Sediment suspension and turbulence in the swash zone of dissipative beaches

This paper investigates mechanisms for stirring and transport of suspended sediment by infragravity-scale swash using high-frequency field measurements of horizontal and vertical flow velocities, water depths and sediment concentration profiles. The field data suggest that vertical velocity fluctuations generated by coherent eddies can be important to the process of sediment suspension and can contribute to asymmetrically distributed sediment transport rates between the uprush and backwash phases of the swash cycle. This result indicates that the traditional and often used proxy for bed shear stress based solely on measurements of the horizontal component of flow, u (i.e. τb ∝ ρfu2 where ρ is the water density and f is a friction factor) may be inadequate when used in energetics-type sediment transport models applied to the swash, which is indeed the case. We demonstrate that a bed shear stress formulation which is more consistent with the physics leads to improved predictive capability for energetics-type transport models. This formulation is based on the correlation between the horizontal and (turbulent) vertical components of the flow (i.e. τb ∝ ρ(uw′). Despite the improvement, it is evident that considerable amounts of calibration data are still required before each application of this model type. Moreover, our data shows that the issue of calibration coefficients is more problematic than generally appreciated. The calibration coefficients for energetics-type models on a given beach not only differ between uprush and backwash, but also vary with position within the swash zone, particularly on the uprush phase.