Linear response of the gate system for protection of the Venice Lagoon. Note II: Excitation of transverse suhharmonic modes

— We show that the transverse subharmonic modes characterizing the free oscillations of the gate system proposed to defend the Venice Lagoon from the phenomenon of high water (see Note I[l]) can be excited when the gate system is forced by plane monochromatic waves orthogonal to the gates with the typical characteristics of large amplitude waves in the Adriatic sea close to the lagoon inlets. A linear stability analysis of the coupled motion of the system sea-gates-lagoon reveals that for typical values of the parameters of the problem modes 4, 5 and 6 may be unstable. The need for a nonlinear analysis of mode competion is pointed out.