Megascale rhythmic shoreline forms on a beach with multiple bars

The study, carried out in 2003 and 2006 at the Lubiatowo Coastal Research Station (Poland), located on the non-tidal southern Baltic coast (tidal range < 0.06 m), focused on larger rhythmic forms (mega-cusps) with wavelengths in the interval 500 m > Lc > 20 m. Statistical analyses of detailed shoreline configurations were performed mostly with the Discrete Wavelet Transform method (DWT). The beach is composed of fine sand with grain diameter D50 ≈ 0.22 mm, which produces 4 longshore sandbars and a gently sloping seabed with β = 0.015. The analysis confirms the key role of bars in hydro- and morphodynamic surf zone processes. The hypothesis was therefore set up that, in a surf zone with multiple bars, the bars and mega-scale shoreline rhythmic forms form one integrated physical system; experimental evidence to substantiate this hypothesis was also sought. In such a system not only do self-regulation processes include swash zone phenomena, they also incorporate processes in offshore surf zone locations. The longshore dimensions of large cusps are thus related to the distances between periodically active large bed forms (bars). The spatial dimension of bar system activity (number of active bars) depends, at a given time scale, on the associated hydrodynamic conditions. It was assumed that such a time scale could include either the development and duration of a storm, or a period of stable, yet distinct waves, capable of remodelling the beach configuration. The indentation to wavelength ratio of mega-cusps for the studied non-tidal dissipative environment may be one order of magnitude greater than for mesotidal, reflective beaches.