Formation of two-section cross-shore profile under joint influence of random short waves and groups of long waves

We show that beach profiles may develop a two-section almost-equilibrium structure under joint impact of short wind waves and groups of long waves with duration of a few minutes and properties that are independent of wind waves. The upper section of the profile is convex and follows the power law h ~ x4/3 at small depths and in the swash zone. Wind waves may cause either erosion or accretion of this section but groups of long waves effectively redistribute the sediment over the convex section and practically do not alter the sediment volume. The ability of short groups of highly energetic waves to regularly build up such profiles from those created by random sea waves is a principally new feature of wave-coastal interaction that may have extensive consequences on the estimates of the wave-induced coastal hazards.