APPARENT CURRENT ROUGHNESS CAUSED BY WAVES AND BEDFORMS ON A SANDY SHOREFACE

摘要

Sand grains, bedforms and the wave boundary layer cause roughness for tidal currents. This paper reports roughness determined from flow velocity measurements in calm weather and storms derived from 1900 hours of detailed flow measurements on a sandy shoreface, 2 km off Noordwijk, The Netherlands. The roughness was determined by fitting logarithmical velocity profiles to data of 3 sensors. The results are compared to observed bedform dimensions and wave conditions. The roughness decreases for increasing current velocity in Hm0 waves <2 m, but increases for larger waves which indicates that apparent wave roughness dominates during storms. Waves opposing the current cause a larger apparent roughness. Individual events show contrasting trends that are probably related to development of current ripples, wave ripples and hummocks.