Cross-Shore Dispersion of Turbid Waters by Wave-Driven Nearshore Flows: Remote Sensing of Rip Currents Off Rocky and Sandy Shores

Abstract

A turbid nearshore zone (TNZ) forms along coasts due to wave-driven flows like rip currents that disperse particulates seaward from the surfzone. While well-studied on sandy beaches, rip currents and associated wave-driven circulation remains understudied in rocky shore environments and have not been groundtruthed in either environment using remote sensing. Here we use Sentinel-2 imagery to quantify the TNZ and wave-driven dispersion off Northern California, using deep-learning package, CoastSeg, to remove image artifacts. Reflectance data serve as a turbidity measure, analyzed alongside wave energy, tidal water level, and high-resolution bathymetry. We create regional maps showing spatial patterns of turbidity and its correlation with wave parameters, revealing that the TNZ expands with wave energy, extending up to several kilometers offshore during high wave events. We characterize the offshore extent of turbidity at 31 sites with varying morphology using an exponential decay function C(x) = C0e−bˣ that separates turbidity source (C0) from cross-shore reach (b−1). Smooth bathymetries, typical of sandy beaches, exhibit high C0 values, consistent with high shoreline turbidity generation, and low b−1 values, indicating limited rip current development. Conversely, rocky shores with high bathymetric roughness show lower C0 values but higher b−1 values, indicating longer rip currents and offshore extent of wave-driven flows. This dichotomy is most pronounced during weak wave forcing and low tides. Turbid waters extend furthest offshore where rocky shores are near sediment-rich estuaries, embayments, headlands, and kelp forests. These findings highlight the need for further research on rip currents along rocky shores and their ecological effects.