The behaviour of nearshore bars on the time scale of years: a conceptual model

Abstract

Abstract Soundings of the sandy Dutch coast have demonstrated the presence of a multiple bar system that exhibits cyclic, offshore-directed migration on the time scale of years (medium-term). A cycle comprises bar generation, interannual offshore migration and decay. In this paper we propose a conceptual model in which we qualitatively outline the relationships between short-term (hourly) process knowledge (waves, currents, sediment transport), based on a 15-week data set of near-bed hydrodynamics obtained in the multiple bar system of Terschelling (the Netherlands), and medium-term bar behaviour. The short-term process knowledge is aggregated to the medium-term by combining probabilistic characterisations of medium-term wave conditions and sediment transports estimated from the short-term field measurements. Essential to the conceptual model is the observed cross-shore change in short-term conditions that dominate medium-term onshore and offshore sediment transport (〈q〉on and 〈q〉off, respectively). In the cross-shore direction, the conditions contributing most to 〈q〉on change from predominantly breaking situations outside the bar zone to mainly non-breaking conditions on the beach. In contrast, 〈q〉off remains restricted to breaking conditions over the entire profile. In the model we qualitatively describe how this cross-shore change in the forcing conditions results in interannual offshore bar migration, bar decay and the onset of the next cycle. As such, our model may serve as a first step towards a more coherent, quantitative model on medium-term periodic bar behaviour.