An experimental study of net sediment transport rate due to acceleration-skewed oscillatory flows over rippled seabeds

Abstract

Wave-driven near-bed oscillatory flows can be acceleration-skewed due to the onshore leaning of shoaling waves in shallow coastal waters. A full-scale experimental study was conducted to investigate the acceleration-skewness-induced net sediment transport rate over coarse-sand rippled beds. A dataset of ripple geometry, migration and net sediment transport rate has been produced for oscillatory flows with different degrees of acceleration skewness. The steepness of equilibrium ripples decreases due to the effect of acceleration skewness and a consistent onshore ripple migration with a very low speed was observed for all the tests involving acceleration-skewed flows. The measured ripple-averaged net sediment transport rate is always in the direction of highest-acceleration (onshore) and increases with acceleration skewness. The magnitude of net transport rate is comparable to that due to velocity skewness, highlighting the necessity of considering acceleration skewness in modeling coastal sediment transport. The measurements can be well predicted by the formula of van der A et al. (2013), of which the calibration dataset does not cover such test conditions. The good model-data agreement suggests that sheet-flow and ripple-bed conditions share similar mechanisms by which acceleration skewness produces net transport rate.