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Hidden Flow Patterns In Coastal Waters Studied
June 29, 2020

Hundreds of people die at sea every year as a result of vessel and airplane accidents. The U.S. Coast Guard performs thousands of search and rescue (SAR) missions at sea, some in perilous conditions, in an attempt to save lives. In such dire circumstances, where every minute matters, a key challenge in reducing the number of fatalities is to SAR algorithms more efficient. For success by first responders, it is critical to have the most efficient ocean modeling data and algorithms at hand.

Now, based on information provided by Virginia Tech, researchers there have joined a multi-institutional group, funded by a $2.8 million National Science Foundation grant, that are using mathematical techniques with ocean models and experiments to better understand near-surface flow patterns and hidden flow structures. Leading the published multi-institutional study is the School of Engineering and Applied Sciences at Harvard University. Joining Harvard and Virginia Tech are investigators from Massachusetts Institute of Technology, UCLA, Woods Hole Oceanographic Institution, the U.S. Coast Guard Office of Search and Rescue, and ETH Zurich. With more accurate modeling data, response teams can better predict the search area grid from the air, and reduce emergency response time when lives are on the line.

Throughout this study, Search and rescue at sea aided by hidden flow structures, published recently in Nature Communications, the research team has uncovered hidden transient attracting profiles – or TRAPs – in ocean-surface velocity data. These transient attracting profiles act as short-term collection zones for all floating objects, debris as well as persons in the water. When incorporated into search and rescue algorithms, the locations of the TRAPs give a more accurate prediction on regions to focus search efforts.

“From the moment they are alerted that someone is lost, search and rescue teams use sophisticated software to try to pinpoint the last known location in the water, factor in how much time has passed, and make their best prediction on how far they have drifted,” explained Shane Ross, professor in the Kevin T. Crofton Department of Aerospace and Ocean Engineering. “By improving the modeling of drifting objects in unsteady currents, search teams will have more efficient probability computations that enable them to set a tighter search grid and make faster, safer rescues.”

Current flow models used in search and rescue operations factor in ocean dynamics, weather prediction, and in-situ observations, such as self-locating datum marker buoys deployed from air. According to the research team, even with high-resolution ocean models and improved weather prediction, search and rescue planning is still based on conventional practices, and rescuers rely on their hunches as much as sophisticated prediction tools.

Computational tools can predict how particles or objects are transported and reveal areas of the flow where drifting objects are likely to converge. In engineering terms, these patterns are called Lagrangian coherent structures. Unfortunately, calculating Lagrangian structures can often be time-consuming and computationally expensive.

For use in disaster response scenarios, transient attracting profiles are easily interpreted and can be computed and updated instantaneously from snapshots of ocean velocity data. This eliminates very expensive and timely computation, especially when short-time predictions are critically important in search and rescue. After six hours, the likelihood of rescuing people alive drops significantly.

In order to prove the predictive influence of transient attracting profiles in coastal waters ?— or identify the regions where objects or people are most likely to accumulate over a two- to three-hour period of time ?— the research team conducted multiple field experiments off the coast of Martha’s Vineyard in Massachusetts.

Using both Coastal Ocean Dynamics Experiment drifters and 180-pound OSCAR Water Rescue Training manikins, targets were released around areas of predicted transient attracting profiles with GPS tracking devices that reported location every five minutes. Even without accounting for wind-drag or inertial effects, the researchers observed that the TRAPs invariably attracted the floating drifters and manikins in the water over a two- to three-hour period.

Identifying transient attracting profiles on ocean surface velocity data can also have significant impact on the containment of environmental disasters, such as catastrophic oil spills. TRAPs provide critical information for environmental hazard response teams and have the potential to limit the spread of toxic materials and reduce damaging impact on the surrounding ecological systems.

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