Coastline sand waves on a low-energy beach at El Puntal spit, Spain: Linear stability analysis

[1] Shoreline sand waves of approximately 125‐150 m of wavelength and � 15 m of amplitude in the offshore direction (reached in a few days) are often observed at the west end of El Puntal spit, Santander, Spain. They tend to last for about 2 months and do not show an evident alongshore propagation during that time. Since wave incidence on that beach is typically very oblique, a shoreline linear stability analysis is performed to test whether those sand waves could be generated by shoreline instability. Because of the low-energy wave climate and the steep mean bathymetric profile, the predicted wavelength (100‐150 m) and growth time (4‐8 days) are much smaller than is typical on open ocean beaches. This provides a unique opportunity to test high-angle wave instability. Thus, the role of the instability on the growth, evolution, and subsequent destruction of the sand waves is examined. A remarkable agreement between predictions and observations regarding the wavelength and growth time is found. Furthermore, by analyzing shoreline instability for a 2 year period where sand waves sometimes occur and sometimes do not, a clear correlation is found between sand wave occurrence and predicted instability growth rate. Intriguingly, results are less satisfactory regarding alongshore migration since the model predicts some migration that is not supported by observations. However, it becomes clear that the instability caused by high-angle waves is crucial for shoreline sand wave formation at El Puntal and should be considered for any future research.