Current roughness over small bedforms and waves

Abstract

Sand grains, bedforms and the wave boundary layer cause roughness for tidal currents. This paper reports roughness and current shear stress in calm weather and storms derived from 1900 hours of detailed flow measurements on a sandy shoreface, 2 km off Noordwijk, The Netherlands. Two methods are employed: fitting logarithmical velocity profiles to data of 3 to 7 sensors, and the inertial subrange method from the spectra collected at 2 Hz. The results are compared to observed bedform dimensions. Various technical problems are discussed. The roughness decreases for increasing current velocity in Hm0 waves <2 m, but increases for larger waves. Individual events show contrasting trends that are probably related to bedform development. Recommendations for future instrumentation and for further analysis of this data are given