Vessel-Generated Edge Waves Along Commercial Shipping Channels

Abstract:

Shallow water field observations in the Savannah River, Georgia, USA revealed that the wake produced by commercial shipping vessels is the dominant source of energy at the shoreline and plays a significant role in shoreline erosion. Low-frequency waves, with periods of 2-3 minutes, were the most energetic component of the wake system produced by container ships and were characterized in shallow water by a water level drawdown, or “Bernoulli Depression” in the vicinity of the vessel, followed by continued water level oscillations after the vessel passed. The Bernoulli Depression is a well-known phenomenon generated by large vessels in constricted channels, but the oscillations that follow, or “trailing waves,” have not been studied as extensively. They have been previously assumed to be cross-channel seiching, but the field measurements revealed alongshore progressive behavior, which contradicts the standing wave characteristic of seiching. To examine the trailing waves in greater detail, a suite of numerical model simulations were conducted in FUNWAVE-TVD, a 2D Boussinesq model, for large vessels traveling parallel to a plane sloping beach. The simulations produced trailing waves behind the vessel that evolved in the shallow regions near the shoreline and propagated with the speed of the ship. Using the dispersion relationship, it was confirmed that these were edge waves.

The model results also revealed that multiple modes were present in the edge wave system, and the relative amplitude of each mode, or “modal amplitude,” was dependent upon the ship speed, with faster ships exciting higher modes. These observations bear resemblance to pressure-generated edge waves, which were discovered when the moving pressure disturbances of offshore hurricanes excited edge waves on the continental shelf of the eastern United States. Despite the difference in scale, the moving Bernoulli depression of large vessels may serve as an offshore pressure source which excites edge waves of various modes along shipping channels. These edge waves are likely to have significant impacts on the sedimentation and ecology along shipping channels.