Wave by Wave Bed Elevation Changes in the Swash Zone

Bed elevation changes in the swash zone depends on the asymmetries of the swash motion and the relative importance of sediment transport in suspension and along the bed. This creates a significant barrier to the prediction of swash zone response to variations in the incident wave field. At the same time, the relativley small changes in bed elevation that occur in response to individual waves have not been measured. This paper describes the use of ultra-sonic distance sensors (Senix toughsonic) to measure the contribution of individual waves and infragravity waves to bed elevation changes in the swash zone. The distance sensors are able to measure bed elevation changes to the thickness of an individual grain. The combination of the distance sensors, pressure transducers and acoustic doppler current meters, allows the hydrodynamic controls on bed elevation changes to be identified. Specifically, bed elevation changes (erosion or accretion) are found to be controlled by the asymmetry of the individual swash, which varies at infragravity frequencies and is affected by wave transformation over the innermost nearshore bar.