Numerical modelling of coastal seiches in Malta

Abstract

Long period waves with periodicities ranging from a few minutes to several tens of minutes are observed throughout the year on the northern coastal area of the Maltese Islands. These coastal seiches, locally known as ‘ milghuba’, closely resemble tsunamis in their frequencies and behaviour, but result predominantly from meteorological forcing rather than seismic events. These waves are present uninterrupted on the sea level trace, and during ‘quiet’ periods they characterise the ‘background’ sea level oscillations. Occasionally the ‘ milghuba’ intensifies to large amplitude fluctuations reaching a range of up to 1 m in extent. The generation and propagation of seiche oscillations that occur in two adjacent wide-mouthed embayments on the northern coast of Malta is investigated by numerical simulations using a 2D version of the free surface non-linear Princeton Ocean Model. The study is targeted to investigate the mechanism by which the coastal sea level responds to signals generated in the open sea. In particular, the numerical experiments permit a full characterization of the dynamics of the seiches in the two coupled embayments and their forcing by the open sea long wave field. The simulations confirm how the topographic features specific to these basins have a predominant role in the transformation and amplification of long waves near the coast. In the case of the wide-mouthed embayments under study, the two-way coupling with the open sea area appears to be most important, and triggers a greater variability in the basin response. The intensity and spectral character of the sea level oscillations inside the embayments are found to be greatly dependent on the nature and incidence of the long period waves in the open sea. Yet the seiche oscillations are still dominated by the fundamental and the first bay modes. Standing oscillations generated along the cross-shore direction on the shelf (leaky waves) are more effective in forcing the extreme oscillations in the embayments rather than edge waves propagating alongshore. The results show the effectiveness of the Princeton Ocean Model in describing the response functions of water level in the embayments to an offshore disturbance, and can describe the associated water currents.