A review of practical models of sand transport in the swash zone

Abstract

The swash zone largely influences nearshore hydrodynamics and morphodynamics through dissipating or reflecting wave energy and controlling whether sediment will be stored on the upper beach or returned to the inner surf zone. It is a region where active beach accretion and erosion occur and beach protection measures such as sand nourishments are often placed. Hence, proper prediction of swash zone beach evolution is required to evaluate beach management scenarios. This paper describes the advances related to swash zone sand transport processes and morphodynamics. We discuss the effects of a variety of physical processes and factors (e.g. bore turbulence, pre-suspended sediment advection, wave-swash interactions, infragravity waves, in−/exfiltration, pressure gradient and bed slope) on sand transport in the swash zone. We then focus on practical models of swash zone sand transport which are appropriate for predicting longer term (days to years) beach evolutions. Three types of practical models, i.e. empirical sand transport formulae, sand transport distribution methods and equilibrium models, are identified. The strengths and limitations of these practical models are discussed. The empirical sand transport formulae include the intra-swash formulae and swash-averaged formulae. The intra-swash formulae are more physics-based and can take physical processes into account more explicitly. However, upscaling of them for modelling sand transport and morphological changes over tidal cycles or longer term is problematic due to the difficulty in obtaining reliable and accurate instantaneous swash hydrodynamics(e.g. flow velocities) and due to the error propagation. Swash-averaged formulae can be more suitable for predicting longer-term morphological changes while they still require better parameterisations of important physical processes (e.g. wave-swash interactions). Sand transport distribution methods generally work reasonably well for beach erosion under energetic wave conditions whereas they have the inability to predict the beach recovery under mild wave conditions. Equilibrium models show a potential for predicting the beach evolution under both erosive and accretive conditions well. The equilibrium slope appears to be an essential factor that largely determines the performance of the equilibrium models. This equilibrium slope should depend on wave conditions and sediment characteristics, and a quantitative relationship between them needs further research in order to make the equilibrium models more predictive.