Depression Waves Generated by Large Ships in the Venice Lagoon

Abstract

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 MalamoccoMarghera 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.

Fast shoreline erosion induced by ship wakes in a coastal lagoon: Field evidence and remote sensing analysis

“…This channel has been recently the subject of discussions as the local port authority is planning to create a new channel connecting the MMC to the passenger terminal in Venice, to redirect cruise ships to the MMC after the central government put a ban on the transit of vessels above 40,000 tons in the historic center of Venice. The proposal to reroute cruise ships from the Lido inlet to the MMC will avoid the historic center of Venice, but will increase the number of large ships in transit in the MMC with S almost twice that of cargo ships currently using the channel [ 40 ]. It is likely that implementation of this solution will result in higher impacts to the lagoon areas in the vicinity of the channel and rise maintenance costs.…”

The effects of ship wakes in the Venice Lagoon and implications for the sustainability of shipping in coastal waters

“…The shape of the depression generated by the steadily sailing ship in such a channel, theoretically, should be a gradually lengthening and almost symmetric “valley” with relatively steep front and end that are separated by more or less constant-level trough 49,50 . This theoretical shape is specific to a weakly nonlinear framework 48,49 and is apparently not created within the MMC as our data show that depression wakes on the channel margin are slightly asymmetric V-like features 26 . The further evolution of such waves is dictated by the properties of nonlinear shallow-water equations 56 .…”

“…tidal flats or water bodies similar to the Venice Lagoon) where the Bernoulli wake may excite strongly nonlinear long-living solitary waves of depression 25 . These waves penetrate to a distance of more than 1 km onto the sub-tidal flat 26 . They may undergo various transformations as they propagate, including shoaling-type effects 27 and formation of bore-like phenomena 27 .…”

“…This not only jeopardizes the safety of smaller vessels in the vicinity but may also seriously damage the coastal environment 23,24 and impact the integrity of the bottom 12 . Depressions originating in the channels of the Venice Lagoon may transform into strongly nonlinear deep solitary waves of depression 25,26 that propagate over a long distance across the lagoon 27 and resuspend large amounts of sediment along their way 28,29 . In addition, the human-induced components of hydrodynamic activity and associated enhanced erosion may affect not only the integrity of the sea bottom but also (pre)historic cultural layers 30 .…”

“…The most significant causal factors are strongly nonlinear local depressions created by vessels in transit 28 . These can reach amplitudes of 2.5 m in the MMC channel 26 and after shoaling on the channel side banks, propagate over a large distance on the sub-tidal flats, causing significant sediment resuspension 29 from the shallow parts of the lagoon and fast erosion of channel sides and shorelines 43 .…”