Behavioral Modes of Shoreline-Sandbar Coupling on a Single-Barred Beach in Baja California

Processes of shoreline-sandbar coupling are key to define morphodynamic beach state transitions. Sandbar patterns, however, can be very variable depending on the alongshore varying wave energy, the angle of wave incidence and sandbar location from the shore. Based on four years of monthly measured bathymetric data, this study describes behavioral modes of shoreline-sandbar coupling on a single-barred and swell-dominated beach in the Pacific coast of the Baja California peninsula. Sandbar location and geometry and shoreline positions were analyzed in relation to the incident wave energy in order to understand the control of sandbar morphometrics over the process of shoreline-sandbar coupling. Four distinct shoreline-sandbar coupling modes are identified depending on the cross-shore bar shape and distance related to the dynamic equilibrium position: I) Complete shoreline-sandbar coupling occurs during low-energy conditions when the sandbar locates within the dynamic equilibrium zone. II) When the sandbar migrates offshore beyond the dynamic equilibrium position (i.e. El Niño 2015-2016 winter), it can split into two sections during low-energy conditions: one section flattens and transforms into a terrace-bar, while the other section remains as a sandbar and migrates onshore but not necessarily coupling to the shoreline. III) If high-energy conditions occur, a terrace-bar located at the dynamic equilibrium point can be reconstructed into a sandbar that will eventually migrate onshore during low-energy conditions. IV) Terrace-bar and sandbar coexistence can happen when a degrading terrace-bar exists beyond the dynamic equilibrium point following a net offshore migration process, and a new sandbar forms and migrates onshore and offshore within the dynamic equilibrium zone.