Sediment Resuspension by Ship Wakes in the Venice Lagoon

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Observations of Bernoulli wakes were recorded and analyzed for 22 passing ships in the industrial Malamocco-Marghera shipping channel in the Venice Lagoon. The wakes were characterized as a progressive N-wave with a dominant, leading trough preceded and followed by small crests. As the wake traveled, water transport under the trough was initially opposite the direction of wake propagation, sending resuspended sediment back toward the channel. Water velocities were documented to reach more than 2ms"' under the trough, and suspended sediment concentrations exceeded 380 mg L~'. Sediment loads were successfully predicted from measurements of current speeds, settling speeds, and sediment flux determined as a function of bottom shear stress. Resuspension rates reached several thousand milligrams per square meter per second. The size of the wake, and the amount of resuspension it can cause, might be parameterized by a nonlinear combination of a blocking coefficient S and a depth-based Froude number F; both parameters depend only on the channel geometry and ship's dimensions and speed. In this case, both the depth of the trough and the maximum suspended sediment concentration depended on S^^P^'^.