VICKSBURG, Miss. - Researchers from the U.S. Army Engineer Research and Development Center’s (ERDC) Coastal and Hydraulics Laboratory (CHL) partnered with the U.S. Army Corps of Engineers, Savannah District and the city of Tybee Island to measure vessel wakes near the island’s north shore in hopes of better understanding which ships and operating conditions are associated with generating large wakes.
Located just 18 miles east of Savannah, Georgia, Tybee Island is a popular vacation spot and tourist destination. The island boasts of five distinct beaches, clean, clear water and includes activities from soaking up the sun to water sports. However, commercial vessels transiting the Savannah entrance channel intermittently generate large wake events on the North Beach, creating a potential hazard for beachgoers.
“The navigation channel for Savannah passes just north of Tybee Island,” said Dr. Richard Styles, a research oceanographer with CHL. “When ships pass by the island, they sometimes generate a large wake. If you’re a beachgoer, and you don’t know about this phenomenon, it can knock you down — it’s hazardous.”
These wake events have been going on for decades and even prompted city officials to post warning signs along the beach.
“The problem is not every ship that comes by generates a wake,” said Styles.
“That’s what stood out to me the most,” said Dr. Rachel Bain, a research physical scientist with CHL. “If you look at it on a superficial level, there’s really no rhyme or reason to which ships seem to generate the large wakes. There really doesn’t seem to be any pattern — at least at a cursory glance.”
“It’s a tricky problem,” she continued. “The city wants some type of warning system for beachgoers, but you don’t want a situation where the lifeguards are like the boy who cries wolf where they evacuate the beach and then nothing happens.”
That’s why city officials and the Savannah District were interested in commissioning a study, and they reached out to the team at ERDC for assistance.
“They found out that we have been involved in other ship wake studies in other areas,” said Styles. “We’ve developed quite a bit of expertise in this, so they reached out to us.”
The team spent four months deploying their instrumentation and collecting data from the site. They used sensors to measure the wake height, obtained broadcast vessel information — which is a U.S. Coast Guard system deployed on every commercial ship that provides the type of ship, how big it is, how fast it’s going and if it is inbound or outbound — and in doing so, they were able to correlate each ship with the wake that it generated while trying to determine what combination of vessel operations, such as size, speed, and environmental conditions like waves, currents and tides, generated the largest wakes.
“It was a rewarding experience being able to work closely with Dr. Bain and Dr. Styles from ERDC and the City of Tybee Island on this study,” said Jared Lopes, water resources planner for the Savannah District. “I was eager to present the final briefing at the Tybee City Council meeting, as the results of this study hold ample potential for helping mitigate the impacts of ship wake on the northern shore of Tybee Island. Despite the fact that wakes generated by commercial vessels tend to be larger than those created by wind, wind waves constitute a more considerable source of continuous energy for moving sediment in the area. This is because large vessels pass the beach infrequently, an average of 12 passages per day, whereas wind waves break on the beach continuously.”
“This is the most comprehensive study to look at this effect on Tybee Island,” said Styles. “We’ve measured longer than anyone else, and we’ve generated more data than has been generated in the past.”
However, even with the massive amount of data collected, the team was still left with some uncertainties.
“We did see that there are correlations between the length of the ship and the drawdown followed by the uprush on the beach as well as the width of the ship and how much water is impacting the beach,” said Bain. “But neither of those variables guarantees a large event. So, you could still have two identical ships, and one would generate a very large event and the next nothing.”
“It looks like the largest ships — the longest and widest — but also the fastest ones tend to generate the largest wake,” said Styles. “However, we cannot unequivocally establish a pattern, but the information we gained is now leading to another planned study.”
“This project really illustrates the value of this type of field study,” said Bain. “Based on mathematical theory, these waves would dissipate by the time they reach the beach. But obviously, we’re seeing these huge events which have the potential to be really impactful for the local community. These field studies are incredibly valuable because they tell us that our observed reality doesn’t always match up with the theory and those are the times where we need to adjust our understanding and improve our predictive ability in those areas.”
The team was able to provide potential mitigation efforts that are currently being considered by the city and the district.
“Our study lists a number of strategies for potentially reducing wave heights,” said Styles. “One is to coordinate with the U.S. Coast Guard and have the ships reduce their maximum speed. We also discussed building a breakwater or refurbishing and extending the jetty. We talked about realigning or modifying the channel, so the wave energy isn’t focused on the North Shore or developing a more advanced warning system. All of those things can be very expensive, so right now, it’s still in the discussion phase.”