Using the Law of the Wall the roughness length in the Marsdiep tidal inlet is inferred from velocity profiles. The spatial and temporal variations across the inlet were related to bathymetry and flow characteristics of the inlet.
Spatial variability of bed roughness in the Marsdiep inlet inferred from velocity profiles
Both in coastal oceanography and in hydrology, hydraulic bed roughness is typically considered to be a scalar quantity, i.e. it has a magnitude and no direction. In tidal environments with bedforms, this view may be false, because the bed roughness may depend on the orientation of bedforms relative to the flow. In particular, the effective bed roughness may be different for ebb and flood currents. One year of acoustic Doppler current profiler data collected from a ferry that operates in the Marsdiep tidal inlet were used to study ebb-flood differences in bed roughness. The objective of this study is to quantify and explain bed roughness values for ebb and flood, as function of the position in the inlet. The ferry does not follow the exact same path for every crossing. Therefore, the data cannot simply be grouped based on a discretisation of a transect across the inlet. Alternative methods were tested to average the velocity profiles in time and space. The most robust approach was to project the data on a horizontal grid, and to fit the law of the wall through the averaged velocity profiles, for ebb and flood data separately in each grid cell. In the northern part of the inlet, the roughness is a factor three larger during ebb than during flood. In the southern part, there was no significant difference. An existing study indicates that bed roughness length not only depends on the height and length of the bed forms, but also on the lee side slope. Roughness was shown to be larger for the flow direction with the steepest lee slope. The shape of the bedforms in the Marsdiep inlet is nearly symmetrical, with a slightly larger lee slope in the flood direction. Despite this, the variation in roughness between ebb and flood is significant, with the largest bed roughness values occurring for ebb. These observations cast doubt about the presumed dominant role of asymmetry in the bedform geometry. The differences between ebb and flood roughness may alternatively be caused by differences in the external flow conditions. For example, density stratification is systematically different between ebb and flood, because the Wadden Sea receives freshwater from lake IJssel.