FREE AND FORCED-OSCILLATIONS OF A GATE SYSTEM AS PROPOSED FOR THE PROTECTION OF VENICE LAGOON - THE DISCRETE AND DISSIPATIVE MODEL

This paper describes the dynamics of a mathematical model for the mobi le barriers designed to close the tidal inlets of the Venice Lagoon an d to defend the city from recurrent high waters. The barriers consist of a large number of steel caissons (hereafter referred to as gates) c onnected to the seabed with hinges. The gates usually rest horizontall y on the sea bottom allowing mass exchange between the sea and the lag oon and are brought into operation during high tides by the inflow of compressed air. When in operation the gates close the lagoon openings even though they may oscillate around their mean position. In the pres ent contribution the barriers are modelled by vertical rigid plates wh ich can slide along the bottom and are subjected to a recoil effect si mulating Archimedes' force acting on the real gates. First, the in-pha se motion of the gates produced by an incoming wave is studied. Then, by means of a linear stability analysis of this basic motion, it is sh own that oscillations of the gates, such that contiguous gates oscilla te out of phase, can be excited for suitable values of the wave and ba rrier characteristics. In particular, the existence of a critical valu e of the amplitude of the incoming waves is pointed out, below which t he in-phase motion of the gates is stable. A quantitative comparison w ith previous studies of the problem which used a simple model is made.