points, and processing steps needed to obtain high accuracy DEMs. Our method results in DEMs with mm resolution in controlled laboratory conditions. New data for 23 dune field DEMs from 6 experiments were obtained and are presented.The dune field DEM time series provide insights on dune interaction processes. In our experiments, merging and defect creation were most common. Merging decreases the number of bedforms, and defect creation is responsible for more bedforms. Dunes continue to interact and jostle after achieving equilibrium. Simple crests are much more frequently observed than irregular, sinuous or double crests. Two-dimensional (2D) crest shapes were observed, although crest shapes were generally of 3D nature. Observed dune migration rates are not steady and vary in time and space. Bedform splitting speeds up dune migration, whilst neighbouring dunes tend to accelerate or decelerate. We identify a cascade of processes and show that 3D DEMs at high frequency are required to resolve observed rapid deformations and interaction of dunes. The identified dune interaction processes are inherent to sediment flux (variability), whereas the temporal component of the underlying sediment transport processes is still not well understood in relation to average transport.