Bed level fluctuations in the inner surf and swash zone of a dissipative beach

• Conductivity concentration profilers quantify bed level variability 1 mm resolution
• Bed level variability for an event exceeds several centimeters
• Event-scale bed level change is nearly normally-distributed.

A field study was conducted on a dissipative beach to quantify inner surf and swash-zone bed level change during periods of inundation and throughout a tidal cycle. Elevation changes were acquired at millimeter resolution with a new conductivity concentration profiler that allowed quantification of the bed level throughout the duration of the wave/swash cycle and also during periods of bed exposure. Bed level change spectra showed the highest energy at low frequencies even though event-scale net bed level changes were observed to exceed the tidal-scale net bed level change. Net bed level change for individual events was nearly normally distributed with most individual events displaying little or no net bed level change. “Large” erosion and accretion events with bed level elevation magnitudes that exceeded net tidal elevation change occurred with similar frequency. The similarity between the frequency of large erosion and large accretion events suggests that a few events may be ultimately responsible for the observed net elevation change over the tidal cycle. The large events displayed different hydrodynamic characteristics. Erosion events had longer duration onshore-directed flow and higher maximum onshore-directed velocity magnitude than offshore-directed velocity magnitude. The opposite was found for accretion events that had longer duration offshore-directed flow and a higher maximum offshore-directed velocity magnitude than onshore-directed velocity magnitude.