With an economic impact of $122 billion in 2019, the Port of Savannah is one of the nation’s largest hubs for maritime trade. It is also undergoing impressive growth. Just as the Savannah Harbor Expansion Project was completed early this year, the Georgia Ports Authority announced plans to increase its capacity by 60%. With this growth comes opportunities to ensure the sustainability of the coastal environment in which the port operates.
As a Ph.D. student in Ocean Science & Engineering at Georgia Tech, I have spent several years studying cargo ship wake and quantifying its role in erosion along the shipping channel in the Savannah River with my research advisors Kevin Haas and Donald Webster. Now, as a Sea Grant Research Trainee mentored by Clark Alexander at the UGA Skidaway Institute of Oceanography, I am investigating how far the effects of cargo ship wake extend beyond the shipping channel.
First, let me describe the wake – it’s unusual. When most people think of wake, they picture a wedge of wave crests like you observe behind a paddling duck. However, I am talking about an entirely different phenomenon that creates a mini tsunami effect on the beach. It is called “Bernoulli wake,” and it is produced when large ships travel in narrow channels. In this scenario, the ship creates a big dip in the water surface surrounding it, similar to the one you create when lying on a mattress. The dip decays outwards from the ship but then deepens again when it reaches shallow water on the edges of the channel; this is where it behaves like a tsunami.
Alexandra Muscalus (left) and Kevin Haas (right) prepare instruments and their frames for deployment on the R/V Sandpiper. Photo by Fran Lapolla
If you stood on the beach as a cargo ship passed by, water would slowly recede for about two minutes as the dip passed over you, dropping the water level as much as one and a half meters. Then, the water would surge back up, exceeding its original level by up to half a meter. Depending on the channel shape, this surge could be a steady rise in water level or a churning wall of water racing along the shore, increasing the water level around you instantaneously.
In either form, Bernoulli wake contains massive “hydrodynamic energy,” the energy of waves and currents, which causes coastal erosion. At our field site directly along the Savannah River shipping channel near Bird and Long Islands, where erosion is especially severe, we found that the Bernoulli wake accounts for 70% of the total hydrodynamic energy – more than we expected.
This was not the only surprise. We expected that the backside of Bird and Long Islands, facing the quieter South Channel of the Savannah River and undergoing slower erosion, was shielded from the effects of cargo ships. However, we discovered that Bernoulli wake accounts for a similarly large portion of energy there, too. This implies that Bernoulli wake – and its effects – can propagate away from the shipping channel like water ripples spreading from a dropped stone.
This motivated my traineeship work: identifying the extent and significance of cargo ship wake that propagates into other waterways. I am thrilled to share that the new data from my traineeship is in hand. After a four-month delay due to channel dredging, this February we successfully deployed instruments measuring wake for five days at 15 sites. The sites were located in several waterways near the intersection of the Savannah River and the Intracoastal Waterway, which we suspect is the main entry point for propagating wake.
Muscalus deploys an instrument used to measure ship wake in the Intracoastal Waterway.
While much analysis is still in the works, there are already two key results. First, cargo ship wake reached every single instrument, demonstrating that it can propagate through at least 10 kilometers of smaller waterways. Second, most locations experienced multiple rounds of wake from a single ship passage. This shows that wake propagates into the waterway network from additional entry points in the Savannah River, all smaller and more distant than the Intracoastal Waterway. Whether or not it is significant in those waterways will be revealed in my dissertation – stay tuned.
As I finish up my research traineeship and my Ph.D., my goal is to provide information about Bernoulli wake to support scientists, engineers and policymakers seeking sustainability for coasts both in Georgia, and worldwide; my measurements come from Georgia, but the physics are universal. Protecting the coast, and its ecological and economic value, is a challenging problem, but approaching it with extensive knowledge will greatly improve the chances of success. I am grateful that the Georgia Sea Grant Traineeship has allowed me to contribute important information about the ship wake problem and a platform to share it with others.