High Resolution Field Observations of Shoreward Sediment Transport Within the Wave Bottom Boundary Layer on a Sandy Beach

Abstract We introduce a data archive containing the first highly resolved profiles of velocity and sediment concentration within the wave bottom boundary layer (WBBL) on a natural sandy beach. Resolution of the thin WBBL was made possible by deployment of a unique “Multi‐Frequency Doppler” (MFDop) bistatic acoustic profiling instrument on an Oregon beach during August‐September 2022. Supporting data are archived and described, including rotary sonar, velocity, pressure, bathymetry, video, and sediment grain size. The MFDop measured velocity and concentration at ∼60 Hz every ∼3.75 mm along a vertical profile extending tens of cm above the bed. Multi‐frequency inversions distinguished sediment from bubble backscatter, improving estimates of suspended sediment concentration and bed elevation. Novel multi‐frequency unwrapping algorithms improved performance of high‐resolution pulse‐coherent techniques in high‐energy flows. During the deployment, offshore significant wave heights varied from 1 to 3 m. With varying wave height and tidal stage, the MFDop location ranged from outside to far inside the surfzone. Flat‐bed conditions predominated, although 0.1‐m‐high megaripples were occasionally observed. Consistent with measured bathymetric accretion onshore of the instrument, MFDop observations revealed shoreward sediment transport very near the bed under strongly skewed velocities. This shoreward transport resulted as dense sediment clouds extending ∼2 cm from the bed were suspended by strong shoreward velocities under wave crests, but typically settled under weaker return flows. These first direct field observations of the wave‐induced shoreward transport responsible for building mobile coastlines may provide a valuable benchmark for testing sediment transport models.