Measuring sediment transport and bed disturbance with tracers on a mixed beach

A new tracer deployment method is applied to the problem of resolving rates and patterns of littoral sediment transport on mixed beaches. We applied the method at the morphologically complex, meso-tidal beach fringing the Elwha River delta, Washington. Clasts were transformed into tracers using Radio Frequency Identifier (RFID) tags. These tracers were deployed in and on the bed along a cross-shore oriented beach transect. Along- and cross-shore variations in tracer velocity were used to estimate sediment transport patterns, including the depth and cross-sectional area of the disturbed bed and bulk alongshore transport rates. We found that the peak bed disturbance averaged 22% of the tidal maximum significant wave height and that tracer velocity declined approximately logarithmically with depth in the bed. Across all deployments the maximum alongshore tracer velocity occurred between 1.0 and 2.0 m above Mean Lower Low Water (MLLW), corresponding to elevations 0.3 m below and 0.7 m above Mean Sea Level (MSL) at this location. Alongshore bulk sediment transport rates estimated from the advection of tracers ranged from 1.1 to 173.1 m3/d under significant wave heights of 0.1 to 2.1 m and these transport rates related linearly to measured wave energy transport. Both the volumetric sediment transport rates and a CERC-type k coefficient derived from the results agreed well with results from other published studies.