Nearshore Flow Dynamics Over Shore‐Oblique Bathymetric Features During Storm Wave Conditions

Shore‐oblique bathymetric features occur around the world and have been statistically correlated with enhanced shoreline retreat on sandy beaches. However, the physical mechanisms that explain a causal relationship are not well understood. In this study, radar remote sensing observations and results from a phase‐resolved numerical model explore how complex morphology alters nearshore hydrodynamics. Observations at selected times during high‐energy storm events as well as a suite of idealized simulations indicate that shore‐oblique features induce strong spatial variations in the water surface elevation, wave breaking patterns, and mean current pathways. Re‐emergent offshore flows and longshore current accelerations occur near the shoreward apex of the oblique nearshore features. The results suggest that complex bathymetric morphology exerts a powerful control on nearshore hydrodynamics and increases the potential for enhanced cross‐shore and alongshore sediment transport. Plain Language Summary Near the shoreline, underwater topography is affected by sea level, waves, currents, tides, and geological characteristics. Typically, sandbars are oriented parallel to the coastline, but shore‐oblique sandbars have also been identified around the world. In many instances, these oblique features have been correlated with zones of enhanced erosion, however the explanation for this statistical relationship is not fully understood. In this study, remote sensing observations and modeling results explore how complex underwater topographies alter coastal wave energy and flow patterns. Selected stormy periods were observed in addition to a set of idealized simulations. Results indicate that shore‐oblique features cause localized changes to wave heights, wave breaking, and current speeds. These alterations contribute to the presence of rapid offshore‐directed rip currents and accelerations in shore‐parallel currents, which could enhance the potential for sand to be transported away from these zones and contribute to erosion. Key Points Radar and numerical modeling indicate similar alongshore spatial variability in surf zone width Re‐emergent offshore‐directed currents flow beyond the surf zone in a region with shore‐oblique sand bars The alignment of shore‐oblique sand bars modifies wave breaking patterns, current pathways, and current speeds