Predicting seiche hazard for coastal harbours along the northern and western coasts of France

Long-period waves propagating inside harbours can lead to the generation of seiche that can affect and significantly disrupt port operations. This study is based on the analysis of a multi-years tide gauge database provided by the sea-level observation network (RONIM). It aims to propose simple models to compute seiche amplitudes at a series of French harbours. The study sites, located along the Atlantic coast, English Channel and North Sea, are representative of the diversity of French harbour configurations and are exposed to different wave regimes. First, the resonant periods of each site are determined by combining analysis of measured water levels and solutions of a linearised mild-slope equation model. Then, the role of incoming bound waves on the generation of seiches is investigated using the solutions of the WAVEWATCH III spectral wave model to compute low frequency spectra using the (Hasselmann in J Fluid Mech 12(04): 481-500, 1962) nonlinear theory. The study shows strong correlation between incoming bound waves and low frequency harbour oscillations for small ports and inside harbour waterway. For a series of sites, empirical formulations based on offshore bulk wave parameters (Bowers in Edge (ed) Proceedings of 23rd international conference on Coastal Engineering, Venice, Italy, 1992) are proposed to predict the long wave significant heights inside the harbour basins generated by short waves. Comparisons with observations show a fairly good agreement. This result suggests that seiche events can be reasonably predicted using such formulations forced by a spectral wave model as WAVEWATCH III. In addition, this study highlights the value of maintaining long-term water-level observation systems to develop operational forecasting system and improve harbour operation management.