Alexandra Muscalus

Cargo ship wake can be a strong and frequent source of energy along shorelines in the vicinity of commercial shipping channels. Large vessels in narrow channels produce low-frequency wake that consists of a positive front wave at the bow of the vessel, a depression (drawdown) spanning the length of the vessel, and an upwards surge produced by the stern wave at the stern. This low-frequency wake is followed by high-frequency wedge-shaped Kelvin wake, as well as continued low-frequency oscillations that may persist in the channel more than 30 minutes after a vessel passage. Low-frequency cargo ship wake with wave heights up to 2.8 meters was identified as the dominant source of erosion at Bird/Long Islands in the Savannah River, Georgia, USA based on the analysis of one month of Aquadopp measurements that captured over 300 large vessel wake events. The islands divide the river into the Main Channel, which contains the shipping channel, and the South Channel, a shallow, sheltered secondary channel trafficked only by small craft. Cargo ship wake was the primary energy source in both channels. The dominance of cargo ship wake energy in the sheltered South Channel demonstrated that the effects of wake extend beyond the shipping channel shorelines. This finding motivated three follow-up studies to quantify the extent and significance of cargo ship wake in the far-field, secondary channels connected directly or indirectly to the shipping channel.

Low-frequency (LF) cargo ship wake is a major source of hydrodynamic energy in shipping channels, where it has increasingly been linked to shoreline erosion. The LF wake consists of a drawdown called the “Bernoulli depression,” a return surge, and finally a series of trailing waves that may persist longer than 30 minutes after the vessel passage. While the Bernoulli depression and surge are well-explained by conservation principles, we presently lack a robust explanation of the trailing waves. They are a ubiquitous feature observed at many locations (e.g. Garrel, Lopez, and Collins, 2008), and have frequently been attributed to cross-channel seiching; however, field measurements from the Savannah River, GA, USA suggested that some of the waves may have alongshore progressive characteristics, which contradicts this assumption.