Wave Period and Grain Size Controls on Short‐Wave Suspended Sediment Transport Under Shoaling and Breaking Waves

Abstract

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.

Surf Zone Turbulence and Suspended Sediment Dynamics—A Review

“…F was calculated for the ≈ 60,000 waves in the data set and averaged within γ-bins (Figure 20). Similar results were obtained in field experiments reported by Christensen et al [74]. In an attempt to obtain common scaling of sediment transport rates at two beaches where suspended sediment loads were very different (Figure 11), near-bed wave-driven sediment flux (q s ; Equation ( 12)) was normalized by the mean sediment concentration, and the ratio was termed a "flux efficiency", F:…”

“…Christensen et al [74] measured the turbulent kinetic energy at two beaches dominated by plunging breakers (Durras Beach, NSW, Australia, which is exposed to swell waves) and spilling breakers (Vejers Beach, Denmark, exposed to mainly wind waves). <k > scaled with γ at both beaches (Figure 5) but with a significant degree of scatter, which is partly due to the different measurement elevations in the water column and uncontrollable bed level changes within and between periods of recording.…”

“…To illustrate near-bed tendencies in more detail, Figure 8 plots <k >-profiles from two field experiments [74]. For strongly breaking wave conditions (γ s = H s /h > 0.5), <k > was maximum at the highest measurement position (z = 50 cm) with a decrease of about 30% close to the bed (z ≈ 5 cm).…”

“…Weakly breaking waves (0.3 < Hs/h < 0.5) exhibited a mixture of the two shapes with <k'> being largest at the highest measurement elevation and likely even larger near the sea surface. The vertical turbulence structure depends not only on the relative wave height but To illustrate near-bed tendencies in more detail, Figure 8 plots <k'>-profiles from field experiments [74]. For strongly breaking wave conditions (γs = Hs/h > 0.5), <k'> maximum at the highest measurement position (z = 50 cm) with a decrease of about close to the bed (z ≈ 5 cm).…”

“…For purely oscillatory-flow experiments, k max emerged under the wave crest, while in cases with a counter-current, k max shifted backwards to the trough phase, because the offshore-directed current added to the wave-generated (bed) shear stress on the trough phase and decreased the shear stress on the crest phase. The implication is that current shear may affect the phasing of sediment suspension and further delay SSC relative to u such that maximum sediment concentrations sometimes may appear at the trough phase under surf bores [74].…”