Estimatin of roughness lengths with a ship-mounted ADCP from Knudedyb, Denmark in October 2009

The hydraulic effect of asymmetric compound bedforms on tidal currents was assessed from field measurements of flow velocity in the Knudedyb tidal inlet, Denmark. Large asymmetric bedforms with smaller superimposed ones are a common feature of sandy shallow water environments and are known to act as hydraulic roughness elements in dependence with flow direction. The presence of a flow separation zone on the bedform lee was estimated through analysis of the measured velocity directions and the calculation of the flow separation line. The Law of the Wall was used to calculate roughness lengths and shear velocities from log-linear segments sought on transect-averaged and single-location velocity profiles. During the ebb tide a permanent flow separation zone was established over the steep (10-20°) lee sides of the ebb-oriented primary bedforms, which generated a consequent drag on the flow. […]

本研究通过丹麦克努德迪布潮汐入口的流速现场测量，对非对称复合床形对潮汐流体的水力效应进行了评估。在大面积的、叠置有较小床形的非对称床形是沙质浅水环境中的常见特征，且已知其作为水力粗糙度元素的作用与流向相关。通过分析测量的流速方向以及计算分离流线，对床形背面的分离流区进行了估算。利用壁面律，从横断面平均和单点流速分布的对数线性段中，计算了粗糙长度和剪切流速。在退潮期间，在退潮方向的初级床形陡峭（10-20°）背侧建立了永久的分离流区，从而对流体产生了相应的阻力。