Robust measurements of bed shear stress under wave runup flows are necessary to inform beachface sediment transport modelling. In this study, direct measurements of swash zone bed shear stress were obtained in medium and prototype-scale laboratory experiments on steep slopes. Peak shear stresses coincided with the arrival of uprush swash fronts and high-resolution measurement of swash surface profiles indicated a consistently seaward sloping swash surface with minimal evidence of a landward sloping swash front. The quadratic stress law was applied to back-calculate time-varying friction factors, which were observed to decrease with increasing Reynolds number on smooth slopes, consistent with theory for steady flows. Additionally, friction factors remained relatively constant throughout the swash cycle (except around flow reversal), with a variation of approximately ±20% from the mean value and with only small differences between uprush and backwash. Measured friction factors were observed to be larger than expected when plotted on the Moody or wave friction diagram for a given Reynolds number and relative roughness, consistent with previous field and laboratory studies at various scales.

A comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic model enabled cross-shore shear stress estimates of up to 21.8Nm-2 with maximum alongshore shear stress estimates of 12.3Nm-2, further highlighting the potential importance of alongshore flows. Friction coefficient estimates, assuming a quadratic drag law, showed no statistical difference between onshore- and offshore-directed motion, with typical values of 0.023 +/- 0.013 (mean +/- SD). Sheet flow concentrations exceeded the maximum measured suspended sediment concentrations of approximately 400kgm-3. Sediment loads in the sheet layer are up to 10 times larger than the sediment loads in the lower suspension layer. Simplified sheet flow sediment transport estimates are 3.6 times larger on average than those in the suspension layer. The latter two findings indicate the importance of sheet flow processes in the swash zone that are generally ignored. (C) 2014 American Society of Civil Engineers.