Spectral analysis of seabed elevation from the Danish Wadden Sea

Bedforms both reflect and influence shallow water hydrodynamics and sediment dynamics. A correct characterization of their spatial distribution and dimensions is required for the understanding, assessment and prediction of numerous coastal processes. A method to parameterize geometrical characteristics using two-dimensional (2D) spectral analysis is presented and tested on seabed elevation data from the Knudedyb tidal inlet in the Danish Wadden Sea, where large compound bedforms are found. The bathymetric data were divided into 20x20 m areas on which a 2D spectral analysis was applied. The most energetic peak of the 2D spectrum was found and its energy, frequency and direction were calculated. A power-law was fitted to the average of slices taken through the 2D spectrum; its slope and y-intercept were calculated. Using these results the test area was morphologically classified into 4 distinct morphological regions. The most energetic peak and the slope and intercept of the power-law showed high values above the crest of the primary bedforms and scour holes, low values in areas without bedforms, and intermediate values in areas with secondary bedforms. The secondary bedform dimensions and orientations were calculated. An area of 700x700 m was used to determine the characteristics of the primary bedforms. However, they were less distinctively characterized compared to the secondary bedforms due to relatively large variations in their orientations and wavelengths. The method is thus appropriate for morphological classification of the seabed and for bedform characterization, being most efficient in areas characterized by bedforms with regular dimensions and directions.床形（bedforms）既能反映浅水环境的水动力与泥沙动力过程，也会对其产生调控作用。准确表征其空间分布与几何尺度，是理解、评估和预测诸多海岸过程的核心前提。本文提出一种基于二维（2D）频谱分析的几何特征参数化方法，并以丹麦瓦登海（Danish Wadden Sea）克努德迪卜潮汐汊道的海底高程数据为测试对象，该区域发育有大型复合床形。研究将水深地形数据划分为20 m × 20 m的分析单元，并对每个单元开展二维频谱分析，提取了二维频谱的能量峰值，并计算了该峰值的能量、频率与延伸方向。针对二维频谱切片的平均值拟合幂律（power-law）模型，并计算该模型的斜率与纵轴截距。基于上述计算结果，研究将测试区域划分为4个特征迥异的地貌单元。二维频谱能量峰值以及幂律模型的斜率与截距呈现出清晰的分布规律：主床形脊部与冲刷坑区域数值较高，无床形发育的区域数值较低，发育次级床形的区域数值则介于两者之间。研究同时计算了次级床形的几何尺度与延伸方向。研究采用700 m × 700 m的区域以提取主床形的特征，但由于主床形的延伸方向与波长存在较大差异，其特征表征效果相较于次级床形稍显不足。综上，该方法适用于海底地貌分类与床形特征表征，在床形尺度与延伸方向均较为规则的区域应用效果最佳。