Intrawave sand suspension in the shoaling and surf zone of a field-scale laboratory beach

Abstract

Short-wave sand transport in morphodynamic models is often based solely on the near-bedwave-orbital motion, thereby neglec ting the effect of ripple-induced and surface-induced turbulence onsand transport processes. Here sand stirring was studied using measurements of the wave-orbital motion,turbulence, ripple characteristics, and sand concentration collected on a field-scale laboratory beach underconditions ranging from irregular nonbreaking waves above vortex ripples to plunging waves and boresabove subdued bed forms. Turbulence and sand concentration were analyzed as individual events and ina wave phase-averaged sense. The frac tion of turbulence events related to suspension events is relativelyhigh (∼50%), especially beneath plunging waves. Beneath nonbreaking waves with vortex ripples, the sandconcentration close to the bed peaks right after the maximum positive wave-orbital motion and shows amarked phase lag in the vertical, although the peak in concentration at higher elevations does not shiftto beyond the positive to negative flow reversal. Under plunging waves, concentration peaks beneaththe wavefront without any notable phase lags in the vertical. In the inner-surf zone (bores), the sandconcentration remains phase coupled to positive wave-orbital motion, but the concentration decreaseswith distance toward the shoreline. On the whole, our observations demonstrate that the wave-drivensuspended load transport is onshore and largest beneath plunging waves, while it is small and can alsobe offshore beneath shoaling waves. To accurately predict wave-driven sand transport in morphodynamicmodels, the effect of surface-induced turbulence beneath plunging waves should thus be included.