The relatively long sandy beaches of El Maresme, and the bimodal wave climate of the Catalan coast, with dominant E-ENE and SSW waves that leads to high angle of incidence, propitiate good conditions for high angle of wave instability. Subtle undulations of the shoreline and the more intense undulations of the bathymetric contour line of 5 meters depth, with wavelengths of about 1300 m, can be found at that stretch of the coast. A morphodynamic model has been used to test if such undulations could be generated by high angle wave instability. Model results, for wave period of 4 s, show that at El Maresme coast high angle wave instability may develop with time rate of about 1 year and with dominant wavelength that ranges from 600 to 1400 m. For the wave climate of the Catalan coast, wave heights of 0.5 – 1 m and a mean peak period of 5.6 s, and at the steep beaches of El Mareme, it has been found that the wavelength of the instability is in good agreement with the observed undulations, and depends in a sensitive way of the mean slope of the bathymetric profile, and on the length of the bathymetric perturbation.