Depression waves generated by large ships in the Venice Lagoon

At present cruise ships use the Lido and Giudecca channels to berth at the Port of Venice, and cargo vessels use the Malamocco-Marghera channel to berth at the Marghera industrial area. Following a government decree to restrict large ship access to the Port of Venice through the Lido and Giudecca channels, the Port authority has been investigating the construction of a new channel between Marghera and the cruise terminal, to re-route all large vessels through the Malamocco lagoon entrance. Ships in the Venice Lagoon travel at low speeds with associated low depth-based Froude numbers (<0.5), and classical Kelvin wave effects are minimal. However, under certain channel geometries, depression waves (also known as Bernoulli wake) can be substantial with significant far-field effects. In the Malamocco-Marghera Channel, depression waves up to 2.5 m, with high (~1.5 m/s) near bed water speeds, transform into asymmetric Riemann waves, able to propagate large distances from the navigation channel into the shallow lagoon. By contrast, depression waves associated with large ships in the Lido and Giudecca channels are much smaller (~0.3-0.4 m). Rerouting large cruise ships entering the Port of Venice through the Malamocco- Marghera channel and a newly constructed channel will have adverse effects on the Venice lagoon with increased water velocities and sediment resuspension associated with the nonlinear Riemann waves. It is therefore essential that the potential effects be thoroughly evaluated using an appropriate methodology before the development of a new lagoon channel.