Wave‐driven setup and alongshore flows observed onshore of a submarine canyon

The effect of alongshore variations in the incident wavefield on wave‐driven setup and on alongshore flows in the surfzone is investigated using observations collected onshore of a submarine canyon. Wave heights and radiation stresses at the outer edge of the surfzone (water depth ≈2.5 m) varied by up to a factor of 4 and 16, respectively, over a 450 m alongshore distance, resulting in setup variations as large as 0.1 m along the shoreline (water depth ≈0.3 m). Even with this strong alongshore variability, wave‐driven setup was dominated by the cross‐shore gradient of the wave radiation stress, and setup observed in the surfzone is predicted well by a one‐dimensional cross‐shore momentum balance. Both cross‐shore radiation stress gradients and alongshore setup gradients contributed to the alongshore flows observed in the inner surfzone when alongshore gradients in offshore wave heights were large, and a simplified alongshore momentum balance suggests that the large [ O (1 kg/(s 2 m)] observed setup‐induced pressure gradients can drive strong [ O (1 m/s)] alongshore currents.