Bedform appearance and dimension in the nearshore under mixed wave and current conditions

Abstract

In autumn 2009 a field campaign was done in the nearshore near a secondary tidal inlet, the Slufter, which is located on the most southern Wadden island Texel of the Netherlands. The aim of this research was to get a better understanding of the behaviour, appearance and geometry, of bedforms in the nearshore under combined wave and current conditions. Due to the presence of the Slufter tidal larger current effects were expected. The wave conditions at the instrument location during the campaign were mostly just prior to breaking having shoaling characteristics, due to its location behind an inner bar at roughly the same height as the bar. The bedforms varied in length between 0.17 and 3 m and between 0.01 and 0.23 m in height. The transition from ripples to larger bedforms is observed to depend on a small range of Hm0 and the mean current, with the mean current playing a key role. 2D bedforms are found for low intensity wave conditions and 3D bedforms for more intense mixed wave current conditions. The wave and mixed wave-current bedforms/conditions are best separated by the criteria of Li & Amos (1998). The occasional observed sheet flow conditions were best separated with a ciritial Shiels number of 0.8 and the wave current Shields number combining method theta(wc) = theta(c) + theta (w) (Allen & Leeder, 1980). The evolution of bedforms is found to differ for increasing versus decreasing condition, hysteresis, being more pronounced for rapid changing conditions. For increasing conditions the bedforms evolve equal rapidly although some relaxation is often present. For decreasing conditions the bedform dimensions delay the conditions significantly and may, for a decrease in Hm0 of 0.2 m and mean current of 0.2 m/s per hour, ‘freeze’. This freeze may be a temporarily result of the process of split. Four bedform predictors are tested against the observed bedform dimensions, two wave and two wave/current based. Little difference between the predictions of wave and wave/current induced bedforms is found. The wave/current predictor of Khelifa & Ouellet (2000) performs the best, even for wave only conditions. Disregarding the bedforms from the first fifth and fifth fifth duration of each tide, which are expected to deviate due to relaxation, did not lead to major improvements of the predictors.