A system of mobile gates has been designed to protect Venice from flooding. Their foundations are made of massive concrete structures and will hold three gates each. The phasing of the construction work requires the analysis of the dynamic forces acting on the caissons due to wave propagation along the channels connecting the Venice Lagoon to the Adriatic Sea. In the first phase of the work the caissons will be sunk to the bottom of an excavated ditch where a system of pedestals will be used to correctly position them and hold until a concrete mix will fill the gaps between the structure walls and the ditch boundaries. During this phase upon an occurrence af a storm a wave oscillatory induced oscillatory flow will occur in the gaps. Here we propose an analytical model to evaluate the dynamic actions due to wave propagation. Firstly the unit response spectra for horizontal and vertical forces are presented, then linear system theory is employed to evaluate the dynamic actions due to an irregular wave train propagating along the canals.