Houser, C., 2013. Flow separation over a prograding beach step at Pensacola Beach, Florida.

Beach steps are a common feature of poorly sorted intermediate and reflective beaches that can develop as an accretionary wedge from foreshore sediment moved seaward by the backwash. Although there have been several studies describing the development and evolution of steps in response to wave and tidal forcing, characterization of the separated flow and development of the backwash vortex has been limited to a small number of laboratory experiments. This study presents the first field data in which flow separation and the backwash vortex are quantified using a pulse-coherent acoustic Doppler profiler over a 0.10-m prograding beach step. As the step migrated beneath the instrument station under a quasi-steady wave forcing it maintained its height and length, allowing for a space-for-time substitution. Flow separation developed under subcritical flows, when the backwash exceeded ∼0.50 m s⁻¹ and extended over the length of the step early in the backwash cycle. The length of the separation cell scales with the brink angle and step height, consistent with previously published field and laboratory steps and low-angle dunes. The relationship between step height and separation length is used as the basis for a predictive model in which step height is dependent on beach slope and wave period through their control on swash excursion length. Predicted step heights show good agreement with observations from both the field and laboratory.