SAVANNAH, Ga. – After four months of data collection, the Engineer Research and Development Center completed the study for the U.S. Army Corps of Engineers, Savannah District, gathering enough facts to provide Tybee Island, Ga., beachgoers with six options to avoid harm from waves the large container ships produce when traveling to and from the port.
This study was accomplished through the Corps Planning Assistance to States Program and was cost-shared 50/50 between the city of Tybee Island and the Corps. Tybee city officials and citizens expressed concerns about increasing vessel traffic and size causing hazardous wakes on the island’s North Beach. Since the completion of the channel deepening for the Savannah Harbor Expansion Project, larger, super-sized vessels are now able to use the channel.
The study evaluates bathymetric data, high-frequency wave and vessel wake measurements, and broadcast vessel identification with the goal of providing a quantitative characterization of vessel wake conditions at Tybee Island. Data from 1,386 cargo vessel passages and 202 tanker passages indicate that vessel dimensions (length and beam) are positively correlated with drawdown magnitude and secondary wake height, although large vessels do not consistently generate large wakes.
“Nearly four months of vessel transits and simultaneous water level measurements were analyzed,” said Jared Lopes, Savannah District Planning Branch water resources planner. “They were looking for links between vessel operations and large wave uprush on the northern shore of Tybee Island.”
Commercial vessels transiting the Savannah entrance channel intermittently generate large wake events at Tybee Island causing erosion and a potential hazard for beachgoers. However, it was determined not all commercial vessels generate large wakes, and the relationship between vessel dimensions, operating conditions, wake height, and drawdown magnitude is unclear. Container ships, which tended to travel faster than tankers, corresponded to the largest wakes in the dataset.
“This study quantitatively explored the relationship between the waves generated by commercial vessels and of vessel dimensions, operating conditions, wake height, and drawdown magnitude,” Lopes went on to explain. “Study findings show vessel dimensions (length and beam), and higher speeds are positively correlated with drawdown magnitude and secondary wake height. The results provide an important baseline for implementing mitigation factors to reduce ship wakes.”
A further theory is the tidally modulated energy dissipation may favor smaller vessel wake uprush at low tide and larger uprush at high tide, according to the study, but this idea cannot be confirmed without additional measurements. Click here for the full report and the six strategy options mentioned in the first paragraph. Implementing many of the strategies requires coordination among various regulatory agencies.