Coarse Sand Transport Processes in the Ripple Vortex Regime Under Asymmetric Nearshore Waves

Large‐scale wave flume experiments are conducted in the ripple vortex regime to study near bed coarse sand transport processes below asymmetric surface waves typical of the coastal nearshore region. For this purpose, a set of complementary acoustic instruments were deployed under regular nearshore wave conditions. Time‐resolved velocity, sand concentration and sand flux profiles are measured across both the dense bedload and dilute suspension layers with an Acoustic Concentration and Velocity Profiler. The equilibrium 2D suborbital ripples are in good agreement in terms of dimensions, shape and onshore migration rate with Wang and Yuan (2018, https://doi.org/10.1029/2018jc013810, 2020, https://doi.org/10.1016/j.coastaleng.2019.103583). Stoss ripple vortex entrainment around the trough‐to‐crest flow reversal (FR+) is found to be more energetic in terms of sand pick‐up into suspension compared to the counter rotating lee side vortex around the FR‐ flow reversal, as a consequence of the onshore skewed wave acceleration. Ripple vortex driven nearbed velocity phase leads around both flow reversals exceed typical bed friction induced values found in turbulent Wave Boundary Layers. Intrawave sand erosion events can be distinguished locally at the two ripple vortex positions around the flow reversals and two events more uniformly distributed along the ripple profile at wave crest and trough. Spatial fields of sand flux reveal the origin of the net onshore directed suspended and bedload transport. Good agreement is found with the mechanism identified under asymmetric oscillatory flows in Wang and Yuan (2020, https://doi.org/10.1016/j.coastaleng.2019.103583). Differences with ripple vortex regime under skewed shoaling waves and symmetric oscillatory flows are highlighted. Plain Language Summary Ripples are common bedform features on sandy beaches. They are formed under specific wave conditions and influence sand transport under waves. Indeed, when the wave passes over the wave crest, a vortex is created on the ripple flank. This modifies sand transport on beaches and makes it harder to predict. Predictions of sand transport on beaches are nevertheless extremely important to study long‐term coastal evolution, which is crucial to solve social and environment issues. In this paper, ripple formation and transport under specific wave conditions are studied. Inside a ripple cycle, two flow reversals are present (i.e., the moments where the direction of the flow changes),