Industrial Ships' Wake Propagation and Associated Sediment Resuspension in the Venice Lagoon

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Gelinas, Morgan Elise
The Graduate School, Stony Brook University: Stony Brook, NY.
Shallow water, breaking waves have been observed along the shoals east of the Malamocco-Marghera canal in the Venice Lagoon after the passage of large (>100 m length) industrial ships. These waves are characterized by a solitary, asymmetrical trough that increases in asymmetry as it propagates over the neighboring shoals. The waves create massive sediment resuspension events due to high water particle velocities. This resuspension is of ecological and economic concern in the Venice Lagoon due to the high level of sediment contamination from the nearby Porto Marghera industrial zone and the necessity for increased dredging, respectively. Water level, water velocity, and suspended particulate matter (SPM) were measured continuously for 9 days in July 2009 with 10 pressure sensors, 1 S4 electromagnetic current meter and an automatic water sampler. The pressure sensors were aligned in three offset transects perpendicular to the channel to obtain wave direction and extent of lateral energy. Ship dimensions and velocity were recorded using an Automatic Identification System (AIS). Wave characteristics were analyzed and correlated to ship parameters by using the ship depth-based Froude number, FrD, and blocking coefficient, S. Of the 22 recorded waves, the largest wake produced in this study was a trough displacement of 0.73 m, and the largest SPM concentrations were >400 mg/L, lasting for minutes after a ship passed. These recorded resuspension events were compared to predicted values using a relationship between sediment flux and boundary shear stress. These waves could be playing a role in the extreme erosion alongside the canal that has occurred over the past 30 years. The shape of these waves is particularly intriguing and empirically a double N-wave is used to model them as they propagate onto the shoals. This equation has been used to model tsunami propagation prior to this study.