Cross-shore sediment transport processes on natural beaches and their relation to sand bar migration patterns
During the last two decades several field studies have shown a clear pattern in the cross-shore sediment transport processes on beaches. Outside the surf zone, the stronger onshore velocities under unbroken (Stokes-type) wave crests, produce a dominant onshore sediment transport. Inside the surf zone, strong offshore-directed mean currents (undertow) drive sediments offshore. It is of great interest for the scientific community to verify further the consistency of this pattern under different morphodynamic conditions, understand the underlying physics and quantify/parameterise this behaviour in order to improve the understanding of cross-shore sediment transport and simplify the modelling of beach profile change. The present investigation addresses this niche by i) analysing cross-shore sediment transport processes with field data spanning the swash, surf and shoaling zones, ii) quantifying (parameterising) the cross-shore structure of such processes, and iii) incorporating the sediment transport parameterisation (shape function) into a model of bar generation and migration. To achieve this, concurrent measurements of velocity, surface elevation and suspended sediment concentration (SSC) were obtained with electromagnetic current meters (EMCM), pressure transducers (PT), and optical backscatter sensors (OBS) on five different beaches across Europe under a wide range of morphodynamic conditions. Results show that the normalised (by the local energy level) net cross-shore sediment transport, expressed as moments of the velocity field (energetics approach), has a remarkably coherent structure across-shore (shape function, SF) in all the data sets. The pattern consists of net onshore transport in the swash zone, offshore transport inside the surf zone, and onshore transport outside the surf zone with a convergence of sediment around the breaking point and a divergence in the inner surf/swash zone. This behaviour is a product of the balance between multiple opposing mechanisms, and a few of them describe the overall pattern, namely short wave skewness outside the surf zone (onshore transport), and the combined effect of undertow and wave stirring at short and long frequencies inside the surf zone (offshore transport). The velocity moments SF represents the cross-shore distribution of the cross-shore sediment transport processes and it is observed to compare well (linear correlation of 0.61) with the cross-shore structure of the measured sediment fluxes. The shapef unction was incorporatedin to a time dependenmt odel of beachp rofile changew ith the aim of reproducing bar migration patterns as observed in the field (Gallagher et al., 1998). The SF-based profile model comprises a simple wave transformation routine that accounts for linear shoaling and assumes a saturation law for wave decay inside the surf zone. An energetics approach (Bailard, 1981) is then used to calculate sediment fluxes with the third and fourth velocity moments parameterised via shape functions. Profile change is calculated by solving numerically the mass conservation equation. When the SF model is forced with measured offshore wave conditions and an initial beach profile, the model can successfully predict bar generation and migration (R2 = 0.86) over 77 days as observed at Duck, North Carolina, a microtidal beach unrelated to the development of the SF. This includes events of bar migration offshore, onshore or no net movement (stable bar). These results show that the convergence of sediment at the breakpoint (breakpoint hypothesis) combined with the morphological feedback can successfully explain the generation and evolution of shore parallel bars over months. The model cannot replicate the whole profile shape, but it is able to produce realistic bar behaviour such as net offshore movement of sandbars, generation close to the shore, volume growth as they travel offshore, bar amplitude decay when continuously subjected to an unbroken wave regime, onshore bar migration, and the subdued morphology of macrotidal beaches.