Formation and evolution of multiple intertidal bars on macrotidal beaches: application of a morphodynamic model

Abstract

A numerical model is presented to simulate the formation and evolution of intertidal bar morphology (‘ridges and runnels’). Cross-shore sediment transport is modeled using a sediment transport shape function, which is a mathematical formulation that expresses the (volumetric) sediment transport rate as a function of cross-shore location. For energetic wave conditions, the shape function predicts offshore sediment transport in the surf zone and onshore transport outside the surf zone. For calm conditions, the shape function predicts onshore transport in the surf zone and zero transport outside the surf zone. The model is forced by measured wave/tide conditions and the time-integrated cross-shore sediment transport rate is obtained by advecting the shape function up and down the beach with changing tide levels. Morphodynamic feedback is included in the model. Specifically, the occurrence of relatively flat beach sections (troughs) leads to the subdivision of the beach profile into distinct cross-shore sediment transport cells, each forced by its own wave breaking conditions. Surf zone sediment transport in the troughs is considered zero and the troughs thus effectively serve as sediment transport barriers. According to the model and in agreement with field observations, intertidal bars form and migrate onshore under the influence of low-wave conditions, while offshore bar migration and flattening of the morphology occurs under high-wave conditions. The model supports the contention that ridge and runnel topography has a surf zone origin, but the ridges are not breakpoint bars in sensu stricto.