Longshore transport estimation and inter-annual variability at a high-energy dissipative beach: St. Trojan beach, SW Oléron Island, France

Abstract

This study investigates the annual and inter-annual longshore transport at a high-energy dissipative beach (St. Trojan, SW Oléron Island, France), over the period 1997–2006. This study is divided in two parts: (1) a short-term study, based on field measurements carried out during, low, moderate and high-energy swell conditions, and permitting to calibrate a wave propagation model and both empirical longshore sediment transport formulas and a hydro-sedimentary numerical model; and (2) a long-term study, combining global optimization methods using WAVEWATCH III (WW3) model outputs and wave numerical modelling to define the nearshore wave climate and then to compute the annual and inter-annual longshore transport. A good agreement between the different methods was obtained and values for the annual longshore transport ranged from about 50,000±20,000 m 3/yr to 140,000±30,000 m 3/yr. The net annual longshore transport displayed an inter-annual variability, whereas values were always 3–10 times less than those proposed in previous studies. Such differences point out the necessity of calibrating empirical transport formulas and computing accurately the nearshore wave climate. The relatively low values for the net annual longshore transport were explained by a weak contribution of the most energetic swells, which systematically approach the shore with a frontal obliquity and also generate episodes of reverse transport. The methodology itself was also discussed, pointing out the limitations of this study but also the advantages of using WW3 data and simplified wave climates.