Numerical Simulation of Wind Waves Induced Oscillations in Harbours of Arbitrary Shape

It is well known that the periods of the natural modes of the inner free surface oscillations of a harbour are determined thoroughly by its plan shape. The amplitude of the induced standing or partially standing waves can be large when the outer sea spectrum shows non negligible energy at those frequencies. Depending on the relative size, the motions of the vessels in the harbour, ships or pleasure boats, can be large too, in particular roll, surge and yaw, so that up-downloading operations must be interrupted or small vessels must abandon their mooring for safety reasons. Thus the design of harbour or a marina takes on a great importance from an economic and safety point of view. In this paper we investigate the so-called harbour oscillations induced by offshore waves by means of a time domain approach. The method proposed here is able to handle an arbitrarily shaped plan view and variable water depth, so that reflection, diffraction and refraction effects can be taken into account. The mathematical model and the numerical scheme employed are presented in synthesis. The preliminary test cases regard 2 harbours with simple rectangular plan shape. Afterwards the method is applied to the harbour of Trieste (Italy-North Adriatic Sea) with regular waves coming from 270° to 330°. The effects on the wave pattern induced by the lengthening of a large pier used to berth modern cruise ships are investigated too.