The influence of bore turbulence on sediment transport in the swash and inner surf zones

The role of bore turbulence for sediment transport in the swash and inner surf zones is investigated using field measurements of (a) cross-shore velocity using an electromagnetic current meter at z=3 cm, (b) turbulent kinetic energy (TKE) and cross-shore velocity using a 3-dimensional acoustic Döppler velocimeter at z=7.5 cm, (c) water depth using a pressure transducer at z=0, and (d) suspended sediment concentration (SSC) using seven miniature optical backscatter sensors at z=1–32 cm, on a steep beach in high energy conditions (Hb≈1.6 m). High SSC and high TKE values were associated with the passing of the bore front while the near-bed flow was undergoing rapid onshore acceleration. Peak TKE values tended to occur whilst the near-bed velocity was still offshore. The SSC peaked only slightly later than the TKE and therefore no evidence exists to support advection of pre-suspended sediment. Correlation tests are performed to assess the predictive capability of (a) a Bagnold-type model based on shear stress, (b) a similar model based on TKE, and (c) a model combining (a) and (b). When the data from the whole time series are used, the combined model improves r2 values over the shear stress model by up to 9%. If only data within bore events are included, then the improvement increases to over 55%, suggesting a significant effect of TKE on sediment transport across the bore.