Wave Period and Grain Size Controls on Short-Wave Suspended Sediment Transport Under Shoaling and Breaking Waves
2019. American Geophysical Union. All Rights Reserved. Beach recovery is key to the continued existence of sandy beaches and is typically driven by the onshore-directed transport of sediment by short waves during low-moderate energy conditions. The physical processes governing beach recovery are not well understood, but the theoretically developed dimensionless fall velocity, Ω = H/wsT, was suggested to be important for separating onshore/offshore sediment motion (Dean, 1973). In this paper, the effect of wave period and sediment grain size on short-wave suspended sediment flux was investigated based on field measurements obtained beneath shoaling and breaking waves at Durras and Vejers beaches. The efficiency of the breaking waves in transporting suspended sediment onshore was roughly the same for the two beaches, despite the wave periods being larger and the mean sediment grain size coarser at Durras beach. The flux efficiencies were, however, shown to be degraded by wave-current interactions and long/short-wave interactions at Durras beach, especially. Excluding time series of strong undertow velocities (< −0.1 m/s) and infragravity wave-energy (>5 m2/s3) resulted in significantly larger flux efficiencies beneath breaking waves at Durras beach compared to Vejers beach. These results indicate that wave-current interactions and long/short-wave interactions should be taken into consideration along with the wave period and mean grain size when estimating short-wave suspended sediment fluxes. The results also showed that plunging breakers were more efficient in suspending sediment and transporting it onshore compared to spilling breakers/surf bores. This finding suggests that wave breaker type also is an important parameter to incorporate when modeling beach recovery.