Field Observations of Intermittent Cross-Shore Bed Load Transport on a Low-Energy Beach

Abstract

Low-energy sandy beaches typically have a rippled bed, and the presence of bed forms can strongly affect net sediment transport rates under combined forcing of waves and currents. In case low-energetic forcing is combined with coarse sediment, bed load transport is an important mechanism to understand transport processes on such beaches. This study presents observations of ripple geometry and migration from a low-energy beach composed of coarse sediment ((Formula presented.) m) in the bed load transport regime. The concurrent hydrodynamics were monitored with free-stream point measurements of velocity and pressure, and with velocity profiles from 15 cm above the bed into the wave boundary layer. The bed was rippled with relic and orbital vortex ripples. Cross-shore bed load transport associated with ripple migration was highly intermittent and alternating in direction. A bed load sediment transport model forced with the measured free-stream velocity signal led to a consistent overprediction of offshore directed transport. Using the measured velocities excluding the mean cross-shore velocity, the model captured the correct direction of all but one observed instance of migration in our data set. Velocity profiles confirmed that mean free-stream velocity was not representative of the magnitude and at times the direction of the mean flow in the wave bottom boundary layer over a rippled bed. Phase coupling between sea-swell and infragravity frequencies in orbital velocity forcing proved essential to capture the cross-shore bed load direction.