A simple model of beach foreshore response to long-period waves

Abstract

A field experiment and computer model have been designed and carried out to test the hypothesis that infragravity and lower-frequency waves influence patterns of erosion and deposition on the beach foreshore. The field data show coherent fluctuations in the foreshore sediment level which can be related to low-frequency wave motions. The fluctuations have heights of up to 6 cm with typical time scales of 8–10 min. They can be characterized in two ways: by the progression of the fluctuation up the foreshore slope (landward), and by the decrease in the root-mean-square (RMS) height of the fluctuations as they progress landward. Analysis of run-up time series obtained by time-lapse photography concurrent with the sediment level measurements reveals that long-period waves of undetermined origin are positively correlated with the sediment level fluctuations. This suggests that the waves are responsible for forcing the initiation of sediment level fluctuations. In order to better understand the characteristics of these sediment level fluctuations, a numerical model of elevation changes on the foreshore was developed. Gradients in sediment transport define erosional and depositional areas on the foreshore, hence will determine the response to an initial beach slope perturbation. The results show that any perturbation (a forced departure from equilibrium) in foreshore topography will progress landward while decreasing in amplitude, thereby matching the field observations. Conceptual relationships between beach slope and the profile response clarified by this model are used to explain the initial formation of the perturbations in sediment level.